Dynamics of Photochemical Electron Injection and Efficiency of Electron Transport in DNA

Abstract

The dynamics of electron injection and quantum yields for photoinduced dehalogenation have been investigated in a series of DNA hairpins possessing an aminopyrene capping group. Aminopyrene serves as an electron donor and is separated from a bromo- or iodoracil electron trap by 1 to 7 A-T base pairs. Broad band femtosecond transient absorption spectra and fluorescence quantum yield measurements are indicative of rapid and efficient charge separation. The resulting charge-separated state decays predominantly via charge recombination on the picosecond time scale. Steady-state irradiation in the presence of 0.1 M i-PrOH results in loss of halogen and conversion of the halouracil-containing conjugates to the corresponding uracil-containing conjugates in high yield. Dehalogenation occurs via a multistep mechanisms consisting of electron injection, electron transport to the halouracil, loss of halide, and trapping of the uracilyl radical. Quantum yields for product formation decrease by a factor of 2 for each additional A-T base pair interposed between the aminopyrene and bromouracil. This distance dependence is similar to that observed in our studies of DNA hole transport in stilbene donor−acceptor capped hairpins; however, the quantum yields for product formation are much lower for the aminopyrene conjugates as a consequence of more rapid charge recombination.