A 13C-N.M.R.-Spectral study of a gel-forming,branched (1→3)-β-d-glucan, (lentinan) from lentinus edodes, and its acid-degraded fractions. Structure,and dependence of conformation on the molecular weight

The structure and conformation of lentinan, an anti-tumor, branched (1→3)-β-d-glucan from Lentinus edodes, and its acid-degraded, lower molecular-weight fractions have been investigated by ¹³C-n.m.r. spectroscopy. It is found that their ¹³C-n.m.r. spectra are considerably changed, depending on the molecular weight. The conformational behavior as studied by ¹³C-n.m.r. spectroscopy is consistent with that revealed by a study of the shift in the absorption maximum of Congo Red complexed with lentinan and its acid-degraded fractions. It is found that the water-soluble fraction II (mol. wt. 3,640) gives rise to well-resolved ¹³C-n.m.r. spectra; the ¹³C-signals are assigned to (1→3)-β-d-glucan and branch points at C-6. The branched structure is also confirmed by examination of the ¹³C-n.m.r. spectra of the compounds in dimethyl sulfoxide. For the gel state of the fractions of higher molecular-weight, lentinan (mol. wt. 1,000,000) and fraction IV (mol. wt. 16,200), however, ¹³C-n.m.r. spectra of considerably attenuated signal-amplitude are observed. The fact that the ¹³C-signals of the β-d-(1→3)-linked main chain and side chains are completely suppressed is explained as a result of immobilization caused by their taking an ordered conformation. The ¹³C-resonances observed in the gel state, which are assigned to β-d-(1→6)-linkages, are unequivocally assigned to the side chains (of disordered conformation). Finally, the ordered conformation of both the β-d-(1→3)-linked main chain and side chains is identified as the single-helix conformation, which tends to form multiple helixes as junction zones for gel structure.