Electro-oxidation of 6-mercaptopurine riboside with special emphasis on the stability of the dimer in aqueous solutions

The electrochemical oxidation of 6-mercaptopurine riboside in aqueous solutions has been studied in the pH range 2.2-9.5 at a pyrolytic graphite electrode. The evidence strongly indicates that the electrochemical oxidation of 6-mercaptopurine riboside occurs in a single well-defined oxidation peak by loss of a proton and an electron to give a free radical, which instantaneously dimerises to give the corresponding disulfide. The disulfide formed was found to be unstable and decayed in a pseudo-first-order reaction with essentially similar k values in the presence and absence of an inert atmosphere. The ultimate products of oxidation in a controlled potential electrolysis have been identified as 6-sulfopurin-9-yl riboside and 6-sulfenopurin-9-yl riboside. A small amount of the starting material was also detected. A tentative mechanism for the formation of these products is suggested. A comparison of the electrochemical behaviour of 6-mercaptopurine riboside with 6-mercaptopurine is also presented.