Oxidation by OH radicals produced by g-radiolysis of single amino acids proline and glutamic acid accompanied by a series of Met-containing model peptides were investigated by a combined study of tandem mass spectrometry and infrared multiple-photon dissociation spectroscopy in the fingerprint region. The thioether group of methionine has been identified as a common target of OH free radicals leading to a Sþ radical cation, which made a 2-center-three electron (2c-3e) bond with any atom having having a lone pair of electrons. However, the final fate of these 2c-3e free radicals is still unknown. These studies led to the identification of the final stable products of oxidation and the localization of the modified sites. We show that the final products contain mostly methionine sulfoxide, whatever the structure of the preceding free radical. Although the distance between two Met residues controls the nature of free radicals formed in the peptides, it has no significant effect on the final products of oxidation. These results thus confirm that the final products are not controlled by free radical structure. Also, although no proline free radical was detected previously, we show that proline residue can be oxidized