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 solut ions has been studied in the pH range 2.2-9.5 at a pyrolytic graphite elect rode. The evidence strongly indicates that the electrochemical oxidation of 6-mercaptopurine riboside occurs in a single well-defined oxidation peak b y loss of a proton and an electron to give a free radical, which instantane ously dimerises to give the corresponding disulfide. The disulfide formed w as found to be unstable and decayed in a pseudo-first-order reaction with e ssentially similar k values in the presence and absence of an inert atmosph ere. The ultimate products of oxidation in a controlled potential electroly sis 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 com parison of the electrochemical behaviour of 6-mercaptopurine riboside with 6-mercaptopurine is also presented.