Intermolecular electronic interactions in the primary charge separation in bacterial photosynthesis

In this paper the intermolecular overlap approximation is used to calculate the relative magnitudes of the electronic transfer integrals between the excited singlet state ({sup 1}P*) of the bacteriochlorophyll dimer (P) and the accessory bacteriochlorophyll (B) and between B{sup {minus}} 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{sub L}{sup {minus}}H{sub L}{sup {minus}}B{sub L}H{sub L}{minus} and for B{sub M}{sup {minus}}H{sub M}{minus}B{sub M}H{sub M}{sup {minus}} was calculated to be 2.1 {plus minus} 0.5, which together with the value of 2.8 {plus minus} 0.7 for the ratio of the transfer integrals for {sup 1}P*B{sub L}-P{sup +}B{sub L}- and for {sup 1}P*B{sub M}-P{sup +}B{sub M}- results in the electronic contribution of 33 {plus minus} 16 to the ratio k{sub L}/k{sub m} of the rate constants k{sub L} and k{sub M} for 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 {sup 1}P* and of structural asymmetry of the P-M and B-H arrangements across the L and M subunits, provides a major contributionmore » 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 {plus minus} 2 of the transfer integrals for {sup 1}P*B{sub L}-P{sup +}B{sub L}- and for B{sub L}-H{sub L}-B{sub L}H{sub L}- was utilized to estimate the energetic parameters required to ensure the dominance of the superexchange mediated unistep electron transfer {sup 1}P*BH {yields} P{sup +}BH{sup {minus}} over the thermally activated {sup 1}P*B {yields} P{sup +}B{sup {minus}} process. 31 refs., 6 figs., 2 tabs.« less