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 s olvent reorganization energy and free energy in the heterogeneous DNA envir onment. DNA is modeled as a low dielectric region that represents the base stack and two regions with more moderate dielectric properties that represe nt the DNA backbone. The DNA is surrounded by a high dielectric medium, whi ch represents water. Model calculations show the importance of including th e reorganization energy and the free energy change and illustrate the diffe rences between the inhomogeneous model and homogeneous single dielectric co nstant calculations using standard Marcus theory. Calculations are performe d 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 electro nic contribution and solvent reorganization energy and free energy contribu tions. For the other set of data,(1) inclusion of the solvent reorganizatio n energy and free energy distance dependences in the analysis of the overal l distance dependent data suggest that the Marcus form of the distance depe ndent rate constant including the Marcus reorganization energy is not Consi stent with the data.