Glycosyldiacylglycerolipids from the Lichen Dictyonema glabratum

Abstract

Three glycolipids (1-3) were isolated from the basidiolichen Dictyonema glabratum. Their carbohydrate and lipid components were structurally characterized using 1D 1H and 13C and 2D NMR spectroscopy, complemented by mass spectrometry, as were the carbohydrate moieties formed on saponification. These were O-alpha-D-Galp-(1''-->6')-O-beta-D-Galp-(1'<-->1)-2, 3-diacyl-D-glycerol (2) and two others not previously found in lichens, O-beta-D-Galp-(1'<-->1)-2,3-diacyl-D-glycerol (1) and O-alpha-D-Galp-(1'''-->6'')-O-alpha-D-Galp-(1' '-->6')-O-beta-D-Galp-(1'<-->1)-2,3-diacyl-D-glycerol (3). Each was saponified to give the free carbohydrates and its fatty acid methyl esters. The most abundant fatty acid esters in 1-3 was palmitic C16:0, but there was a wide variation of ester composition. Others present were C8:0 and C14:0 in 1, C14:0, C15:0, C17:0, C18:0, C18:1 (oleic), C18:2 (linoleic), C22:0, and C24:0 in 2, and C8:0, C14:0, C18:0, C18:1 (oleic), C18:2 (linoleic), and C18:3 (linolenic) in 3. As in ascolichens, the glycolipids appear to arise from the phycobiont.

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

“…The carbohydrate region also contained one signal at N 62.0 (3.750), corresponding to non-O-substituted C-6 (external K-Galp). Based on previous assignments [6], those at N 69.6 (3.860, 3.920) and 67.1 (3.680, 3.968) arose from 6-O-substituted units of internal K-Galp and L-Galp, respectively. The presence of the D-glycerol moiety was also con¢rmed by on HMQC experiment [6], which showed two doublets arising from C-3 and C-1.…”

“…Extracts of lipid-containing compounds, obtained from D. glabratum (see below), were evaporated at 6 40³C under reduced pressure [6]. Thin layer chromatography (TLC) was performed on silica gel plates (DC-Alufolien Kieselgel 60 from Merck), solvent: CHCl 3 /MeOH/H 2 O, 65:25:4 (v/v), and the resulting glycolipids were detected by spraying with carbohydrate-speci¢c orcinol-H 2 SO 4 , followed by heating at 100³C [8].…”

“…The organic layer was evaporated to dryness and the residue was then dissolved in CHCl 3 and chromatographed on a column of silicic acid, particle size 0.1 mm, column dimensions 5.6U12 cm. Elution was carried out using CHCl 3 (600 ml) and with mixtures of CHCl 3 /MeOH (600 ml each), with successive MeOH concentrations of 10, 12, 15, 20, 25, 30, 35, 40, and 50% [6]. The glycolipid was eluted with 40% MeOH (neutral glycolipid fraction D, NGD), and stored in a sealed tube below 310³C.…”

“…Acid hydrolysis (100 Wg) of NGD was performed with 2 M tri£uoroacetic acid at 100³C for 8 h, followed by evaporation to dryness, the residual material being successively reduced with aqueous NaBH 4 (1 mg) and acetylated with Ac 2 O-pyridine at 25³C for 15 h [6]. The resulting alditol acetates were analyzed by GC^MS as described above.…”

“…The most abundant glycolipids in plants and lichenized fungi are the glycosyldiacylglycerides [2,4]. Their structures have been completely characterized in the ascomycetous Ramalina celastri [5], and in the basidiolichen Dictyonema glabratum [6] using spectroscopic methods. We now examine the glycolipid extracts from Dictyonema glabratum (Sprengel) D. Hawks [7], in which we ¢nd another neutral trigalactosyl lipid, containing one O-acyl group linked to the glycerol moiety.…”