Distance dependence of electron transfer in DNA: The role of the reorganization energy and free energy

A model of the distance dependence of photoinduced donor-acceptor electron transfer in DNA is presented that includes the distance dependence of the solvent reorganization energy and free energy in the heterogeneous DNA environment. DNA is modeled as a low dielectric region that represents the base stack and two regions with more moderate dielectric properties that represent the DNA backbone. The DNA is surrounded by a high dielectric medium, which represents water. Model calculations show the importance of including the reorganization energy and the free energy change and illustrate the differences between the inhomogeneous model and homogeneous single dielectric constant calculations using standard Marcus theory. Calculations are performed for comparison to published experimental work (Science 1997, 277, 673;(1) J. Am. Chem. Sec. 1992, 114, 3656 (2)). Fits to one set of data(2) permit the previously reported distance dependence to be separated into an electronic contribution and solvent reorganization energy and free energy contributions. For the other set of data,(1) inclusion of the solvent reorganization energy and free energy distance dependences in the analysis of the overall distance dependent data suggest that the Marcus form of the distance dependent rate constant including the Marcus reorganization energy is not Consistent with the data.