Carbohydrate Chemistry

Structure and Isomerism

Structure of Monosaccharides

Fischer projection• The

straight chain structural formula

Haworth projection• Cyclic

formula or ring structure

X-ray diffraction analysis• Boat and

chair conformation

Straight chain

Ring structure

Chair form

Isomerism

• The compounds possessing identical molecular formula but different structures are called isomers.

Various types of isomerism1. Structural isomerism2. Stereoisomerism

Structural Isomerism• Same molecular formulae but differ

from each other by having different structures.

Stereoisomerism • Same molecular formula and same

structure but they differ in configuration.

• That is, they differ in the arrangement of their atoms in space.

• Presence of asymmetric carbon atoms allow the formation of stereoisomerism

Stereoisomerism • The important types of

stereoisomerism associated with glucose are

D and L enantiomerism

Optical isomerism

Epimerism

α and β anomerism

D and L Enantiomerism“Handedness”

• Stereoisomers that are nonsuperimposable mirror images of each other.

• Handedness (D and L forms) is determined by the configuration at the high-numbered chiral carbon.

D and L Enantiomerism“Handedness”

Optical Isomerism• Optical activity is the capacity of a

substance to rotate the plane polarized light passing through it.

Clockwise direction• Dextrorotatory (d) or (+)

Counterclockwise direction• Levorotatory (l)or (-)

Optical Isomerism

Chiral compounds rotate polarized light clockwise or counter clockwise through a certain angle

Epimerism

• Epimerism is the stereoisomerism if two monosaccharides differ from each other in their configuration around a single specific carbon (other than anomeric) atom.

Epimerism

Anomerism • These are isomers obtained from the

change of position of hydroxyl group attached to the anomeric carbon e.g. and glucose are 2 anomers.

• Also and fructose are 2 anomers.

Anomerism

• Mutarotation is defined as the change in the specific optical rotation by the interconversion of α and β forms of D glucose to an equilibrium mixture

Mutarotation

Structure of Oligosaccharides

• Disaccharides

Disaccharides

Reducing

MaltoseLactose

Isomaltose

Nonreducing

Sucrose

Disaccharides• These are glycosides formed by the

condensation of 2 simple sugars.

• If the glycosidic linkage involves the carbonyl groups of both sugars (as in sucrose) the resulting disaccharide is non-reducing.

• On the other hand, if the glycosidic linkage involves the carbonyl group of only one of the 2 sugars (as in maltose and lactose) the resulting disaccharide is reducing.

Polysaccharides

• These are formed by the condensation of n molecules of monosaccharides with the removal of n-1 molecules of water. Since condensation involves the carbonyl groups of the sugars, leaving only one free carbonyl group at the end of a big molecule, polysaccharides are non-reducing.

• They are of 2 types:1. Homopolysaccharides (e.g. starch, glycogen, cellulose).

2. Heteropolysaccharides (e.g. glycosaminoglycans, glycoproteins)

- 1,4 linkage between two glucose units

-1,6 linkage between two glucose units

The ability to digest cellulose is found only in microorganisms that contain the enzyme Cellulase.

Certain animal species (e.g. Cow) utilize such organisms in their digestive tracts to digest cellulose

Aldehyde groupH-C=O

Monosaccharides

Enantiomers

if they are

Nonsuperimposable mirror images of each other

can link to form

Disaccharidese.g.,

Sucrose = glucose + fructoseLactose = galactose + glucoseMaltose = glucose + glucose

Oligosaccharides Polysaccharides

can be

Homo-e.g.,

Starch, glycogen,cellulose

Hetero-e.g.,

GAGs

Epimers

Differ in configuration around one specific carbon atom

Isomers

if they have

Same chemical formula

Ketoses

Ketone groupC=O

Can be classified as

if theyif they containif they contain

Aldoses

Tri-

Tetra-

Penta-