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200 POLYSACCHARIDES Vol. 11

POLYSACCHARIDES

Introduction

Polysaccharides are naturally occurring polymers in which the repeat unit consistsof monosaccharides linked through glycosidic linkages by a condensation type re-action; the example of cellulose is given in Figure 1. They exist in plants, animals,or microbial worlds where their roles as energy storage or structural materials oras source of biological activity are recognized. Many general books published canbe taken into consideration (1–13).

Monosaccharides are most readily obtained from natural sources; in thatrespect D-glucose plays a central role in the biochemistry of carbohydrates.

In addition, because of the presence of OH groups, natural polysaccharidesmay be modified by controlled chemical reaction to give derivatives with newspecific properties; industrial derivatives are mainly obtained from cellulose (qv),starch (qv), and chitin and chitosan (qv). The presence of these OH functionalgroups is also the origin of interaction with water molecules (hydrophilic characterof oligo- and polysaccharides) and intra- and interchain H-bond network formationplaying a role in reactivity control, swelling, or dissolution rate.

This article describes the structure of the principal polysaccharides from nat-ural sources, some methods for their characterization, and some physical proper-ties. Few derivatives are described and this article is extended to synthetic poly-mers having a sugar entity in the basic structure.

Carbohydrate Nomenclature

The international rules of carbohydrate nomenclature adopted by the Interna-tional Union of Pure and Applied Chemistry and the International Union ofBiochemistry have been published (14). A brief summary of the structural basisfor naming oligo- and polysaccharides is given in the following.

Structure of Monosaccharides. Monosaccharides (general formulaCn(H2O)n) are the basic constitutional units of oligo- or polysaccharides. Amonosaccharide is a polyhydroxycarbonyl compound classified as tetrose, pentose,hexose, etc, according to the number of carbon atoms in the molecule; a prefix in-dicates the nature of the carbonyl group which is an aldehyde or a ketone. Thus,aldohexose has six carbons and one end of the molecule (position 1) has an alde-hydic group with a reducing character. If the reducing group is a ketone in thesecond position, a six-carbon chain is called a ketohexose (Table 1) (9,10).

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Vol. 11 POLYSACCHARIDES 201

Fig. 1. Cellulosic chain formed from condensed D-glucose.

Fig. 2. The two tetrahedral representations of glyceraldehydes.

Configuration of Monosaccharides. Different representations wereproposed to show the structure of the monosaccharide. The original Fischer repre-sentation allows description of different structures and especially demonstratesthe chiral relation between monosaccharides, eg, that D-galactose is the C-4 epimerof D-glucose.

The chirality of monosaccharides is related to the presence of asymmet-ric carbon in the molecule; aldohexose contains four asymmetric carbons (seeTable 1) and consequently 16 stereoisomers can be described.

The first member of the polyhydroxycarbonyl series is the aldotriose glyc-eraldehyde which contains one asymmetric carbon, and thus two stereoisomers.Figure 2 shows the two different molecules, which are mirror images of each other,in the tetrahedral representation with the asymmetric carbon in the center. Theprojection on a plane of these molecules corresponds to the Fischer representations(Fig. 3). Initially, one of the forms was found to be dextrorotary (+) and was named

Table 1. Acyclic Forms of D-Aldoses and D-Ketosesa

Trioses Tetroses Pentoses Hexoses

Aldoses

Ketoses

aAsterisk (∗) indicates the presence of asymmetric carbons.


Fig. 3. Fisher projection of D- and L-glyceraldehyde.

D-glyceraldehyde; the other one was levorotary (−) and called L-glyceraldehyde.From the triose, in the Fischer representation, a series of stereoisomers aregenerated as indicated in Figure 4. The D-series refers to molecules in which the

OH on the last asymmetric carbon is on the right. The D- or L-identification doesnot mean that it is dextro (+) or levo (−) (Rosanoff convention). Physical investi-gation, later after Fischer, concluded that the D- and L-attribution represents theabsolute configuration as demonstrated for (+)-D-glyceraldehyde. The majority ofnatural monosaccharides involved in polymeric systems belongs to the D-series.

Cyclic Conformation of Monosaccharides. The crystalline form ofglucose was shown as cyclic (six-membered, oxygen-containing ring) from X-raydiffraction (15); infrared spectroscopy detects no carbonyl group, confirming a

Fig. 4. Acyclic forms of D-aldoses in their Fischer projections.


Fig. 5. Representation of the α- and β-cyclic anomers of D-glucopyranose.

Fig. 6. Haworth representation of the cyclic forms (pyranoid and furanoid) of D-glucose.

cyclic hemiacetal structure. In the hemiacetalic form, the carbonyl carbon changesfrom sp2 to sp3 hybridization and thus becomes chiral with two anomeric forms(α and β) on carbon C-1 as shown in Figure 5. Among common hexoses,six-membered (pyranose) and/or five-membered (furanose) rings can be formed.Haworth introduced more realistic pictures of the cyclic forms (Fig. 6). The repre-sentation ∼OH means that the two configurations α and β exist in equilibrium.For the D-series, the α-anomer has the hydroxyl at the anomeric center (position1) downwards in the Haworth representation; the β-anomer is upwards.

Considering the six-membered ring, different conformations can be distin-guished, taking as reference the cyclohexane. The favored conformations arechairs in the 4C1 and 1C4 conformations. The representation of α-D-glucose is

Fig. 7. Two chair conformations for α-D-glucose.


Fig. 8. The different representations for D- glucose in the α conformation for the cyclicform: (a) Fischer, (b) Haworth, (c) 4C1 chair corresponding to the energy minimum.

given in Figure 7; the 4C1 conformation, with all but the OH at C-1 in the equa-torial orientation, is preferred compared with the 1C4 conformation. In the α-D

geometry, the anomeric hydroxyl (on C-1) is axial but it is equatorial in the β-D

conformer.For D-aldopyranose in general, the conformation 4C1 is usually preferred.

A summary of the different representations of D-glucose and the most probableconformation of α-D-glucose are given in Figure 8.

Mutarotation in Solution. In solution, equilibrium exists between thelinear conformation and the two anomeric forms of pyranose and furanose cyclichemiacetal forms (Fig. 9); the percentage of each form as well as the ratio α/β ischaracteristic of the monosaccharide considered.

A few values of the percentage at equilibrium are given in Table 2. Becauseof this equilibrium the reducing nature of the sugar is maintained.

1H NMR in D2O allows characterization of the anomeric contents in equi-librium after a short time at 25◦C; an example is given in Figure 10: the H-1

Fig. 9. Mutarotation in solution: representation of the different species in equilibrium forD-glucose (see Table 2).


Table 2. Conformation Equilibrium for Some Monosaccharidesa

Sugar T◦C α-Pyran, % β-Pyran, % α-Furan, % β-Furan, % Acyclic, %

Arabinose 31 60 35.5 2.5 2 0.03Glucose 31 38 62 — 0.14 0.02Galactose 31 30 64 2.5 3.5 0.02aReference 10.

signal corresponding to the α-anomer is located at 5.12 ppm with a narrow doublet(J = 3.7 Hz) but the signal for the β-anomer is located 0.6 ppm upfield with a cou-pling constant J = 7.9 Hz. 13C NMR also allows identification of the equilibriumin solution; in addition, no C O signal is seen, confirming the cyclic configura-tion. The NMR characteristics given for D-glucose in water are recalled in Table 3(16,17).

From this example, it is shown that the coupling constants and the locationof the signals allow identification of the nature of the sugar and its configuration(16–19). The NMR technique is one of the most powerful tools for characterizingmonosaccharides but also for establishing the structure of polysaccharides. Theanomeric equilibrium only affects the reducing end of the chain; for a high molarmass, the spectrum is not perturbed by this effect and only one series of signalsappears.

Uronic Acid and Other Monosaccharides. Formation of a uronosiderequires the oxidation of the primary hydroxyl group of a hexopyranoside. Thistype of unit is very important in many natural polysaccharides such as algi-nates, pectins, or some hemicelluloses. Some important monosaccharides are rep-resented in Figure 11.

Some chemical or enzymic methods are recognized to allow the specific oxi-dation of primary hydroxyls in the C-6 position. The TEMPO method applied topolysaccharides was used to oxidize the N-acetylglucoamine unit in hyaluronan(20); galactose oxidase was used to modify the galactose side groups in galac-tomannan (21,22).

Table 3. NMR Spectrum Characteristics of D-Glucose in D2O

H-1 H-2 H-3 H-4 H-5 H-6 H-6′

δ (ppm)α 5.09 3.41 3.61 3.29 3.72 3.72 3.63βa 4.51 3.13 3.37 3.30 3.35 3.75 3.60

J (Hz)α 3.6 9.5 9.5 9.5 2.8 5.7, 12.8β 7.8 9.5 9.5 9.5 2.8 5.7, 12.8

C-1 C-2 C-3 C-4 C-5 C-6

δ (ppm)α 92.9 72.5 73.8 70.6 72.3 61.6βb 96.7 75.1 76.7 70.6 76.8 61.7

aMeasured at 400 MHz in D2O at 296 K relative to acetone (2.12 ppm) 16.bGiven in Table 1, Ref. 17.


Fig. 10. 1H and 13C NMR spectra for D-glucose after a short time in D2O at 25◦C at 400and 100 MHz respectively (low content of β-form).

Important monosaccharides are the amino sugars which have an aminogroup at any position other than anomeric carbon: 2-amino-2-deoxy-D-glucoseor D-glucosamine also called chitosamine is the monomeric unit constitutive ofchitosan, the polymer obtained by deacetylation of chitin which is based on theN-acetyl derivative of D-glucosamine (see Fig. 11). The sugar 2-amino-2-deoxy-D-galactose is found in dermatan and chondroıtin sulfate, constituents of mam-malian tissues and cartilage.

Oligosaccharides. These oligomers result from partial acid or enzymatichydrolysis of polysaccharides and they are prepared for structural analysis ofpolysaccharides. Few disaccharides exist naturally, or are produced by enzymes;


Fig. 11. Representation of some monosaccharides involved in most of the polysaccharides.


the linkage between two monosaccharides is an acetal bond also called an osidicbond. The acetal bond imparts a nonreducing character to the anomeric carbon.For common di- or trisaccharides, trivial names are normally used instead of sys-tematic names. The nonreducing elements of an oligosaccharide are designatedby the suffix -yl. In the case of nonreducing disaccharides the monomers are listedin alphabetical order.

Examples of reducing disaccharides are such as lactose and maltose. Lactoseoccurs in milk of mammals and is named systematically as β-D-galactopyranosyl-(1→4)-D-glucopyranose or 4-O-β-D-galactopyranosyl-D-glucopyranose and abbre-viated Galp-β-(1→4)-Glc (the italic p indicates the pyranoid form). Maltose, ob-tained by hydrolysis of amylose, is named (4-O-α-D-glucopyranosyl-D-glucose orα-D-glucopyranose-(1→4)-D-glucose). The -ose suffix in the systematic name de-notes a reducing disaccharide, eg, one with one anomeric carbon in a hemiacetalform. Sucrose or saccharose is a nonreducing disaccharide. It is extracted fromsugarbeet or sugar cane, is named β-D-fructofuranosyl-α-D-glucopyranoside, andabbreviated as β-D-Fruf -(2↔1)-α-D-Glcp. The biosynthesis and structure of dis-accharides are developed in Reference 11. The -ide suffix denotes a nonreducingdisaccharide wherein both anomeric carbons are in the acetal linkage.

In Figure 13, the 13C NMR spectra of cellobiose (β-D-Glcp-(1→4)-D-Glc)(Fig. 12a) and maltose (α-D-Glcp-(1→4)-D-Glc) (Fig. 12b) are given; the anomericequilibrium of D-glucose at the reducing end is demonstrated (23); the signalscorresponding to the C-1 of the nonreducing D-glucose (indicated as ′) are locatedat a different chemical shift depending on the anomeric configuration: C-1′β isat 105.3 ppm (cellobiose) but C-1′α is at 102.5 ppm (maltose) (in D2O at 60◦C).NMR spectroscopy data are given in the literature for mono- and oligosaccharides(16–19).

Cyclodextrins (qv) represents a series of cyclic α-(1→4)-linked D-glucopyranose oligomers obtained by enzymic degradation of starch using a cy-clodextrin glucotransferase (11). The most common cyclodextrins are called α, β,γ cyclodextrins with 6, 7, 8 sugars respectively resulting in intrasaccharide α-1,4-glycosidic bonds.

They have a toroıdal structure with hydrophilic exteriors and a hydrophobiccavity in which hydrophobic organic compounds can be trapped.

They are used to stabilize vitamins against temperature or increase solu-bility of pharmaceutical products. They also show an interesting selectivity inrelation with the dimensions of the cavities (Fig. 13).

