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 B-A of the A branch than between D and B-B of the inactive B branch. We have been able to localize the coupling to the acetyl group of ring I of D-B and the methyl group of ring III of B-A. The corresponding contact between D-A and B-B has less coupling, apparently due to a distorting hydrogen bond between the acetyl group on D-A and the imidazole side group of HisL168. The coupling between B-A and bacteriopheophytin Phi(A) is accomplished by two methyl groups, directly connected to the conjugated pi system of the chromophore.