| The DNA helix presents a novel medium in which electron transfer (ET) mediated by a molecular pi-stack can be investigated. To probe ET through DNA, we have constructed duplexes modified with probe molecules and applied spectroscopy and electrochemistry to study this phenomenon.; Photoinduced ET between two intercalators, ethidium and Rh(phi) 2bpy3+, was examined in a series of DNA duplexes. At distances up to 35 A, ET occurs on the subnanosecond timescale. However, the efficiency of ET is decreased in the presence of intervening base mismatches, confirming that the pathway proceeds through the pi-stack. These results provided the first demonstration that DNA-mediated ET between intercalators, is weakly dependent on distance, but highly sensitive to intervening base stacking.; To investigate a DNA base as a reactant, ethidium-modified duplexes containing deazaguanine were synthesized. The photooxidation of deazaguanine by ethidium also proceeds on a subnanosecond timescale and exhibits a shallow distance dependence. The overall distance dependence is sensitive to the stacking of deazaguanine. These studies again showed that the DNA base stack can mediate extremely fast, long-range ET, and elucidated that reactant stacking modulates the efficiency of this phenomenon.; Using base-base photochemistry, ET through DNA was probed directly without external donors and acceptors. Using fluorescent adenine analogues, ET was investigated as a function of reactant stacking and energetics. Small variations in each of these factors lead to remarkable changes in the kinetics of DNA-mediated ET and values of beta, which reflects the exponential dependence of ET on distance, were measured ranging from 0.1 A-1 to 1.0 A-1. Hence, the DNA base stack can exhibit insulator to "wire"-like properties, depending on molecular probes employed.; We also investigated DNA-mediated ET using electrochemical methods. With daunomycin crosslinked to DNA films, efficient electron transfer over distances greater than 30 A was observed. Base mismatches also attenuate this reaction, providing a new method for the electrochemical detection of genomic mutations.; As evidence increasingly supports the notion that ultrafast ET can occur in DNA, the implications for biological systems can now be considered. Our findings point to the DNA pi-stack as not only a carrier of genetic information, but also a pathway conducive to charge transport. |
