Structural characterization of a heterogalactan purified from fruiting bodies of Ganoderma atrum

A water-soluble heterogalactan, designated as PSG-2 was isolated and purified from the fruiting bodies of Ganoderma atrum. The molecular weight of PSG-2 was 6.9 × 104 Da by HPSEC determination. Its structure was analyzed using monosaccharide composition, partial acid hydrolysis, methylation analysis, combined with FT-IR spectra and 1D/2D NMR spectroscopy. PSG-2 was composed of galactose, fucose and glucose with molar percentage of 75.87%, 11.83% and 6.02%, respectively. Low content of mannose was also detected. Acetyl group was found in PSG-2 by FT-IR and NMR spectra. The backbone of PSG-2 consisted of 1,6-Galp (55.29%), and the short branched chains were attached to its O-2 position (1,2,6-Galp, 15.76%). The terminal residues contained T-Fucp (8.71%), T-Galp (5.97%) and T-Manp (1.83%). Three sugar residues, 1,2-Galp, 1,4-Glcp and 1,3-Glcp were also found to be with comparable amounts. Based on these results and NMR spectroscopy, a structure of PSG-2 was proposed as follows: where R would be one of the following fragments: T-α-D-Galp(1 → 6)-2-OAc-α-D-Galp-(1→, T-α-D-Galp(1 → 6)-α-D-Galp-(1 → 4)-β-D-Glcp-(1→, T-α-D-Galp or T-α-D-Manp.