On‐Line Mass Spectrometric Insights Into Electrochemical Reactions: Oxidation of Thiopurines

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Abstract
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 +0.30V, 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 > +0.30V, 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 +0.40V, 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.

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