## Abstract

In this paper we utilize the intermolecular overlap approximation to calculate the relative magnitudes of the electronic transfer integrals between the excited singlet state (^{1}P*) 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_{L}H_{L}^{-}and for B_{M}^{-}H_{M}-B_{M}H_{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 ^{l}P*B_{L}-P^{+}B_{L}~ and for ^{1}P*B_{M}-P^{+}B_{M}' results in the electronic contribution of 33 ± 16 to the ratio k_{L}/k_{m}of the rate constants k_{L}and k_{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 ^{1}P* 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 ^{1}P*B_{L}^{-}P^{+}B_{L}^{-}and for B_{L}^{-}H_{L}-B_{L}H_{L}^{-}was utilized to estimate the energetic parameters required to ensure the dominance of the superexchange mediated unistep electron transfer ^{1}P*BH → P^{+}BH^{-}over the thermally activated ^{1}P*B → P^{+}B^{-}process.

Original language | English |
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Pages (from-to) | 7279-7285 |

Number of pages | 7 |

Journal | Journal of the American Chemical Society |

Volume | 110 |

Issue number | 22 |

DOIs | |

State | Published - Oct 1988 |