Why is electron transport in the reaction centers of purple bacteria unidirectional?

Although the two electron-transfer branches in the reaction centers (RC) of purple bacteria are virtually symmetric, it is well known that only one of them is functionally active (the A-branch). The mechanisms of functional a symmetry of structurally symmetric branches of the electron transport syste m are analyzed in this work within the framework of the theory of bimolecul ar charge-transfer complexes (CTC). CTC theory is shown to provide an expla nation of this phenomenon. According to the CTC theory, the dominance of on e branch is required to implement the CTC state in special bacteriochloroph yll pairs of RC, in which more than 30% of the excited electron density in the CTC is shifted toward one of the bacteriochlorophyll molecules. This ca uses a significant increase in the efficiency of further electron transfer to the primary quinone acceptor as compared to a system with two absolutely symmetric electron transfer branches. Specific features of dielectric asym metry near the RC special pair are discussed. it is emphasized that a stron g CTC is able to provide effective trapping of electronic excitation energy from antenna chlorophyll, which is a main function of the RC. Hypothetical stages of CTC formation in other classes of photosynthesizing bacteria dur ing evolution are discussed.