Structural features of (glucurono)arabinoxylans extracted from wheat bran by barium hydroxide

A (glucurono)arabinoxylan extract obtained from water-unextractable cell wa ll material of industrial wheat bran was fractionated by means of anion-exc hange chromatography and graded ethanol precipitation. Methylation analysis and enzymic degradability of various fractions were used to elucidate thei r structures. Approximately one third of thr extracted (glucurono)arabinoxy lan was very lowly substituted (Ara/Xyl approximate to 0.2). Substitution o ccurred predominantly by arabinose at the O-3 position of xylose residues. Enzymic degradation showed a random distribution of the substituents, which are probably interrupted by stretches of 6 or more contiguous unsubstitute d xylose residues. More than half the extracted (glucurono)arabinoxylan was highly substituted (Ara/Xyl greater than or equal to 1). The complexity of the structure was shown by poor enzymic degradability and the presence of considerable amounts of branched arabinose and terminal xylose. Substitutio n of xylose occurred not only through O-3 mono-, O-2 and O-3 disubstitution by terminal arabinose and O-2 monosubstitution by (4-O-methyl)glucuronic a cid, but also through dimeric arabinose, xylose and possibly galactose cont aining branches as well as through 2,3-linked arabinose. The remainder of t he extracted (glucurono)arabinoxylan (15%) was either intermediately substi tuted (Ara/Xyl approximate to 0.5) or very highly substituted (Ara-Xyl appr oximate to 1.2) and associated with protein and lignin. (C) 1999 Academic P ress.