ABSTRACT
The SAC-CI calculations clarify the natures of the excited states and the electron-transfer (ET) processes in the photosynthetic reaction center (PSRC) of Rhodobactor (Rb.) sphaeroides. The absorption spectrum was assigned with the averaged error of 0.11 eV. The electronic factors calculated from the SAC-CI wave functions clarified the mechanism of the unidirectionality of the ET in Rb. sphaeroides. It is controlled by the ET step from bacteriochlorophyll (B) to bacteriopheophytin (H), not from the special pair (P) to B as in the Rhodopseudomonas (Rps.) viridis reported previously: the electronic factor of the Abranch ET is 20 times larger than that of the B-branch. An analysis clarified that the unidirectionality originates from the inter-chromophore distances, and further that the hyperconjugations of the methyl groups with the π electrons of the chromophores have primary contributions to the electronic factor.
