Isolation and structural characterization of water unextractable arabinoxylans from Chinese black-grained wheat bran

Water unextractable polysaccharide from bran of a Chinese black-grained wheat variety was isolated with saturated barium hydroxide as extraction medium. The fraction composed mainly of arabinose (36.53%) and xylose (61.31%), indicating the presence of pure arabinoxylans. The arabinoxylan isolates exhibited the high average molecular weight (Mw) of 3.81 × 105 and an intermediate Ara/Xyl (A/X) ratio of 0.60. The fraction consisted of a β-d-(1→4)-linked xylan backbone, which was mono- or disubstituted with α-l-arabinofuranosyl (Araf) groups at position O-3, O-2 or at both O-2 and O-3 positions. The xylan backbone contained a relatively high amount of unsubstituted xylopyranosyl (Xylp) residues (57.71%) and low amount of disubstituted Xylp residues (6.22%). About 22.0% of Xylp residues were monosubstituted at O-3 position. α-l-Araf side groups were prevalently (1→3)-linked to β-d-Xylp residues as single units. Moreover, small quantities of 2-, and 5-linked α-l-Araf indicated the presence of short arabinan side chains.