Dissacharides

Dissacharide

-A carbohydrate that contains two monosaccharide units covalently bonded to each other.

- An acetal formed when two monosaccharides are linked together by a glycosidic bond.

- Are crystalline, water soluble substances.

-Hydrolysis of a disaccharide produces two monosaccharide units.

Lactose

- A disaccharide, also know as milk sugar.

- Composed of:

~ Beta D-Galactose + Alpha D-Glucose

~ Beta D-Galactose + Beta D-Glucose

- Glycosidic Linkage: Beta (1->4)

Glycosidic Linkage (definition)

The bond between two monosaccharides resulting from the reaction between the hemiacetal carbon atom —OH group of one monosaccharide and an —OH group on the other monosaccharide.

Glycosidic Linkage Notation

Sucrose

- A disaccharide, also known as table sugar.

- Composed of: Alpha D-Glucose + Beta D-Fructose

~Remember that glucose is an aldose and fructose is a ketose.

- Glycosidic Linkage: Alpha, Beta (1->2)

Maltose

- A disaccharide known as malt sugar. Produced whenever a polysaccharide starch breaks down.

- Composed of:

~ Alpha D-Glucose + Beta D-Glucose.

~ Alpha D-Glucose + Alpha D-Glucose

- Glycosidic linkage: Alpha (1->4). The two —OH groups that form the linkage are attached to the C1 Alpha Glucose and C4 of the second glucose (an Alpha or Beta)

- Is a reducing sugar

Reactions of Dissacharides

Disacharides can also go through mutarotation.

Acetal and Hemiacetal Carbon of Disaccharides

Most Disaccharides have an acetal carbon and hemiacetal carbon. Sucrose is an exception, the hemiacetal carbon of both monosaccharide monomers are participating in the glycosidic linkage. This is also what makes sucrose a non-reducing sugar.

What group must be present for a mutarotation to occur in a dissacharide?

A hemiacetal group.

How to determine is a disaccharide is alpha or beta (not linkage)

Look to the hemiacetal carbon (C1) of the second monosaccharide unit.