Identification of monohydroxy fatty acids by electrospray mass spectrometry and tandem mass spectrometry

Negative-ion electrospray mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS) were used to characterize saturated and unsaturated monohydroxy fatty acids and fatty acid metabolites formed following incubation with soybean lipoxygenase. Ions corresponding to [M-H]- of eicosanoids were readily observed using ESI-MS, but double bond migration precluded the use of MS to localize double bonds or the position of hydroxyl moieties; however, by following MS analysis with negative-ion ESI-MS/MS of precursor ions, the position of oxygenation could be determined for picogram quantities of underivatized monohydroxy fatty acids. Loss of 46 mass units from the precursor ion of saturated monohydroxy compounds was explained in some cases by resonance stabilization of enolate ions, but this product ion was found in spectra of compounds where resonance was not possible. Spectra of deuterated analogs supported charge-driven vinylic processes as the most common mechanism of fragmentation. The utility of low-collision-energy ESI-MS/MS to examine biological samples was shown by examining the products formed by the metabolism of linoleic (18:2omega6) and arachidonic (20:4omega6) acids by soybean lipoxygenase using aerobic and anaerobic incubation conditions that generated increasingly complex mixes of metabolites.