Charge transfer and transport in DNA.

We explore charge migration in DNA, advancing two distinct mechanisms of charge separation in a donor (d)-bridge ([Bj])-acceptor (a) system, where [Bj] = B1,B2, . , BN are the N-specific adjacent bases of B-DNA: (i) two-center unistep superexchange induced charge transfer, d*[Bj]a --> d[Bj]a+/-, and (ii) multistep charge transport involves charge injection from d* (or d+) to [Bj], charge hopping within [Bj], and charge trapping by a. For off-resonance coupling, mechanism i prevails with the charge separation rate and yield exhibiting an exponential dependence approximately exp(-betaR) on the d-a distance (R). Resonance coupling results in mechanism ii with the charge separation lifetime tau approximately Neta and yield Y approximately (1 + Neta)-1 exhibiting a weak (algebraic) N and distance dependence. The power parameter eta is determined by charge hopping random walk. Energetic control of the charge migration mechanism is exerted by the energetics of the ion pair state dB1+/-B2 . BNa relative to the electronically excited donor doorway state d*B1B2 . BNa. 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.