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‐Cn‐Cm (where Cn and Cm 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 C10‐C10 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 © 2000 John Wiley & Sons, Ltd.