Radiation chemical studies of methionine in aqueous solution: understanding the role of molecular oxygen.

The oxidation of methionine is an important reaction in the biological milieu. Despite a few decades of intense studies, several fundamental aspects remain to be defined. We have investigated in detail the gamma-radiolysis of free methionine in the absence and presence of molecular oxygen followed by product characterization and quantification. The primary site of attack by HO(*) radicals and H(*) atoms is the sulfur atom of methionine. We have disclosed that HO(*) radicals do not oxidize methionine to the corresponding sulfoxide in either the presence or the absence of oxygen; the oxidizing species is H(2)O(2) derived either from the radiolysis of water or from the disproportionation of the byproduct O(2)(*-). 3-Methylthiopropionaldehyde is the major product of HO(*) radical attack in the presence of molecular oxygen. Together with the direct oxidation at sulfur as the major product, the potential of H(*) atoms is also proven to be highly specific for sulfur atom attack under anoxic and aerobic conditions. The major products derived from the H(*) atoms attack are found to be alpha-aminobutyric acid or homoserine, in the absence or presence of oxygen, respectively. All together, these results help clarify the fate of methionine related to a biological environment and offer a molecular basis for envisaging other possible pathways of in vivo degradation as well as other markers.