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 inv olves charge injection from d (or d(+)) to {B-j}, charge hopping with in {B-j}, and charge trapping by a. For off-resonance coupling, mechan ism i prevails with the charge separation rate and yield exhibiting an exponential dependence proportional to exp(-beta R) on the d-a distan ce (R), Resonance coupling results in mechanism ii with the charge sep aration lifetime tau proportional to N-eta and yield Y similar or equa l to (1 + <(delta)over bar> N-eta)(-1) exhibiting a weak (algebraic) N and distance dependence. The power parameter eta is determined by cha rge hopping random walk, Energetic control of the charge migration mec hanism 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 dB1B2... B(N)a, The realization of charge separation via supere xchange or hopping is determined by the base sequence within the bridg e. Our energetic-dynamic relations, in conjunction,vith the energetic data for d/d(-) and for B/B+, determine the realization of the two di stinct mechanisms in different hole donor systems, establishing the co nditions 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 molecul ar-scale systems, for proteins, and for DNA.