Mass spectrometric analysis of pentafluorobenzyl oxime derivatives of reactive biological aldehydes

We recently demonstrated that a battery of reactive aldehydes can be generated by human neutrophils through the action of myeloperoxidase on α-amino acids. To determine the aldehydes, we formed the pentafluorobenzyloxime (PFBO) derivatives by reacting them with pentafluorobenzylhydroxylamine (PFBHA) and submitting the derivatives to gas chromatography electron-capture mass spectrometry (GC/EC/MS). Two geometric isomers are formed for each of the aldehydes, and they are separable by gas chromatography (GC) and exhibit distinguishable electron-capture (EC) mass spectra. Major fragment ions include [M - HF]^•, which probably has a six-membered ring formed via HF loss from the molecular radical anion. Subsequent decomposition of this intermediate yields other characteristic ions (e.g. those formed by elimination of NO and the anion of m/z 178). We proposed fragmentation pathways and mechanisms that are consistent with the data derived from collisionally-activated dissociations (CAD) coupled with tandem mass spectrometry, exact-mass measurements, studies of deuterium-labeled analogs, and calculations by PM3 semiemperical and ab initio methods. The generality of the structurally informative fragmentation pattern together with GC separation is the basis of a powerful means for the identification of reactive aldehydes in biological processes and the pathogenesis of disease.