Introduction to chemistry of carbohydrates
Carbohydrates are the most abundant macromolecules in nature. They are the main source and storage of energy in the body. They serve also as structural component of cell membrane. The general molecular formula of carbohydrate is CnH2nOn or (CH2O)n, where n > 3.
Chemically, they contain the elements like carbon, hydrogen and oxygen. Thus, they are Carbon compounds that contain large quantities of Hydroxyl groups (-OH).
Carbohydrates in general are polyhydroxy aldehydes or ketones or compounds which give these substances on hydrolysis.
1. Chemistry of Carbohydrates
1.1. Monosaccarides
Monosaccharides
Monosaccharides also called simple sugars. They consist of a single polyhydroxy aldehyde or ketone units. The most abundant monosaccharides in nature are the 6-carbon sugars like D-glucose and fructose.
Structure
Monosaccharide has a backbone, which is un-branched, single bonded carbon chain. One of the carbon atoms is double bonded to an oxygen atom to form carbonyl group. Each of the other carbon atoms has a hydroxyl group.
For example, observing the structure of glucose below;
Classification of monosaccharides
Monosaccharides having aldehyde groups are called Aldoses and monosaccharides with Ketone group are Ketoses.
Depending on the number of carbon atoms, the monosaccharides are named trioses (C3), tetroses (C4), pentoses (C5), hexoses (C6), heptoses (C7).
Physical properties of Monosaccharides
- They are colorless crystalline compounds
- readily soluble in water.
- Their solutions are optically active and exhibit the phenomenon of mutarotation.
- Carbohydrates spontaneously change between the α and β configuration.
- Exhibit asymmetric center and stereoisomerism
Asymmetric carbon is a carbon that has four different groups or atoms attached to it and having optically activity in solution.
All the monosaccharides except dihydroxyacetone contain one or more asymmetric or chiral carbon atoms and thus occur in optically active isomeric forms.
Optical Activity Monosaccarides
The presence of asymmetric carbon atom causes optical activity. When a beam of plane-polarized light is passed through a solution of carbohydrate it will rotate the light either to right or to left. Depending on the rotation, molecules are called dextrorotatory (+) (D) or levorotatory (-) (L).Epimers
When sugars are different from one another, only in configuration with regard to a single carbon atom (around one carbon atom) they are called epimers of each other. For example glucose and mannose are epimers. They differ only in configuration around C2. Mannose and Galactose are epimers of Glucose.
Oligosaccharides
Oligosaccharides contain 2 to 10 monosaccharide units. The most abundant oligosaccharides found in nature are the Disaccharides.