Photoinduced electron transfer occurs with different rate constants upon picosecond laser pulse excitation of the stereoisomeric (+)-trans- and (-)-cis-benzo[a]pyrene diol epoxide-N2-deoxyguanosine covalently linked adducts (BPDE-N2-dG, both with 10S absolute configuration) in polar solvents (N,N′-dimethylformamide (DMF), and the hydrogen-bonding liquids H2O, D2O, formamide (FA), and N-methylformamide (NMF)). In the case of (+)-trans-BPDE-dG in DMF, photoinduced electron transfer occurs in the normal Marcus region, from dG to the pyrenyl residue singlet with a rate constant ks = (9.1 ± 0.9) × 109 s-1, which is followed by a slower recombination (kr = (1.8 ± 0.5) × 109 s-1) in the inverted Marcus region. In the cis-stereoisomeric adduct, both rate constants are enhanced by a factor of ∼5. The presence of the hydrogen-bonding network in NMF and FA exerts opposite effects on these rate constants, decreasing ks and increasing kr by factors of 2-5. In aqueous solutions these effects are even more pronounced, and radical ions are not observed since kr ≫ ks. A kinetic isotope effect on the decay of the pyrenyl singlets in H2O and D2O (ks(H2O)/ks(D2O) = 1.3-1.5) suggests that a proton-coupled electron transfer mechanism may be operative in aqueous solutions.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry