TOPIC: 22. Carbohydrates

SUBTOPIC: 224. Polysaccharides

5160

(+)-Raffinose, a non-reducing sugar found in beet molasses, has the formula C₁₈H₃₂O₁₆. Some chemical properties of (+)-raffinose are described below:

(a) Hydrolysis by acid gives D-fructose, D-galactose and D-glucose.

(b) Hydrolysis by the enzyme a-galactosidase gives D-glactose and sucrose. (a-galactosidase cleaves a-galactose linkages).

(c) Hydrolysis by invertase gives D-fructose and the disaccaride melibose. (Invertase cleaves b-fructose linkages).

(d) Hydrolysis by maltase yields fructose and an unnamed disaccaride. (Maltase cleaves a-glucose linkages).

(e) Methylation of (+)-raffinose, followed by hydrolysis gives

1,3,4,6-tetra-O-methyl-D-fructose, 2,3,4,6-tetra-O-D-galactose and 2,3,4-tri-O-methyl-D-glucose.

Provide a structure for raffinose and show how the data supports your proposed structure.

5270

Polysaccharides known as dextrans have been used as substitutes for blood plasma in transfusions; they are made by the action of certain bacteria on (+)-sucrose. Interpret the following properties of a dextran:

• Complete hydrolysis by acid yields only D-(+)-glucose

• Partial hydrolysis yields only one disaccharide only one trisaccharide, which contain only a-glycoside linkages

• Upon methylation of the dextran and hydrolysis, there is obtained chiefly 2,3,4-tri-O-methyl-D-glucose, together with a very small amount of 2,4-di-O-methyl-D-glucose and 2,3,4,6-tetra-O-methyl-D-glucose