Sequential solvent extraction and structural characterization of polysaccharides from the endosperm cell walls of...

The objective of this study was to examine the composition and molecular structure of the endosperm cell walls (CW) derived from barley grain grown in three environments in Canada, and differing in grain hardness, protein, and total β-glucan contents. The endosperm CW were isolated from barley, cv. Metcalfe, grown in Davidson, SK (Sample A), Hythe, AB (sample B), and Hamiota, MB (sample C). The CW were sequentially extracted with water at 65(o)C, saturated Ba(OH)2, again with water at 25(o)C, and 1M NaOH, resulting in fractions designated WE65, BaE, Ba/WE, and NaE, respectively. The monosaccharide analysis indicated the presence of β-glucans, arabinoxylans, and small amounts of arabinogalactans, glucomannans, and xyloglucans. Cellulose was detected in the CW remnants. The CW of sample A, exhibiting a lower grain hardness than sample B, contained the lowest amount of β-glucans, but the highest amount of arabinoxylans and the mannose-containing polysaccharides. The CW of sample C, characterized by very high protein content in the grain, contained the highest amount of β-glucans and the lowest amount of other polysaccharides. Polysaccharides in the CW of sample B, exhibiting the highest grain hardness, were characterized by the highest weight average molecular weights (Mw). β-Glucans in the CW of Sample B showed the highest ratio of DP3/DP4 and the longest cellulosic fragments in the polymeric chains. Of the three barley samples, arabinoxylans in the endosperm CW of sample A exhibited the lowest degree of branching, the highest amount of unsubstituted Xyl residues, and the highest ratio of singly to doubly substituted Xylp. The highest water solubility of the CW of sample C was associated with the highest concentration of β-glucans, the lowest DP3/DP4 ratio, and the lowest Mw of the polymeric constituents. Arabinoxylans with the lowest amount of doubly substituted but the highest amount of unsubstituted xylose residues and long sequences of unsubstituted xylan regions were found in the NaE fractions. The NaE fractions showed a high ratio of →4)-Glcp-(1→ to →3)-Glcp-(1→ linkages and some →4)-Manp-(1→ linkages, indicating a high level of long cellulosic regions in β-glucan chains and the presence of glucomannans.