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 aeruginosa 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 α‐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 ²H‐ and ¹⁸O‐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 α‐cleavage at the oxidized carbons with formation of carboxylate anions. Our results demonstrate that C₁₆–C₂₀ fatty acids with a 7,10‐dihydroxy‐8(E) functionality undergo charge‐driven fragmentation after charge migration to the ω‐end, whereas the main ions of 7,10‐(HO)₂‐18:2 retain charge at the carboxyl group. Copyright © 2010 John Wiley & Sons, Ltd.