Charge transfer and transport in DNA

We explore charge migration in DNA, advancing two distinct mechanisms of charge separation in a donor (d)-bridge ({B(j)})-acceptor (a) system, where {B(j)} = B₁,B₂,..., B(N) are the N-specific adjacent bases of B-DNA: (i) two-center unistep superexchange induced charge transfer, d*{B(j)}a → d(±){B(j)}a(±), and (ii) multistep charge transport involves charge injection from d* (or d⁺) to {B(j)}, charge hopping within {B(j)}, and charge trapping by a. For off-resonance coupling, mechanism i prevails with the charge separation rate and yield exhibiting an exponential dependence ∞ exp(-βR) on the d-a distance (R). Resonance coupling results in mechanism ii with the charge separation lifetime τ ∞ N(η) and yield Y ≃ (1 + δ N(η))^-1 exhibiting a weak (algebraic) N and distance dependence. The power parameter η is determined by charge hopping random walk. Energetic control of the charge migration mechanism is exerted by the energetics of the ion pair state d(±)B₁(±)B₂...B(N)a relative to the electronically excited donor doorway state d*B₁B₂... B(N)a. The realization of charge separation via superexchange or hopping is determined by the base sequence within the bridge. Our energetic-dynamic relations, in conjunction with the energetic data for d*/d^- and for B/B⁺, determine the realization of the two distinct mechanisms in different hole donor systems, establishing the conditions for 'chemistry at a distance' after charge transport in DNA. The energetic control of the charge migration mechanisms attained by the sequence specificity of the bridge is universal for large molecular-scale systems, for proteins, and for DNA.