J. Chem. SOC., Faraday Trans. I, 1983,79, 1303-1309

Enthalpies and Heat Capacities of Transfer of Some Sugars from Water to Aqueous Urea Solutions

BY R A K S H V. J A S R A AND J A G D I S H C. AHLUWALIA*

Department of Chemistry, Indian Institute of Technology, New Delhi 110 016, India

Received 8th November, 1982

Integral enthalpies of solution at 298.15 and 308.15 K for glucose, sucrose, cellobiose and maltose monohydrate were measured in 2,4 and 6 mol kg-I aqueous urea solutions using an isoperibol calorimeter. These data were used to determine enthalpies and heat capacities of transfer from water to aqueous urea solutions. The observed exothermic enthalpies of transfer and positive heat capacities of transfer are explained in terms of hydrogen-bonding interactions between the sugars and urea molecules. An attempt is made to explain the differences in enthalpies of transfer of the sugars in terms of the compatibility of the disaccharide aqueous conformation with the tetrahedral structural order existing in water.

The involvement of heterosaccharides, present' at the periphery of plasma mem- branes, in highly stereospecific processes such as cell-cell interactions is being increas- ingly recognised.2-10 Various thermodynami~ll-l~ and spectroscopic16 studies show that the hydration of a sugar is not only dependent on the number of hydroxy groups, potential hydrogen-bonding sites, but also on their relative orientation. Therefore, it is likely that the carbohydrate part of the heterosaccharide provides a recognisable pattern on the cell surface and is responsible for its stereospecific interactions. In our earlier study17 of the thermodynamics of transfer of sorbitol and mannitol from water to aqueous urea solutions, it was observed that non-planar sorbitol, which was expected to be less compatible with water, was found to interact more extensively with urea molecules than planar mannitol. It was thought that a study of the interactions of small organic compounds like urea with sugars of different stereochemistries might be a prelude to an exploration of the nature and specificity of interactions of similar structures present on cell membrane surfaces with other molecules. Urea was chosen as a cosolute because its interactions with sugar might throw some light on the interactions of the peptide group (-CONH) with sugar molecules.

The present work on the transfer of sugars from water to aqueous urea solutions is a continuation of our earlier study.17 The thermodynamic properties reported are the calorimetrically determined enthalpies of solutions and heat capacities of dissolu- tion derived from enthalpy data. The sugars studied include glucose, sucrose, cellobiose and maltose monohydrate.

E X P E R I M E N T A L The isoperibol submarine calorimeter used for measuring the precise values of integral

enthalpies of solution at very low concentrations of the solute was essentially similar to that described earlier.'* The calorimeter consisted of a glass Dewar-type flask (capacity 550 cm3) immersed in a water bath maintained at a constant temperature (to f0.005 K) by a Tronac PTC 40 proportional temperature controller. A 3 SZ thermistor (Y.S.I. Ohio, U.S.A.), acting as a temperature-sensing probe, formed one arm of a Wheatstone bridge, which was constructed from precision decade resistors. The output voltage from the bridge was amplified by a Keithley model 140 nV d.c. amplifier and then recorded on a Bryans 28000 potentiometric strip-chart

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1304 E N T H A L P I E S A N D HEAT CAPACITIES OF TRANSFER

TABLE 1 .-INTEGRAL ENTHALPIES OF SOLUTION OF GLUCOSE, SUCROSE, CELLOBIOSE AND MALTOSE MONOHYDRATE IN AQUEOUS UREA SOLUTIONS AT 298.15 AND 308.15 K

298.15 K 308.15 K urea

molalityl m/ 1 OP3 AHs/ m/ 103 A&/ mol kg-' mol kg-' kJ mol-l mol kg-' kJ mol-1

4

6

D - g l u c o s e

2 1.44 9.87 2.63 9.94 2.69 10.00 3.41 10.04 3.75 9.84

AH," = 9.94 k 0.11 1.59 8.89 1.79 8.93 1.89 8.88 2.04 8.97 2.55 9.04

- -

- - - -

AH? = 9.36 k 0.08 1.19 8.54 1.98 8.68 2.06 8.66 2.41 8.52

AH," = 8.60 f 0.08 - -

1.71 11.57 1.81 11.51 2.05 11.25 2.61 11.42 4.20 11.52 3.25 11.39

AH? = 11.45f0.12 1.35 10.65 1.55 10.95 2.1 1 10.83 2.23 10.92 3.05 10.94 4.16 10.78 4.20 10.65