Fructosans are oligomers or polymers of β-D-fructofuranose and belong to theinulin-type ((2→1)-β-linked) or to the phlein-type (2→6 linked); inulin oligomersare found in Jerusalem artichoke or dahlia tubers where they are carbohydrate re-serves. They are usually low molecular weight polymers (degree of polymerization,DP ≤ 50). The high yield in fructosans in Jerusalem artichoke may be considered asa source of fructose for the food industry. The juice extracted from grounded tuberswas isolated and studied by liquid chromatography (HPLC). The oligosaccharidedistribution was obtained by size exclusion chromatography (SEC) and by normal-phase HPLC (24–29). After separation of the different constituents and NMRanalysis, it is concluded that the juice consists of an oligomer series formed of oligo-β-fructofuranosyl units (2→1) ended by one α-D-glucopyranosyl unit (Fig. 14).

Fig. 12. 13C NMR of (a) cellobiose and (b) maltose at 60◦C in D2O at 62.86 MHz. The anomeric equilibrium is clearly observed on the C-1position. Reproduced from Ref. 23, with the permission of John Wiley & Sons (Copyright 2003).

209


Fig. 13. Representation of the α-cyclodextrin (six glucose units) in the cyclic and toroidalforms showing the hydrophobic cavity and the hydrophilic faces.

Oligosaccharins. It has been demonstrated that specific oligosaccharidesare used to control biological processes in plant. Albersheim who was a pioneer inthis work recognized that specific oligosaccharide components of plant cell walls(named oligosaccharins) can have biochemical activity which results in regulatoryresponse (30). α-(1→4)-linked D-galacturonic acid oligosaccharides obtained frompectins have regulatory effects on plant growth and development.

In connection with the biological activity of mono- and oligosaccharides,it is important to mention plant lectins: lectins are proteins possessing atleast one noncatalytic domain which binds reversibly to specific mono- andoligosaccharides (31,32); lectins are carbohydrate-binding (glyco)proteins ofnonimmune origin capable of specific recognition of carbohydrates and re-versible binding to them, without altering their covalent structure. Manyof plant-based food ingredients contain lectins, some with striking biologicalactivities.


Fig. 14. Fructans, as reserve polysaccharides, from the inulin-type like in Jerusalemartichoke.

Glycans. The generic name for polysaccharides is glycan using the suf-fix “an.” The names of polysaccharides should be D-glucan, or D-mannan forhomopolymers constituted of D-glucose or D-mannose respectively. Certain his-toric names have been retained such as cellulose (β-(1→4)-D-glucan) or amylose(α-(1→4)-D-glucan). The homopolymers are also named homoglycans, whereaspolysaccharides composed of two or more monosaccharides are named heterogly-cans. An example is given with the bacterial polysaccharide called Fucogel® basedon a trisaccharide repeating unit:

3)α−L−Fucp−(1→3)−α−D−Galp−(1→3)−α−D−GalpA−(1→

in which the sequence, the name of the constitutive sugars, the type of bonds, andthe configuration are all given (33). A labile acetyl substituent was located on theGalpA unit.

Single words such as xylans or mannans may also be used to refer to re-lated groups of polysaccharides without implying that they are homopolymers.The name xylan is used for arabinoxylan and glucuronoarabinoxylan, heteroge-neous polysaccharides found in plants. The term mannan is used for glucomannan,galactomannan, and galactoglucomannan.

The extract from a natural source is often a mixture of different polymers, es-pecially from plant sources; the extract must be fractionated but each fraction stillcontains molecules with minor structural features (partial acetylation or lengthof side chains). The fraction is polydisperse in molecular structure; the differencein local structure is also termed the microstructure.

Bacterial polysaccharides produced by a specific strain are usually perfectlyhomogeneous; the only difference between molecules being in the DP (or the


molecular weight); the sample is characterized by the polydispersity index, whichis defined as the ratio between the mass-average molar mass (Mw) and the number-average molar mass (Mn) (34) (see MOLECULAR WEIGHT DETERMINATION).

Molecular Modeling

The physical and biological properties of macromolecules depend on their three-dimensional structures. Knowledge of the different stable conformations is re-quired to better understand and to predict their behavior in different environ-ments. This knowledge is also important to control and modify the role played bythese macromolecules in recognition and interaction processes. Conformationaldata can be obtained by both experimental and computational methods. Experi-mental studies give data that are often incomplete, dependant on conformationalequilibra or specific for a frozen state. An alternative method is molecular mod-eling where results depend unfortunately on human decisions. However, conjunc-tion, these two approaches lead to a good understanding of the three-dimensionalarrangements of the studied molecule.

Monosaccharides. Most of the monosaccharides exist as pyranose rings.The most stable conformation of such six-membered ring systems is usually oneof the chair forms (C). In principle the pyranoid ring can also adopt energeticallyless favorable conformations. Six different skew conformers (S) separated by sixdifferent boat conformers can be identified on the pseudorotational itinerary (35).Three puckering parameters define unambiguously the position of the individualforms of the pyranoid ring on the conformational sphere (Fig. 15), Q is the maxi-mum puckering amplitude, the parameters θ and φ are angles in the range 0◦ <θ

<180◦ and 0◦ <φ <360◦, and can be thought as polar and azimutal angles for asphere of radius Q. The two poles φ = 0◦ and 180◦ represent the energy wheels ofthe chair conformations; 1C4 and 4C1.

All 12 flexible forms are located at the equator. In unsubstituted cyclohexane,the two chair forms are the prominent species. Substitution of heteroatom in thering and addition of hydroxyls or other exocyclic substituents further stabilizeor destabilize ring conformers in reference to cyclohexane. As a general rule, theequatorial position of bulky substituents would be preferred because of 1, 3 syn–diaxial interaction that causes steric clashes. The 4C1 of glucopyranose having allring substituents in the equatorial position is preferred to the 1C4 conformer inwhich all the substituents are in axial orientation. However, at high temperatureconformational transition to this form can arise spontaneously as demonstratedby the formation of levoglucosan (1,6-anhydro-β-D-glucopyranose). Besides, theα-L-iduronate ring, which is a constituent of the glycosaminoglycans (heparin,heparan sulfate, and dermatan sulfate), shows conformational mobility. Threeforms, namely 1C4, 2S0, and 4C1, of this ring have been suggested to be responsiblefor the biological activities of these compounds.

Very important furanose molecules (five-membered rings) are commonlyfound in nature. For example, D-ribose and D-deoxyribose are found as the build-ing units of nucleic acids, and fructose is a constituent of sucrose. These ringsare not planar, either one (envelope form) or two (twist form) atoms are out ofthe plane containing the others. The different envelope and twist conformations


Fig. 15. Pseudorotation sphere of pyranoid rings.

are of similar energy, and the barrier to their interconversion is small, thereforemixtures of different conformations are expected in solution. The different con-formations of the furanoid ring are described by a pseudorotation circle (Fig. 16).Two puckering parameters are needed to define a conformation for those rings,the puckering amplitude ν and the phase angle P (35).

Several low energy conformations are accessible for the primary hydroxylexocyclic groups. The energies of the different ring conformations are affectedby the orientations of the hydroxymethyl group. This group usually exists inthree staggered positions called gauche–gauche, gauche–trans, and trans–gauche(Fig. 17).

In this terminology, the torsion angle ω(O-5 C-5 C-6 O-6) is statedfirst, followed by the torsion angle ω′(C-4 C-5 C-6 O-6). It is known fromcrystallographic studies, NMR measurements, and theoretical calculations thatthe conformational equilibrium around the C-5 C-6 bond in aldopyranoses de-pends significantly upon the configuration at C-4. For the “gluco” configuration(O-4 equatorial) the trans–gauche is high in energy and the remaining two confor-mations are almost equally populated, while the trans–gauche and gauche–transpositions are preferred for those having a “galacto” configuration (36).

The secondary hydroxyl groups undergo almost free rotational transitions.All these hydroxyl groups can participate in the creation of hydrogen bonds. As aresult of the many possible orientations of such groups, prediction of the hydro-gen bonding network is a difficult task. Most carbohydrates offer an exceptionally


Fig. 16. Pseudorotation wheel of furanoid rings.

high ratio of hydroxyl groups per saccharide residue. Such a hydrogen-bondingpotential is satisfied by association with neighboring carbohydrate molecules, gly-coproteins, or surrounding water molecules.

Anomeric Effect. The anomeric effect describes the axial preference foran electronegative substituent of the pyranose ring adjacent to the ring oxygen,whereas the exo-anomeric effect describes the rotational preference of the glyco-sidic C-1 O bonds (37). These stereoelectronic effects are of general importancefor all molecules having two heteroatoms linked to a tetrahedral center. Surveyof X-ray crystallographic data reveals that these effects have geometrical conse-quences. The most obvious feature of the experimental data on both the α and β

configuration is a marked difference in the molecular geometry around the acetalgroup. By way of example, in the axial configuration one observes a general short-ening of the C-1 O-5 bond, a lengthening of the C O-X bond (for a 1→X link-age), and an increase in the O-5 C-1 O-X bond angle value. Molecular orbital


Fig. 17. The staggered orientations of the hydroxymethyl group. GG = gauche–gauche;GT = gauche–trans; TG = trans–gauche.

theory accounts for these observations. The magnitude of the anomeric effectvaries with the nature of the electronegative group, the polarity of the solvent,and the location of the other substituents in the molecule.

The exo-anomeric effect influences the rotations around the glycosidic C-1 Obond and is therefore important in determining the relative orientations of saccha-ride units in carbohydrate chains. The exo-anomeric effect is a balance betweenelectronic and steric effects. The three staggered orientations for rotation aboutthe glycosidic bond are not equivalent; the exo-anomeric effect causes preferencefor the +synclinal orientation of the aglycone group in the α series and −synclinalfor the β series (see Fig. 18 for the definition of the torsion angle domains).

Disaccharides. Monosaccharides can be condensed to produce oligomericstructures through glycosidic bond formation. Water is eliminated between theanomeric hydroxyl and any one of the hydroxyls of a second monosaccharide oroligosaccharide. The glycosidic linkage is constituted by two bonds, the glycosidicC-1 O-X and the aglycone O-X C-X. In the case of (1→6) linkages between twopyranosic units, three bonds connect the successive sugar rings.

The low energy conformers of a disaccharide can be estimated using molecu-lar mechanics. The conformational parameters that occur for a simple disaccharideare as follows:

OH rotationCH2 OH rotationAglycone rotationGlycosidic bondPuckering

7 torsion angles2 torsions1 torsion2 torsions6 variables

If we assume three minima that should be explored for each variable, thereare 318 combinations to consider, far beyond the available computer resources.


Fig. 18. Definition of angle domains.

It is therefore important to recognize that the global shape of a disaccharidedepends mainly on rotations around the glycosidic linkages, because the flexi-bility of the pyranose ring is rather limited and the different orientations of thependant groups do not participate in the description of the backbone trajectories.The relative orientations of saccharide units are therefore expressed in terms ofthe glycosidic linkage torsional angles � and � which have the definition � =O-5 C-1 O-X C′-X and � = C-1 O-X C′-X C′-(X + 1) for a (1→X) linkage. The�,� space is then explored in a systematic fashion. Both torsions are sequentiallyrotated in small increments over the full 360◦ range. At each point on the grid,energy contributions calculated from the force field in use are evaluated. It isthen possible to represent the energies of all the conformations available as acontour map in the �,� space. These contour maps enable graphical descriptionof energy change as related to the relative orientation of the monosaccharides;this is a three-dimensional cross section of the complex conformational space ofdisaccharides. They indicate the shape and position of minima, the routes forinterconversion between conformers, and the heights of the transitional barriers.Among the many different methodologies for calculating contour maps, adiabaticprocedures are now widely accepted. In such procedures, the strain produced bysteric interactions inherent from rotation of monosaccharide residues is relievedby the inclusion of bond length and angle adjustment in the form of minimization,with respect to all degrees of freedom of the system (except �,�), at each point ofthe space. However, as minimization will only lead to conformations “downhill”from the starting structure, the torsional dimension where most conformationalvariation occurs is limited to only one orientational well. It is possible that rotationof pendant groups over torsional barriers could produce lower energy conforma-tions at that point in �,� space. Ideally at each point in �,� space investigationof all possible combinations of pendant group orientations is required. Adiabatic


Fig. 19. The potential energy surface of mannobiose.

maps attempt to represent the lowest energy of all possible pendant group orien-tations at each point in the �,� space. An example of an adiabatic map is given inFigure 19.

Solvation. It is important to recognize that most procedures are designedto treat molecules in the isolated state. Omission of the effect of the environmentfrom the calculation results in a neglect of the fraction of the energy contributionthat arises from these interactions.

