ONLINE MASS-SPECTROMETRIC INSIGHTS INTO ELECTROCHEMICAL REACTIONS - OXIDATION OF THIOPURINES

Electrochemistry was used on-line with high-performance liquid chromatography with mass spectrometric and UV-vis spectrophotometric detection to characterize the electrochemical oxidation pathways of 6-thiopurine and 6-thioxanthine. At low potentials, the electrochemical oxidation of 6-thiopurine proceeds via one electron resulting in disulfide formation. It is proposed that purine-6-sulfenic acid is formed at potentials > + 0.50V. Further oxidation of this unstable sulfenic acid presumably results in the formation of purine-6-sulfinate and purine-6-sulfonate. At potentials > + 0.50V purine-6-sulfinate, purine-6-sulfinamide, and purine-6-sulfonate have been identified as the final products of 6-thiopurine oxidation. On-line electrochemical studies indicate that at potentials less than +0.30 V, oxidation of 6-thioxanthine results in disulfide formation. The disulfide readily disproportionates to regenerate the original thiol plus small amounts of a sulfinic acid. At potentials > +0.30 V, it is proposed that the thiol group of 6-thioxanthine is further oxidized to a sulfinic acid. Xanthine and 2-hydroxypurine presumably form as a result of nucleophilic and electrophilic attack, respectively, on the sulfinic acid. At potentials greater than +0.40 V, both the thiol group and the purine ring of 6-thioxanthine undergo oxidation. Subsequent hydrolysis reactions produce an imine alcohol, the same intermediate which forms during uric acid oxidation, providing proof that oxidation of the purine ring occurs at potentials > + 0.40V. © 1990, The Electrochemical Society, Inc. All rights reserved.