Chemical structure of the water-insoluble polysaccharide isolated from the fruiting body of Ganoderma lucidum

Abstract

Water-soluble and water-insoluble polysaccharide fractions were isolated from fruiting bodies of Ganoderma lucidum (rheishi) by extraction with phosphate buffer and 1 N sodium hydroxide solutions at different temperatures and investigated by high-performance liquid chromatography (HPLC), one- and two-dimensional 1H and 13C NMR, infrared (IR) spectroscopy, and other techniques. This work was prompted by Sone et al.'s earlier conclusion that similarly extracted, water-insoluble polysaccharides are (1→3)-β-D-glucans with very low degrees of branching though hardly hydrolyzed by the enzyme exo-(1→3)-β-D-glucanase of Basidiomycete QM 806. HPLC on the acid hydrolysates from our extracts showed that the water-soluble fractions were minor constituents and mostly heteropolysaccharides, while the water-insoluble ones (i.e., the alkali-extracts at 25 and 65°C) were essentially pure glucans. It was found from NMR and viscosity measurements in dimethylsulfoxide containing 0.25M LiCl that these glucans consist of linearly linked (1-→3)-α-D-glucose residues and differ only in molecular weight. The (1→3)-α-structure of the water-insoluble fractions was confirmed by IR and optical rotation and also by comparison with the NMR spectra taken for standard (1→3)-β-D-glucan and (1→3)-α-D-glucan samples.