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 ( V 1 ) 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 V 1 B 2 is probably smaller by 3 orders of magnitude than V 1 A 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 V 1 A / V 1 B 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.