Solvent reorganization energy of charge transfer in DNA hairpins

We report calculations of solvent reorganization energies and the energy gap analysis for charge transfer in synthetic DNA hairpins consisting of two complementary strands linked by a stilbene chromophore. Reorganization energies are calculated for the processes of photoinduced hole injection from the linker to a neighboring guanine (G) through variable numbers of adenine/thymine (AT) pairs and for the hole transfer between adjacent G sites connected by AT bridges of different length. Twelve molecules with varying donor-acceptor separation have been analyzed. The solute structure is taken from crystallographic data with atomic resolution, and the polarization response of water is modeled in terms of polarization structure factors accounting for the molecular nature of the polarization fluctuations of the solvent. Both the magnitudes of reorganization energies and the parameter of the rate constant falloff with the donor-acceptor separation turn out to be much higher than those obtained from the Marcus-Levich-Jortner analysis of the experimental kinetic data. The discrepancy is resolved by applying a new model (Q-model) of charge-transfer accounting for different reorganization energies for charge separation and charge recombination reactions.