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-beta-D-ribofuran osylpurine) 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/pul se/cm(2)) results in the consecutive two-photon ionization of the aromatic 2APr (or 2AP) residues. In neutral solutions, the 2APr (or 2AP) radical cat ions rapidly deprotonate (<100 ns). The 2APr(-H)(.) (or 2AP(-H)(.) neutral radicals thus formed reversibly oxidize 2-deoxyguanosine 5'-monophosphate ( dGMP) on mu s time scales, resulting in the formation of dGMP(-H)(.) neutra l radicals. Transient absorption measurements show that a remarkable solven t 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 c onstants of dGMP(-H)(.) formation, as well as the rate constants of the rev erse 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 th e electron-transfer reaction from dGMP to 2APr(-H)(.) (or to 2AP(-H)(.)), a nd: the: reverse electron transfer from 2APr (or 2AP) to dGMP(-H)(.), is co upled to a deprotonation of the primary electron-transfer radical cation pr oducts, dGMP(.+) and 2APr(.+) (or 2AP(.+)). Therefore, these reactions, inv olving redox equilibria between different nucleobases, can be considered in terms of proton-coupled electron-transfer reactions.