Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Chemistry of Carbohydrates
Oligosaccharides

Oligosaccharides are CARBOHYDRATES whose molecules contain from 2 to 10 monosaccharide residues linked by glycosidic bonds. Accordingly, they are classified into Disaccharides, trisaccharides, and so forth.

Disaccharides are complex sugars whose molecules yield two monosaccharide molecules upon Hydrolysis. Alongside Polysaccharides, disaccharides are among the primary sources of dietary carbohydrates for humans and animals. Structurally, disaccharides are Glycosides in which two monosaccharide molecules are joined by a glycosidic bond.

Among disaccharides, maltose, lactose, and sucrose are the most widely known. Maltose, which is a-glucopyranosyl-(1→4)-a-glucopyranose, is formed as an intermediate product during the action of amylases on starch (or Glycogen) and contains two a-D-glucose residues (the name of the sugar whose hemiacetal hydroxyl group participates in The formation of the glycosidic bond ends in "-yl").

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In the maltose molecule, the second glucose residue features a free hemiacetal hydroxyl group. Such disaccharides exhibit reducing properties.

One of the most common disaccharides is sucrose, or ordinary table sugar. The sucrose molecule consists of one D-glucose residue and one D-fructose residue. Consequently, it is a-glucopyranosyl-(1→2)-Р-fructofuranoside:

Unlike most disaccharides, sucrose lacks a free hemiacetal hydroxyl group and does not exhibit reducing properties. The hydrolysis of sucrose yields a mixture known as invert sugar. This mixture is dominated by strongly laevorotatory fructose, which inverts (reverses) the sign of Rotation of the dextrorotatory solution of the original sucrose.

The disaccharide lactose is found exclusively in milk and is composed of D-galactose and D-glucose. It is Р-galactopyranosyl-(1→4)-glucopyranose:

Due to the presence of a free hemiacetal hydroxyl group in its molecule (within the glucose residue), lactose is classified as a reducing disaccharide.

Among natural trisaccharides, raffinose is the best known, containing residues of fructose, glucose, and galactose. Raffinose is found in large quantities in sugar beets and many other plants. In general, oligosaccharides present in plant Tissues are more diverse in composition than those in animal tissues.