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-1B(2) is probably smaller by 3 orders of magnitude than V-1A(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 Rd further support this conclusion Zhang and Friesner (Proc. Natl. Acad. Sci. U.S.A. 1998, 95, L3603-13605), using more elaborate calculations, showed that V-1A/V-1B, 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.