Estimate of the reorganization energy for charge transfer in DNA

The solvent reorganization energy λ_s can have a significant effect on the activation energy for charge transfer in DNA and its dependence on donor (D)-acceptor (A) distance R_DA. To estimate λ_s and the resulting effective contribution β_s to the falloff parameter β for the overall transfer rate constant, the Poisson equation was solved numerically for several systems representing DNA duplexes, 5′-GGGT_nGGG-3′, in a realistically structured heterogeneous dielectric, as determined by molecular dynamics (MD) simulations. The charge transfer was modeled primarily for holes localized on single guanine bases. Effects of thermal fluctuations on λ_s were taken into account via structures for a given duplex sampled from MD trajectories. Calculated values of λ_s were found to be rather insensitive to thermal fluctuations of the DNA fragments but depended in crucial fashion on details of the dielectric model (shape and dielectric constants of various zones) that was used to describe the polarization response of the DNA and its environment to the charge transfer. λ_s was calculated to increase rapidly at small R_DA values (<15 Å), and accordingly the falloff parameter β_s (defined as a local function of R_DA) decreases appreciably with increasing R_DA (from 1.0 Å^-1 with only one intermediate base pair between D and A to 0.15 Å^-1 for systems with five intervening pairs). Calculated λ_s values were accurately fitted (standard deviation of ∼0.5 kcal/mol) to a linear function of 1/R_DA, including all cases except contact (R_DA = 3.4 Å), where some overlap of D and A sites may occur. A linear fit to an exponential (of form exp(-β_sR_DA)) gave comparable accuracy for the entire R_DA range. λ_s based on D and A holes delocalized over two adjacent guanine bases was uniformly ∼12 kcal/mol smaller than the corresponding results for holes localized on single guanines, almost independent of R_DA. The internal reorganization energy for hole transfer between GC pairs was calculated at 16.6 kcal/mol at the B3LYP/6-31G* level.