Distribution pattern of uronic acid units in 4-O-methyl-D-glucurono-D-xylan of beech (Fagus sylvatica L.)

Abstract

4-O-Methyl-D-glucurono-D-xylan isolated from beech sawdust was characterized: its main chain consists of D-xylopyranose units linked by a glycosidic β(1 → 4) bond; uronic acid units are bound to C(2) of D-xylose units of the main chain as monomeric side units by a glycosidic α(1 → 2) bond. The distribution pattern of uronic acid units along the macromolecule chain was investigated in the water-soluble polysaccharide fraction of the D-xylose to uronic acid 6.1 to 1 molar ratio. The method based on interpretation of the activity coefficient of calcium counterions γCa2+ as determined with the calcium salt of glucuronoxylan was applied. The activity coefficient is a function of the linear charge density of the acid polysaccharide. It has been evidenced that the 4-O-methyl-D-glucurono-D-xylan under investigation is irregularly branched; it consists of longer segments of the macromolecule, each second D-xylose unit of which binds a uronic acid unit as a monomeric side terminal; these segments alternate with those composed of only D-xylan units.