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

Abstract

Photoexcitation of 2-aminopurine riboside (2APr, 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/cm2) results in the consecutive two-photon ionization of the aromatic 2APr (or 2AP) residues. In neutral solutions, the 2APr (or 2AP) radical cations rapidly deprotonate (<100 ns). The 2APr(−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 2APr(−H)• (or 2AP(−H)•) radicals in H2O and D2O solutions. In H2O, the rate constants of dGMP(−H)• formation, as well as the rate constants of the reverse reaction of the 2APr (or 2AP) oxidation by dGMP(−H)• is larger than in D2O by a factor of 1.5−2. This kinetic isotope effect indicates that the electron-transfer reaction from dGMP to 2APr(−H)• (or to 2AP(−H)•), and the reverse electron transfer from 2APr (or 2AP) to dGMP(−H)•, is coupled to a deprotonation of the primary electron-transfer radical cation products, dGMP•+ and 2APr•+ (or 2AP•+). Therefore, these reactions, involving redox equilibria between different nucleobases, can be considered in terms of proton-coupled electron-transfer reactions.