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

► The IR SO bond signature is established by IRMPD and DFT calculations around 1000 cm−1. ► Not only the 2-electron oxidation, but also one-electron oxidation by OH radicals of a peptide containing methionine leads to the sulfoxide form. ► 2 the one electron oxidation of the dipeptide Met Lys was studied by mass spectrometry.