AH," = 10.80fO.11 1.39 10.27 1.61 10.24 1.64 10.32 2.44 10.44 2.48 10.53

AH," = 10.36k0.15

2

4

6

sucrose 1.29 4.03 1.48 4.18 1.80 4.18 1.82 4.05 3.28 4.10

AH," = 4.1 1 f0.09 0.13 2.93 0.88 2.80 1.40 2.82 1.75 2.89

AH," = 2.86k0.10 1.54 2.08 1.66 2.01 1.96 2.05 2.16 2.05

AH," = 2.04 f 0.06

- -

- -

1.01 6.40 1.30 6.58 1.31 6.56 1.40 6.56 2.25 6.56

AH? = 6.52 k0.09 1.06 5.55 2.32 5.56 2.45 5.70 2.46 5.60 2.62 5.64

AH," = 5.61 k0.07 2.04 4.83 2.14 4.86 2.20 4.92 2.48 4.80 2.09 5.12

AH," = 4.93f0.11

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R. V. J A S R A AND J . C. A H L U W A L I A 1305

maltose monohydrate 0.66 9.49 0.74 9.40 0.8 1 9.44 1.08 9.54 1.40 9.36 1.98 9.51

AH," = 9.45 f0.06 0.40 7.50 0.41 7.51 0.47 7.48 0.53 7.42

AH," = 7.48 f0.06 0.47 6.40 0.48 6.49 0.52 6.43 0.8 1 6.54 0.88 6.51

AH? = 6.47 f 0.06

- -

2

4

6

2 cellobiose

0.40 6.00 0.50 6.10 0.68 6.10 0.72 5.93

4 AH," = 6.03 f0.08 0.28 4.21 0.61 4.36 0.66 4.29 0.67 4.30

AH," = 4.28 & 0.08 6 0.52 2.98

0.67 2.90 0.75 2.93

AH," = 2.94f0.07 - -

0.62 11.99 0.75 11.88 0.77 11.92 0.78 11.89 0.94 12.02

AH," = 11.95 f0.09 0.51 10.73 0.55 10.82 0.59 10.90 0.74 10.88 0.83 10.80

AH," = 10.83 f0.09 0.45 9.71 0.48 9.76 0.61 9.70 0.80 9.81 0.8 1 9.67

AH," = 9.73 f 0.07

- -

0.59 8.86 0.73 8.74 0.85 8.92 0.92 8.74 0.93 9.00 1.10 9.01 1.06 8.97

AH," = 8.89 f 0.09 0.46 7.30 0.58 7.31 0.59 7.28 0.68 7.29 0.70 7.36

AH? = 7.31 f0.04 0.53 6.28 0.63 6.18 0.72 6.34 0.82 6.31

AH," = 6.28 fO.11

recorder (Bryans Southern Instruments Ltd). The calorimeter was calibrated electrically immediately before and after each experiment by passing a constant (ca. 50mA) current through a non-inductively wound manganin calibration heater. A Keithley model 227 constant-current source coupled with a digital timer (model DET 203, Thermadyne, New Delhi) was used as a power source for the calibration heater.

Resolution of the temperature measurement system was 1 x lop5 K over the whole range. The operation of the calorimeter was checked by measuring the enthalpies of solution of KC1 in water and of THAM (tris-hydroxyaminomethane) in 0.1 mol dm-3 HC1. The AH? value for KC1 at 298.15 K agreed to within 1 % with the 2o values and the AH? value of THAM in 0.1 mol dmP3 HCl agreed to within f 0.2% with reportedz0 values under similar conditions.

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1306 E N T H A L P I E S A N D HEAT CAPACITIES OF TRANSFER

TABLE 2.-sTANDARD ENTHALPIES AND HEAT CAPACITIES OF DISSOLUTION FOR GLUCOSE, SUCROSE, CELLOBIOSE AND MALTOSE MONOHYDRATE IN AQUEOUS UREA SOLUTIONS~

urea AH,"/kJ mol-l Ac; l molality/ J K-l mol-* mol kg-l 298.15 K 308.15 K 303.15 K

a-D-glucose 0 10.70 f 0.04 1 2.04 f 0.1 2 134f 12 2 9.94f0.11 11.45 f 0.12 151 f 16 4 9.36 f 0.08 10.80 f 0.1 1 144f 13 6 8.60f9.08 10.36 f 0.15 176f9

sucrose 0 5.76 f 0.14 7.63 f 0.15 187f20 2 4.1 1 f 0.09 6.52 & 0.09 241 & 12 4 2.86 f 0.10 5.61 f0.07 275f 12 6 2.06 f 0.05 4.93 f 0 . 11 287f 12

cellobiose 0 7.25 f 0.06 9.98 f 0.07 273 f 6 2 6.03 f 0.08 8.89 f 0.09 286f 12

4.28 f 0.08 7.3 1 f 0.04 303 f 8 4 6 2.94 f 0.07 6.28 f 0.1 1 334k 13

maltose monohydrate 0 9.56 f 0.09 11.97 +_O. 13 241 +_ 12 2 9.45 f 0.06 1 1.95 f 0.09 250 f 10

6 6.47 f 0.06 9.73 f 0.07 326 f 9 4 7.48 & 0.06 10.83 f 0.09 335 f 10

a AH? and AC; for glucose and disaccharides in water have been taken from ref. (1 1) and (27), respectively.

a-D-ghCOSe, sucrose, cellobiose and maltose monohydrate of highest available purity (Sigma and P.L. Biochemicals) were used without further purification. Analytical reagent grade urea (B.D.H.) was used after drying for 48 h at 333 K.

Deionised water obtained by passing distilled water through a Barnstead mixed-bed ion-exchange resin column was used for enthalpy measurements. The urea solutions were made up by weight.

