| Interest in directly studying excess electron transport between electron donors and acceptors in DNA at room temperature motivates us to link covalently the electron donor pyrene to C5 of 2′-deoxyuridine (dU) via an ethylenyl linker (PEdU). To produce reversible electron acceptors, we also link two different pyridiniums to C5 of dU. This dissertation presents the synthesis of PEdU as well as of four 5′- O-DMT-protected 5-(N-methylpyridiniumyl)-dU conjugates using either ethynyl or ethylenyl linkers. Time resolved absorbance (TA) studies on PEdU and time resolved emission studies on both PEdU and PEdU-substituted DNA hairpins are carried out with 25-ps Nd3+/YAG laser pulse (FWHM) at 355 nm. The TA results show that intramolecular electron transfer (ET) from pyrene to dU occurs in less than 30 ps in PEdU in McOH. Both TA and time resolved emission studies show that the weighted average lifetime (<τ>) of the Py·+/dU·− ET product for PEdU is 2.5 ns in deoxygenated McOH. In contrast, the time resolved emission studies show that the <τ> for the Py·+ /dU·− ET product in each of four DNA hairpins in deoxygenated buffer is shorter than the PEdU ET product lifetime: respectively, 2.38, 0.99, 1.34, and 0.73 ns. The rank order of <τ> ET products for PEdU and the four hairpins also agrees with the rank order of relative ET product emission quantum yields: 0.10 for PEdU and, respectively, 0.25, 0.025, 0.035, and 0.022 for the same hairpin series. Lack of delayed charge recombination with a long lifetime in hairpins compared to that seen in PEdU (ca. 10 ns) suggests that the excess electron on dU·− does not move far from PEdU. In fact a new mechanism for Py·+/dU ·− charge recombination is present in the hairpins, that is not present in PEdU itself. These results show that some amount of short-circuit charge recombination in which an excess electron on dU·− hops to a neighboring thymine base and then directly recombines with Py·+ is responsible for the shorter ET product lifetimes in the DNA hairpins. |
