Intermolecular Electronic Interactions in the Primary Charge Separation in Bacterial Photosynthesis

In this paper we utilize the intermolecular overlap approximation to calculate the relative magnitudes of the electronic transfer integrals between the excited singlet state (¹P*) of the bacteriochlorophyll dimer (P) and the accessory bacteriochlorophyll (B) and between B^-and bacteriopheopytin (H), along the L and M subunits of the reaction center (RC) of Rps. viridis. The ratio of the electron-transfer integrals for B_L^-H_L-B_LH_L^-and for B_M^-H_M-B_MH_M^-was calculated to be 2.1 ± 0.5, which together with the value of 2.8 ±0.7 for the ratio of the transfer integrals for ^lP*B_L-P⁺B_L~ and for ¹P*B_M-P⁺B_M' results in the electronic contribution of 33 ± 16 to the ratio k_L/k_mof the rate constants k_Land k_Mfor the primary charge separation across the L and M branches of the RC, respectively. The asymmetry of the electronic coupling terms, which originates from the combination of the asymmetry in the charge distribution of ¹P* and of structural asymmetry of the P-B and B-H arrangements across the L and M subunits, provides a major contribution to the unidirectionality of the charge separation in bacterial photosynthesis. A significant contribution to the transfer integrals between adjacent pigments originates from nearby methyl groups through hyperconjugation. The ratio 6 ± 2 of the transfer integrals for ¹P*B_L^-P⁺B_L^-and for B_L^-H_L-B_LH_L^-was utilized to estimate the energetic parameters required to ensure the dominance of the superexchange mediated unistep electron transfer ¹P*BH → P⁺BH^-over the thermally activated ¹P*B → P⁺B^-process.