RESULTS The integral enthalpies of solution, AHs, of glucose, sucrose, cellobiose and maltose

monohydrate were measured at 298.15 and 308.15 K in aqueous urea solutions. The concentrations ofthe sugars ranged from 1 0-5 to mol kg-l. Since the measurements of integral enthalpies, AHs, were performed at very dilute concentrations of the solute, the average value of AHs was treated as the integral enthalpy of solution at infinite dilution, AH?. AH? values along with AHs at various molalities of urea for glu- cose, sucrose, maltose monohydrate and cellobiose are given in table 1 . The reported uncertainties are computed as 95 % confidence limits. The heat capacities of dissolution at infinite dilution, ACF, of sugars in aqueous urea solutions at 303.15 K were calculated using the integral heat method.'* The standard thermodynamic parameters AH? and ACF for all the compounds in water and aqueous urea solutions are presented in table 2.

The enthalpies of transfer, AHt,, AHt, = AH," (urea+water)-AH? (water), for

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R. V. J A S R A A N D J. C. A H L U W A L I A

[urea] /mol kg-'

1307

FIG. 1.-Enthalpies of transfer of glucose (O), sucrose (A), cellobiose (0) and maltose monohydrate (0) from water to aqueous urea solution as a function of concentration of urea at 298.15 K.

[urea] /mol kg-'

FIG. 2.-Heat capacities of transfer of glucose (O), sucrose (A), cellobiose (0) and maltose monohydrate (0) from water to aqueous urea solution as a function of concentration of urea at 303.15 K.

glucose, sucrose, cellobiose and maltose monohydrate at 298.15 K are plotted as a function of urea molality in fig. 1 . The heat capacities of transfer, ACp, tr, at 303.15 K are plotted against urea molality in fig. 2.

DISCUSSION E N T H A L P I E S OF T R A N S F E R

It is seen from fig. 1 that the enthalpies of transfer values for glucose and the three disaccharides show exothermicity at both 298.15 and 308.15 K. This behaviour is similar to that for the transfer of ionic hydrophilic 22 and p01yols.l~ However, this is in contrast to the behaviour of hydrophobic s o l ~ t e s , ~ ~ - ~ ~ which show endothermic enthalpies of transfer from water to aqueous urea solutions.

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1308 E N T H A L P I E S A N D HEAT CAPACITIES OF TRANSFER

Since both urea and sugars possess potential sites for hydrogen bonding, exothermic enthalpies of transfer can be attributed to hydrogen-bonding interactions between urea and sugar molecules. This is further supported by the observation that the exothermicity increases with an increase in urea concentration. The possibility of hydrogen-bond complex formation between urea and polyols17 and urea and peptides or amino acids26 has also been suggested to explain the negative enthalpies of transfer.

Comparison of AH,, values for disaccharides with those of monosaccharide glucose and the alcohols sorbitol and mannitol17 shows that the magnitude of the exothermicity is higher for disaccharides. This is probably due to the higher number of hydroxy groups in a disaccharide molecule, reaffirming the view that there might be hydrogen- bonding interactions between sugar OH/O groups and urea molecules. AH,, data for glucose, sorbitol and mannitol show that glucose has a higher exothermicity at both the temperatures. This, perhaps, reflects that hydrogen-bond interactions are more extensive for glucose. This is not unexpected if one compares the heat capacity of dissolution ACF in water for glucose (1 34 J K-l mol-l), sorbitol17 (194 J K-l mol-l) and mannitoP7 (232 9 J K-l mol-l); the higher AC; values for sorbitol and mannitol show that the acyclic and hence flexible polyol structures are more compatible with water than the rigid ring structure of glucose and thus less available for interactions with urea molecules.

From fig. 1, comparison of AH,, for cellobiose and sucrose shows that up to 2 mol kg-l of urea, sucrose shows more exothermicity than cellobiose. However, at higher concentrations of urea the trend is reversed and is more prominent at 308.15 K. The higher ACP,,, values for sucrose appears to be consistent with the view that cellobiose, being more compatible with the water structure (as reflected in its higher ACF value in water), has its hydroxy group less available for interaction with urea molecules. However, at higher concentrations of urea this explanation does not seem to be tenable, some other factors like urea-urea interactions becoming significant.

HEAT CAPACITIES OF TRANSFER

Heat capacities of transfer, AC,,,,, as seen in fig. 2 are significantly positive for all the sugars. Positive AC,, ,, from water to aqueous urea solutions have also been observed for aminoacids , peptides26 and polyols. l7 However, hydrophobic show a decrease in ACF on transferring from water to aqueous urea solutions. Positive AC,, ,, values may be indicative of hydrogen-bond interactions between urea and sugar molecules. Higher ACP, ,, values for disaccharides compared with glucose, sorbital and mannitol reflect that these thermally labile interactions are more extensive in disaccharides. Higher ACP, ,, values for sucrose than cellobiose is in agreement with the above discussion on the enthalpy results.

G. M. W. Cook and R. W. Stoddart, Surface Carbohydrates of the Eukaryotic Cell (Academic Press, New York, 1973), chap. 2.

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R. V. JASRA A N D J . C. A H L U W A L I A 1309

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