Electrochemical Oxidation of Uric Acid and Xanthine at the Pyrolytic Graphite Electrode

The quantitative course of electrochemical oxidation of uric acid and xanthine at the pyrolytic graphite electrode has been examined between p H 1–7, the most commonly encountered p H range in biological systems. Basically, uric acid is oxidized in a primary oxidation of the bond to give a readily reducible and highly reactive bis‐imine. The bis‐imine can be detected as a cathodic peak by fast sweep cyclic voltammetry. Complete hydration of the bis‐imine gives rise to uric acid‐4,5‐diol that breaks down to alloxan and urea at p H 1, allantoin and a small amount of urea at p H 7, and at intermediate p H values mixtures of alloxan, allantoin, urea, and occasionally traces of parabanic acid are formed. Xanthine undergoes a oxidation that proceeds by two stages. The first involves oxidation of the bond to give uric acid which, being more easily oxidized than xanthine, is immediately further oxidized to the uric acid bisimine. This then hydrates and fragments in essentially the same way as described for uric acid, the same products being observed. Mechanisms are developed which explain the observed electrochemistry and over‐all formation of products from this reaction. The nature and yields of products formed electrochemically and the effect of p H on the reaction parallels the biological (enzymatic) oxidation of these compounds.