Characterization of lyso-galactolipids, C-2 and C-3 O-acyl trigalactosylglycerol isomers, obtained from the lichenized fungus Dictyonema glabratum

Abstract

A mixture of two lyso isomers of a galactolipid was obtained from Dictyonema glabratum. Aqueous hydrolysis gave rise to galactose and glycerol in a 3:1 molar ratio. ESI–MS spectroscopy gave, in the positive-ion mode, a pseudomolecular ion at m/ z 839 and daughter ions with m/ z 677, 600, 515 and 353, suggesting three galactosyl units linked to a glycerol moiety, substituted by one O-acyl group. 1D and 2D NMR experiments were used to characterize the glycolipid, and HMQC examination showed three anomeric signals, corresponding to two α-Gal p and one β-Gal p residue liked to glycerol. The glycolipid structure was shown to be O-α- D-Gal p-(1→6)- O-α- D-Gal p-(1→6)- O-β- D-Gal p-(1↔1)-2- and -3-monoacyl- D-glycerol, the latter structures not having been previously found in nature. The fatty acid composition was determined by GC–MS of derived methyl esters: that of palmitic acid C 16:0 was the most abundant, although the presence of C 12:0, C 14:0, C 16:1 and C 18:0 esters was observed.