SUBSTITUTION PATTERNS OF WATER-UNEXTRACTABLE ARABINOXYLANS FROM BARLEY AND MALT

Intact arabinoxylan polymer fractions from barley and malt differing in their ratio of arabinose to xylose (ara:xyl ratio) were degraded with a purified endoxylanase (E.C. 3.2.1.8) from Aspergillus awamori. Enzymic degradability decreased with increasing ara:xyl ratio. No differences in degradability between corresponding fractions from barley and malt could be observed. The fragments liberated by enzymic action were characterized using HPAEC. Fragments consisting of less than seven pentose residues contained relatively low amounts of O-2- and O-2,3-substituted xylose, compared to the original substrate. Xylose residues substituted at O-2 or both O-2 and O-3 appeared to be concentrated in the larger fragments. From these results and from our knowledge of the mode of action of the xylan degrading enzyme it was concluded that substituted xylose residues were not randomly distributed in the polysaccharide, but were arranged according to a pattern in which isolated unsubstituted residues are separated by one or two substituted residues. This pattern was interrupted by sequences of contiguous unsubstituted xylose residues. From the available data it was concluded that those sequences could reach a length of at least four residues, but the presence of longer unsubstituted sequences could not be excluded. Simulations of arabinoxylan structure and its enzymic degradation supported this model.