Chemical structure of a partially 3-O-methylated mannofucogalactan from edible mushroom Grifola frondosa

An unusual heteropolysaccharide was isolated from the fruiting bodies of the medicinal mushroom Grifola frondosa, via successive cold aqueous extraction, followed by fractionation through freeze-thawing, precipitation with Fehling solution and dialysis using a membrane with a size exclusion cut-off of 500 kDa. Its chemical structure was determined based on total acid hydrolysis, methylation analysis and NMR studies. The mannofucogalactan had a molar mass of 15.9 x 10(3) g mol(-1), which was determinate by HPSEC-MALLS. This heteropolymer showed to have a main chain of (1 -> 6)-linked alpha-D-Galp partially substituted at O-2 by 3-O-alpha-D-mannopyranosyl-alpha-L-fucopyranosyl groups and in a minor proportion with alpha-L-Fucp single-unit side chains. Moreover, the presence of 3-O-Me-Galp units could also be observed in the main chain of the G. frondosa mannofucogalactan.