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)} = B(1),B(2), … , 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 + δ̄ 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(∓)B(1)(±)B(2) … B(N)a relative to the electronically excited donor doorway state d*B(1)B(2) … 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.