Quantum chemical calculation of donor-acceptor coupling for charge transfer in DNA

The electronic coupling V-da is the parameter which determines most strongly how the charge-transfer rate between donor and acceptor depends on the distance between the sites and the mutual orientation of donor and acceptor moieties. We discuss quantum chemical procedures to estimate electronic coupling matrix elements of hole transfer in DNA. The two-state model was shown to be quite reliable when applied to the coupling between neighboring Watson-Crick pairs. However, one has to be careful when employing the two-state model to estimate V-da in systems where donor and acceptor are separated by a bridge of base pairs. We considered the gross features of base-pair specificity, directional asymmetry, and conformation sensitivity of the couplings. Matrix elements between base pairs are found to be extremely sensitive to conformational changes of DNA. This strongly suggests that a combined QM/MD approach should be best suited for estimating V-da within DNA fragments. Comparison of the effective couplings mediated by pi-stack bridges TBT and ABA (B = A, (z)A, G, T, C) demonstrate that the efficiency of charge transfer is considerably affected by the nature of B; in turn, the effect of B strongly depends on the neighboring pairs. Especially large effects are due to the variation of the oxidation potential of guanine and adenine (B = G, A). Chemical modification of these species or changes of their environment strongly influence the efficiency of charge transfer. We conclude with a discussion of several open questions and problems concerning the calculation of electronic couplings in DNA-related systems.