Electrochemical Oxidation of 6-Thiopurine at the Pyrolytic Graphite Electrode

Abstract

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 1M 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 ale 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-6sulfonamide (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. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 141.217.58.200 Downloaded on 2015-04-10 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 141.217.58.200 Downloaded on 2015-04-10 to IP

The Cathodic Stripping Voltammetric Determination of 6-Mercaptopurine at a Silver Electrode

“…As expected the peak current changed with deposition potential, the optimum being in the range of 0.1-0.2 V. At more positive potentials the peak height decreased rapidly due to the formation of silver oxide and the possible oxidation of 6MP [14]; at more negative potentials the deposition of 6MP was less effective.…”

“…Likewise electrolysis at the anodic peak produced no visible change, partly because of blocking by the adsorption film. Since no change occurred with both anodic and cathodic electrolysis, the anodic wave cannot be ascribed to the oxidation of 6MP to bis(6-purinyl)-disulfide or purine-6-sulfinic acid [14] and the cathodic peak cannot be attributed to the reduction of 6MP or its oxidation products formed at more positive potentials. Thus the formation and reduction of silver 6MP thiolate can be suggested as an alternative.…”

“…Following separation by liquid chromatography 6MP may be detected amperometrically at a cobalt phthalocyanine chemically modified electrode [7] or spectrophotometrically after thin-layer separation and removal of spots by measurement at 325 nm [8]. Because some work has suggested a parallel between electrochemical behavior and biological action [9][10][11], electrochemical techniques have been extensively employed for these investigations [3,4,[12][13][14][15][16]. Previously, the electrochemical behavior of 6MP at mercury, pyrolytic graphite, and chemically modified electrodes has been studied and reaction mechanisms have been proposed.…”

Voltammetric Behavior of 2-Mercaptopyrimidine at a Silver Electrode

“…Thus voltammetric techniques are useful for their study. Much attention has been paid both to the electrochemical mechanisms and the analytical applications of the electrochemical redox of these compounds [11,12].…”

“…Thiouracils are minor components of transfer RNA and have been used for treatment of hyperthyroidism in man; thiopurines are usually used in cancer therapy [13][14][15][16]. Their electrochemical behavior at mercury and some solid electrodes was investigated by Dryhurst [1,12] and Wrona et al [13,17]. Although they are all thiol derivatives of pyrimidine (or purine), quite different electrochemical behavior has been observed for them.…”