2.10 Glycosidic Bonds in Disaccharides

This section details the formation, naming, and types of covalent bonds that link monosaccharide units together to form larger carbohydrates like disaccharides and polysaccharides.

Glycoside

A ring-structured sugar molecule (hexose or pentose) that is typically attached to another sugar molecule.

Glycosidic Bond

A covalent chemical bond formed between two monosaccharide units through a condensation reaction. This bond is crucial for the formation of disaccharides, oligosaccharides, and polysaccharides.

• Formation: A hydroxyl (-OH) group from one sugar reacts with a hydroxyl (-OH) group of another sugar. This process releases one molecule of water (${\mathrm{H}}_2\mathrm{O}$), forming a C-O-C bridge between the two sugars.
• Naming Convention: Glycosidic bonds are named based on two factors:
  1. The carbon numbers of the two interacting carbons (e.g., C-1 and C-4).
  2. The anomeric form (alpha or beta) of the first sugar involved in the linkage.
  • Example: An α-1,4 glycosidic linkage forms when the first sugar is an alpha anomer and its C-1 links to the C-4 of the second sugar.

Reducing vs. Non-Reducing Sugars

• Reducing Sugar: A carbohydrate that can be oxidized and cause the reduction of other substances. They have a free anomeric carbon (open chemical structure).
  • Examples: All monosaccharides, maltose, lactose.
• Non-Reducing Sugar: A carbohydrate that cannot be oxidized. Both anomeric carbons are involved in the glycosidic bond, so no free anomeric carbon is available.
  • Examples: Sucrose, all polysaccharides.

Types of Glycosidic Linkages in Disaccharides

The structure and properties of a disaccharide are determined by its constituent monosaccharides and the specific glycosidic bond that links them.

1. Glycosidic Linkage in Sucrose

• Monomers: Formed by the condensation of α-glucose and β-fructose.
• Reaction: The -OH group at C-1 of α-glucose reacts with the -OH group at C-2 of β-fructose.
• Bond Type: α-1,2-glycosidic linkage (also written as α-1, β-2-glycosidic linkage).
• Reducing status: Non-reducing sugar (both anomeric carbons are involved in the bond).
• Diagram:

2. Glycosidic Linkage in Maltose

• Monomers: Formed by the condensation of two α-glucose molecules.
• Reaction: The -OH group at C-1 of the first α-glucose reacts with the -OH group at C-4 of the second α-glucose.
• Bond Type: α-1,4-glycosidic linkage.
• Reducing status: Reducing sugar (one anomeric carbon remains free).
• Diagram:

3. Glycosidic Linkage in Lactose

• Monomers: Formed by the condensation of β-galactose and β-glucose.
• Reaction: The -OH group at C-1 of β-galactose reacts with the -OH group at C-4 of β-glucose.
• Bond Type: β-1,4-glycosidic linkage.
• Reducing status: Reducing sugar (the anomeric carbon of glucose remains free).
• Diagram:

Summary Table

Biological Significance: The formation of glycosidic bonds allows for the synthesis of complex carbohydrates essential for energy storage (e.g., starch, glycogen) and structural support (e.g., cellulose, chitin).