The proton-coupled electron-transfer reactions of guanine and other modified purine nucleobases have been investigated by electrochemistry and stopped-flow spectrophotometry. The rate of oxidation of guanine by Ru(bpy)(3)(3+) varied linearly with the fraction of D2O in H2O/D2O mixtures (bpy = 2,2'-bipyridine), suggesting the involvement of a single proton, consistent with earlier studies (Weatherly, S. C.; Yang, I. V.; Thorp, H. H. J. Am. Chem. Soc. 2001, 123, 1236-1237). The oxidation of the tetrabutylammonium salt of 2'-deoxyguanosine-5'-monophosphate was investigated in acetonitrile. The oxidants were polypyridyl complexes of Ru(III) and Fe(III) that ranged in oxidation potential from 0.83 to 1.6 V versus SCE. The values of (RT ln k) obtained for these reactions increased with driving force with a slope of 0.5 +/- 0.1, consistent with a simple electron-transfer reaction. A conventional slope of 0.5 was not observed in aqueous solution where deprotonation of the guanine nucleobase was feasible. In addition to the driving force dependence, there was a dramatic suppression in the overall rate of guanine oxidation in acetonitrile compared to water both for dilute solution and for modified electrodes. The driving force dependence for the rate of oxidation of 7-deazaguanine and 7-deazaadenine was also investigated; plots of (RT ln k) versus E-1/2 showed slopes of 1.1 in aqueous solution, and these reactions gave large isotope effects ranging from 2.2 to 10, depending on the nucleobase and the oxidant. In contrast, the base 7,8-dihydro-8-oxoguanine did not exhibit an isotope effect, because the pK(a) of this nucleobase is known to be near 7. Taken together, these results suggest a picture where oxidation of guanine in aqueous solution by these oxidants proceeds via concerted proton-coupled electron transfer.