Long-range and very long-range charge transport in DNA

Abstract

We present a kinetic–quantum model for the mechanisms of hole transport in DNA duplexes, which involves a sequence of hole hopping processes between adjacent guanines (G) and/or hole hopping/trapping via GG or GGG, all of which are separated by thymine (T)–adenine (A) bridges. Individual hole hopping processes between G sites fall into two distinct parallel mechanisms, i.e., unistep superexchange mediated hopping via `short' (T–A) n bridges and thermally induced hopping (TIH) via `long' (T–A) n ( n>3–4) bridges. The bridge specificity for TIH via (A) n chains pertains to the energetics, with the G +A energy gap Δ=0.20±0.05 eV being sufficiently low to warrant endothermic hole excitation from G + to (A) n , and to the electronic couplings, with the nearest-neighbor A–A couplings being unique in the sense that the intrastrand and interstrand couplings are close and large (V( A– A)≃0.30–0.060 eV) . Accordingly, both effective intrastrand and interstrand (zigzagging) hole transport via (A) n chains will prevail, being nearly invariant with respect to the nucleobases ordering within the (T–A) n duplex. We treated the `transition' between the superexchange and the TIH mechanism in 5 ′-G +(T–A) n G-3 ′ duplexes to predict that the crossover occurs at n x ≃3–4, with n x exhibiting a moderate bridge specificity and energy gap dependence. n x is in accord with the experimental data of Giese et al. [Nature 412, 318, 2001]. We assert that the kinetic–quantum mechanical model for the chemical yields and elementary rates cannot be reconciled with the experimental TIH data, with respect to the very weak bridge size dependence of the relative chemical yields and the ratios of the rates. Configurational relaxation accompanying endothermic hole injection from G + to (A) n may result in the gating (switching-off) of the backrecombination, providing a reasonable description of TIH dynamics and very long-range hole transport in long (A) n chains.