Several different approaches have been proposed to treat solvation effects(38). In the simplest one, the effect of the solvent is achieved by increasing thedielectric constant for calculations of electrostatic interactions or by the use ofa distance-dependant dielectric constant. Another possible way is to explicitly


include a limited number of solvent molecules in the calculation. By way of ex-ample, in aqueous solution the potential importance of hydrogen bonding andthe great strength and directionality of hydrogen bonds requires explicit consid-eration of the water molecules surrounding the solute. An example obtained onmaltose–water interaction is given in Reference 39. To account for such a solventeffect, molecular dynamics with the inclusion of explicit water molecules aroundthe carbohydrate can be used. An alternative approach is to treat the solvent asa dielectric continuum. The conformational free energy of a given conformer in aparticular solvent may be described as arising from the contribution of the energyof the isolated state and the solvation free energy. The latter is evaluated fromthe continuum model and is partitioned into cavity, electrostatic, and dispersioncontributions.

Probing Potential Energy Surfaces. The accuracy of the predicted po-tential energy surfaces has to be evaluated. Indeed, many observable properties ofoligosaccharides can be calculated and have been shown to be sensitive to the de-tails of the conformational energy surface. Many crystal structure determinationsof carbohydrates, nucleosides, and nucleotides are available in the CambridgeCrystallographic Database. X-ray analysis gives the best data for the conforma-tion of an oligosaccharide. Precise atomic coordinates are provided, along with anexplicitly defined environment. Comparisons with crystal structures are amongthe most precise test of modeling available for carbohydrate molecules, providedthat packing forces are taken into account. These surveys of crystalline conforma-tions are indicative of the structural variations and the conformational flexibilitythat the glycosidic linkages exhibit in the solid state.

In solution, the method of choice to study the three-dimensional structureof saccharides is NMR, through the parameters represented by chemical shifts,coupling constants, nuclear Overhauser effects (nOe), and also relaxation timemeasurements. Although the conformational dependence of the carbon chemicalshifts is far from understood, coupling constants can be used to evaluate the mag-nitude of the torsion angles, and nOe measurements can provide estimations ofdistances between protons located in rather close proximity. In addition, relax-ation time measurements give information on the mobility and the behavior ofmolecules in solution.

Three bond coupling constants provide a source of information with regardsto torsional angles about the bonds. They can be interpreted by trigonometricformulas with suitable parameters derived from empirical studies on model com-pounds. 3JH−H coupling constants allow the extraction of information about thesugar ring pucker. For the exocyclic group of pyranose, the vicinal coupling con-stants between H-5 and H-6 define the torsional angle about C-5 C-6, providedthat the stereospecific assignment of the two H-6 protons is possible. The relativemagnitudes of the coupling constants, 3JH5−H6 and 3JH5−H6′ can be used to deter-mine the distribution of conformers about the ω angle in the case of (1→6)-linkedpyranoses. As for the torsional angles � and � about the glycosidic linkage, onlyheteronuclear coupling constants 3JC−H and 1JC−H can be used, and empiricaltrigonometric formulas relating 3JC−H to torsional angles have been proposed.The angular dependence that incorporates the solvent effects of 1JC−H have beenestablished in α- and β-oligosaccharides (40,41).


The nOe data may be obtained by either one- or two-dimensional meth-ods. These through-space effects may be used to estimate internuclear distances.However, nOe cross-peak volumes are inversely proportional to the sixth powerof the distance between the correlated protons only in the case of rigid spheri-cal molecules whose tumbling is isotropic. The simplest method that has beenproposed for the interpretation of nOe-derived distance data is the “isolated spinpair approximation.” The matrix treatment is more rigorous, although relyingalso on several approximations. A set of equations describing the cross-relaxationpathways of all the protons in the molecule is cast into a matrix form andsolved.

All these NMR parameters represent average values, because of the acknowl-edged flexibility of carbohydrates and of all the rapidly interconverting confor-mations accessible to the molecule on the NMR time scale. This means that astraightforward interpretation of experimental data leads only to the definitionof a “virtual conformation” without real physical meaning. Conformational aver-aging based on the abundances of the predicted structures gives averaged valueswhich can be compared safely to the experimental ones.

The Disordered State of Polysaccharides. The polysaccharide chainsin solution tend to adopt a more or less coiled structure. Such a dissolved coil wouldfluctuate between local and overall conformations. Polysaccharides are able to as-sume an enormous variety of spatial arrangements around the glycosidic linkagesbecause these molecules have extensive conformational freedom; they may sam-ple a large part of the space around the glycosidic and aglycone bonds. Theoreticalpolysaccharide models are based on studies of the relative abundance of the var-ious conformations, of a given polysaccharide, in conjunction with the statisticaltheory of polymer chain conformation (42). Possible interactions between residuesof the polysaccharide chain that are not nearest neighbors in the primary sequenceof the polymer are ignored.

The Monte Carlo sample then reflects the number of conformations of poly-mer molecules. This means that observable parameters describing the solutionbehavior of polysaccharides are averages of the properties of individual conforma-tions. This approach yields properties corresponding to the equilibrium state ofthe chain. Results refer to a model for an unperturbed chain that ignores the con-sequences of the long range excluded volume effect, because only nearest-neighborinteractions are accounted for in the computation of the �,� surfaces.

Given a sufficient Monte Carlo sample of unperturbed polysaccharide chains,it is possible to assess many mean properties of the polymer in question simplyby computing unweighted arithmetic averages over the chains of the sample. Forexample, the mean-square end-to-end distance, the average square radius of gyra-tion, the mean persistence length, and dipole moment, are all average geometricproperties readily computed from a knowledge of the coordinates of the atoms oratomic groups making up the Monte Carlo sample.

Models of polymer chain extension were first used to compare the effect ofthe glycosidic linkage geometry of simple polysaccharide chains, eg cellulose andamylose (43). Both polymers are 1,4-linked glucans; the only difference is in theanomeric configuration on the C-1 atom of the monomeric unit, α for amyloseand β for cellulose. The calculated data show a remarkable pseudohelical chain


trajectory of the amylosic chains, the characteristic ratio of 5 denotes a moder-ately compact chain configuration. This behavior seems to be the consequence ofthe glycosidic bond geometry because changing this geometry from the α to the β

configuration has a dramatic effect on the character of the chain trajectory. Rela-tive to amylose, the cellulosic characteristic ratio of 100 is an increase of a factorof 20. This reflects the extended character of the cellulosic chains. Investigationof the solvent effects on those two representative polysaccharides has also beencarried out (44). In good concordance with the experimentally observed solvent de-pendence, significant changes in the unperturbed chain dimensions were found;the characteristic ratio is larger in water than in vacuum for amylose, whereasfor cellulose it is smaller.

The answer to the question of how reliable is the predicted conformationalbehavior of the polysaccharide chains lies again in the comparison of theoret-ically and experimentally measured properties. A few examples were analyzed(chitin, chitosan, hyaluronan, galactomannan) and the local stiffness of the chainreflected by the persistence length Lp has been compared with the value obtainedexperimentally using multidetection SEC (45–47).

Molecular Structure

The polymer may be linear (such as hyaluronan) (Fig. 20a) or branched with a longside chain (as in xanthan) (Fig. 20b) or with many short chains or monosaccharideside groups (as in galactomannan) (Fig. 20c) or branched with side chains them-selves branched giving rise to a tree-like structure (as in amylopectin) (Fig. 20d).

The determination of the structure is a very difficult task especially in het-eroglycans. The structure must be established on a pure sample isolated from itsnatural source with a minimum degradation.

Fig. 20. Structural patterns of polymer chains.


Considering the repeat unit of a polysaccharide represented by the scheme,

the answers to the different following questions are required:

(1) What is the nature and relative proportion of the A, B, C, D monosaccha-rides involved in this oligomer, including ring size and their configuration(D- or L-)?

(2) At which positions (u, v, x) are A, B, D engaged in the osidic linkage andwhat is the anomeric geometry (α or β) of each glycosidic linkage ?

(3) Unit C is substituted at w and y positions but which position correspondsto the attachment of the side chain and which to the main chain linkage?

(4) Which is the exact sequence of the four sugars in the repeat unit?

The general methods are reviewed in many books (3) and in this article onlygeneral overview will be given.

If there are any substituents on the rings as is frequent in plant or bacterialpolysaccharides (acetyl, pyruvyl, succinyl groups), they must be located on a spe-cific position, but often they are labile in acidic or mainly basic conditions and thestructure determination becomes even more difficult.

Extraction–Purification. Purification of polymeric molecules is often eas-ier than for small molecules. It is relatively easy to separate small (impurities)from large molecules on solubility criteria, gel filtration, or by dialysis.

If the polymer is water soluble as in exocellular bacterial polysaccharides, itis often a polyelectrolyte (containing uronic acid); it can be precipitated using anonsolvent such as ethanol or 2-propanol (in a ratio around 1:1 water/alcohol) afterclarification of the extract by filtration and addition of monovalent salt excess toexchange the multivalent counterions. The presence of bivalent counterions leftin the sample will prevent resolubilization after drying. For a strongly interactingpolymer with calcium (such as pectin) the use of oxalate as chelating agent isuseful.

Before extraction of polysaccharides from plants or woods, it is recom-mended

(1) to extract these materials with organic solvents (toluene, chloroform,methanol, etc) to remove pigments, waxes, or oils;

(2) then, to eliminate Lignin (qv), a highly cross-lilnked polyaromatic com-pound, by chemical treatment (chlorite, ozone, sulfite, etc).

For neutral polymers (cellulose being excluded), extraction can be performedusing DMSO (48), N-methylmorpholine N-oxide (49), NaOH, or KOH at differentconcentrations. Over pH 12, alcoholate is formed on the OH groups, increasingthe solubility of polysaccharides. When hemicelluloses have to be extracted froma plant source, KOH 24% or NaOH 18% are often used; these hydroxides produce


swelling of the cellulosic matrix by rupturing hydrogen-bonding associations, al-lowing better diffusion of other polymers (50).

The extract may contain different polysaccharides and it is useful to res-olubilize this material after first purification and to perform SEC in addition toion-exchange chromatography before establishment of the structure. This allowsseparation of fractions with different average molecular weights and differentaverage charge densities.

NMR Spectroscopy. The structure can be established as mentioned be-fore using 1H and 13C NMR; many examples are given in the literature. NMRis very useful especially for analyzing polysaccharides with a regular structure.Figure 21 shows the NMR analysis of a bacterial polysaccharide (hyaluronan)based on a 2 sugars repeat unit. On the 1H NMR spectrum, the two anomeric C-1are identified as well as the acetyl of the N-acetyl-D-glucosamine unit. On the 13CNMR spectrum, two different C O appear in the range of 175 ppm due to the car-boxylic group and to the acetyl group. The attribution of all the signals was givenby Haxaire (51). NMR is also useful for estimating the population of substituents,the number of sugars in the repeat units, and to determining the anomery as wellas the position of linkage and the sequence.

Composition. The most usual method to determine the composition of apolysaccharide (nature of the sugar units) is the complete hydrolysis by heating inacidic conditions (1 M H2SO4 followed by neutralization with barium carbonate, orwith 2 M trifluoroacetic acid, which is volatile and easily eliminated after reaction)(52,53). Nevertheless, no recommendation can be proposed a priori; especially,for heteropolysaccharides, the rate of hydrolysis of the osidic linkage depends onthe sugars involved: glycosidic bonds of deoxysugars and furanose sugars (likearabinose and fructose) are more labile whereas the glycosidic bond of the uronicmoiety is more stable. In the latter case, it is better to reduce the polymer beforehydrolysis (54,55). Methanolysis is sometimes preferred to simple acid hydrolysis,leading to methyl-glycoside derivatives (56).

Quantitative determinations of sugars are achieved by liquid chromatogra-phy (HPLC) or gas chromatography (GC). HPLC has the advantage of utilizingsugars directly but each sugar can give two or more peaks corresponding to theanomeric equilibrium; an example of HPLC chromatogram obtained on C18-µBondapak on maltodextrins is given in Figure 22. The peaks corresponding to DP= 4–6 show the anomeric equilibrium (26).

Reduction of the saccharide gives alditols and only one peak per sugar. Thesame occurs in GC; but, in addition, as sugars are not volatile, they must betransformed into alditol acetates by reduction and peracetylation to get a singlepeak per sugar in GC (57).

Methylation. Methylation involves the complete conversion of all free hy-droxyl groups in a polysaccharide into methoxyl groups ( OH → OCH3). Themost used method is the Hakomori procedure using DMSO, sodium hydride,and methyl iodide (58). To give significant results, methylation must be perfectlycompleted; then, total hydrolysis yields monomeric units with different degreesof methylation. The position of nonetherified OH indicates the position of theglycosidic bond. Thus, a tetra-O-methylhexose is representative of a terminalunit (nonreducing end). In a linear oligo- or polysaccharide, the fraction of theseunits can be used to estimate the number-average degree of polymerization.


Fig. 21. (a) 1H and (b) 13C NMR spectra for hyaluronan in D2O at 85◦C in the presenceof DMSO as standard at 300 and 75 MHz respectively (51).


