The diffusion-controlled reaction of semioxidized tryptophan with the superoxide radical anion

From pulse radiolysis measurements in oxygenated aqueous solution, the semioxidized tryptophan radical (Trp·— formed by the one-electron oxidation of Trp by Br2- radical—has been shown to oxidize the superoxide radical anion with a rate constant of k = 2 × 109 M−1 s−1. Proof of this reaction is found in addition of superoxide dismutase (SOD) to the system, which totally eliminates the contribution of the Trp· + O2- mechanism to Trp· decay. Little, if any, reaction of molecular oxygen with Trp· may be observed on the time scale of the pulse radiolysis experiment.