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In 1999, Wan et al. [Proc. Natl. Acad, Sci. USA 96, 6014-6019] published a pioneering paper that established the entanglement between DNA base pair motions and the transfer time of the charge carrier. The DNA assemblies contained an ethidium covalently bound via a flexible alkyl chain to the 5' hydroxyl group of the DNA backbone. Although covalently attached, the loose way in which the ethidium was linked to DNA allowed for large degrees of conformational freedom and thus raised some concern with respect to conformational inhomogeneity. In this letter, we report studies on a different set of ethidium DNA conjugates. In contrast to the 'Caltech systems,' these conjugates contain ethidium tightly incorporated (as a base pair surrogate) into the DNA base stack, opposite to an abasic site analog. Despite the tight binding, we found that charge transfer from the photoexcited ethidium base pair surrogate across two or more base pairs is several orders of magnitude slower than in case of the DNA systems bearing the tethered ethidium. To further broaden the scope of this account, we compared (oxidative) electron hole transfer and (reductive) electron transfer using the same ethidium chromophore as a charge donor in combination with two different charge acceptors. We found that both electron and hole transfer are characterized by similar rates and distance dependencies. The results demonstrate the importance of nuclear motions and conformational flexibility and underline the presence of a base gating mechanism, which appears to be generic to electronic transfer processes through [pi]-stacked nucleic acids. conformational gating | electron transfer | ultrafast spectroscopy | hole transfer

The mechanism and dynamics of photoinduced charge separation and charge recombination are investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes separated by one to seven A:T base pairs. The observation of crossover from a superexchange to a hopping mechanism provides a 'missing link' in the analysis of DNA electron transfer and requires reevaluation of the existing literature for photoinduced electron transfer in DNA.

Research has been conducted on the methylenecyclopropane, bicyclopropylidene and spirocyclopropanated methylenecyclopropanes. The bromine additions to these compound carried out in methanol have been investigated and the results indicate the enhanced bromination rate with the increased number of spiroannelated three-membered rings.

The dynamics of electron injection and quantum yields for photoinduced dehalogenation are examined in a series of DNA hairpins possessing an aminopyrene capping group. Dehalogenation has occurred via a multistep mechanisms consisting of electron injection, electron transport to the halouracil, loss of halide, and trapping of the uracilyl radical and the quantum yields for product formation have decreased by a factor of 2 for each additional A-T base pair interposed between the aminopyrene and bromouracil.

The excited state behavior of synthetic DNA dumbbells possessing stilbenecarboxamide linkers is investigated. This is the first example of symmetry breaking in an A-B-A system via bridge mediated hole-hopping mechanism.

The synthesis, structure, and electronic properties of a series of tertiary di-and polyarylureas possessing pyrene and nitrobenzene end groups separated by a variable number of internal phenylenediamine bridging groups are reported. The change in the directionality of electron transfer upon addition of a second bridging phenylenediamine is a consequence of a decrease in the bridge oxidation potential as well as a decrease in the rate constant for single-step superexchange electron transfer.

Charge transfer in supramolecular assemblies of DNA is unique because of the notion that the [Pi]-stacked bases within the duplex may mediate the transport, possibly leading to damage and/or repair. The phenomenon of transport through [Pi]-stacked arrays over a long distance has an analogy to conduction in molecular electronics, but the mechanism still needs to be determined. To decipher the elementary steps and the mechanism, one has to directly measure the dynamics in real time and in suitably designed, structurally well characterized DNA assemblies. Here, we report our first observation of the femtosecond dynamics of charge transport processes occurring between bases within duplex DNA. By monitoring the population of an initially excited 2-aminopurine, an isomer of adenine, we can follow the charge transfer process and measure its rate. We then study the effect of different bases next to the donor (acceptor), the base sequence, and the distance dependence between the donor and acceptor. We find that the charge injection to a nearest neighbor base is crucial and the time scale is vastly different: 10 ps for guanine and up to 512 ps for inosine. Depending on the base sequence the transfer can be slowed down or inhibited, and the distance dependence is dramatic over the range of 14 [Angstrom]. These observations provide the time scale, and the range and efficiency of the transfer. The results suggest the invalidity of an efficient wire-type behavior and indicate that long-range transport is a slow process of a different mechanism.