Temperature Dependence of the Electric Field Modulation of Electron-Transfer Rates: Charge Recombination in Photosynthetic Reaction Centers

The rates of electron-transfer reactions can be systematically varied by application of external electric fields. The electric field effect on the charge recombination rate constant of the state P+QAin photosynthetic reaction centers has been measured in polymer film samples at a number of temperatures. This extends an earlier study at 80 K in which a small field-induced slowing of the rate constant was reported [Franzen, S.; Goldstein, R. F.; Boxer, S . G. J. Phys. Chem. 1990, 94, 5135-51491. At temperatures greater than 160 K, evidence is presented for an electric-field-induced activated recombination pathway similar to that observed in reaction centers which have nonnative quinones of low reduction potential substituted for ubiquinone. However, it is shown that this pathway alone cannot account for the electric-field-induced changes in kinetics observed at higher temperatures. The data analysis used to obtain the log(ket) vs AGet curve at 80 K is extended to include changes in the steady-state population of P+QAobserved at higher temperatures with a continuous wave probe beam. This field dependence is analyzed in terms of electron-transfer theory including the entropy of reaction and the temperature dependences of the reorganization energy and the local field correction. The data and analysis suggest that a temperature-dependent reorganization energy is associated with solvation of the P Q A dipole.