PERIODATE-OXIDATION AND DEGRADATION STUDIES ON THE MAJOR WATER-SOLUBLE ARABINOXYLAN IN RYE GRAIN

The major water-soluble arabinoxylan from rye grain has previously been shown to contain a main chain of 4-linked beta-D-xylopyranosyl residues in which, on average, every second is substituted at position 3 with terminal alpha-L-arabinofuranosyl residues. Periodate oxidation, reduction and fragmentation by mild acid hydrolysis produced a series of glycerol xylosides containing 4-linked xylopyranosyl residues linked at the reducing end to position 2 of glycerol. It was shown that a one-step periodate oxidation was incomplete due to the formation of relatively stable hemiacetal linkages. A sequential oxidation and reduction procedure was used to bring about complete oxidation of arabinose and unbranched xylose residues in the intact polysaccharide. Quantitative analysis of the products liberated by mild acid hydrolysis revealed the presence of glycerol xylosides with one, two or three xylose residues in the molar ratio of 1.00:0.86:0.02. The xylose residues must have originated from branched residues in the main chain of the arabinoxylan. The units or small blocks of two residues are therefore distributed mainly as isolated branched residues and not randomly as previously reported.