Mechanism of the OH. radical induced oxidation of methionine in aqueous solution

The reaction mechanism of the OH. radical induced oxidation of methionine in aqueous solutions has been investigated by radiation chemical methods, mainly ns-ms pulse radiolysis. The initial step is a competitive process between addition of the OH. radical to the sulfur atom and hydrogen abstraction at a 4:1 ratio in favor of the former. In strong acid solutions (pH approx. less than or equal to 3) the subsequent mechanism is practically identical with the oxidation mechanism of aliphatic thioethers. The intermediate of most interest in this case is a sulfur-centered radical cation complex with an absorption maximum at 480 nm. At higher pH in slightly acid, neutral, and basic solutions the mechanism is more complex, essentially involving a fast (k > 4 x 106 s-1) intramolecular and probably sterically assisted oxidation of the amino group by the primarily oxidized sulfur function. This process includes irreversible decarboxylation of the amino acid. Both the CO2 and the remaining α-amino radicals have been identified and their yields determined. The N-oxidation depends on the reduction potential of the amino group and takes place with both the basic and the zwitterionic form of methionine - in the latter case on account of the rapid proton exchange between the NH3+ and COO- groups. Complete protonation of the carboxyl group in very acid solutions, or by the addition of powerful proton donors, e.g., H2PO4-, or the introduction of an electron-withdrawing substituent such as in N-acetylmethionine lowers the reduction potential to such an extent that the oxidation site may effectively remain at the sulfur. A detailed mechanism with the associated reaction kinetics and some physicochemical properties of the intermediates are presented and discussed