Fig. 22. Separation of maltodextrins (the degree of polymerization is indicated by thenumber over the peaks) by gel permeation chromatography on Biogel P-2 (T = 15◦C; eluent:water). Anomeric equilibrium is shown under peaks 4–6. Reproduced from Ref. 26, bycourtesy of Marcel Dekker, Inc. (Copyright 2003).

A tri-O-methylhexose is an in-chain unit; di-O-methylhexose indicates the ex-istence of a branched point. The partially methylated sugars can be fractionatedby HPLC. This technique allowed determination of the distribution of methyl sub-stituents in methylcellulose (59); Figure 23 shows the chromatogram obtained forpartially methylated glucose after total hydrolysis of the methylcellulose. Eightmodified D-glucose derivatives are obtained including unsubstituted glucose.

Usually, the methylated sugars are derivatized to alditol acetates and sepa-rated by GC combined with mass spectroscopy (60–63).

Partial Acid and Enzymic Hydrolysis. Partial hydrolysis with acid un-der mild conditions gives a mixture of oligosaccharides. Homoglycan gives a seriesof homologous oligomers, whereas heteroglycan gives a series of products depend-ing on the relative acid labilities of the different glycosidic bonds. The fragmentspermit the sequencing of the monosaccharide residues in the polysaccharides.

Establishment of the polysaccharide structure can be obtained by the use ofspecific enzymes. The most developed studies concern the amylopectin molecularstructure using α- and β-amylase and debranching enzymes (isoamylase, pullu-lanase, glucoamylase). These enzymes hydrolyze specifically the α(1→6) and/orα(1→4) linkages, allowing separation of the side chains. The different sizes of sidechains in amylopectins were separated by SEC (64,65). Endo- and exoglycanasesspecific for the cellulose (β(1→4) linkage) were also investigated; a cocktail of thesetypes of enzymes is also used to disrupt the plant cell walls. Bacteriophages pos-sess a highly specific endoglycanase for a single glycosidic bond in each repeat unitof the corresponding bacterial polysaccharide, thus giving a single oligosaccharidemotive in good yield, identified as the repeat unit, easier to characterize (66).


Fig. 23. HPLC analysis of methycellulose (DS = 1.7) after total hydrolysis on a NucleosilC18 reverse-phase column. Eluent: water; ambient temperature. Reproduced from Ref. 59,with permission from Elsevier (Copyright 2003).

Molecular Weight Determination. Whenever the polysaccharide is per-fectly water soluble it is easy to perform SEC; but the polyhydroxyl nature ofpolysaccharides causes specific problems: owing to the large number of OHgroups in the molecules, cooperative interaction may form aggregates; the as-sociated molecules are connected by a network of H-bonds, often created by theconditions of isolation and drying of the pure polysaccharide. The presence ofaggregates disturbs the chromatogram which becomes difficult to interpret.

In addition, when uronic acid or pyruvic acid substituents are present, thepolysaccharide becomes a polyelectrolyte (see POLYELECTROLYTES). It is necessaryto shield the polymer from electrostatic interactions; this implies that the polymerhas to be solubilized in the presence of an external 1–1 electrolyte (67–70). A mul-tidetection experimental device in which a complete molecular weight distributioncan be obtained without any calibration has been developed (71).

Another important parameter is the intrinsic viscosity [η] (expressed inmL/g). It is obtained by extrapolation to zero polymer concentration of the re-duced viscosity (ηred) of the polymeric solution plotted as a function of the polymerconcentration C (g/mL) in accordance with the Huggins relation:

ηred = (η − η0)/η0C = [η] + k′[η]2C (1)

in which η is the viscosity of the polymer solution, η0 is the viscosity of the solvent,and k′ is the Huggins constant. There is a relative relation between the intrinsicviscosity and the molecular weight of the polymer following the Mark–Houwinkrelationship:

[η] = K M av (2)


Table 4. Mark–Houwink Parameters for Cellulose and Some Cellulosic Derivatives andfor Starch and Starch Derivatives

K, 10− 3

Polymer Solvent T, ◦C mL/g a Reference

Cellulose Cupriethylenediamine 25 10.1 0.9 92Cellulose Cupriethylenediamine 25 7.62 0.936 72Cellulose nitrate Tetrahydrofuran 25 1.32 1.01 73Cellulose acetate acetone 30 16.0 0.82 74Cellulose acetate butyrate acetone 25 13.7 0.85 75Cellulose triacetate acetone 25 14.9 0.82 76Sodium Aqueous NaCl (0.1 M) 25 12.3 0.91 77

carboxymethylcelluloseMethylcellulose Water 25 316 0.55 78Methylcellulose Water 20 280 0.55 79Amylose Dimethyl sulfoxide 25 15.1 0.70 80Amylose Dimethyl sulfoxide 25 30.6 0.64 81Amylose Water 20 13.2 0.68 82

Amylose Aqueous KOH (0.15 M) 25 8.36 0.77 80Amylose triacetate Chloroform 30 1.06 0.92 83

in which Mv is the viscometric average molecular weight; K and a are two param-eters depending on the polymer, the solvent, and the temperature. Some valuesare given in Table 4.

Plant Polysaccharides

Polysaccharides from the biomass consist largely of renewable polymers whichhave been used for a long time in many industrial applications. The main polysac-charides concerned are cellulose (qv) and starch (qv) (84).

Cellulose is the main structural polymer in wood (representing around 50%of the dried matter) or agricultural resources (normally between 35 and 75% of thedried matter depending on the source; in the case of cotton (qv), values are between90 and 99%) (85–88). Cellulose is found in a very organized fibrous structure. Incontrast, starch constitutes the reserve polysaccharide organized in a granularstructure. Both of these polymers are semicrystalline polymers (qv) based respec-tively on β-(1→4)-D-glucose and α-(1→4), α-(1→6)-D-glucose (89,90).

Derivatization may be used to extend the domains of application for thesenatural polymers; nevertheless, their relative lack of solubility and their highdegree of organization lead usually to heterogeneous chemical modifications. Het-erogeneity of the substitution gives bad reproducibility of the samples, dependingalso on the sources of polymers. Because of this problem, a large variety of pa-rameters relating the degree of substitution and molecular weight to the physicalproperties can be found in the literature.

The supramolecular structure of the initial materials is a very impor-tant point. The swelling and/or dissolution in different conditions dependon the morphology of the native substrate. Cellulose in cellulosic fibers and


amylose/amylopectin in starch granules are tightly packed in a more or less regu-lar organization, depending on their source but also on the presence of other addi-tional components. Thus, the accessibility of the reactive groups ( OH) depends onthis supramolecular structure in addition to the degree of crystallinity (diffusioncontrol of the reactants) and on the intra- or interchain H-bonds modulating the

OH reactivity. This is why cellulose and starch are not thermoplastic materialsbut in presence of water playing the role of plasticizer; especially starch becomesthermoplastic. Only a complete destructuration passing through a solution stateavoids a memory effect of the original state of organization and attainment ofregular chemical modification.

The two main characteristics of cellulose and starch derivatives are the dis-tribution in molecular weight (the weight average DPw or Mw) in addition tothe degree of substitution represented by DS (DS is the average number of sub-stituents per glucose unit; 0 < DS < 3).

Cellulose and Cellulose Derivatives. The solubilization of cellulose todetermine its molecular weight is delicate; one of the most powerful solvents iscupriethylenediamine for which the Mark–Houwink parameters relating the in-trinsic viscosity to the molecular weight ([η] = KMa) have been determined (seeTable 4). Some authors previously prepared cellulose derivatives (nitrocellulose orcellulose tricarbanilate) to determine the molecular weight and molecular weightdistribution, assuming no polymer degradation (72–83,91,92).

Cellulose derivatives are the most important derivatives obtained from nat-ural sources. Chemical modifications of cellulose are very useful to enhance sol-ubility in organic or aqueous solvents or to obtain new thermoplastic materials.The main derivatives are described in Table 5.

In designing derivatives for various properties and applications the degree ofsubstitution is a very important parameter, as well as the chemical structure of thesubstituent. Recent studies have been done in selective cellulose functionalizationobtaining regioselectively substituted esters and ethers (93–97).

Table 5. Main Substituents in Cellulose Derivatives

Product Functional group (R=)a DSb MSc

Cellulose nitrate NO2 1.8–2.8Cellulose acetate C(O)CH3 0.6–3.0Carboxymethylcellulose CH2COONa 0.5–2.9Methylcellulose CH3 0.4–3.0Hydroxyethycellulose [CH2CH2O]x H (x = 1, 2, 3, . . .) 0.8–1.2 1.7–3.0Hydroxypropylcellulose [CH2CH(CH3)O]xH (x = 1, 2, . . .) <4.6Ethylcellulose CH2CH3 0.5–4.6

a

b0 < DS<3 is the degree of substitution.cMS is the molar substitution (gives the average number of monomoles of reactant addedto one anhydroglucose unit).


Among the large variety of cellulose derivatives, two main groups of productsare prepared: cellulose esters and cellulose ethers. Worldwide, around 1.4 × 106

t/year of cellulosic dissolving pulp is used in the production of cellulose esters,whereas 5.4 × 105 t/year is used in the production of cellulose ethers (98).

Cellulose ethers (qv) are obtained by substitution of hydroxyl groups by ethergroups, and depending on the substituent they can be soluble in water or organicsolvents. Carboxymethylcellulose (CMC) is the ether of cellulose of largest pro-duction. Approximately 300,000 t/year is produced each year. It is a water-solublepolymer in the sodium salt form and when DS is larger than 0.5. It is produced byreaction of the alkalicellulose with sodium chloroacetate or reaction with sodiumin liquid nitrogen in the presence of chloroacetic acid (99–101). On a laboratoryscale, soluble CMC with DS varying from 0.5 up to 3 has been produced. Typicalraw materials for cellulose-ether production are wood pulp or cotton linters. Cot-ton linters are used for the production of high viscosity ethers because of theirhigher DP. CMC sodium salt is a white, odorless, and nontoxic solid. Viscositiesrange in aqueous solution from 10 to over 50,000 mPa·s (=cP) for 2% aqueoussolutions. The DS of commercial samples may be between 0.3 and 1.2 althoughclear and fiber-free CMC solutions require a minimum DS value of about 0.5. Themajority of commercial products have a DS between 0.65 and 0.85. Most CMCsolutions are highly pseudoplastic (102,103).

Methylcellulose (MC) is produced, as in the case of CMC, by the Williamsonsynthesis, ie, by reaction of alkali cellulose with methyl chloride. World annualproduction is around 80,000 t/year (98). Other hydroxyalkyl derivatives or mixedethers such as hydroxyethylmethylcellulose, hydroxypropylmethylcellulose, hy-droxybutylmethylcellulose, ethylmethylcellulose, and carboxymethylmethylcellu-lose have similar properties to MC.

MC is a white to slightly off-white, essentially odorless and tasteless powder.The solubility of MC varies as a function of DS. In fact, commercial MC is dividedinto two types of products according to DS. MC with a degree of substitutionbetween 1.4 and 2.0 are soluble in cold water, whereas lower substituted materialsare soluble in dilute alkali. MC is also soluble in ethanol, acetone, ethyl acetate,benzene, and toluene for higher DS. Just as in the case of CMC, viscosity valuesof MC vary as a function of (average) molecular weight. Apparent viscosity at 2%and 20◦C can vary between 10 and 19,000 mPa·s (=cP) in a range of average molarmass between 13,000 and 140,000. MC and its derivatives form gels at specifictemperatures when heated (104).

The mechanism of gelation is related to the distribution of substituents alongthe chain; two steps in the gelation mechanism have been demonstrated. The firststep of gelation is the formation of a clear gel related to the existence of blocksof highly substituted glucose units (105–108). The second step is the phase sepa-ration giving a turbid gel when temperature is larger than 60◦C. Below their gelpoint, solutions of MCs exhibit pseudoplastic rheology, but appear to be Newtonianat low shear rates. MCs have excellent water retention properties.

The two main types of commercial hydroxyalkylcelluloses are hydroxyethyl-cellulose (HEC) and hydroxypropylcellulose (HPC). HEC is produced in largeramount with around 60,000 t/year, but investments are planned in order to in-crease the capacities of HPC, in future years. HEC and HPC are synthesized byreaction of alkali cellulose with ethylene oxide or propylene oxide respectively in a


slurry process in an organic medium. In the case of HPC it is possible to use liquidpropylene oxide as a medium of reaction; after etherification the crude product ispurified by washing with hot water (109–111).

HEC is available in a wide range of viscosities (from 10 to 100,000 mPa·s(=cP) in 2% aqueous solution at ambient temperature). It is soluble in cold andhot water or some mixtures of water and water-miscible organic solvents. Thenonionic chemical structure and its solubility in both cold and hot water are themain advantages for the utilization of HEC. HPC has a thermal gel point like MCand it is thermoplastic. Because of its high level of substitution (molar substitutionabout 4) and its remarkable hydrophobic character it is soluble in a number oforganic solvents as well as water.

