Polysaccharides of tropical pasture herbage. II. A xylan from the leaf of spear grass (Heteropogon contortus)

Abstract

Alkaline extraction of spear grass holocellulose gave a xylan which was purified by fractional precipitation as a copper complex. Acid hydrolysis yielded L-arabinose, D-xylose, and an aldobiouronic acid consisting of D-xylose and 4-O-methyl-D-glucuronic acid. Hydrolysis of the methylated xylan gave 2,3,5-tri-O-methyl-L- arabinose, 2,3,4-tri-, 2,3-di-, and 2-O-methyl-D-xylose, and 3-O-methyl-2-O-(2,3,4-tri-O-methyl-D-glucopyranosyluronic acid)-D-xylose. The conditions for isolation of the methylated sugars for gas chromatography were investigated with particular reference to difficulties arising from volatility of the tri-O-methyl pentoses. The xylan is composed of chains of 1,4-linked ß-D-xylopyranose residues to which are attached single units of L-arabinofuranose at position 3 and 4-O-methyl-D-glucuronic acid at position 2 of the basal residues.