Futher insights into the electrochemical oxidation of uric acid

Abstract

The electrochemical oxidation of uric acid has been studied between pH 1.5 and 9.5 in phosphate buffers using thin-layer spectroelectrochemistry to generate and study UV-absorbing intermediates. Some intermediates and all important products have been separated and analyzed by gas chromatography—mass spectrometry. It is concluded, on the basis of this and preceding studies, that uric acid is initially oxidized in a 2 e-2H + reaction to a very unstable quinonoid diimine (half-life \\ ̌ 22 ms). At pH ⩾ 6 the anion of the latter species is attacked by water to give an anionic imine-alcohol that undergoes a ring contraction reaction to give 1-carbohydroxy-2,4,6, 8-tetraaza-3,7-dioxo-4-ene-bicyclo-(3,3,0)-octane (BCA). This then decomposes to allantoin. At pH 3–5.6 a neutral quinonoid diimine is generated upon 2 e-2H + oxidation of uric acid. In high-phosphate buffers H 2PO 4 − attacks the diimine, whereas in low-phosphate buffers solvent (H 2O) attacks the diimine. In high-phosphate buffers analysis of absorbance vs. time curves obtained following oxidation of uric acid in a thin-layer cell allows three intermediate species to be inferred. In low-phosphate buffers only two intermediates may be inferred. Mechanisms are advanced to rationalize these observations and to account for the end products formed, i.e. allantoin, 5-hydroxyhydantoin-5-carboxamide and, at pH 3, alloxan.