Abstract
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[sup 2]-deoxyguanosine covalently linked adducts (BPDE-N[sup 2]-dG, bond with 10S absolute configuration) in polar solvents (N,N[prime]-dimethylformamide (DMF), and the hydrogen-bonding liquids H[sub 2]O, D[sub 2]O, 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[sub s] = (9.1 [+-] 0.9) x 10[sup 9] s[sup [minus]1], which is followed by a slower recombination (k[sub r] = (1.8 + 0.5) x 10[sup 9] s[sup [minus]1]) in the inverted Marcus region. In the cis-stereoisomeric adduct, both rate constants are enhanced by a factor of approximately 5. The presence of the hydrogen-bonding network in NMF and FA exerts opposite effects on these rate constants, decreasing k[sub s] and increasing k[sub r] by factors of 2-5. In aqueous solutions these effects are even more pronounced, and radical ions are not observed since k[sub r] [much gt] k[sub s]. A kinetic isotope effect on the delay of the pyrenyl singlets in H[sub 2]O and D[sub 2]O (k[sub s](H[sub 2]O)/k[sub s](D[sub 2]O) = 1.3-1.5)more » suggests that a proton-coupled electron transfer mechanism may be operative in aqueous solutions. 51 refs., 10 figs., 2 tabs.« less
Published Version
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