Many general books develop the structure and reactivity of cellulosic mate-rials and the preparation of their derivatives (112–115).

Starch and Starch Derivatives. The starch granule dimensions dependon the sources as well as the composition and degree of crystallinity. The molec-ular structure of the principal component in the starch granules consists of alinear (1→4)-D-glucose chain, amylose. Amylose is a flexible polymer having a rel-atively low thickening power; its molecular weight seems to remain relatively low(less than 1 million). The second major component is a highly grafted polymer,amylopectin. It is a very compact molecule with a very high molecular weight(10–100 millions have been cited in the literature). Also, in some materials athird component is a slightly grafted amylose called the intermediate material(116,117).

The main techniques to characterize starch molecules consist of iodine or bu-tanol complex formation to estimate the fraction of linear amylose (118). A coloredcomplex is formed with iodine; its wavelength of absorption is characteristic of thelength of the helicoıdal segments involved in the complex formed with iodine. Thistechnique has been examined by Salemis (119). The second important character-istic is the molecular weight or better, the molecular weight distribution. It isoften difficult to solubilize starch in aqueous solution. In one SEC investigation ofstarch in aqueous systems containing KOH (120), it was shown that amylopectinhas a low solubility. SEC studies were also done in DMSO (121,122). The deter-mination of the molecular weights can be performed using the empirical relationrelating the intrinsic viscosity to the viscometric average molecular weight. Someparameters are given in Table 4.

Nowadays, starch is used in industry as native starch (world production:(10–12) × 106 t/year), gelatinized starch, modified starch (7 × 106 t/year), or aspartially or totally degraded starch ((16,17) × 106 t/year), (98). First, the charac-teristics of the products depend on the parent starch sample.

The hydroxyl groups of starch can be derivatized as esters or ethers, as inthe case of cellulose. Table 6 gives the main substituents for starch.

Starch esters are synthesized by reaction of a carboxylic acid or an acylchloride or an acid anhydride with the hydroxyl groups of amylose or amylopectinchains. As in the case of cellulose derivatives, there are organic and inorganicesters depending on the origin of the substituent.

Acetate of starch is the most important ester of starch industrially produced.Commercial products are traditionally low substituted derivatives (DS < 0.3)(123–125). In this range of DS it is possible to derivatize preserving the granule


Table 6. Main Substitutents in Starch Derivatives

Product Functional group DS MS

Starch acetate C(O)CH3 0.04–3.0Starch phosphate OPO3HNa 0.02–0.25Hydroxyethylated starch [CH2CH2O]xH (x = 1, 2, 3, . . .) 0.05–1.25Hydroxypropylated starch [CH2CH(CH3)O]xH (x = 1, 2, . . .) 0.1–1.0Quaternary ammonium OCH2CH(OH)CH2N(CH3)3Cl 0.1–0.7

starch esters and othersStarch alkenylsuccinates OC(O)CH2CH2C(O)ONa 0.02

structure, to purify by washing with water, and to recover by centrifugation or fil-tration. The acetylating agent is commonly acetic anhydride in aqueous mediumin the presence of dilute sodium hydroxide. Derivatization eases the formationand stability of colloidal dispersions of starch and the adjustment of the colloidalproperties to the requirements of the application. Products obtained have high pu-rity and low ash content. The principal uses are in food (paste clarity and viscositystability) and the pharmaceutical and textile industry (warp sizing, finishing op-erations). Other higher acetylated starches are those obtained with a DS of 2–3(126,127). Their characteristics of solubility and thermoplasticity permit their uti-lization as materials. Nevertheless, this type of material has not been developedcommercially because they could not compete with similar cellulose derivativesin terms of strength and cost. The glass transition (Tg) stabilizes around 150◦C.These polymers have been proposed to prepare hydrophobic thermoplastic ma-terials or to coat hydrophilic films (128–130). Even up to DS ∼ 3, the polymersremain biodegradable (129).

Syntheses of longer chain organic esters (C3–C18) have been carried out on alaboratory scale, but have not yet been industrially produced because at presentthey are not profitable (125,130–133).

Starch can be etherified by reaction of an alkyl halide or an epoxy in alkalinemedium. Depending on the substituent it is possible to obtain anionic, hydrox-yalkyl, and cationic products.

The two main types of hydroxyalkyl ethers industrially produced are hy-droxyethyl starch and hydroxypropyl starch. These compounds are synthesizedby reaction of ethylene oxide or propylene oxide respectively with starch underalkaline conditions (134).

Low MS hydroxyalkyl starch products (MS < 0.1) are prepared by reactionof the epoxy in an aqueous and alkaline starch slurry. A salt such as sodiumsulfate is often added to protect the starch from swelling so that relatively highlevels of caustic can be added to increase the reaction efficiency. In this case usualtemperatures are between 25 and 50◦C. If the temperature is increased and thesalt eliminated, it is possible to do the reaction in a starch solution and obtainhigher molar substitutions.

High MS hydroxyalkyl products are prepared by reaction of the epoxy inalcohol slurry and alkaline conditions. The alcohols typically used are methanol,ethanol, and 2-propanol. Another possibility is to combine starch with a low level


of moisture (5–15%) with ethylene or propylene oxide under pressure in presenceof an alkaline catalyst. In this process the starch retains its granular form.

Low MS hydroxyalkyl starches are very similar in appearance to unmodifiedstarch. Nevertheless, there are two main differences in their properties owingto the presence of hydroxyalkyl groups. On one hand, these groups weaken thehydrogen bonding between starch molecules and facilitate the reduction of pastingtemperature. On the other hand, when the product is solubilized, retrogradationof the starch chains is inhibited, resulting in a more fluid paste with improvedclarity, viscosity stability, freeze–thaw stability, and cohesiveness. Although thesubstituted starch paste thickens when it cools, reheating will return it to itsoriginal hot viscosity and clarity.

When the MS increases the pasting temperature decreases, until the productis soluble in cold water. These products have excellent viscosity stability over awide range of conditions. The viscosity of high MS hydroethyl starch solution isquite stable to shear changes, changes in pH, and enzymatic attack. Salts havelittle effect on these products.

Hydroxyethyl starch is used in papermaking, textile manufacturing, medicaland pharmaceutical industries. Hydroxypropyl starch is used in the food industry,textile and paper products, building materials, cosmetics, and pharmaceuticals(134).

Cationic starches are obtained by reaction of starch with reagents containingamino, imino, ammonium, sulfonium groups. The two main types of commercialproducts are the tertiary amino and quaternary ammonium starch ethers (135).

Quaternary Ammonium Starch Ethers. Among the reagents that canadd quaternary ammonium groups to starch probably the most popular is 2,3-epoxypropyltrimethylammonium chloride. Ethers of this type with different DShave been prepared and tested for their role in adsorption on calcium carbonatein relation with the mechanisms of dispersion and flocculation of small particles(136).

Cross-linking. Starch polymer chains can be cross-linked with difunctionalreagents to form diethers or diesters. These derivatives are distinguished fromstarch ethers and starch esters because the properties obtained by the cross-linking of starch are specific. In general terms cross-linking reinforces granuleintegrity, modifies the water retention capacity, and provides higher mechanicalresistance and improves film properties.

Cross-linking is employed when a stable, high viscosity starch paste isneeded, and particularly when the dispersion is to be subjected to high tempera-ture, high shear, and/or low pH (137).

Hemicelluloses. Hemicelluloses constitute a series of polysaccharides as-sociated with cellulose in plants. In wood, in addition to lignin and cellulose, hemi-celluloses represent around 25% by weight (138). Some structures are given inFigure 24. The composition of hemicelluloses depends on the source: deciduoustrees (hardwoods) have mainly glucuronoxylan; coniferous trees (softwoods) yieldmore glucomannans, O-acetylgalactoglucomannans, and xylans having both ara-binofuranosyl and 4-O-CH3-galacturonic side chains. Some softwoods also havesignificant amounts of arabinogalactan (eg in larch wood) (138).

Hemicelluloses from annual plants (cereals crops) are mainly of the xy-lan type; mixed-linked β-1,3-1,4 glucans are also found. The structure has been


Fig. 24. Partial chemical structure of some hemicelluloses: A = O-acetyl-(4-O-methyl-glucurono)xylan from hardwood; B = arabino-(4-O-methylglucurono)xylan from softwood;C = O-acetyl-galactoglucomannan; D = arabinogalactan from larch wood (138).

reviewed (138) and it is clear that they represent a large variety of molecules.There are no important applications for hemicelluloses extracted from plants.

Galactomannans and Glucomannans. Galactomannans are neutralpolysaccharides isolated from seeds (carob, guar, locust bean, tara, etc) (140). Themain chain is made of →4-β-D-Man-(1 (M) with different degrees of substitutionon O-6 with α-D-galactopyranosyl units (G) (Fig. 25). The structure is easily de-termined by 1H NMR.

The solubility of the galactomannan depends on the ratio M/G and on thedistribution of galactose units along the mannan chain; the larger the galactosecontent, the higher the solubility in water. The ratio M/G varies from 1:1 to 1:5in Mimosa scrabella to locust bean gum respectively. In addition, the rheologicalproperties of solutions depend on M/G because of an increase of loose interchaininteractions when G content increases (141–144).


Fig. 25. Representation of the galactomannan structure. ∗ represents the C-6 positionwhere OH can be specifically modified.

Cooperative interactions may be stabilized between xanthan and galac-tomannan depending on the conformation of xanthan (helical or coiled). Two typesof gels can be identified: one around 20◦C involving disordered xanthan and galac-tomannan whatever its content in galactose; the second one, around 60◦C, engageshelical xanthan and it is the usual gel identified when these two polymers aremixed (145–152). Molecular modeling was applied for the determination of thepersistence length of a galactomannan model for which M/G ≈ 1 (47); the valuepredicted as a function of the DP is given in Figure 26. The role of the microstruc-ture of the polymer (random or blockwise distribution of galactose, content ofgalactose) was also analzsed by molecular modeling (153).

Fig. 26. Persistence length of galactomannan (for G/M = 1) as a function of the degreeof polymerization x. L∞ is equal to 9.6 nm. Reproduced from Ref. 47, with permission fromElsevier (Copyright 2003).


Fig. 27. Schematic representation of the pectin structure. Reproduced from Ref. 157 withthe permission from American Chemical Society. (Copyright 2003).

Glucomannans are also obtained from seeds or roots; the structure is a ran-dom arrangement of →4)-β-Glc-(1 and →4)-β-Man-(1. The ratio Glc/Man dependson the source and varies from 1:1 in Iris bulbs to 1:5 in certain gymnosperms.Konjac glucomannan has been developed mainly in Japan for food applications(154,155).

Pectins. Pectins are located in the middle lamella and primary cell wallsof plant tissues. They are extracted from apple skins and citrus peels. Pectinsconsist primarily of 1,4-linked α-D-galacturonic units or their methyl esterswith some interruption by a rhamnogalacturonan region kinking the linearpolygalacturonic backbone (156). They also contain branched chains composedof neutral sugars such as galactose or arabinose. These neutral sugars amountto 10–15% of the pectic dried weight and are concentrated in blocks (called thehairy region; Fig. 27) (157).

In nature, they occur often with different degrees of esterification (DE)around 70%. Because of enzymic degradation, they can have a lower DE; in thatcase (if DE < 50%), they are insolubilized by calcium cross-linkage. Extractionrequires the use of chelating agent.

The interaction between calcium ions and carboxyl groups of the pectinsforms gels; the interaction with calcium is very cooperative owing to the stereo-chemistry of the 1,4-linked monomeric units, leading to the formation of a polarcavity that can be occupied by calcium or related cations. The mechanism of inter-action is described by the egg-box model. A series of galacturonic acid oligomerswas investigated to test the minimum carboxylic group necessary to get a stablejunction (158–160). A two-step process leads to gelation: first dimer formationfollowed by aggregation forming junction zones (161–163).

The ability to gel can be determined from the dependence of viscosity as afunction of counterions added (164). At a given concentration, the critical amountof cations at the gel point is directly related to the distribution of carboxyl groupsalong the chain (Fig. 28).

It is shown that the divalent counterions form gels with a sequence of affinityBa > Sr > Ca, but Mg does not form gels nor dimers. The interaction with counte-rions as well as change in conformation have been identified by X rays (165) andby circular dichroism (162).


Fig. 28. Change in reduced viscosity and scattered light during addition of CaCl2 topectins (polymer concentration: 0.2 g/L; 5 × 10− 3 M CaCl2). Role of the carboxyl groupsdistribution: (a) DE = 30% blockwise distribution obtained by pectinesterase hydrolysis;(b) DE = 30% random distribution obtained by alkaline hydrolysis. Reproduced from Ref.164, with permission from John Wiley & Sons, Inc., on behalf of the Society of ChemicalIndustry (Copyright SCI 2003).

