Question

Raffinose is a trisaccharide (C₁₈H₃₂O₁₆) isolated from cottonseed meal. Raffinose does not reduce Tollens reagent, and it does not mutarotate. Complete hydrolysis of raffinose gives D-glucose, D-fructose, and D-galactose. When raffinose is treated with invertase, the products are D-fructose and a reducing disaccharide called melibiose. Raffinose is unaffected by treatment with a-galactosidase, but an α -galactosidase hydrolyzes it to D-galactose and sucrose. When raffinose is treated with dimethyl sulfate and base followed by hydrolysis, the products are 2,3,4-tri-O-methylglucose, 1,3,4,6-tetra-O-methylfructose, and 2,3,4,6-tetra-O-methylgalactose. Determine the complete structures of raffinose and melibiose and give a systematic name for melibiose.

Short answer

The systematic name for melibiose is 6-O-(α -D-galactopyranosyl)-D-glucopyranose.

Step-by-step solution

Raffinose to melibiose conversion:

Raffinose is a trisaccharide (C₁₈H₃₂O₁₆), and when it is treated with invertase, it gives products D-fructose and disaccharide melibiose which is reducing in nature. In raffinose, glucose acts as a monosaccharide bridge between galactose and fructose sugar.

It has both alpha and beta-glycosidic bonds. Raffinose sugar is not digestible by humans.

Systematic name for melibiose:

Melibiose is a reducing dissacharide. Disaccharides are named as glycosyl glycosides, means glycosyl sugar is named first followed by glycoside or glycose sugar. The glycosidic linkage present is named in the bracket before naming the sugar. Alpha linkage is present in melibiose thus, the systematic name for melibiose is 6-O-(α -D-galactopyranosyl)-D-glucopyranose.