The Kinetic Deuterium Isotope Effect as a Probe of a Proton Coupled Electron Transfer Mechanism in the Oxidation of Guanine by 2-Aminopurine Radicals

Photoexcitation of 2-aminopurine riboside (2AP r, 2-amino-9-β- d -ribofuranosylpurine) and 2-aminopurine (2AP) in oxygenated aqueous buffer solutions (pH 7.0) with 308 nm XeCl excimer laser pulses (fwhm = 12 ns, ca. 70 mJ/pulse/cm 2 ) results in the consecutive two-photon ionization of the aromatic 2AP r (or 2AP) residues. In neutral solutions, the 2AP r (or 2AP) radical cations rapidly deprotonate (<100 ns). The 2AP r (−H) • (or 2AP(−H) • neutral radicals thus formed reversibly oxidize 2-deoxyguanosine 5‘-monophosphate (dGMP) on μs time scales, resulting in the formation of dGMP(−H) • neutral radicals. Transient absorption measurements show that a remarkable solvent isotope effect is observed on the kinetics of oxidation of dGMP by 2AP r (−H) • (or 2AP(−H) • ) radicals in H 2 O and D 2 O solutions. In H 2 O, the rate constants of dGMP(−H) • formation, as well as the rate constants of the reverse reaction of the 2AP r (or 2AP) oxidation by dGMP(−H) • is larger than in D 2 O by a factor of 1.5−2. This kinetic isotope effect indicates that the electron-transfer reaction from dGMP to 2AP r (−H) • (or to 2AP(−H) • ), and the reverse electron transfer from 2AP r (or 2AP) to dGMP(−H) •, is coupled to a deprotonation of the primary electron-transfer radical cation products, dGMP •+ and 2AP r •+ (or 2AP •+ ). Therefore, these reactions, involving redox equilibria between different nucleobases, can be considered in terms of proton-coupled electron-transfer reactions.