Abstract
Structural influences on the direction of electron transfer in the charge separation process of the photosynthetic reaction centers of Rhodopseudomonas viridis and Rhodobacter sphaeroides are studied using quantum chemical models to calculate the electronic factor. Our results support the sequential mechanism for the primary charge separation. Using crystallographic coordinates refined in 1994, we find a larger coupling between the special pair D and the accessory bacteriochlorophyll BA of the A branch than between D and BB of the inactive B branch. We have been able to localize the coupling to the acetyl group of ring I of DB and the methyl group of ring III of BA. The corresponding contact between DA and BB has less coupling, apparently due to a distorting hydrogen bond between the acetyl group on DA and the imidazole side group of HisL168. The coupling between BA and bacteriopheophytin ΦA is accomplished by two methyl groups, directly connected to the conjugated π system of the chromophore.
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