We explore charge migration in DNA, advancing two distinct mechanisms of charge separation in a donor ( In 1962, Eley and Spivey proposed (1) that -interactions between stacked base pairs in double-strand DNA could provide a pathway for rapid, one-dimensional charge separation. In spite of subsequent theoretical and experimental effort in this intriguing field (2-7), experimental evidence for such ''molecular wire'' type conduction in DNA remained elusive. The studies of Warman et al. (8) in 1996 of radiation-induced conductivity in hydrated DNA argued against one-dimensional conduction confined to the base pair core. Interest in this fascinating subject (9-31) was triggered recently by the studies of Barton and her colleagues (9-19), which seemed to indicate the occurrence of long-range, almost distance-independent charge separation in DNA, manifesting ''chemistry at a distance'' (17). The problem of charge separation in DNA (9-31) is pertinent for the realization of a particular DNA repair mechanism as an alternative to the DNA-photolyase (20-23), which rests on long-range charge transfer to the defect site, i.e., a thymine dimer followed by concurrent or sequential bond breaking. Moreover, a deeper understanding of charge migration processes and of the effects of electronic excess charges localized at specific nucleic bases has wide range implications for (i) protein binding to DNA. Because electrostatic interactions are primarily responsible for the association of proteins to nucleic bases, changes in the charge density at the DNA core induced by charge separation may affect the specificity of protein binding; (ii) DNA sequencing. The control of duplex formation via charge migration may be important for specific DNA sequencing; and (iii) DNA-based biosensors. The development of biosensors, which depend on specific long-range charge separation along duplex structures in solution and preferentially at electrodes, is of considerable potential.The interpretation of the early experiments of Barton, Turro, and their colleagues (9-13) on charge separation between donor and acceptor complexes attached to DNA was fraught with some difficulties because of the possibility of aggregation effects (24). The recent data of Dandliker, Holmlin, and Barton (17-19) on hole migration between the electronically excited metal intercalator Rh(phi) 2 DMB ϩ3 and the thymine dimer, both of which are specifically incorporated in a 16-bp DNA duplex, provide evidence for long-range hole separation (over a distance scale of r ϭ 19-26 Å) with the yield being independent of donor-acceptor distance (R). These results (17)(18)(19) are in dramatic conflict with other experiments on charge separation in DNA (25-27), as well as with the standard electron transfer theory (32-40). For a donor (d)-bridge-acceptor (a) system, the theory (33-40) predicts an exponential (donor-acceptor) distance R dependence of the hole (or electron) transfer rate, k ϭ (2͞)V 2 F of the MarcusLevich-Jortner equation (33-40):Here, F is the thermally averaged n...

The fundamental mechanisms of charge migration in DNA are pertinent for current developments in molecular electronics and electrochemistry-based chip technology. The energetic control of hole (positive ion) multistep hopping transport in DNA proceeds via the guanine, the nucleobase with the lowest oxidation potential. Chemical yield data for the relative reactivity of the guanine cations and of charge trapping by a triple guanine unit in one of the strands quantify the hopping, trapping, and chemical kinetic parameters. The hole-hopping rate for superexchange-mediated interactions via two intervening AT base pairs is estimated to be 10 9 s ؊1 at 300 K. We infer that the maximal distance for hole hopping in the duplex with the guanine separated by a single AT base pair is 300 ؎ 70 Å. Although we encounter constraints for hole transport in DNA emerging from the number of the mediating AT base pairs, electron transport is expected to be nearly sequence independent because of the similarity of the reduction potentials of the thymine and of the cytosine.

In this paper we consider a theory for intramolecular radiationless transitions in an isolated molecule. The Born–Oppenheimer zero-order excited states are not pure in view of configuration interaction between nearly degenerate zero-order states, leading to the broadening of the excited state, the line shape being Lorentzian. The optically excited state can be described in terms of a superposition of molecular eigenstates, and the resulting wavefunction exhibits an exponential nonradiative decay. The linewidth and the radiationless lifetime are expressed in terms of a single molecular parameter, that is the square of the interaction energy between the zero-order state and the manifold of all vibronic states located within one energy unit around that state. The validity criteria for the occurrence of an unimolecular radiationless transition and for exponential decay in an isolated molecule are derived. Provided that the density of vibrational states is large enough (i.e., exceeds the reciprocal of the interaction matrix element) radiationless transitions are expected to take place. The gross effects of molecular structure on the relevant molecular parameters are discussed.