Physicochemical properties and structural characterization by two-dimensional NMR spectroscopy of wheat β-D-glucan—comparison with other cereal β-D-glucans

The first isolation, purification and characterization of a wheat β-D-glucan is reported. Alkaline extraction of wheat bran prepared by a novel pre-processing gave a non-starch polysaccharide fraction. Purification was carried out by hydrolyzing the arabinoxylans of this fraction with β-D-xylanase (T. Viride, EC 3.2.1.8), which had no effect on the molecular weight of the β-D-glucan. The structure of the purified wheat β-D-glucan was characterized by NMR spectroscopy, and additional structural detail was revealed by hydrolysis with (1→3)(1→4)-β-D-glucan-4-glucanohydrolase (lichenase, EC 3.2.1.73) and analysis of the oligosaccharide reaction products. Direct and long-range homo- (1H/1H) and hetero- (13C/1H) nuclear shift correlations were used to make complete assignments of both the 13C and 1H spectra as well as to confirm sequences and linkage sites. The 13C NMR spectrum of wheat β-D-glucan appeared identical to that of other cereal β-D-glucans, such as those from oat and barley. However, the ratio of 3-O-β-cellobiosyl- and 3-O-β-cellotriosyl-D-glucose obtained from wheat β-D-glucan by lichenase hydrolysis was significantly higher than ratios from other cereal β-D-glucans (approximately 4, 3 and 2 for wheat, barley and oat, respectively). The predominant molar proportion of trisaccharide (72.4%) from wheat β-D-glucan suggests that it will have a more regular structure than the other β-D-glucans, which might be responsible for its greater gelling ability and poorer solubility in water compared to other cereal β-D-glucans.