Accessory chlorophylls (B-A/B) in bacterial photosynthetic reaction center play a key role in charge-separation. Although light-exposed and dark-adapted bRC crystal structures are virtually identical, the calculated B-A redox potentials for one-electron reduction differ. This can be traced back to different orientations of the B-A ester-group. This tuning ability of chlorophyll redox potentials modulates the electron transfer from SP* to B-A. (C) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.