Distribution of 4-O-methyl-D-glucuronic acid units in xylan of the bark of white willow (Salix alba L.)

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Abstract
Homogeneous, water soluble 4-O-methyl-D-glucurono-D-xylan was isolated from the bark of white willow (Salix alba L.). The equivalent weight of the polymer was found to be 990, i.e. each sixth unit of D-xylose on average was substituted by 4-O-methyl-D-glucuronic acid. The single-ion activity coefficient of calcium counterions γCa2+ was estimated in a molecular disperse solution of calcium 4-O-methyl-D-glucurono-D-xylan. The mean distance of adjacent carboxyl groups, b = 1.1 nm, was obtained from the experimentally determined value γCa2+ = 0.39 using the relation γCa2+ = f(b). Comparison of the value b = 1.1 nm with the D-xylose unit length (0.50 nm) and the calculated value of the equivalent weight indicated the presence of blocks with a close arrangement of uronic acids in the D-xylan chain. In this case, as it follows from the conformation of the macromolecule, at least each second D-xylose on average bears the side monomeric 4-O-methyl-α-D-glucuronic acid unit at C(2). Blocks with a high concentration of uronic acid alternate with approximately three times greater blocks of unsubstituted, or very little substituted D-xylose units.

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