ANALYSIS OF LIPID HYDROPEROXIDES AND LONG-CHAIN CONJUGATED KETO ACIDSBY NEGATIVE-ION ELECTROSPRAY MASS-SPECTROMETRY

Lipid hydroperoxides are important products of enzymatic processes and autooxidation products of polyunsaturated fatty acids. Analysis of su ch compounds has proved difficult in the past, but negative ion electr ospray ionization mass spectrometry was found to be suitable for direc t analysis. Abundant [M-H](-) ions were observed in full scan mode for hydroperoxyeicosatetraenoic (HPETE), hydroperoxyoctadecenoic acid iso mers, and 5,12-diHPETE. Loss of water was observed for all species. Co llisional activation and tandem mass spectrometry generated unique and characteristic spectra that shared some common features such as loss of small neutral molecules. More importantly, fragment ions that were indicative of the position of the hydroperoxide were observed. Collisi on-induced decomposition (CID) of [M-H2O](-) for the HPETE isomers was found to be virtually identical to the CID mass spectra of the [M-H]( -) anions from corresponding keto-eicosatetraenoic acids, which sugges ts that the hydroperoxide anions decompose via a dehydration intermedi ate that resembles the keto acid molecular anion. Cleavage of the doub le bond allylic to the hydroperoxide formed structurally characteristi c ions at m/z 129 from 5-HPETE, m/z 153 from 12-HPETE, and m/z 113 fro m 15-HPETE. Charge-driven allylic fragmentation led to formation of m/ z 203 from 5-HPETE, m/z 179 from 12-HPETE, and m/z 219 from 15-HPETE. Mechanisms consistent with the decomposition of stable isotope analogu es are proposed for the formation of these and other characteristic io ns. These specific decompositions can be used in multiple reaction mon itoring to measure picomolar concentrations of hydroperoxides by direc t high performance liquid chromatography tandem mass spectrometry.