Proton-coupled electron transfer in the oxidation of guanines by an aromatic pyrenyl radical cation in aqueous solutions

Electron transfer reactions between nucleic acid residues in DNA and strong oxidants are often the critical initial steps that initiate oxidative, irr eversible DNA damage. Employing laser flash photolysis transient absorption spectroscopic techniques, we investigated the characteristics of electron transfer reactions in aqueous solutions between the 2'-deoxynucleoside 5'-m onophosphates, dGMP, dAMP, dCMP and dTMP and a representative one-electron oxidant. The latter was a radical cation of a pyrene derivative with enhanc ed water-solubility, 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT). T he BPT radical cation BPT.+, was generated by intense nanosecond laser puls e (308 or 355 nm, 50-70 mJ pulse(-1) cm(-2)) by a non-linear consecutive tw o-photon absorption process. No electron transfer reactions were observed w ith dAMP, dTMP and dCMP, consistent with their unfavorable redox potentials . However, BPT.+ efficiently oxidized dGMP with a rate constant k(b) = 1.7 +/- 0.1) x 10(9) M-1 s(-1), which is smaller than the diffusion-controlled value by a factor of only similar to 3. The dGMP(-H)(.) neutral radicals fo rmed on time scales of a few microseconds, were identified by their charact eristic transient absorption spectrum (lambda(max) similar to 310 mn). The rate constant of electron transfer from dGMP to BPT.+ was smaller in D2O by a factor of similar to 1.5 than in H2O. This kinetic isotope effect indica tes that the electron transfer reaction from dGMP to BPT.+ is accompanied b y the deprotonation of dGMP(.+), and therefore appears to be a proton-coupl ed electron transfer reaction.