Unidirectionality of charge separation in reaction centers of photosynthetic bacteria

Time-resolved spectroscopy in conjunction with X-ray structural data for reaction centers of Rhodopseudomonas viridis and Rhodobacter sphaeroides reveal a branching ratio a > 5 for the primary electron-transfer rates, favouring one of the two, almost symmetrical pigment/protein branches, L and M. In this paper we explore the origins of this unidirectionality of electron transfer between the excited singlet state of the bacteriochlorophyll dimer (¹P*) and the bacteriopheophytin (H) along the L protein subunit. Nonadiabatic electron-transfer theory is applied to analyze the asymmetry of the electron-transfer rates, k_L and k_M across the L and M branches. The asymmetry originates from the cumulative contributions of the nuclear Franck-Condon factor and the electronic coupling, both of which enhance the electron transfer rate across the L branch. The nuclear Frank-Condon factors are modified by the energy difference ΔE_LM between the states P⁺H^-_L and P⁺H^-_M, which is induced by the electrostatic interactions of these ion-pair states with the protein polar groups, as well as by asymmetric Coulomb and medium polarization interactions. The computation results in ΔE_LM = -(0.09 ± 0.04) eV, which yields a nuclear enhancement contribution at 300 K of 1.5 (+0.8, -0.3) to k_L k_M and therefore is insufficient to explain alone the observed asymmetry in reaction centers of Rps. viridis. Another contribution to the unidirectionality originates from electronic superexchange coupling for ¹P*-B-H via the virtual states of the accessory bacteriochlorophyll (B). The ratio of the intermolecular ¹P*-B_L and ¹P*-B_M electronic interaction terms was evaluated utilizing the tight-binding approximation with SCF-MO wavefunctions, together with the structural data for the prosthetic groups and for the polar amino acid side chains of the protein in reaction centers of Rps. viridis. The contribution to the enhancement of k_L k_M by the electronic superexchange is approx. 8 ± 4. This asymmetry was traced to the combination of an excess negative charge density on the M-dimer component P_M, together with structural asymmetry, which enhances the P_M-B_L electronic overlap. Consequently, the ¹P*-B_L-H_L superexchange is favoured over the ¹P*-B_M-H_M interaction. The combined effects of asymmetric nuclear Franck-Condon factors and electronic couplings yield a branching ratio of the electron-transfer rates along the two pigment branches in reaction centers of Rps. viridis of an approx. 12 (-7, +15). This is sufficiently large to explain the experimentally observed unidirectionality.