Disaccharides consist of two ringed mono-saccharides.
The bonds that unite neighboring mono-saccharides are called glycosidic bonds and are formed by the condensation of a hydroxyl group of carbon atom number 1 of one monosaccharide with the hydroxyl group of either the number 2, 4, or 6 carbon atom of another.
The formation of the common disaccharide maltose from two molecules of glucose is shown in Figure 5-9.
In maltose, the oxygen bridge is formed between the number 1 carbon atom of one a-d- glucose unit and the number 4 carbon atom of the other. The bond formed is referred to as an α1→4 glycosidic bond.
Another important disaccharide is sucrose (i.e., ordinary “table” sugar), which is formed by the condensation of a-d-glucose and 0-d-fructose (Fig. 5-10). Milk contains the disaccharide lactose, which consists of the hexoses α-D-galactose and β-D-glucose (Fig. 5- 11). In lactose, the glycosidic bond is of the beta variety; i.e., β1 → 4 (compare with maltose).
