Spontaneous autoxidation of dopamine

A detailed kinetic study has been carried out of the reaction of dopamine, 2-(3,4-dihydroxyphenyl)ethylamine, with dioxygen over the pH range 7–9 where it reacts spontaneously without the necessity of metal ion catalysis. The reaction was found to be accurately first-order in O2 and in dopamine and first-order in H+–1 and, furthermore, stoichiometric amounts of H2O2 were shown to be produced. The other product of oxidation is, initially, the pink dopaminochrome which, however, is not stable and reacts further (without the consumption of dioxygen) to form the insoluble polymeric material known as ‘melanine’. The rate-determining step is assumed to be hydrogen atom abstraction from the monodeprotonated species by O2(as with many other catecholamines, dopamine is stable towards oxidation in acidic media in the complete absence of metal ions) with a second-order rate constant of k1= 0.47 ± 0.05 dm3 mol–1 s–1 at 25 °C in a solution of ionic strength 0.1 mol–1 dm–3(KCl).