The hemicelluloses of lucerne

Abstract

Fractionation of the polysaccharides obtained from lucerne stems by extraction with dilute alkali furnishes a xylan containing ca. 11% of 4-O-methyl-D-glucuronic acid residues and ca. 1% of rhamnose residues. Partial hydrolysis of the polysaccharide affords 4-O-β-D-xylopyranosyl-D-xylose, the polymer-homologous tri-, tetra-, penta-, and hexa-saccharides, 2-O-(4-O-methyl-α-D-glucopyranosyluronic acid)-D-xylose, O-(4-methyl-α-D-glucopyranosyluronic acid)-(1 → 2)-O-β-D-xylopyranosyl-(1 → 4)-D-xylose, a series of higher acidic oligosaccharides, and small amounts of 3-O-xylopyranosylrhamnose and O-xylopyranosyl-(1 → 4)-O-xylopyranosyl-(1 → 3)-rhamnose. Hydrolysis of the methylated polysaccharide gives, as the major products, 2,3-di-O-methyl-D-xylose and 3-O-methyl-2-O-(2,3,4-tri-O-methyl-α-D-glucopyranosyluronic acid)-D-xylose. It is concluded that the polysaccharide contains essentially linear chains of 1 → 4 linked chains of β-D-xylopyranose residues with side-chains of 4-O-methyl-α-D-glucopyranosyluronic acid residues attached to C-2 of approximately every ninth residue; the main chain also contains occasional rhamnose residues. A similar xylan is present in lucerne leaves.