There are different categories of pectins: HM pectins (high methoxyl; DE >

50%) form gels in acid conditions and in the presence of sucrose to decrease the wa-ter activity. The gelation is based on H-bond association and is thermoreversible.Only, LM pectins (low methoxyl; DE < 50%) form gels in presence of calcium.

HM pectins can be extracted by water, mineral acids, or bases. LM pectinsimmobilized in situ via metallic ions need a sequestering agent to displace thecounterions. Alkaline treatment also has a role in the deesterification. Differenttreatments were compared on sugar beet and potato pulps. HM pectins are natu-rally occurring in sugar beet pulp; they were extracted in alkaline conditions (50mM NaOH, pH 12) and precipitated with ethanol after neutralization to pH 6.5–7.A pretreatment of the plant material is necessary to inactivate enzymes (water at85◦C, 20 min). On the contrary, for potato pulp containing an LM pectin the extrac-tion is performed in acid conditions (pH 3.5 in presence of 0.75% hexametaphos-phate, 75◦C, 1 h). The pectins are precipitated at pH 2 with HCl. The precipitateis redispersed in water with NaOH up to pH 6.5–7 and reprecipitated by ethanol.Potato pulp was also extracted in alkaline conditions with hexametaphosphate,precipitated at pH 2, redispersed in water to pH 6.5–7, and precipitated by ethanolas previously. The LM pectins obtained in alkaline conditions have a higher gellingability due to deesterification and partial deacetylation (166,167).

Seaweed Polysaccharides

Algae are an important sources of polysaccharides with different structures.Among the different groups of algae, green algae (Chlorophycea), red algae


Fig. 29. Structure of alginate consisting of guluronic (G) and mannuronic (M) blocks orMG blocks.

(Rhodophycea), and brown algae (Phaeophycea), only the last two series are usedon an industrial scale (168). Many different polysaccharides may be extracted butalginates and carrageenans are the more developed especially as physical gel for-mers (169). Fucoidans also are now under investigation because of the importanceof sulfated polysaccharides for biological applications (170,171).

Alginates. Alginates are linear copolymers of α-L-guluronate (G) and α-D-mannuronate (M) (Fig. 29). They are also called polyuronides and are extractedfrom brown algae. Their gelling properties derive from the cooperative binding ofdivalent cations on the G blocks in the egg-box model (similar to that proposed inLM pectins).

Alginates behave as polyelectrolytes in the presence of monovalent counteri-ons, and in 0.1 M NaCl, the intrinsic viscosity allows determination of the averagemolar mass (172).

Mechanical properties of calcium alginate gels are related to the ratio M/Gand also to the distribution of the G units along the chain and to the molar massof the polymer (173). Alginate is often used for encapsulation of pharmaceuticals,bacteria, or yeasts in biotechnological processes. The beads of gel are formed by adripping technique; drops of alginate solution falling in 1 N CaCl2 solution formbeads which are washed in water after a few hours maturation. Smaller and moreregular particles can be obtained by emulsification (174,175); the porosity of thesegels is smaller than for that obtained by the dripping method.

Agarose and Carrageenans. These polysaccharides represented a fam-ily of alternating (AB) type copolymers with different degrees of sulfation; agaroseis the neutral polymer form. They are extracted from red algae. Their structuresare given in Figure 30. They consist of alternating (1→3)-β-D-galactose and (1→4)-α-(3,6)anhydro-D-galactose or (1→4)-α-(3,6)anhydro-L-galactose (in agarose).


Fig. 30. Structure of red algae polysaccharides. (A) R = H in κ-carrageenan; R = SO3−

in ι-carrageenan; (B) λcarrageenan; (C) agarose.

Agarose, κ-carrageenans, and ι-carrageenans are recognized as gelling poly-mers; the conditions of gelation as well as the mechanism of gelation have beenabundantly discussed in the literature. The gels are thermoreversible based onH-bond stabilized conformations. Gelation is based on the formation of double he-lices which associate forming an aggregate or a junction zone which is the basis ofthe network formation; a two-step gelation process has been demonstrated clearly.Gelation and melting of the gel present a hysteresis in temperature directly re-lated with the degree of aggregation of double helices and to the charge density ofthe polymers (176–178). The rigidity of the gels follows the same trend. A sequenceof solubility has been established : Agarose < κ-carrageenan < ι-carrageenan <

λ-carrageenan following the order of sulfate density: No sulfate < 1 sulfate < 2sulfates < 3 sulfates The stiffness of the gels formed in presence of K+ counterionsvaries in the following order: Agarose > κ-carrageenan > ι-carrageenan The poly-mer λ-carrageenan never forms gels or helical conformation.

For charged polysaccharides, the formation of double helices and gelation de-pends on the ionic concentration, nature of electrolyte, and temperature. A phasediagram relating the total ionic concentration and the inverse of the tempera-ture for conformational change (Tm

− 1) has been established for each of them. Forκ-carrageenan, the ionic selectivity among monovalent cations is very important(179,180); the sequence is as follows:

Rb+ > K+,Cs+ > Na+ > Li+,NH+4 > R4N+


The mechanical properties of the gels formed in the presence of differentcounterions follow the same order: Rb-carrageenan gel has a higher modulus anda higher melting temperature than those of Na-carrageenan gel. The molecularweight also plays a role on the elastic modulus which increases up to a limitaround M ∼ 250,000 (181). The mechanism of gelation in two steps is describedin Reference 182.

In the series of anions, a characteristic behavior was observed with I − ; itstabilizes the double helix but prevents gelation (180).

Microbial Polysaccharides

Fungi and bacteria are sources of polysaccharides and especially of exopolysac-charides which can be produced in culture media on an industrial scale. They area source of new additives for cosmetic or food applications but also for biologicalactivity. Many of them are now in development; a review will be published in thesecond edition of Reference 12. Many of these polysaccharides are water solubleand able to compete with natural polysaccharides as described before (alginates,carrageenans, galacto- and glucomannans, chitosans, pectins, etc) especially inthe domain of food additives. Many books discuss their applications (183–187).

The best examples of fungal polysaccharides are the β(1→3) glucans with afew β(1→6) glucose units as side groups. They often form a triple helical conforma-tion with a tendency to give physical gel. There have been studies on scleroglucan,lentinan, schizophillan, etc (139,188,189). Because they are neutral, solubility de-pends on the fraction of side glucose units and their distribution.

Bacterial polysaccharides represent a large variety of polymers biosynthe-sized by bacteria. Their chemical structures and also their physical propertiesin solution or in the solid state may vary widely. They often contain uronic acidand then become polyelectrolytes (190). Many new polysaccharides have beendeveloped from bacteria for industrial purposes. Exocellular polysaccharides areproduced on a large scale by the usual techniques of microbiology and fermenta-tion. This procedure allows good control of the characteristics of the polymers andallows purification of the polysaccharides more easily than from other naturalsources (191–194). Extension of such production also allows reducing the priceand extends the range of applications. A good example remains the hyaluronanpreviously produced by extraction from animal sources but in which some fractionof proteins remained. Bacterial hyaluronan can be prepared in a very pure form(195).

In general, polysaccharides are especially important in the domain of water-soluble polymers; they play an important role as thickening, gelling, emulsifying,hydrating, film forming, and suspending polymers. Especially important is thefact that some polysaccharides give physical gels in well-defined thermodynamicconditions. They constitute a very important class of materials in food, cosmetics,or pharmaceutical applications.

Specific methods for the purification of the polysaccharides have been devel-oped. They are usually isolated in their sodium salt forms by precipitation fromaqueous solution with ethanol or 2-propanol. This step is the most important oneto get reproducible experimental characteristics. In fact, because of the presence


of many OH groups in the molecule, the polysaccharides have a tendency to formcooperative intra- and interchain H-bonds, causing some insolubility or at leastthe presence of aggregates when solutions are prepared. Because of their stere-oregularity, they are often able to form helical conformations in solution. Theirordered conformation has a semirigid character and its stability depends on tem-perature and ionic concentration if the polysaccharide structure contains uronicacid unit or ionic substituents; an example is given for succinoglycan (196,197).

Details of the chemical structure of bacterial polysaccharides have been givenpreviously (198,199). In the following, the main industrial bacterial polysaccha-rides (xanthan, succinoglycan, gellan) are described briefly.

Succinoglycans are produced by many soil bacteria of the species Pseu-domonas, Rhizobium, Agrobacterium, and Alcaligenes. These polysaccharideshave a general chemical structure based on an height sugar repeat unit (D-glucode/D-galactose ratio equals 7:1); this structure is given in Figure 31.

Different substituents are also present in the molecule such as acetyl, suc-cinyl, and pyruvyl groups, depending on the strain and on the conditions offermentation and/or isolation (200–203). The content in substituents is easily de-termined by 1H NMR in the presence of an internal standard to calibrate thesignals and get a quantitative determination.

Succinoglycan, whatever its origin, is water soluble and, being stereoregular,it adopts a single-chain helical conformation at least in dilute solution. As shownby different techniques, the conformational transition is reversible and very co-operative at least in the presence of some salt excess. The stability of the orderedconformation depends on the salt concentration in solution as is usual for chargedpolysaccharides. On the two sides of the transition, the stiffness is completely dif-ferent. In the helical conformation, the chain behaves as a semirigid chain havingan intrinsic persistence length (Lp) estimated to be 35 nm and becoming moreflexible at higher temperature in the coiled conformation with Lp ∼ 5 nm. Thepersistence length in the helical conformation has been confirmed by AFM fromthe analysis of chain curvature (204).

The rheological behavior in dilute and semidilute solution is quite usual andtypical of viscoelastic systems (197).

Xanthan was the first bacterial polysaccharide produced by the strain Xan-thomonas campestris and developed on a large scale. The chemical structure ofxanthan is recalled in Figure 31; it was confirmed by NMR. The 13C spectrumallows identification of the different sugar units as well as the substituents (205).1H allows quantification of the yield in substituents. Xanthan is a semirigid poly-mer for which, in the native conformation, a persistence length around 40 nm wasfound (206–208). It has been demonstrated that when the native form is heatedto coil conformation (with a much lower stiffness) and renatured by cooling, theconformation is changed. Xanthan has many industrial applications due to itssuspending character (stabilizer of solid suspensions or foams), its pseudoplasticand thickening properties, good stability of the rheological properties as a functionof pH, temperature, and salt concentration, and also its ability to be cross-linkedand gelled. It is used in paints and coatings, cosmetics, ceramics, etc. In Figure32 the rheological behavior characteristic for these semirigid polysaccharides isshown. The dependence of the viscosity as a function of shear rate and polymerconcentration demonstrates the existence of a Newtonian plateau for low shear


Fig. 31. Examples of some bacterial polysaccharide structures: (A) succinoglycan; (B)xanthan; (C) gellan in the native form and after deacylation.

rate followed by a decrease of the viscosity due to the viscoelastic character of thesolution (209).

Gellan is produced by the bacteria Sphingomonas elodea (210). The nativegellan as produced by the bacteria contains two additional substituents (acetyland L-glyceryl) compared to the commercial polymer named Gelrite® (Fig. 31).


Fig. 32. Relative viscosity–shear rate curves for xanthan at different polymer concentra-tion. Solvent: 0.1 M NaCl; T = 25◦C; Mw = 7 × 106. Reproduced from Ref. 209, with thepermission from Springer-Verlag GmBh & Co. (Copyright 2003).

The native gellan gives highly viscous solutions with a loose gel-like behavior.After deacylation, gellan gives a strong and rigid gel; a comparison between thenative and deacylated gellan has been done (211,212). The chemical structuresof the repeat units of the native and deacylated gellans are easily controlled by1H NMR.

The mechanism of gelation of deacylated gellan was examined and shown asa two-step process similar to the mechanism proposed for κ-carrageenan (182,213).Deacylated gellan adopts an ordered double-helix conformation in salt excess andthe temperature for conformational change Tm increases when the salt concen-tration increases as is usual for stereoregular Polyelectrolytes (qv). The helicalconformation is more stable in the presence of divalent counterions but no ionicselectivity appears among monovalent counterions (Li, Na, K, TMA) on one sideand among divalent counterions (Ca, Mg) on the other side (212). When the saltand polysaccharide concentrations increase, the double helices interact to give aphysical gel with strong ionic selectivity. The sequence of selectivity is

K+ > Na+ > Li+

K+ promotes gel formation and gives the higher elastic modulus.Many new bacterial polysaccharides are now under development when they

have specific physical behavior.


Mammalian Polysaccharides

Two homopolysaccharides are found in animals: glycogen and chitin. The oth-ers are heteroglycans based on disaccharide repeat units (they are also namedglycosaminoglycans).

Glycogen. It is a storage polysaccharide able to produce D-glucose de-pending on the nutritional level. It is a highly branched neutral polysaccharide,analogue to amylopectin but more highly branched; thus, its solubility in water islarger.

