Collect. Czech. Chem. Commun. 1981, 46, 1405-1412

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

Rudolf Toman, Rudolf Kohn, Anna Malovíková and Jozef Rosík

Institute of Chemistry, Slovak Academy of Sciences, 809 33 Bratislava


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.