ELECTROCHEMICAL OXIDATION OF 6-THIOPURINE AT PYROLYTIC GRAPHITE ELECTRODE

The electrochemical oxidation of 6-thiopurine (6-TP), which gives three well defined voltammetric waves at the pyrolytic graphite electrode (PGE), has been investigated by linear sweep and cyclic voltammetry, macroscale controlled electrode potential electrolysis at the PGE in aqueous IM acetic acid (pH 2.3), ammonia buffer pH 9 and carbonate buffer pH 9, and by determination of reaction products and possible intermediates. The electrochemical oxidation of 6-TP appears to follow a pathway quite different from the major enzymatic oxidative route. The first pH dependent wave is an adsorption wave due to oxidation of 6-TP to give an adsorbed layer of product bis(6-purinyl) disulfide (PDS). The second pH-dependent wave is a le process to give PDS both at pH 2.3 and pH 9; however at pH 2.3 further slow chemical oxidation to a disulfone or disulfoxide occurs, while in ammonia pH 9 the PDS decomposes rapidly to 6-TP, purine-6-sulfinic acid (P-6-Si) and purine-6-sulfonamide (P-6-Sm), with possibly some purine-6-sulfonic acid (P-6-So), with the over-all result that somewhat in excess of 4e are transferred. The third pH dependent wave in carbonate pH 9 gives P-6-So as the final product; in ammonia pH 9 wave III gives rise to a mixture of P-6-Si, P-6-Sm and possibly some P-6-So; PDS appears to be an unstable intermediate in these oxidations at high pH capable of being chemically oxidized by dissolved atmospheric oxygen. © 1969, The Electrochemical Society, Inc. All rights reserved.