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. Gas phase 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 S=O bond was clearly identified at around 1000 cm (1). Oxidation of methionine-lysine dipeptide by (OH)-O-center dot radicals in the presence of catalase revealed the formation of methionine sulfoxide upon one-electron oxidation. (C) 2010 Elsevier B.V. All rights reserved.