A partially 3-O-methylated (1→4)-linked α-?-galactan and α-?-mannan from Pleurotus ostreatoroseus Sing

The two main water-soluble extracellular polysaccharides produced by the basidiomycete fungus Pleurotus ostreatoroseus Sing were isolated and purified. They were characterized using 13C, 1H, and 1H, 13C HMQC NMR spectroscopy, methylation analysis, and Smith degradation. One was a mannan having a main chain of (1-->6)-linked alpha-D-mannopyranosyl residues, almost all of which were branched at O-2 with side chains of different lengths, containing 2-O- and 3-O-linked mannopyranosyl units. The other was a partially 3-O-methylated (1-->4)-linked alpha-D-galactopyranan, a structure that has not been previously described.