The sequence statistics and solution conformation of a barley (1----3, 1----4)-beta-D-glucan

The sequence statistics and aqueous solution conformation of the 40 degrees water-soluble (1----3,1----4)-beta-D-glucan isolated from barley (Hordeum vulgare) have been modeled realistically using the known sequence-distributions of (1----3) and (1----4) linkages, theoretical conformational analysis, and the statistical mechanical theory of polymer-chain conformation. This barley beta-glucan fraction consists of (1----4)-beta-glucooligosaccharides, predominantly of d.p. 4 or less, joined by single beta-(1----3) linkages. Approximate treatments of the sequence statistics which do not take into account the small mole fraction (approximately 2%) of (1----4)-beta-glucooligosaccharides of d.p. approximately 10 significantly underestimate the chain extension in solution. A correct prediction of the observed chain extension is achieved when these longer, highly extended (1----4)-beta-glucooligosaccharide blocks are included in a model which randomly incorporates all (1----4)-beta-glucooligosaccharide segments in the proportions observed experimentally. Chain flexibility in the 40 degrees water-soluble beta-glucan fraction is shown to arise principally from the isolated beta-(1----3) linkages; blocks of two or more contiguous beta-(1----3) linkages provide a source of additional flexibility which may influence the properties of barley beta-glucan fractions containing a significant proportion of such sequences.