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.
Denekamp
C
C
University of Antwerp (UIA), Department of Pharmaceutical Sciences 1, Belgium.
Claeys
M
M
Pocsfalvi
G
G
D000468:Alkalies; D005227:Fatty Acids / Q000032:analysis; D006017:Glycolipids / Q000737:chemistry*; D006460:Hemolysin Proteins / Q000032:analysis; D013058:Mass Spectrometry; D011550:Pseudomonas aeruginosa; D012210:Rhamnose / Q000032:analysis
D000468:Alkalies; D005227:Fatty Acids; D006017:Glycolipids; D006460:Hemolysin Proteins; C051310:rhamnolipid hemolysin, Pseudomonas aeruginosa; D012210:Rhamnose