Structural characterization of a polysaccharide and a β-glucan isolated from the edible mushroom Flammulina velutipes

Abstract

Two polysaccharides were isolated from the basidiomycete Flammulina velutipes, via successive hot extraction with water, 2% and 25% aq. KOH, and then submitted to freeze-drying. The precipitate formed by repeated freeze-thawing from the 2% aq. KOH extraction PK2 was analyzed by determination of its monosaccharide composition, as well as by methylation analyses using GC–MS, mono- ( 13C, 1H NMR) and bidimensional ( 1H (obs.), 13C HMQC) spectroscopy, and controlled Smith degradations. It was established to be a branched β-glucan, with a main chain of (1 → 3)-linked-Glc p residues, substituted at O-6 by single-unit β-Glc p side chains. The precipitate formed by repeated freeze-thawing from the 25% KOH extraction PK25 contained Xyl, Man, and Glc and was heterogeneous by HSPEC and extraction with DMSO gave a soluble xylomannan (XM). It was homogeneous with a molar mass 30.8 × 10 4 g/mol (d n/d c = 0.186). Using the above chemical analyses, it was a xylomannan with Man and Xyl in a 3:2 molar ratio. Its main chain consisted of (1 → 3)-linked α-Man p units, mainly substituted at O-4 by β-Xyl p units or with some β-Xyl p-(1 → 3)-β-Xyl p groups.