Mechanism of cross-ring cleavage reactions in dirhamnosyl lipids: effect of the alkali ion

Liquid secondary ion mass spectrometry and high-energy collision-induced dissociation were used to analyze a dirhamnosyl lipid mixture. The negative fast-atom bombardment spectrum reveals a mixture of four homologous dirhamnosyl lipids with the following general structure: Rha-Rha-C-n-C-m (wbere C-n and C-m denote 3-hydroxy fatty acid moieties), The mass region 450-600 u in the collision-induced dissociation spectra of the negative [M - H](-) ions shows product ions that can be rationalized by terminal loss of a 3-hydroxyalkanoic acid residue; these ions can be used for the characterization of the fatty acid substituents, A unique effect of alkali-metal ions on the course of fragmentation of dirhamnosyl lipid attachment ions was observed. The strong chelation of sodium is revealed from the stability of the [M - H + 2Na](+) ion that does not lose a sodium ion with the eliminated neutrals, contrary to what is observed for the dilithium adduct, Cross-ring cleavages occur during high-energy collision-induced dissociation of both positively and negatively charged precursor ions. The results suggest a concerted decomposition pathway involving the six-membered rings of the monosaccharide residues. The formation of cross-ring cleavage products, which retain the C-10-C-10 moiety during high-energy collision-induced dissociation of all the precursor ions that contain sodium or lithium, strongly supports a retro [2 + 2 + 2] mechanism. Copyright (C) 2000 John Whey & Sons, Ltd.