A novel trisaccharide glycolipid biosurfactant containing trehalose bears ester-linked hexanoate, succinate, and acyloxyacyl moieties: NMR and MS characterization of the underivatized structure

A Gram-positive actinomycete growing on n-hexadecane secreted a family of anionic glycolipid surfactant homologs. The major homolog, with a molecular weight of 1210.6347, had the formula C58H98O26. Following mild alkaline saponification, 1H and 13C NMR spectroscopy were used to characterize the non-reducing trisaccharide backbone: β-Glcp-(1→3)-α-Glcp-(1↔1)-α-Glcp (‘laminaratrehalose’). Hexanoate, succinate, 3-hydroxyoctanoate, and 3-hydroxydecanoate were found in 3:1:1:1 molar ratio using GC–EIMS analysis of fatty acid methyl esters (FAME) prepared by transesterification. We found that the β-hydroxy acids bore secondary hexanoate chains in 3-O-ester linkage, giving acyloxyacyl anions of appropriate m/z in FABMS and FABMS/MS spectra. COSY, HETCOR, HMBC, and HMQC NMR experiments established the acylation pattern: succinate at C-2 of the terminal α-glucopyranose ring; hexanoate at C-3′′ of the β-glucopyranose ring; 3-hexanoyloxyoctanoate and 3-hexanoyloxydecanoate at the 2′- and 4-positions. In FABMS spectra, the homologs flanked the molecular ion by ±14 and ±28 amu, suggesting heterogeneity in acyl chain length.