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 (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.