ELECTRON-TRANSFER MODEL FOR THE ELECTRIC-FIELD EFFECT ON QUANTUM YIELD OF CHARGE SEPARATION IN BACTERIAL PHOTOSYNTHETIC REACTION CENTERS

The effect of an electric field on a photoinduced charge separation pr ocess is treated theoretically. The system considered is a reaction ce nter (RC) of photosynthetic bacteria, involving an electron transfer ( ET) from the electronically excited singlet state of the bacteriochlor ophyll dimer (P) to the bacteriopheophytin (II) and quinone (Q). In co ntrast to formulations which focus only on the forward steps and do no t explain the major effect on the quantum yield of P(+)Q(-) or, in Q-d epleted samples, of P+H-, the present study includes the effect on the back reactions, an effect which we find to be large. The low-frequenc y medium and high-frequency intramolecular vibrational modes are inclu ded in the calculation of the various ET rates. Recent experimental re sults on the ET energetics, including the estimated effect of static h eterogeneity in RCs, are incorporated. The rate equations for the popu lation densities of distinct states are solved for both oriented and r andomly oriented (isotropic) RC samples, and the results are compared with experimental data for the field-induced reduction of the quantum yield of formation of charge-separated state P(+)Q(-). A simple (quasi equilibrium) model calculation illustrates the essential features of t his analysis of the electric field effect and compares reasonably well with these numerical results of the more detailed model. The question of the electric field effect on the fluorescence quantum yield is als o addressed, and a suggestion is made for consistency with the data on the formation of P(+)Q(-).