Oxidation potentials of guanine, guanosine and guanosine-5′-monophosphate: Theory and experiment

Abstract

Guanine, having lower one-electron oxidation potential than other nucleobases, is of relevance to oxidative degradation of nucleic acids in mutagenesis, carcinogenesis, and aging. Here we compare oxidation potentials of guanine (G), guanosine (Guo), deoxyguanosine (dGuo), guanosine -5′- monophosphate (GMP) and 2′- deoxyguanosine -5′- monophosphate (dGMP) obtained by theoretical and experimental methods. Structures of G species were optimized and the identities of minima were verified by vibration frequency calculations. Redox equilibria were modelled in terms of corresponding thermochemical cycles. The changes in free energy were calculated at DFT level using the two different functionals: (i) general purpose B3LYP functional, and (ii) more specific ωB97X-D functional (both with 6-31 + G(d) basis set). Experimental oxidation potentials of all G analogues were measured voltammetrically on a polymer pencil graphite electrode (pPeGE) providing the best results from all carbon electrodes used (glassy carbon electrode, basal and edge plane pyrolytic graphite electrodes). The oxidation process is strongly dependent on the pH value and with increasing pH a linear shift of G oxidation peaks (Epa) towards negative potentials is observed. The theoretically and experimentally obtained oxidation potentials were compared for the pH 5. Anodic peak potentials increase in the order G « dGMP ≤ GMP < dGuo ≤ Guo and correlate with the calculated thermodynamic redox potentials as well as with NBO charges in purine moiety. The oxidation of deoxy analogues was predicted theoretically to occur at lower potentials than that of corresponding parent compounds and this fact was experimentally verified. The assumption that due to negatively charged phosphate group of GMP or dGMP their oxidation potentials could be observed at lower positive potential has not been confirmed and the significant difference (more than 200 mV) between the oxidation potentials of G nucleobase and its nucleosides and nucleotides is discussed. Moreover, conformity of theoretical and experimental data for radicals (cation, neutral) indicates that while the deprotonation process of G differs from its analogues, the oxidation process of all species takes place on imidazole ring.