Characterization by mass spectrometry and IRMPD spectroscopy of the sulfoxide group in oxidized methionine and related compounds

Methionine protein residues are prone to oxidation. To unravel the controversy about the mechanism of its one-electron oxidation, we characterised the main biological product, methionine sulfoxide, using mass spectrometry and IR multiple photon dissociation spectroscopy. ase IR spectra in the 800–2000 cm−1 range of protonated methionine and its sulfoxide were recorded and compared to those computed for the lowest energy structures. The signature of the SO bond was clearly identified at around 1000 cm−1. Oxidation of methionine–lysine dipeptide by OH radicals in the presence of catalase revealed the formation of methionine sulfoxide upon one-electron oxidation.