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.

Plant cryptochromes are blue light photoreceptors that regulate key responses in growth and daily rhythm of plants and might be involved in magnetoreception. They show structural homology to the DNA repair enzyme photolyase and bind flavin adenine dinucleotide as chromophore. Blue light absorption initiates the photoreduction from the oxidized dark state of flavin to the flavin neutral radical, which is the signaling state of the sensor. Previous time-resolved studies of the photoreduction process have been limited to observation of the decay of the radical in the millisecond time domain. We monitored faster, light-induced changes in absorption of an algal cryptochrome covering a spectral range of 375-750 nm with a streak camera setup. Electron transfer from tryptophan to flavin is completed before 100 ns under formation of the flavin anion radical. Proton transfer takes place with a time constant of 1.7 micros leading to the flavin neutral radical. Finally, the flavin radical and a tryptophan neutral radical decay with a time constant >200 micros in the millisecond and second time domain. The microsecond proton transfer has not been observed in animal cryptochromes from insects or photolyases. Furthermore, the strict separation in time of electron and proton transfer is novel in the field of flavin-containing photoreceptors. The reaction rate implies that the proton donor is not in hydrogen bonding distance to the flavin N5. Potential candidates for the proton donor and the involvement of the tryptophan triad are discussed.

The kinetics and energetics of the electron transfer reactions occurring in the enzyme·substrate complex of DNA photolyase have been studied by transient absorption spectroscopy with picosecond time resolution in the temperature range 275−90 K. The results can be summarized as follows:  (i) The lifetime of 1(FADH-)* depends not only on the presence of the substrate but also on its nature, e.g. the electron transfer from 1(FADH-)* to thymine dimers T<>T and to thymine-uracil dimers T<>U is slower by a factor of 2 to 3 compared to that of U<>U and U<>T. This feature is attributed to the enlarged electronic coupling between 1(FADH-)* and the respective dimers in the absence of the methyl group in the C5 position of the 5‘-uracil. (ii) The temperature dependence of the quantum yield was studied, and from this the activation energy for the overall repair process has been estimated to be E A= 0.45 ± 0.1 eV.

The high efficiency of charge separation in photosynthetic reaction centers arises from the interplay of energetics, electronic couplings, and reorganization energies relevant for the fast charge separation and slow recombination processes. All these parameters can be determined unambiguously only from magnetic-fielddependent measurements of the recombination dynamics of the intermediate radical pair P + H A -and the lifetime of the recombination product 3 P*. Results obtained on Q A -depleted reaction centers of Chloroflexus aurantiacus are compared with those for the well-characterized reaction centers of Rhodobacter sphaeroides. In contrast to Rb. sphaeroides, the magnetic field dependence of the triplet yield in Cf. aurantiacus has a pronounced resonance structure, allowing the direct determination of the exchange interaction of P + H A -, J ) 21 G. The recombination rate k T is slightly larger for Cf. aurantiacus and shows a different temperature dependence. All these differences can be explained by the free energy of P + H A -, found to be larger by 0.04 eV in Cf. aurantiacus compared to Rb. sphaeroides. We propose that this different energy arises largely from the different amino acid at position L104, which is glutamic acid in the case of Rb. sphaeroides and glutamine in the case of Cf. aurantiacus. The electronic couplings and the reorganization energies, on the other hand, are very similar in both reaction centers. Implications for the mechanism of primary charge separation are discussed. The pronounced nonexponential kinetics of charge separation in Cf. aurantiacus is explained by the energetic inhomogeneity of the primary radical pair P + B A -.

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