Kinetic study of the oxidation of some catecholamines by digital simulation of cyclic voltammograms

Electrochemical oxidation of some catecholamines such as dopamine (1), L-dopa (2), and methyldopa (3) has been studied in various pH values, using cyclic voltammetry. The results indicate participation of catecholamines (1-3) in intramolecular cyclization reaction to form the corresponding o-quinone derivatives (1d-3d). In various pHs, based on ECE mechanism, the observed homogeneous rate constants (k(obs)) of cyclization reaction were estimated by comparing the experimental cyclic voltammetric responses with the digital-simulated results. Also, the cyclization rate constants (k(cyc)) were calculated using microscopic acidic dissociation constant of ammonium groups. The significant differences in electrochemical behavior, k(obs) and k(cyc), of L-dopa (2) and methyldopa (3) with dopamine (1) are due to the effects of the side chain carboxyl group. (C) 2004 Wiley Periodicals, Inc.