© E.V. Blackburn, 2008 Carbohydrates Saccharides - saccharum (Latin) - sugar.

© E.V. Blackburn, 2008

Carbohydrates

Saccharides - saccharum (Latin) - sugar


Photosynthesis

The detailed mechanism of the transformation is not fully understood. There are many enzyme-catalyzed reactions which take place, the first of which involves absorption of light by the extended system of chlorophyll.

xCO2 +h

chlorophyll

(CH2O)x + xH2O xO2

Carbohydrates are synthesized by green plants via the process of photosynthesis. This involves the chemical combination of carbon dioxide and water which results from the absorption of visible light.


Chlorophyll-a

N N

N N

CH3

CH2CH3

CH3

OH

C

H3C

H3CH

H

H3C H H3C H

Mg

CO2CH3

HCH2=

CH2

CH2

C=O

O

CH2

Synthesis - R.B. Woodward, 1960


Carbohydrate metabolismCarbohydrates “store” solar energy which is released when they are metabolized to give CO2 and water:

Cx(H2O)y + xO2 xCO2 + yH2O + energy

Much of the energy released is conserved in a new chemical form through reactions that are coupled to the synthesis of adenosine triphosphate from adenosine diphosphate.


Carbohydrate metabolismN

N N

N

NH2

OH

H

OH

H

OHCH2-

H

O O

O- O-

O

O-

N

N N

N

NH2

OH

H

OH

H

OHCH2-

H

O O

O- O-

O

O-

O-P-O-P-OH + HO-P-O-adenine

ribose phosphate ion

ADP

O-P-O-P-O-P-O-

ATP

Plants and animals use the conserved energy of ATP to perform energy-requiring processes such as muscle contraction. The energy is released and the ATP hydrolyzed to ADP.


D-glyceric acidCO2H

CH2OHH OH

enzymes

simple sugars starch cellulose

polymers

(C6H10O5)n


D-glyceric acidCO2H

CH2OHH OH

This pre-dates Cahn-Prelog-Ingold by many years! This system was suggested by Rosanoff in 1906 and works as follows:

the configuration is D, if the hydroxyl group of the stereogenic centre is to the right in the Fischer projection formula. If it is to the left, the letter L is used.

D???? The letter D designates the absolute configuration of this acid.


D, L Sugars

The configuration of (+)-glyceraldehyde was arbitrarily assigned as (D)!

Monosaccharides have the D configuration if the OH group attached to their highest numbered stereogenic centre is to the right in the Fischer projection formula; L is used if it is to the left.

In 1951, Bijvoet determined the absolute configuration of L-(+)-tartaric acid and hence that of D-(+)-glyceraldehyde. The D assignment was correct!

CHO

CH2OHH OH

(R)-(+)-glyceraldehyde


D-Sugars

CO H

H OHHHO

H OHH OH

CH2OH

D-glucose

CO H

HO HHHO

H OHH OH

CH2OH

D-mannose

CH2OHOHHO

H OHH OH

CH2OHD-fructose

Virtually all naturally occurring monosaccharides are D sugars.


EpimersExamine D-(+)-glucose and D-(+)-mannose:

CO H

H OHHHO

H OHH OH

CH2OH

D-glucose

CO H

HO HHHO

H OHH OH

CH2OH

D-mannose

They only differ in the configuration about one stereogenic centre, C-2.Two diastereoisomers which only differ in the configuration about one stereogenic carbon are called epimers.


Epimers

CO H

H OHHHO

H OHH OH

CH2OH

CO H

H OHHHO

HO HH OH

CH2OH

D-(+)-glucose D-(+)-galactose


Classification of carbohydrates

“monosaccharides” - sugars that cannot be hydrolyzed into smaller molecules (monomer units)

“aldoses” - sugars with an aldehyde carbonyl group

“ketoses” - sugars with a ketone carbonyl group

“sugars” - the general name used for monosaccharides, disaccharides, oligosaccharides (low molecular weight polymers) and polysaccharides. (saccharum, Latin, sugar)


(+)-Glucose

• present in the free state in fruits, plants, honey, blood and urine of animals

• an aldohexose

• C6H12O6

• there are 4 stereogenic centres and therefore 24 (16) optical isomers are possible! All are known!

• the most abundant monosaccharide


CHOCHOHCHOHCHOHCHOHCH2OH

****

Emile Fischer, at the end of the 19th century, successfully identified glucose among the 16 possible isomers!

(+)-Glucose

CO H

H OHHHO

H OHH OH

CH2OH

1

5

The configuration at C-5 was identical to that of D-(+)-glyceraldehyde.


Glucose reduces -Fehling’s solution (Cu(II) in Cu(I))

Tollens’ reagent (Ag(I) in Ag(0))

Reactivity of glucose

O

H

Ag(NH3)2+

O-

O+ Ag

When it reacts with amines, it does not always form imines (C=N-R). Glucose is not an aldehyde?

Glucose is an aldehyde!


Glucose does not form a dimethyl acetals! It forms two different monomethylated derivatives:

RCHO + 2CH3OH R-C-OCH3

H

OCH3

+ H2O

methyl- and -D-glucoside

Reactivity of glucoseAldehydes react with two equivalents of methanol to form dimethyl acetals:

CH3OH+C6H12O6 (C6H11O5)OCH3 H2O


Cyclic glucose

CO H

H OHHHO

H OHH OH

CH2OH

..

