Characterisation of free radical spin adducts of the cyclic β-phosphorylated nitrone DEPMPO using tandem mass spectrometry

The application of the spin trapping technique coupled with mass spectrometry for the detection of free radicals using the nitrone diethyl-(2-methyl-1-oxido-3,4-dihydro-2H-pyrrol-2-yl)phosphonate (DEPMPO) is described. In a first step, the fragmentation pathway of this β-phosphorylated nitrone was studied by MS/MS. Then, DEPMPO was used to trap the free radicals OH, CH2OH and CH3, and structure elucidation of the adducts obtained was performed by tandem mass spectrometry. The fragmentation pathway of the two carbon-centred radical adducts mainly proceeds via the loss of the diethoxy(oxido)phosphoranyl radical. In contrast, the nitroxide formed after OH trapping exclusively dissociated via the release of the radical initially trapped. This last decomposition pathway was unexpected in terms of radical stability but could be accounted for when considering the OH elimination could be favoured by an intramolecular hydrogen transfer, leading to the formation of a stable oxazirane intermediate. This spin trapping/mass spectrometry technique using DEPMPO provides a powerful tool for the identification of short-lived free radicals in complex mixtures, without preliminary chromatographic separation.