Photoinduced Electron Transfer and Enhanced Triplet Yields in Benzo[a]pyrene Derivative-Nucleic Acid Complexes and Covalent Adducts

The carcinogenic and mutagenic benzo[a]pyrenediol epoxide derivative 7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,- 10-tetrahydrobenzo[a] pyrene (BPDE) binds via its C-10 position predominantly to the exocyclic amino group of guanine residues in native DNA. In such DNA adducts, the fluorescence of the pyrenyl moieties is strongly quenched by physicochemical interaction with the DNA bases. Using nanosecond time scale transient absorption techniques, products of the fluorescence quenching have been examined in two model systems in aqueous and polar organic solvents. The first is a monomeric, covalently linked (+)-trans-BPDE-N²-2′-deoxyguanosine adduct (BPDE-dG), the most abundant adduct when (+)-BPDE binds to native DNA; the second consists of mixtures of 7,8,9,10-tetrahydroxytetrahydrobenzo [a] pyrene (BPT) with dG, which was used to study the role of noncovalent interactions in solvents of different hydrophobicities. At moderate laser pulse energies (≤25 mJ/cm²/pulse, 347 nm), the primary products of the fluorescence quenching reaction are pyrenyl residue triplet excited states, with greatly enhanced yields (by factors of 3–10 or more relative to the yields expected from simple intersystem crossing). The primary quenching reaction involves photoinduced electron transfer from dG to pyrenyl residues, followed by efficient recombination to form triplet excited states. Consistent with this mechanism, pyrenyl radical anions are observed in solutions of BPT and dG (0.1 M) in polar organic solvents. However, radical ions are not observed in any of the covalently linked adducts nor in aqueous solutions of BPT and dG on time scales > 10 ns; triplet excited states are unique products of the fluorescence quenching reaction. These effects are attributed to noncovalent fluorophore-dG interactions in aqueous BPT/dG solutions and to the close proximities of electron donor/acceptor pairs in the covalently linked systems. Both of these circumstances favor rapid ion pair recombination, limiting their escape into the bulk of the solution.