Chapter 6: Carbohydrates – Structure, Glycosidic Bonds, and Disaccharides

Carbohydrates: Multiple Sugars Per Compound

Overview of Carbohydrates

Carbohydrates are essential biomolecules composed of carbon, hydrogen, and oxygen. They serve as energy sources and structural materials in living organisms. Carbohydrates are classified based on the number of sugar units present: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

• Monosaccharides: Single sugar units (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined together (e.g., maltose, lactose, sucrose).

• Oligosaccharides: Short chains of monosaccharide units.

• Polysaccharides: Long chains of monosaccharide units (e.g., starch, cellulose).

Ring Forms of Monosaccharides

Monosaccharides often exist in a cyclic (ring) form due to the reaction between the carbonyl group and a hydroxyl group within the same molecule, forming a hemiacetal (for aldoses) or hemiketal (for ketoses) ring. This process is illustrated by rotating the Fischer projection 90° clockwise to visualize the ring structure.

• Anomeric Carbon: The carbon derived from the carbonyl group becomes the anomeric carbon in the ring form.

• Alpha (α) and Beta (β) Anomers: Two possible ring forms exist, differing in the orientation of the hydroxyl group on the anomeric carbon.

Example: D-Glucose forms both α-D-glucose and β-D-glucose anomers.

Disaccharides: Formation and Structure

Disaccharides are formed when two monosaccharides are joined through a condensation reaction, which involves the removal of a water molecule. The bond formed between the two sugar units is called a glycosidic bond.

• Condensation Reaction: The hydroxyl group on the anomeric carbon of one monosaccharide reacts with a hydroxyl group on another monosaccharide, releasing water and forming a glycosidic bond.

• Glycosidic Bond: A covalent bond that links two monosaccharides. It can connect sugars to other alcohols as well.

Equation:

• $\text{Monosaccharide}_1 + \text{Monosaccharide}_2 \rightarrow \text{Disaccharide} + H_2O$

Naming Glycosidic Bonds

It is important to specify the type of glycosidic bond formed between monosaccharides. This includes the anomeric form (α or β) and the carbon atoms involved in the bond.

• Alpha (α) or Beta (β): Indicates the configuration of the anomeric carbon.

• Bond Position: Specified by the numbers of the carbons joined (e.g., α(1→4)).

Example: In maltose, the glycosidic bond is α(1→4), meaning the α-anomeric carbon 1 of one glucose is bonded to carbon 4 of another glucose.

Examples of Glycosidic Bonds

Disaccharide	Monosaccharide Units	Glycosidic Bond
Maltose	D-glucose + D-glucose	α(1→4)
Cellobiose	D-glucose + D-glucose	β(1→4)

Important Disaccharides: Maltose, Lactose, and Sucrose

The three most common disaccharides formed by condensation reactions are maltose, lactose, and sucrose. Each has a unique combination of monosaccharide units and glycosidic bond type.

Disaccharide	Monosaccharide Units	Glycosidic Bond	Reaction Equation
Maltose	D-glucose + D-glucose	α(1→4)	$\text{D-glucose} + \text{D-glucose} \rightarrow \text{maltose} + H_2O$
Lactose	D-galactose + D-glucose	β(1→4)	$\text{D-galactose} + \text{D-glucose} \rightarrow \text{lactose} + H_2O$
Sucrose	D-glucose + D-fructose	α,β(1→2)	$\text{D-glucose} + \text{D-fructose} \rightarrow \text{sucrose} + H_2O$

• Maltose: Formed from two glucose units via an α(1→4) bond; found in malted grains.

• Lactose: Formed from galactose and glucose via a β(1→4) bond; found in milk.

• Sucrose: Formed from glucose and fructose via an α,β(1→2) bond; common table sugar.

Additional info: The configuration and position of glycosidic bonds determine the properties and digestibility of disaccharides. For example, humans can digest α(1→4) bonds in maltose but not β(1→4) bonds in cellobiose.