Carbohydrates chemistryDr. Mohammad Waseem Kausar

Assistant professor.Imdc. biochemistry

Some properties of m/s with reference to glucose

chiral Carbon:

ISOMERS=

2n.

Chirality rules.

1. Monosaccharides contain one or more asymmetric C-atoms: get D- and L-forms, where D- and L- designate absolute configuration

2. D-form: -OH group is attached to the right of the asymmetric carbon

3. L-form: -OH group is attached to the left of the asymmetric carbon

4. If there is more than one chiral C-atom: absolute configuration of chiral C furthest away from carbonyl group determines whether D- or L-

Enantiomersisomerism the pairs of structures that are

mirror images of each other. The vast majority of the sugars

in humans are D-sugars. In the D isomeric form, the OH

group on the asymmetric carbon (a carbon linked to four different atoms or groups) farthest from the carbonyl carbon is on the right,

in the L-isomer it is on the left. Enzymes known as racemases

are able to interconvert D- and L-isomers.

Racemic solution = equal amount of d and lno optical activity

Resolution = the separation of optically active isomers from a racemic mixture.

Ring formation / Ring structure

An aldose: Glucose

A ketose: Fructose

Ring Structure (Cyclization)Linear known as “Fischer” structure”Ring know as a “Haworth projection” 3 D representation Cyclization via intramolecular hemiacetal (hemiketal)

formationC-1 becomes chiral upon cyclization - anomeric carbonAnomeric C contains -OH group which may be a or b (mutarotation a b)m/s having 5 or more C occurs predominantly in cyclic

(ring)form owing to a reaction b/w carbonyl C & an alcohol.

These structures have lower energies than the straight chain forms. Aldohexose in cyclic form have 5 asymmetric C so 32 isomers.

The two cyclic compounds α and β have different optical rotations.

the α and β forms are not mirror images, they are referred to as diastereomers.

MutarotationMutarotation is a term given to the change in the observed optical rotation of a substance with time. Glucose, for example, can be obtained in either its a or b-pyranose form. The two forms have different physical properties such as melting point and optical rotation.When either form is dissolved in water, its initial rotation changes with time. Eventually both solutions have the same rotation.

Mutarotation of Glucose

H

H

HOO

HC

O

C

CH2OH

OH

OHH

OH

HO

O

CH2OH

OH

OH

OR

OHHO

O

CH 2OH

OH

OH

C

H

HO H

OH

CO

H OH

CH2OH

C

CH OH

CH ....

or

+

[+18.7°][+112.2°]

anomericcarbon

atom

-D-glucose-D-glucose

Mutarotation of Fructose

HOCH2 OH

CH2OH

OH

HO

OCH2OH

CH OH

CH2OH

CHO H

CH OH

C OHOCH2 CH2OH

OH

OH

HO

O

D-Fructose -D-Fructose-D-Fructose

OPTICAL ACTIVITYThe presence of asymmetric carbon

atoms also confers optical activity on the compound. When a beam of plane-polarized light is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+); or to the left, levorotatory (−). The direction of rotation is independent of the stereochemistry of the sugar, so it may be designated D(−), D(+), L(−), or L(+). For example, the naturally occurring form of fructose is the D(−) isomer.

EPIMERS•Carbohydrate isomers that differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon) Epimers: Isomers differing as a result of variations in configuration of the -OH and -H on carbonatoms 2, 3, and 4 of glucose are known as epimers. . For example, glucose and galactose are C-4 epimers ,their structures differ only in the position of the -OH group at carbon 4

Reducing sugars If the oxygen on the anomeric carbon of

a sugar is not attached to any other structure, that sugar can act as a reducing agent and is termed a reducing sugar. Such sugars can react with chromogenic agents

the anomeric carbon of the sugar becoming oxidized.

the other hydroxyl groups on the molecule are not involved.]

SOME PROPERTIES OF MONOSECCHARIDES1. formation of polyols or alcoholwhen carbonyl gp is reduced to a

hydroxyl gpKeto sugar of more then 3 c can

yeild more then one sugar alcohal because of creation of new asym- center at c2. e.g fructose yeild a mixture of sorbitol & mannitol.

Glyceradehyde & DHA = glycerolMannose = mannitol sorbitol (DM)

Sugar alcohol though derived from reducing m/s are not reducing agents, exist only in straight chain form