Shallow traps for thermally induced hole hopping in DNA

The theory of thermally induced hopping (TIH) in donor-bridge-acceptor systems for hole transport in DNA duplexes in solution is extended to include energetic theoretical data for the effects of inter-nucleobase interactions. The extended theory incorporates the site specificity of the energetic stabilization of the radical cation of guanine (G), which acts as a resting site for the hole, and of the radical cations of adenine (A), which are accessible by thermal excitation from G⁺ (Δ = 0.20-0.25 eV). The modified TIH model properly accounts for the flat bridge size dependence of the relative chemical yields for hole transport in G(A-T)_nGGG duplexes (n = 4-16). This flat, non-ohmic, bridge length dependence is attributed to an energetic gating mechanism, which is induced by energy barriers (∼0.1 eV) exerted by the proximal and by the terminal edge A groups in the (A)_n bridge, while the interior A groups act as shallow traps for the hole. Our 'molecular polaron' model for incoherent, hopping charge transport in solvated DNA is supported by independent theoretical evidence for hole localization induced by intrabase configurational distortions and by polar solvent effects.