Photoinduced electron transfer occurs with different rate constants upon picosecond laser pulse excitation of the stereoisomeric (+)-trans- and (-)-cis-benzo[a]pyrene diol epoxide-N-2-deoxyguanosine covalently linked adducts (BPDE-N-2-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 k(s) = (9.1 +/- 0.9) x 10(9) s(-1), which is followed by a slower recombination (k(s) = (1.8 +/- 0.5) x 10(9) s(-1)) in the inverted Marcus region. In the cis-stereoisomeric adduct, both rate constants are enhanced by a factor of similar to 5. The presence of the hydrogen-bonding network in NMF and FA exerts opposite effects on these rate constants, decreasing k(s) and increasing k,by factors of 2-5. In aqueous solutions these effects are even more pronounced, and radical ions are not observed since k(r) much greater than> k(s). A kinetic isotope effect on the decay of the pyrenyl singlets in H2O and D2O (k(s)(H2O)/k(s)(D2O) = 1.3-1.5) suggests that a proton-coupled electron transfer mechanism may be operative in aqueous solutions.