Fundamentals of Medicinal Chemistry

1.4.4 Polysaccharides

Polysaccharides (glycans) are carbohydrates whose structures consist of mono-

saccharide residues joined together by oxygen glycosidic linkages (Figure 1.18).

The links between between monosaccharide residues of a polysaccharide

molecule are usually referred to in terms of the type of the numbers of the

carbon atoms forming the link and the stereochemistry of the anomeric pos-

ition. For example, the glycosidic link formed in maltose is refered to as an

a-1,4-link (Figure 1.19) because the anomeric carbon of ana-D-glucose residue

is linked to carbon number 4 of the other (second) glucose residue in the

structure. The anomeric carbon atom of the second glucose residue can undergo

mutarotation and so maltose will exist as two isomers in aqueous solution. The

prefix (a,b) is used for residues that can undergo mutarotation.

O

O

O

HO

H

H

OH

H

H

O CH 2 OH

H

O

H

O

HO

H

OH

H

H

O CH^2 OH

O

HO

H

H

OH

H

H

CH 2 OH

H

H

O

HO

H

OH

H

H

CH 2 OH

A B C D

Figure 1.18 A fragment of theb-D-glucose chain in cellulose

HO

H

O

H O

HO

H

H

OH

H

H

CH 2 OH

O

HO

H

OH

H

H

CH 2 OH (H, OH)

O

HO

H

H

OH

H

H

CH 2 OH

O

HO

HO

H

H

OH

H

H

CH 2 OH H

(H, OH)

3

2

1

(^14)
Maltose
4-(α-D-glucopyranosyl)-α,β-D-glucopyranoside
O
1
3
2
1
4
Lactose
4-(β-D-galactopyranosyl)-α,β-D-glucopyranoside
Figure 1.19 Examples of simple disaccharides. Appropriate systematic names are given in
brackets. The structures are normally drawn so that the oxygen atom forming the glycosidic link is
above or below the plane of the ring system. This sometimes requires the structure of a residue to be
turned around and/or over in order to obtain the correct alignment of the oxygen atom. Hexagonal
cardboard cutouts can be useful in determining how a particular glycosidic link was formed
CARBOHYDRATES 17