Asymmetric Electron Transfer in Reaction Centers of Purple Bacteria Strongly Depends on Different Electron Matrix Elements in the Active and Inactive Branches

We have re-examined the contribution of electronic matrix elements (V1) between the primary electron donor and the accessory bacteriochlorophylls in the active (A) and inactive (B) branches of bacterial reaction centers (RC) to the unidirectional light-induced electron transfer (ET) (a preliminary report was recently given by Kolbasov and Scherz in Photosynthesis: Mechanisms and Effects; Garab, G., Ed.; Kluwer Academic Publishers: Dordrecht, 1998; Vol. II; pp 719-722). Our calculations showed that V1B is probably smaller by 3 orders of magnitude than V1A 2 in Rb. sphaeroides and by at least 1 order of magnitude in Rps. Viridis. These phenomena reflect the quantum interference and mutual cancellation of the resonance integrals corresponding to different ET pathways between atoms of P and the accessory bacteriochlorophyll in the inactive branch. The calculated values of V and the corresponding ET rate constants for mutated RC further support this conclusion. Zhang and Friesner (Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 13603-13605), using more elaborate calculations, showed that V1A/V1B for Rps. Viridis can reach a value of 14 for same reason, indicating that differences in the overlap matrix elements are key factors in the unidirectional electron flow in both organisms.