SPONTANEOUS AUTOXIDATION OF DOPAMINE

A detailed kinetic study has been carried out of the reaction of dopam ine, 2-(3,4-dihydroxyphenyl)ethylamine, with dioxygen over the pH rang e 7-9 where it reacts spontaneously without the necessity of metal ion catalysis. The reaction was found to be accurately first-order in [O- 2] and in [dopamine] and first-order in [H+](-1) and, furthermore, sto ichiometric amounts of H2O2 were shown to be produced. The other produ ct 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 ra te-determining step is assumed to be hydrogen atom abstraction from th e monodeprotonated species by O-2 (as with many other catecholamines, dopamine is stable towards oxidation in acidic media in the complete a bsence of metal ions) with a second order rate constant of k(1) = 0.47 +/- 0.05 dm(3) mol(-1) s(-1) at 25 degrees C in a solution of ionic s trength 0.1 mol(-1) dm(-3) (KCl).