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

Liquid secondary ion mass spectrometry and high-energy collision-induced di ssociation were used to analyze a dirhamnosyl lipid mixture. The negative f ast-atom bombardment spectrum reveals a mixture of four homologous dirhamno syl 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 i n the collision-induced dissociation spectra of the negative [M - H](-) ion s shows product ions that can be rationalized by terminal loss of a 3-hydro xyalkanoic acid residue; these ions can be used for the characterization of the fatty acid substituents, A unique effect of alkali-metal ions on the c ourse of fragmentation of dirhamnosyl lipid attachment ions was observed. T he 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 cleavage s occur during high-energy collision-induced dissociation of both positivel y and negatively charged precursor ions. The results suggest a concerted de composition 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 th e precursor ions that contain sodium or lithium, strongly supports a retro [2 + 2 + 2] mechanism. Copyright (C) 2000 John Whey & Sons, Ltd.