Describe the formation of a glycosidic bond by condensation, with reference to disaccharides, including sucrose, and polysaccharides

Sourav PanOctober 28, 2024

Answered

A glycosidic bond forms when two monosaccharides join together through a condensation reaction (also called dehydration synthesis), where a molecule of water is released. This bond can form between various hydroxyl groups of the participating monosaccharides, creating either an α-glycosidic or a β-glycosidic bond, depending on the orientation of the linkage.

Formation of a Glycosidic Bond in Disaccharides

1. Example with Sucrose:
   • In the formation of sucrose (a non-reducing disaccharide), an α-glucose molecule and a β-fructose molecule join together.
   • The glycosidic bond forms between the hydroxyl group on the C1 of glucose (α-1) and the C2 of fructose (β-2), resulting in an α(1→2) glycosidic bond.
   • As a result of this linkage, both the anomeric carbons are involved in the bond, making sucrose a non-reducing sugar with no free aldehyde or ketone groups.
2. Example with Maltose:
   • In maltose (a reducing disaccharide), two α-glucose molecules join together.
   • The glycosidic bond forms between the C1 of one glucose and the C4 of the second glucose, creating an α(1→4) glycosidic bond.
   • Unlike sucrose, maltose retains a free anomeric carbon (C1 of the second glucose unit), allowing it to act as a reducing sugar.

Formation of Glycosidic Bonds in Polysaccharides

• Polysaccharides are formed by linking multiple monosaccharides through repeated glycosidic bonds. The specific types of glycosidic linkages and the sequence of monosaccharides determine the structure and function of the polysaccharide.
• For instance, starch (in plants) and glycogen (in animals) are storage polysaccharides composed of α-glucose units connected primarily by α(1→4) glycosidic bonds, with branching points formed by α(1→6) glycosidic bonds.
• Cellulose, a structural polysaccharide in plant cell walls, consists of β-glucose molecules linked by β(1→4) glycosidic bonds. This arrangement allows cellulose chains to form strong, rigid fibers due to hydrogen bonding between adjacent chains.