Chitin. It is found in bacteria, fungi (such as Aspergillus niger), and crus-taceans but only crustaceans are important commercial sources (see CHITIN AND

CHITOSAN). Especially crab or shrimp shells are the main sources of chitin; in situchitin is embedded in calcium carbonate and proteins. Chitin is a linear polymermade of →4(β-D-N-acetyl-D-glucosamine)-1. Treatment of shells with acid and al-kali allows isolation of chitin partially deacetylated. When chitan in decetylatedto about 50% of the free amine form it is refered to as chitosan. Chitin is therenewable polysaccharide of most importance in quantity produced per year af-ter cellulose. It has found many applications, at least on laboratory scale in food,medical, cosmetic applications in which their original structure is appreciated.After partial deacetylation, the polymer becomes soluble in acidic conditions byprotonation of the amino group in C-2 position; the intrinsic pK of the NH2groups is around 6 (214,215). Methods for characterizing chitosans with differentdegrees of acetylation (DA) have been re-examined using infrared spectroscopyand NMR (216–218). The molar mass distribution was examined by SEC (219).This polymer is the only natural water soluble polycation; it may be used for floc-culation in water purification. It is recognized for its mucoadhesive properties.Unlike cellulose or starch, chitosan is able to be chemically modified on the spe-cific C-2 position of the glucosamine unit. Several derivatives have been prepared.Alkylation gives alkylchitosans (220,221), amphiphilic polymers which are goodthickeners. Carboxymethylation gives an anionic polymer (222) and quaterniza-tion of NH2 gives a cationic polymer (223,224). Grafted cyclodextrin–chitosanhas also been prepared (225,226). Another original property of chitosan is itschelating properties; a specific complex is formed with Cu2+ and its structure hasbeen proposed (227,228). The applications and biochemical properties of chitosanhave been reviewed in two books (229,230).

Proteoglycans. They are carbohydrate–protein polymers in which manylong polysaccharide chains are covalently linked to the protein core. The per-centage of protein is low. They are also classified as glycoproteins, but the nameproteoglycan is now preferred to the historical term mucopolysaccharide. Thesepolymers are widely distributed in connective tissue, cartilage, tendon, or synovialfluid. Carbohydrate components (with an alternating structure (AB)n dividedinto six groups) based on disaccharide repeat units (Table 7) are joined to theprotein core in different ways by a N- or O-glycosidic linkage. Depending on theircomposition, glycoaminoglycans are known as hyaluronan, chondroıtin sulfate (in4- or 6-position), dermatan sulfate, heparin, heparan sulfate, and keratan sulfate.Much research is in progress on these biopolymers; a review on the structure,properties, medical and biological applications of hyaluronan has been published(231).


Table 7. Disaccharide Units Constitutive of Proteoglycans

Basic disaccharide unit Common names

GlcNAcβ1,4-GlcAβ1,3 Hyaluronic acidGalNAc(6 or 4-SO3

− )β1,4-GlcAβ1,3 Chondroitin sulfateGalNAc(4-SO3

− )β1,4-L-IduAα1,3 Dermatan sulfateGlcNSO3(6-So3

− )α1,4-L-IduAα1,4 HeparinGlcNSO3β1,4-L-IduAα1,4 Heparan sulfateGlcNAc(6-SO3

− )β1,3Gal(6-SO3− )β1,4 Keratan sulfate

Glycoconjugates

Glycoconjugates, eg glycoproteins and glycolipids, are biopolymers; the name in-dicates that the carbohydrate is the minor component. In the glycoconjugates,oligosaccharide moieties consist of up to 20 monomers. They are basic compo-nents of all the cell membranes. Thus, certain oligosaccharides can be associatedwith degenerative cell growth and can be used in cancer diagnostics as so-calledtumor-associated antigens.

Glycoproteins. They are proteins containing one or several oligosaccha-ride side chains. The linkages are N- or O-glycosidic linkages or by ethanolaminephosphate. The O-glycoproteins seem to be more involved in the stabilization andprotection of protein structures rather than as signaling molecules in cell commu-nication.

Glycolipids. They are amphiphilic molecules of low molecular weight.The lipophilic part consists of 1,2-di-O-diacylglycerol or N-acylsphingosin; thehydrophilic part is a phosphate group and/or a carbohydrate moiety. There aremany glycolipids with different covalent structures with specific biological roles.

Another series of glycoconjugates (but usually not joined in this classifica-tion) is represented by natural plant gums such as Arabic gum which is recognizedas very specific to stabilize emulsion. In these polymers, the glycosidic part is themost important in weight fraction compared to the protein fraction.

Glycopolymers

Under this term we refer to synthetic polymers in which at least one of themonomers engaged contains a saccharidic unit.

The extensive participation of saccharides in recognition processes hassparked the synthesis of new polymeric materials expected to have sophisticatedfunctions similar to or even superior to natural glycoconjugates. Numerous syn-thetic polymers carrying various kinds of saccharide residues as information el-ements have therefore been developed. It has been shown that such glycocon-jugates may have enhanced binding capacity with lectins based on a polymersugar-cluster effect whereas the monomeric carbohydrate derivatives exhibit onlyweak affinity to the same lectins (232–235). These “glycopolymers” have thus di-verse potential uses, such as multivalent inhibitors of cell or virus binding, cell-specific culture substrata, artificial antigens, targeted drug delivery systems. The


term “glycopolymers” was first introduced by Roy and co-workers (236) as a re-placement for “pseudo-polysaccharide” previously used, to designate water-solubleas well as insoluble polymers bearing covalently bound carbohydrates. In the fol-lowing, the term glycopolymer refers to synthetic (or natural) linear polymerspossessing sugar moieties as pendant or terminal groups irrespective of whetherthey are water soluble or not. Glycopolymers can be obtained by polymerization ofsugar-carrying monomers. An alternative synthetic method consists of the chem-ical modification of preformed polymers using carbohydrate-containing agents.Nevertheless, both strategies generally rely on the same carbohydrate precursors.

In accordance with the great attention that these glycoconjugates have at-tracted for the last 10 years, several reviews on the synthesis and biological func-tions of these macromolecules have been published (237–243). In fact, the selec-tion and design of a glycopolymer for a given application is a challenging taskbecause of the inherent diversity of structures and to the impact of the latter andsolution conformation of the sugar-containing polymers on their biorecognition.Here in is considered glycopolymers with an emphasis on their structures, theirphysicochemical and biological properties, and their applications.

Synthesis Strategy. Polymer chemistry offers the possibility to prepareglycopolymers with custom-designed properties since several groups with specificproperties can be easily introduced along the polymer chain, as shown in thescheme:

Indeed, besides the saccharide moieties which are expected to act, in mostcases, as recognition signals, various molecules such as hydrophobic chains, fluo-rescent probes, and drugs can be used to modulate the solution properties of thepolymer, label the cells, or deliver a given drug on site. In addition, the polymerbackbone can be adjusted to confer desired physical or biological properties.

The most commonly used strategies for the syntheses of such glycopolymersinvolve direct homopolymerization of sugar-bearing monomers or copolymeriza-tion of the latter with one or two other monomers. The next most used strategyinvolves chemical modification of preformed polymers with suitably functionalizedcarbohydrate residues. Generally, both methodologies require the same glycosyn-thons and several methods leading to activated carbohydrate derivatives suitableto be covalently linked to either a preformed polymer or a polymerizable anchorhave been described. These methods belong to five main categories, discussedbelow and depicted in Figures 33–37.

The first one relies on standard glycosylation chemistry, providing O-, C-, orS-glycosides having, for example, alkene, styrene or amine groups in the aglyconportion (Fig. 33) (236,244–255) The intermediate amine-containing glycoside canbe used in graft copolymerizations or acryloylated to give a monomer precursor.


Fig. 33. Examples of glycosynthons obtained by standard glycosylation reactions.

Fig. 34. Synthetic routes to carbohydrate monomers based on the reductive aminationreaction.

The second approach is based on the reductive amination of the sac-charide (lactose, maltose, N-acetylchitooligosaccharides, etc) by ammonium ac-etate, hydrazine, N-methylamine, or 4-acrylamidobenzylamine, followed byN-acryloylation (256,257), addition of 4-vinylbenzyl isocyanate (258) orN-acetylation (259,260) (Fig. 34).

This method is quite easy and rapid; however, it leads to an altered saccha-ride, the reducing sugar being transformed into a linear polyol amine derivative.Therefore, it is rather applied to oligosaccharides since at least, the terminal sugaris preserved.

Fig. 35. Preparation of suitable glycosynthons for polymer synthesis from carbohydratelactones.


Fig. 36. Synthetic routes to polymerizable glycosylamides.

In the third approach, the reducing end of the carbohydrate (maltose, lactose,maltotriose, etc) is oxidized by hypoiodite and then, the resulting lactone can becondensed with p-vinylbenzylamine (261) or 1,4-diaminobutane (262) (Fig. 35).For the same reasons as those described above, this reaction is rather applied tooligosaccharides.

In the fourth approach, the carbohydrate can be reversibly converted intoa glycosylamine by treatment with an ammonium salt (263,264) or an aliphaticamine (265,266). However, these derivatives are quite unstable in slightly acidicmedium or neutral medium. They are further stabilized by acylation with, forexample, anhydride acetic (265), p-vinylbenzoyl chloride (267), acryloyl chloride(263), or chloroacetic anhydride, followed by ammonolysis (268) (Fig. 36). The N-linked structure of these glycosynthons is distinct from the open chain structureof the carbohydrate derivatives described above. This method may be useful tointroduce complex oligosaccharides.

These different methodologies generally provide unprotected activatedglycosynthons. Carbohydrate hydroxyl or amine residues at positions otherthan the anomeric center can also be used to introduce functional groups fordirect or graft polymerization. These carbohydrate precursors can be fullyprotected or unprotected. However, in the former case, the resulting glycopoly-mers must undergo a final deprotection step of the carbohydrate moietiesunder the usual conditions. Some examples of these glycosynthons such asN-methacryloylglycylglycylgalactosamine (MA-GG-GalN) (269,270), diacetoneglucofuranose acrylate (271), p-vinylbenzylether (272) are given in Figure 37.

Fig. 37. Examples of carbohydrates monomers bearing a polymerizable group at a posi-tion other than the anomeric center.


In this last approach, although it has been shown in some cases that thecarbohydrate–protein interaction remains efficient (269,270), the carbohydratemodification at a position other than the anomeric center may decrease its affinitytoward the protein.

Glycopolymer Syntheses by Homo- or Copolymerization. The rad-ical polymerization of N-acryloylated carbohydrate precursors with acrylamideand/or acrylamide derivatives with specific properties is likely one of the mostcommonly used strategies for the syntheses of glycopolymers. Indeed, this is aversatile method which can be easily and efficiently performed in aqueous solu-tion from unprotected sugar-carrying monomers. Moreover, it generally leads towater-soluble derivatives. This method has been extensively used for the prepara-tion of sialic acid bearing polyacrylamides as inhibitors of influenza virus adhesion(251–253,273). Several custom-designed glycopolymers made of two or more com-ponents were also synthesized according to this methodology. Various applicationsof such copolymers have been envisaged as discussed in the following section.

Numerous carbohydrate styrene monomers such as (p-vinylbenzamido)-β-lactose and N-(p-vinylbenzyl)-4-O-β-D-galactopyranosyl-D-gluconamide have beenhomopolymerized (or copolymerized) in an organic solvent (dimethyl sulfox-ide) using 2,2′-azobisisobutyronitrile (261,267). The resulting polystyrenes, hav-ing amphiphilic structures, were shown to have strong affinity toward specificlectins. Moreover, the hydrophobic effect of the styrene moiety allowed prepa-ration of polymeric nanospheres with a polystyrene core and a glycopolymercorona. These nanospheres were synthesized by free radical copolymerization ina polar solvent (ethanol/water mixture) of styrene with a hydrophilic carbohy-drate macromonomer such as the glucosyloxyethyl methacrylate macromonomer(Fig. 38) (274). The concanvalin A lectin recognized the glucose on the nanosphereswith a better binding activity than with the monomeric glucose.

Living polymerization techniques have been successfully used for the syn-thesis of glycopolymers with controlled molecular weights and molecular weight

Fig. 38. Structure of the glucosyloxyethyl methacrylate (GEMA) macromonomer.


Fig. 39. Example of an amphiphilic block copolymer obtained by living cationicpolymerization.

distributions. They have also served for the preparation of well-defined blockcopolymers. These methods are based on “living” radical polymerizations suchas nitroxide-controlled polymerization (271,275) and atom transfer radical poly-merization (276), living cationic (277–279) or anionic (280) polymeriation (seeLIVING POLYMERIZATION, CATIONIC; ANIONIC POLYMERIZATION; CARBOCATIONIC POLY-MERIZATION). Thus these polymerization strategies allowed preparation of newamphiphilic block copolymers, the hydrophilic segment having pendant sugarmoieties (see Fig. 39). However, one of the drawbacks of the approach is thatit involves protected carbohydrate monomers and thus, a final deprotection stepof the glycopolymer is necessary.

