A 13C-nuclear magnetic resonance study of polysaccharide gels. Molecular architecture in the gels consisting of fungal, branched (1 → 3)-β- d-glucans (lentinan and schizophyllan) as manifested by conformational changes induced by sodium hydroxide

Abstract

To gain further insight into the architecture of the gel network of some branched (1 → 3)-β- d-glucans, a 13C-n.m.r. study of sodium hydroxide-induced, conformational change was performed. The branched d-glucans examined were lentinan from Lentinus edodes, a lower-molecular-weight fraction thereof, and schizophyllan from Scilizophyllum commune; these (1 → 3)-β- d-glucans have two branches for every five d-glucopyranosyl residues (lentinan), or one for every three or four (schizophyllan) at 0-6. In contrast to the gel oflinear (1 → 3)-β- d-glucan (curdlan), all of the 13C signals due to the β- d-(1 → 3)-linked d-glucosyl residues were completely suppressed in the gel state. As the peak intensity and line width of the 13C-resonance peaks for the gel state are strongly influenced by the degree of cross-linking, such a complete loss of the peak areas can be explained in terms of a higher degree of cross-linking than that of the linear d-glucans. As demonstrated previously, the cross-links involve physical association of the helical segments, such as the double- or triple-stranded helices. These helix forms were found to be converted, at 0.2 M sodium hydroxide, into the random-coil form (gel-to-sol transition), which gives rise to full peak-areas, because of complete breaking of the physical cross-links. Also, in contrast to the linear d-glucan, such helix-coil transition of the branched d-glucans proceeded in a noncooperative way: the peak intensity and line width gradually changed with the concentration of sodium hydroxide. This behavior is best interpreted in terms of distribution of the various degrees of cross-linking. Some loose cross-links are readily broken in the lower range of concentration of alkali (0.09 M), and others are resistant until complete conversion into the random coil occurs (0.2 M). This result is consistent with the view that the primary structure of the branched (1 → 3)-β- d-glucans is hi-highly branched, as in a tree-like structure.