Disaccharides: Structure, Linkages, and Properties

Disaccharides

Definition and General Structure

Disaccharides are carbohydrates composed of two monosaccharide units covalently linked by an O-glycosidic bond. They are a common class of oligosaccharides and play important roles in energy storage and metabolism.

• Monosaccharide: The simplest form of carbohydrate (e.g., glucose, fructose, galactose).

• O-glycosidic bond: A covalent bond formed between the anomeric carbon of one monosaccharide and a hydroxyl group of another.

The most abundant disaccharides include maltose, cellobiose, lactose, and sucrose.

Common Disaccharides and Their Properties

Disaccharide	Sugar 1	Sugar 2	Linkage	Degradable?
Maltose	Glucose	Glucose	α-1,4 linkage	Yes
Cellobiose	Glucose	Glucose	β-1,4 linkage	No (in humans)
Lactose	Galactose	Glucose	β-1,4 linkage	Yes
Sucrose	Glucose	Fructose	α-1,2 linkage	Yes

Additional info: Cellobiose is not digestible by humans due to the lack of the enzyme β-glucosidase.

Key Glycosidic Linkages

• α-1,4 linkage: Found in maltose; connects the anomeric carbon (C1) of one glucose to the C4 of another glucose in the alpha configuration.

• β-1,4 linkage: Found in lactose and cellobiose; connects the anomeric carbon (C1) of one sugar to the C4 of another in the beta configuration.

• α-1,2 linkage: Found in sucrose; connects the anomeric carbon (C1) of glucose to the C2 of fructose in the alpha configuration.

Examples and Applications

• Lactose: Found in milk; composed of galactose and glucose with a β-1,4 linkage.

• Sucrose: Table sugar; composed of glucose and fructose with an α-1,2 linkage.

• Maltose: Product of starch digestion; composed of two glucose units with an α-1,4 linkage.

Practice Questions and Explanations

• Which disaccharide contains galactose as one of the sugar subunits? Answer: Lactose

• Which disaccharide forms a 1,1-glycosidic linkage? Answer: Trehalose (not shown in the main table, but trehalose is a glucose-glucose disaccharide with an α,α-1,1 linkage).

• What is the identity of the disaccharide below? Given the structure, the answer is Lactose (galactose + glucose, β-1,4 linkage).

Ring-Chain Equilibrium

• Both sugar rings in a disaccharide can be in equilibrium with their linear chain forms, especially when the anomeric carbon is free (not involved in glycosidic bond).

Identifying Monosaccharide Units and Linkages

• To name the monosaccharide units and glycosidic linkage in a disaccharide, identify the ring structure and the carbons involved in the bond.

• Example: In lactose, the linkage is β-1,4 between galactose (C1) and glucose (C4).

Haworth Projections

• Haworth projections are used to represent the cyclic structure of sugars.

• To draw a disaccharide, connect the appropriate carbons of each monosaccharide according to the linkage type.

Summary Table: Common Disaccharides

Name	Monosaccharide 1	Monosaccharide 2	Linkage	Source
Maltose	Glucose	Glucose	α-1,4	Starch hydrolysis
Lactose	Galactose	Glucose	β-1,4	Milk
Sucrose	Glucose	Fructose	α-1,2	Plants (table sugar)
Cellobiose	Glucose	Glucose	β-1,4	Cellulose hydrolysis

Key Equations

• General formula for a disaccharide (after condensation): $C_{n}H_{2n}O_{n} + C_{n}H_{2n}O_{n} \rightarrow C_{2n}H_{4n-2}O_{2n-1} + H_2O$