An alternative method developed by Kiessling and co-workers is ring-openingmetathesis polymerization (ROMP) (281–285). A significant advantage of thisstrategy is that living polymerization can be performed from unprotected car-bohydrate monomers including, for example, sulfated sugar-carrying monomers.Moreover, besides the possibility of preparing block copolymers, specific endla-bels can be introduced. However, although the molecular weight distribution ofglycopolymers obtained by the ROMP strategy is narrower than polymers pre-pared through classical radical polymerization approaches, the control of molecu-lar weights remains difficult.

Glycopolymer Syntheses by Reactions on Preformed Polymers.Compared to the methodologies based on polymerizations of sugar-carryingmonomers, the chemical modification of preformed polymers using carbohydrate-containing reagents is generally advantageous as it requires fewer reaction stepsand it allows easy control of the number of sugars along the polymeric chain.Moreover, large quantities of preformed polymers can be synthesized with thedesired molecular weight. Therefore, the same polymer backbone can be used toprepare glycopolymers with different carbohydrate contents and/or carbohydrateresidues. Of course, the preformed polymers must have reactive functionalitiessuch as amine, carboxylic acid, or alcohol groups. These polymers may be acti-vated or not. For instance, polyacrylates with p-nitrophenyl (249,268) or succin-imidyl esters (252,255) react at room temperature with carbohydrate amines inaprotic polar solvents (DMSO or DMF) to give glycopolymers after the quench-ing of excess active esters with sodium hydroxide, ammonia, or ethanolamine.


Carbohydrate amines also react with polymers possessing pendant hydroxylgroups partially functionalized with p-nitrophenyl carbonate groups (286). Maleicanhydride copolymers have the advantage of being directly modified by carbohy-drate amines under mild conditions. This method allowed efficient preparation ofvarious glycopolymers useful for solid-phase assays or new drug delivery systems(262,287–289). Various carbohydrate derivatives have also been grafted to naturalpolymers such as chitosan. The free amine group at the C-2 position of the glu-cosamine unit can easily react with aldehydes which allows preparation of a widevariety of compounds such as L-fucose-, D-fucose-, D-mannose-branched chitosan(290), cyclodextrin-grafted chitosans (225,291,292), and a sialic acid dendron-grafted chitosan (293).

Moreover, the pendant carbohydrate chains of synthetic glycopolymers havebeen shown to be further elongated by enzymatic reactions. Thus, by this approach,Nishimura and co-workers have been able to prepare a glycopolymer having a sia-lyloligosaccharide using glycosyl transferases and transglycosidases (294). Morerecently, this strategy allowed the selective modification of a glycopolymer possess-ing both lactose and mannose residues (254). Indeed, using a galactosyl trans-ferase, a galactose unit could be introduced specifically on the pendant lactosemoieties.

Role of Glycopolymer Structure. The properties of glycopolymers ofcourse depend on the nature of the pendant saccharide chain as a biorecog-nizable element, but also on the whole chemical structure. Indeed, the latterinfluences conformation, charge, biodegradability, etc, which are important pa-rameters for the glycopolymer functions. For example, glycosylated polystyreneshave been shown to exhibit strong binding affinities to lectins, viruses, and cells(267,295,296) The strong binding affinity has been attributed to the characteristicconformations based on the amphiphilic structure of glycopolystyrenes. Indeed,the hydrophobic main chain tends to form a hydrophobic core that is sheltered fromwater, and hence the saccharide chains gather on the outside of the polymer in wa-ter. In addition, as mentioned above, the hydrophobic effect of the styrene moietyallowed preparation of original polymeric nanospheres having a polystyrene coreand a glycopolymer corona. Furthermore, the significant enhancement of protein–carbohydrate interactions due to hydrophobic associations in water has been wellevidenced by Kopecek and co-workers (269) who have synthesized glycopolymerswith photoresponsive benzospiropyran side chains. Upon irradiation by UV light,benzospiropyrans change to a red colored polar (zwitterionic) merocyanine form.On exposure to visible light, merocyanine converts back to the nonpolar (hydropho-bic) spiro form. Thus, under visible light, the photoresponsive glycopolymers showa high lectin binding due to contraction of the polymer chains, which tends toincrease the probability of formation of clustered saccharide chains. After irradi-ation, the binding decreases as a result of the expansion of polymer chains.

The flexibility of glycopolymers may also have a large influence on theirbiorecognition. In the case of flexible glycopolymers, the increased content of car-bohydrate side chains generally increases biorecognition. This is not confirmedat all in the case of rigid ones. Indeed, rigid cylindrical phenyl isocyanide (PPI)glycopolymers have been shown to exhibit little specific interactions with lectins(297). In these systems, the saccharide arrays are crowded too thickly to be acces-sible or to be induced-fit to the binding sites. The decrease of saccharide density


by insertion of comonomer units into the rigid PPI glycopolymer backbone wasfound to increase the binding affinity to the lectin.

Applications of Glycopolymers. Most applications of glycopolymersarise from the specific molecular and cell recognition of saccharides. Therefore,many bio- and immunochemical properties have been demonstrated for a largenumber of glycopolymers. The synthesis of sugar-based hydrogels from carbohy-drate monomers and cross-linking agents or by covalent attachment of carbohy-drates to commercially available affinity supports made of agarose, cellulose, orpolyacrylamide allowed preparation of new bioaffinity supports (298–300). Thelatter have been used for the purification of antibodies, enzymes, lectins, andmyeloma proteins. Sugar-based hydrogels also show potential as substrates forcell growth. Indeed, the replacement of agar, used in culture plates, by hydro-gels containing specific sugars may enhance the adhesion of cells. Such propertieshave also been evidenced by a lactose-carrying styrene homopolymer (301,302).The latter has been shown to be adsorbed on the surface of glass culture dishesand the resulting polymer-coated dishes have been found to be useful for hepaticcell cultures.

Moreover, glycopolymers have been shown to constitute very sensitive coat-ing antigens in solid-phase enzyme-linked immunosorbent assays (ELISA). Incomparison to protein glycoconjugates, glycopolymers offer several advantagessuch as lower cost productions and improved thermal and biological stabilities.Therefore, many examples of applications of glycopolymers in the detection ofcarbohydrate-binding proteins including serodiagnosis of bacterial antigens havebeen reported. These have been summarized in several reviews (240–242).

Glycopolymers can also be advantageously used to inhibit cell adhesion. Typi-cal examples are polymers bearing pendant α-sialoside groups which show potentanti-influenza activity (249,251–253,268,273,303). Indeed, it is now well estab-lished that human infections by influenza viruses are mediated by the binding ofthe viral membrane protein hemagglutinin (hyaluronan) to sialosides on cell sur-face glycolipids and glycoproteins. The inhibitory properties of such glycopolymersillustrate the value of cooperative polyvalent interaction in the design of potentinhibitors of viral adherence to the host cell.

Glycopolymers also show promising properties as drug-targeting deliveryagents. One of the most successful polymer–drug–saccharide conjugate sys-tems reported are the N-(2-hydroxypropyl)methacrylamide (HMPA) copolymer–adriamycin (ADR)–sugar conjugates, in which both ADR and sugar were attachedto a polymer backbone via lysosomally cleavable oligopeptide spacers (304). Theconjugates containing galactosamine moieties were reported to accumulate in theliver selectively (305). Other glycopolymers such as lactose-carrying styrene poly-mers were also found to be very efficient liver-specific targeting materials (306).

Besides these fields of applications, Wulff and co-workers (307,308) investi-gated the potential ability of glycopolymers to modify surface properties. Thus,they copolymerized several isopropylidene-protected vinyl sugars with styrene,methyl methacrylate, and acrylonitrile. After eliminating the isopropylideneprotecting groups from the copolymer surfaces by acid hydrolysis, these sur-faces were shown to become hydrophilic with improved dyeability and surfaceconductivity.


Synthesis of Oligo- and Polysaccharides

The chemical synthesis of oligosaccharides or their analogues is well developednow; nevertheless, it is a difficult task. Protection of labile groups of the OHposition which are not engaged in an osidic linkage is needed. Then, reaction withspecific groups of the anomeric and nonanomeric position must be done with acontrol of the stereochemistry of the anomeric position. Oligosaccharides in whichglycosidic oxygen atoms are replaced by sulfur atoms can be routinely synthesizedby iterative or convergent approaches (309,310) and these nonnatural compoundsare not hydrolyzed by various glycosidases (311,312).

Polysaccharides having a regular structure were first obtained by polymer-ization of anhydrosugars or step-by-step elongation. For synthesis of irregularstructures, a step-by-step elongation by polycondensation is the usual method atleast to produce the repeating unit which is then polymerized. These routes havebeen discussed previously in the review of Kochetkov (313).

A different but interesting approach comes from the enzymatic polymer-ization of the corresponding mono- or disaccharide precursors with controlledstructure.

Synthesis of Regular Polysaccharides. The first attempt at such syn-thesis appears with the work of Haq and Whelan in 1956 (314); they proposed,without success, the polycondensation of 2,3,4,-ti-O-acetyl-α-D-glucosyl bromidein the presence of Ag2O and CaSO4 to get a (1–6)-β-D-glucan. Other negativeattempts have been published (313).

The most fruitful results came with the synthesis of polysaccharides by poly-merization of anhydrosugars. Schuerch (315–317) investigated the mechanismof cationic polymerization in solution of 1,6-anhydro-hexopyranoses wherein the2,3,4 positions were protected by benzyl groups. He got high molecular weight(1,6)-α-D-glucan and the benzyl groups were easily removed at the end of polymer-ization. The 1,4; 1,3; 1,2 anhydrosugar polymerizations were also performed withless success. However, (1–3)-α-D-glucopyranans and (1–3)-α-D-mannopyrananswere obtained with high stereoregularity. These results are discussed in the re-view by Kochetkov (313).

Chemoenzymatic Synthesis. Original oligosaccharides bifunctional-ized with fluorescent entities were synthetized in the presence of a mutant ofHumicola insolens endoglucanase (318). Hemithiocellodextrins with degree ofpolymerization from 4 to 14 were synthetized in the presence of a cellulase inbuffer/organic solvent (319).

Homopolymers were also obtained with mutant of Barley (1,3)-β-D-glucanendohydrolase producing (1,3)-β-D-glucan and with mutant of cellulase producingβ-(1→4)-oligo- and polysaccharides (320,321).

A very active research is lead by the Kobayashi group in Japan; in a recentreview, cellulose, chitin, and xylan syntheses were described using EnzymaticPolymerization (qv) (322).

Hydrolases were most often used as catalyst forming the glycosidic bondsin vitro. Recently, synthetic polysaccharides were synthetized using other en-zymes such as glycosyltransferase, phosphorylases, oxidoreductases, and lipases(323).


Copolymers may also be produced using combinations of chemically modi-fied mono- or disaccharides and enzymatic condensation. Such a route has beenproposed to prepare an artificial hyaluronan, GlucA-β-(1→3) GlucNAc oxazolinemonomer reacting with hyaluronidase (324).

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11825 (2001).

M. RINAUDO

R. AUZELY

K. MAZEAU

Centre de Recherches sur les Macromolecules Vegetales,Affiliated with Joseph Fourier University


POLYSILANES. See Volume 7.

POLYSULFIDES. See Volume 3.

POLYSULFONES. See Volume 4.

POLYTETRAFLUORETHYLENE. See PERFLUORINATED POLYMERS.

POLYTRIACETYLENE. See DIACETYLENE AND TRIACETYLENE POLYME.

POLY(TRIMETHYLENE TEREPHTHALATE). See Volume 3.

POLYURETHANES. See Volume 4.

POLY(VINYL ACETATE). See VINYL ACETATE POLYMERS.

POLY(VINYL ALCOHOL). See VINYL ALCOHOL POLYMERS.

POLYVINYLCARBAZOLE. See VINYLCARBAZOLE POLYMERS.

POLY(VINYL CHLORIDE). See VINYL CHLORIDE POLYMERS.

POLY(VINYL ETHER). See VINYL ETHER POLYMERS.

POLY(VINYL FLUORIDE). See VINYL FLUORIDE POLYMERS.

POLY(VINYLIDENE CHLORIDE). See VINYLIDENE CHLORIDE

POLYMERS.

POLY(VINYLIDENE FLUORIDE). See VINYLIDENE FLUORIDE

POLYMERS.

POLYVINYLPYRROLIDINONE. See VINYL AMIDE POLYMERS.

POM. See POLYOXYMETHYLENE.

POSS NANOCOMPOSITES. See THERMOSETS.

PRESSURE-SENSITIVE ADHESIVES. See Volume 4.

'Polysaccharides'. In: Encyclopedia of Polymer Science and ... · of monosaccharides linked through...

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