Identification of carbon-carbon double bond stereochemistry in unsaturated fatty acids by charge-remote fragmentation of fixed-charge derivatives

Separation and identification of fatty acid (FA) isomers in biological samples represents a challenging problem for lipid chemists. Notably, FA regio- and stereo-isomers differing in the location or (cis/trans) geometry of carbon-carbon double bonds are often incompletely separated and ambiguously assigned in conventional chromatography-mass spectrometry analyses. To address this challenge, FAs have been derivatized with the charge-switch derivatization reagents N-methyl-pyridinium-3-methanamine and N-(4-aminomethylphenyl)pyridinium and subjected to reversed-phase liquid chromatography-tandem mass spectrometry. Charge-remote fragmentation of the fixed-charge derivatives leads to characteristic product ions arising from dissociation at allylic positions that enable assignment of position(s) of unsaturation, while a newly discovered dihydrogen neutral loss was found to be dominant for double bonds with cis-stereochemistry. The structure of the [M - 2]+ product ions was probed by gas-phase ozonolysis revealing the presence of two new carbon-carbon bonds on either side of the initial position of unsaturation consistent with an electrocyclic mechanism of 1,4-dihydrogen elimination. Charge-remote fragmentation pathways diagnostic of double bond position and stereochemistry were found to be generalized for FAs of different carbon-chain lengths, double bond positions, and degrees of unsaturation and were effective in the unequivocal assignment of the FA structure in complex mixtures of FA isomers, including bovine milk powder.