(Carbohydrates Part 3: Properties, Structure and Examples of Disaccharides)
What are Disaccharides?
Disaccharides are carbohydrates which contain two covalently linked monosaccharide units. Sucrose, Maltose, Lactose, Trehalose and Cellobiose are naturally occurring disaccharides. The individual monosaccharide units in a disaccharide are called ‘residues’. All disaccharides are soluble in water
Glycosidic bonds links monosaccharide units
The monosaccharide units in disaccharides (and also in polysaccharides) are linked through a special type of covalent bond called Glycosidic bond (specifically O-glycosidic bond). O-glycosidic bond is formed by the reaction between the hydroxyl group of one monosaccharide with the anomeric carbon atom of the other. During the glycosidic bond formation, one molecule of water is eliminated as given in the diagram. Glycosidic bonds are strong covalent bonds and they can be hydrolyzed by treating with mild acids. The hydrolysis of the glycosidic bond of a disaccharide releases its corresponding monosaccharide units.
Chemical Structure, Characteristics, Examples & Classification
Monosaccharides are Simplest Sugars
Monosaccharides are the simplest carbohydrates. They are polyhydroxy aldehydes or ketones with a carbon backbone. The carbon backbone in monosaccharides usually consists of 3 – 6 carbon atoms. The simplest monosaccharides are glyceraldehyde and dihydroxyacetone (with 3 carbons). The most abundant monosaccharide in nature is a 6 carbon sugar called glucose. Majority of the monosaccharides follow the empirical formula C(H2O)n. Monosaccharide with five or more carbon can predominantly exist as cyclic structures in the aqueous condition. All monosaccharides are colourless, crystalline solids and that are readily soluble in water but insoluble in nonpolar solvents. Most of the monosaccharides are sweet in taste.
Chemical Structure of Monosaccharides
Ø All monosaccharides are polyhydroxy (contain many hydroxyl groups) aldehydes or ketones.
Ø The hydroxyl groups are attached to the carbon backbone.
Ø The number of carbon atoms in the backbone of monosaccharides varies from 3 to 6.
Ø The carbon backbone of monosaccharides is unbranched and individual carbon atoms are connected by single bonds.
Ø Monosaccharides are broadly classified into Aldoses and Ketoses.
Ø In the open chain conformation of a monosaccharide, one of the carbon atoms of the backbone is double bonded to an oxygen atom to form the carbonyl group (C=O).
Ø If the carbonyl group is at the end of the carbon chain it will be an aldehyde group (R – COH) and thus the sugar formed will be an Aldose sugar.
Ø Similarly, if the carbonyl group is inner to the carbon chain, it will be a keto group (C=O) and the sugar formed will be a Ketose sugar.