Electric field effects on the primary charge separation in bacterial photosynthesis

In this paper we present a theoretical study of the effects of external electric fields on the primary charge-separation process from the electronically excited singlet state (P*) of the bacteriochlorophyll dimer (P) to the bacteriophytin (H) in reaction centers of photosynthetic bacteria. The role of the accessory bacteriochlorophyll (B) was considered within the framework of three distinct processes, i.e., the superexchange, the sequential, and the nonadiabatic/adiabatic mechanisms for the primary electron-transfer process. The electric field dependence of the rate-controlling electron-transfer rate, the quantum yield for the charge separation, and the fluorescence quantum yield were evaluated for both oriented and isotropic samples. The most interesting conclusions are traced to the manifestation of level crossing, whereas the crossing between the P⁺B^- and P⁺H^- levels for the sequential mechanism predicts the unveiling of the P⁺B^- intermediate at high external fields, while the level crossing between the P*B and P⁺B^- levels for the nonadiabatic/adiabatic mechanism predicts an asymmetric field-dependent low-temperature retardation of the charge separation. These predictions have to be subjected to experimental scrutiny, which requires the establishment of the interrelationship between the external field and the internal field acting on the ion-pair states in the reaction center.