Sulfonoglycolipids from the lichenized basidiomycete Dictyonema glabratum: isolation, NMR, and ESI-MS approaches

Two distinct sulfonoglycolipid fractions were isolated from the basidiolichen Dictyonema glabratum by chromatography on Diethylaminoethyl (DEAE)-Sepharose, which resulted in rapid elution, followed by partition between aqueous sulfuric acid and an ethyl acetate-methanol-chloroform mixture and the content of the organic layer chromatographed of a column of silicic acid. The products were examined by nuclear magnetic resonance spectroscopy in their native rather than their acetylated forms, as in previous investigations. Each was methanolyzed to give the same methyl glycoside, Me-G. On electrospray ionization mass spectrometry (ESI-MS), it provided a pseudomolecular ion at m/z 303 in the positive-ion mode and a molecular ion at m/z 257 with a daughter ion at m/z 146 in the negative-ion mode, showing the presence of a sulfonate group S-linked to a hexosyl ring. An exclusively alpha-glucopyranosyl configuration was indicated by (1)H, (1)H correlation spectroscopy (COSY) and (1)H, (1)H total correlation spectroscopy (TOCSY). S-substitution occurred at CH(2)-6, because a high-field (13)C signal at delta 52.6 gave an inverted distortionless enhancement by polarization transfer (DEPT) signal and (1)H, (3)C heteronuclear multiple quantum coherence (HMQC) showed correlation with two H-6 signals. This 6-sulfono-alpha-quinovopyranoside group was present in the glycolipid fractions, O-alpha-D-Quip-6-sulfono-(1'<-->1)-2,3-diacyl-D-glycerol (polar fraction 1a; PF1a) and O-alpha-D-Quip-6-sulfono-(1'<-->1)-2- or -3-monoacyl-D-glycerol (polar fraction 1b; PF1b), the monoacyl derivatives not having been previously completely characterized in other systems. All components are typical of plant glycolipids. The most abundant fatty acid ester in PF1a and PF1b was C(16:0). Other esters present in PF1a were C(14:0), C(17:0), C(18:0), C(18:1) (oleic), C(20:0), C(21:0), and C(24:0), in contrast with C(14:0), C(17:0), C(18:0), and C(20:0) in PF1b. HMQC and TOCSY data can be used as fingerprints for detection of glycosylacylglycerides and sulfonoglycolipids and the positive ESI-MS ions at m/z 329 and 271 for identification of the latter.