D-(+)-glucose

C

CH2OH

H OHHO H

H OHH

HOH

O

a hemiacetal

O OH

O

H

OH


Anomers

There are two different D-(+)-glucose molecules!

Stereoisomers which only differ in the configuration about the hemiacetal carbon are called anomers.

The OH at C-1 is to the right of the chain in the anomer and to the left in the anomer.

CH2OH

H OHHO H

H OHH

H OH

O

CH2OH

H OHHO H

H OHH

HO H

O

* *

-D-Glucose -D-Glucose


Haworth Projections

-D-Glucose -D-Glucose

O

OHOH

OHOH

CH2OH

1

O OH

OH

OHOH

CH2OH

1

The CH2OH is drawn above the ring for D sugars and below the ring for L sugars. In D sugars, the OH at C1 is drawn below the ring for the anomer and above the ring for the anomer.


Conformations of monosaccharides

-D-Glucose

H O

H

HO

H

HO

OHOHH

HOH2C

H

-D-Glucose

H O

H

HO

H

HO

HOHH

HOH2C

OH


Ring Size

O O

pyran furan

-D-glucopyranose

H O

H

HO

H

HO

HOHH

HOH2C

OH

O OH

CH2OH

HOH2C

H H

HO H

OH

-D-fructofuranose


Mutarotation

20o

60o

52.7o

18.7o - -D-glucose

112o - -D-glucose


Mutarotation

H OH

H

HO

H

HO

HOHH

HOH2C

O

A-

HB+

H O

H

HO

H

HO

OOHH

HOH2C

H

A-

H-B+

H

64%

30%

H O

H

HO

H

HO

HOHH

HOH2C

O:H

-D-glucopyranose

:B

H:A

H O

H

HO

H

HO

O:HOHH

HOH2C

H

-D-glucopyranose

:B

H:A


Reactions of Glucose With Amines

H O

H

HO

H

HO

HOHH

HOH2C

O:H

H+H O

H

HO

H

HO

HOHH

HOH2C

OH2

+

H O

H

HO

H

HOOHH

HOH2CH O

H

HO

H

HO

HOHH

HOH2C

OH2

+

H

+


Reactions of Glucose With Amines

anomers of N-phenyl-D-glucosamine

H O

H

HO

H

HOOHH

HOH2C

H O

H

HO

H

HO

HOHH

HOH2C

NHG

H

+

:NH2G

+ H O

H

HO

H

HO

NHGOHH

HOH2C

H


Glycosides

OHO

HOOH

HOH2C

OH

+ CH3OH HCl

cyclic acetals

OHO

HOOH

HOH2C

H

OCH3

methyl -D-glucopyranoside

OHO

HOOH

HOH2C

OCH3

Hmethyl -D-glucopyranoside


Osazones

CO H

H OHHHO

H OHH OH

CH2OH

C6H5NHNH2

CH NNHC6H5

H OHHHO

H OHH OH

CH2OH

CH NNHC6H5

NNHC6H5

HHOH OHH OH

CH2OH

+

NH2

+ NH3phenylosazone of glucose


C=O

(CHOH)3

CH2OH

CH2OH

fructose

CHO

(CHOH)4

CH2OHglucose andmannose

phenylhydrazine

excessCH=NNHC6H5

C=NNHC6H5

(CHOH)3

CH2OHphenylosazone of glucose, mannose andfructose

Osazones


Kiliani-Fischer Synthesis

CH O

HO HH OH

OHHCH2OH

D-arabinose

+ NaCNpH 8

H2O

HO HH OH

OHHCH2OH

CH OH

N

HO HH OH

OHHCH2OH

CHO H

N

+

D-gluconitrile D-mannonitrile

1 : 1,8

HO HH OH

OHHCH2OH

CH OH

N

+ H2 + H2OPd/BaSO4

60 psi, pH 4,5HO H

H OHOHH

CH2OH

CH OH

H O


Ruff Degradation

CH O

HO HH OH

OHHCH2OH

D-arabinose

Fe(OAc)3

H2O2 30%+ CO2

HO HH OH

OHHCH2OH

CH OH

H O

1. Br2/H2O

2. Ca(OH)2

HO HH OH

OHHCH2OH

CO2- Ca2+

H OH

calciumgluconate


(+)-Lactose - a Disaccharide

OOH

HO

HO

R

R = CH2OH

galactose

HO OHO

OH

R

OH

glucose

HCl (1M)

D-galactose + D-glucose

-glycosidicbonding


(+)-Sucrose

OHO

OO

R

R = CH2OH

HH

HO

OC

CH2OH

H

HOH

H OHHO-H2

fructose

glucose


Cellulose

OHO

HO

R

R = CH2OH

HO

n

D-glucopyranose

Cellulose contains about 3,000 monomer units. It is essentially linear. Cotton fiber is almost pure cellulose. Wood and straw is made up of about 50% of this polysaccharide.


Starch

Starch is a polyglucose containing more than 1,000 units connected by acetal bonds. It is easily hydrolyzed by acid to give glucose.


Adenosine - a NucleosideA nucleoside is a glycosylamine in which the amino residue is a pyrimidine or a purine:

OC

H

H

OH

H

OH

N

H

H2HO

N

N

N

H2N

D-ribose

adenine

(a purine)

© E.V. Blackburn, 2008 Carbohydrates Saccharides - saccharum (Latin) - sugar.

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Transcript of © E.V. Blackburn, 2008 Carbohydrates Saccharides - saccharum (Latin) - sugar.