One-electron oxidation reactions of some purine and pyrimidine bases in aqueous solutions. Electrochemical and pulse radiolysis studies

The reduction potentials of some purine and pyrimidine bases and the guanine nucleoside and nucleotide at pH values between 7 and 13 were investigated using the techniques of cyclic voltammetry, differential pulse polarography, and pulse radiolysis. The results clearly show that the lowest reduction potentials, in volts vs NHE, at pH 7 are those of xanthine, 0.88 V, and 1-methylguanine, 1.06 V (NHE). The extrapolated value of guanine is ca. 1.0 V. We also studied the one-electron oxidation reaction of the azide radical with the above compounds. We monitored their transient absorption spectra and determined their formation second-order rate constants. The decay kinetics of these radicals was followed. Radicals derived from the bases decayed via a radical-radical mechanism with a second-order rate constant. However, the guanine nucleosides and nucleotide radicals have shown at all pHs two consecutive processes (first order followed by a second order). The first-order reaction is pH dependent. Formation of a new transient was observed at pH greater than or equal to 9 for guanosine, pH greater than or equal to 11 for 2'-deoxyguanosine, and at pH 13 for 5'-GMP. The observed new transient spectra were similar to that observed for the oxidized guanine radical. Therefore, we suggest that in these oxidized guanine nucleosides and nucleotide the oxidized guanine radical has been released. As previously suggested our results imply that the radiation-induced base release from nucleosides in alkaline solution is due to the preferred reaction of the nucleoside sugar with O-.- Using our results, those of Steenken et al. (J. Am. Chem. Sec. 1992, 114, 4701-4709), and Symons model (Symons, M. C. R. J. Chem. Sec., Faraday Trans. 1, 1987, 83, 1-11) we suggest a proton assisted mechanism for a double-strand break in DNA.