Liquid chromatography/tandem mass spectrometric analysis of 7,10-dihydroxyoctadecenoic acid, its isotopomers, and other 7,10-dihydroxy fatty acids formed by Pseudomonas aeruginosa 42A2

Pseudomonas aerugthosa is an opportunistic pathogen, which oxidizes oleic acid to 7(S),10(S)-dihydroxy-8(E)-octadecenoic acid (7,10-(OH)(2)-18:1) of biological and industrial interest. Electrospray tandem mass spectrometric (MS/MS) analysis of hydroxylated fatty acids usually generates characteristic fragments containing the carboxylate anion and formed by a-cleavage at the oxidized carbon. These fragments indicate the positions of the hydroxyl group. In contrast, liquid chromatography (LC)/MS/MS analysis of 7,10-(OH)(2)-18:1 yielded a series of other ions with structural information. To study the fragmentation mechanism, we prepared H-2- and O-18-labeled isotopomers. We also performed MS3 analysis of the major ions, and for comparison we generated the corresponding 7,10-dihydroxy metabolites of 16:1n-7, 18:2n-6, and 20:1n-11 with a protein extract of P. aeruginosa. The MS/MS spectra of 7,10-(OH)(2)-18:1 and its isotopomers, 7,10-(OH)(2)-16:1, and 7,10-(OH)(2)-20:1, contained a series of prominent fragments that all hold the omega end. The 8,9-double bond was not essential for this fragmentation, as 7,10-(OH)(2)-18:0, and its isotopomers, formed essentially the same fragments in the lower mass range. In contrast, 7,10-dihydroxy-8(E),12(Z)-octadecadienoic acid (7,10-(OH)(2)-18:2) fragmented by a-cleavage at the oxidized carbons with formation of carboxylate anions. Our results demonstrate that C-16-C-20 fatty acids with a 7,10-dihydroxy-8(E) functionality undergo charge-driven fragmentation after charge migration to the omega-end, whereas the main ions of 7,10-(HO)(2)-18:2 retain charge at the carboxyl group. Copyright (C) 2010 John Wiley & Sons, Ltd.