Electrochemical oxidation of uric acid and xanthine: An investigation by cyclic voltammetry, double potential step chronoamperometry and thin-layer spectroelectrochemistry

Abstract

The electrochemical oxidation of uric acid and xanthine has been studied at various graphite electrodes and at a gold electrode by a variety of techniques including cyclic voltammetry, double potential step chronoamperometry, and thin-layer spectroelectrochemistry. It has been confirmed that the primary electrooxidation product formed from uric acid or xanthine is a diimine species. This diimine is very unstable in solution and is rapidly hydrated to an imine-alcohol in a first order reaction having an apparent rate constant of ca. 30 s −1 at pH 8.0. Both below and above pH 8.0 the rate of the latter reaction appears to increase. Although the diimine primary electrooxidation product is very unstable in homogeneous solution, having a half-life at pH 8.0 of ca. 23 ms, a voltammetric reduction peak due to reduction of the diimine may be observed on cyclic voltammetry of uric acid or xanthine at quite slow sweep rates (e.g., 200 mV s −1 at pH 8.0) at graphite electrodes provided that the electrode has a rough surface (large surface area). This is so because the diimine is strongly adsorbed at the rough electrode surface and in the adsorbed state is appreciably more stable than in solution. The imine-alcohol, formed by partial hydration of the diimine, may be observed as a further, unstable, u.v.-absorbing intermediate upon electrochemical oxidation of uric acid or xanthine at pH 7–9 by means of thin-layer spectroelectrochemical studies. The imine-alcohol is hydrated in a first order reaction to give uric acid-4,5-diol, characterized by an observed rate constant of 8×10 −3 s −1 between pH7 and 9. Uric acid-4,5-diol further decomposes to allantoin and CO 2 in neutral and weakly alkaline solution.