Electronic coupling for charge transfer and transport in DNA

We calculated electronic matrix elements for hole transfer between adjacent nucleobases in DNA. Calculations of the matrix elements for intrastrand and interstrand transfer were performed at the Hartree-Fock level employing the 6-31G* and 6-311G** basis sets. The matrix elements for intrastrand hole transfer, for which a wealth of experimental solution data is available, are almost independent of the basis set and exhibit an exponential interbase distance dependence, sensitivity to the donor-acceptor geometry, and dependence on 5′ → 3′ direction base sequence. The calculated intrastrand hole transfer matrix elements between adjacent thymines, v₊(T, T) = 0.16 eV, is in good agreement with the experimental estimate, v₊(T, T) = 0.18 eV, inferred from hole hopping in G⁺(T)_mGGG (m = 1-3). The features of the nucleobase bridge specificity for superexchange-induced hole hopping between guanines in G⁺XY...G (X, Y = T or A) were elucidated, with the prediction of enhanced efficiency of thymine relative to adenine as mediator. Information on superexchange-mediated intrastrand and direct interstrand hole hopping between guanine bases was also inferred. Our results for interstrand, adjacent G⁺G coupling predict the existence of zigzagging pathways for hole hopping, in line with experiment.