Charge transfer and transport in DNA

Abstract

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.