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

Water-soluble and water-insoluble polysaccharide fractions were isolated from fruiting bodies of Ganaoderma lucidum (rheishi) by extraction with phosphate buffer and IN sodium hydroxide solutions at different temperatures and investigated by high-performance liquid chromatography (HPLC), one- and two-dimensional H-1 and C-13 NMR, infrared (IR) spectroscopy, and other techniques. This work was prompted by Sone el nl.'s earlier conclusion that similarly extracted, water-insoluble polysaccharides are (1-->3)-beta-D-glucans with very low degrees of branching though hardly hydrolyzed by the enzyme exo-(1-->3)-beta-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 degrees C) were essentially pure glucans. It was found from NMR and viscosity measurements in dimethylsulfoxide containing 0.25 M LiCl that these glucans consist of linearly linked (1-->3)-alpha-D-glucose residues and differ only in molecular weight. The (1-->3)-alpha-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)-beta-D-glucan and (1-->3)-alpha-D-glucan samples.