Electric field effects on the initial electron-transfer kinetics in bacterial photosynthetic reaction centers

The effect of an applied electric field on the kinetics of the initial picosecond electron-transfer reaction in Rb. sphaeroides reaction centers has been measured in isotropic samples at 77 K. The net rate of formation of H_L^•- is reduced upon application of an electric field of 10⁶ V/cm, consistent with the previously observed increase in the quantum yield of the competing prompt fluorescence. The observed magnitude of the effect on the initial reaction is compared with the predictions of various models, and the consequences of including indirect electronic coupling between the initial and final states through a third state (superexchange) are investigated. It is found that the treatments of the initial electron-transfer reaction commonly in use greatly overestimate the magnitude of the field effect because they are based on a dependence of the rate of electron transfer on the free energy change which is steeper than appears to be appropriate for this process. No evidence was found for electron transfer down the M side of the reaction center at the highest applied field, indicating that unidirectionality of the initial electron transfer is not due to small energetic differences between charge-separated states involving the chromophores on the L and M sides.