On the formation of conjugated linoleic acid diagnostic ions with acetonitrile chemical ionization tandem mass spectrometry.
Acetonitrile chemical ionization tandem mass spectrometry has recently been shown to be a rapid method for the identification of double-bond position and geometry in methyl esters of conjugated linoleic acids (CLAs); however, the structures of intermediate and diagnostic ions and their mechanisms of formation are not known. A mechanism is proposed here in which the m/z 54 ion, (1-methyleneimino)-1-ethenylium (MIE), undergoes nucleophilic attack preferentially by the cis double bond in CLAs with mixed geometry (cis/trans, trans/cis), favoring the observed C--C cleavage vinylic to the trans double bond. The [M+54](+) addition product intermediate is consistent with a heterocyclic six-membered ring resulting from the two-step addition of MIE to the CLA. Experiments with isotopically labeled CLAs and acetonitrile, and from MS/MS/MS experiments, yield data consistent with this proposal. The proposed mechanism is also consistent with known ion-molecule chemistry in smaller compounds, and explains most phenomena associated with MIE-CLA ion chemistry.
Michaud
Anthony L
AL
Division of Nutritional Sciences, Cornell University, Savage Hall, Ithaca NY 14853, USA.
Lawrence
Peter
P
Adlof
Richard
R
Brenna
J Thomas
JT
D000097:Acetonitriles / Q000737:chemistry*; D000838:Anions / Q000737:chemistry*; D044243:Linoleic Acids, Conjugated / Q000032:analysis / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
D000097:Acetonitriles; D000838:Anions; D044243:Linoleic Acids, Conjugated; C099033:acetonitrile anion