Mechanisms of the Photosensitized Oxidation of Tyramine

Abstract— …According to the criteria of enhancement in D 2 O and inhibition by sodium azide, the oxidation of tyramine photosensitized by methylene blue is largely a singlet oxygen or Type II process. Its quantum yield approximates 0.3 in D 2 O at pH 10. There is a less efficient reaction not quenched by azide, which is assigned to a dye‐substrate or Type I process. It gives rise to products with distinct bands at 320 and 285nm. Products of the Type I reaction are further oxidized by singlet oxygen and thereby compete with tyramine for this reagent. Kinetic parameters were estimated by computer simulation of the dependence of quantum yield on extent of reaction. The rate constant for reaction of O 2 ( 1 Δ g ) with tyramine was estimated to be 2.8 × 10 8 M ‐1 s ‐1 ± 20% at pH 10. The reaction was also sensitized by hypericin in what appears to be a Type II process.