The mechanism of electron transfer between the primary and the secondary quinone of the photosynthetic reaction center of the Rhodobacter Sphaeroides has been studied theoretically, on the basis of the x-ray structure obtained under illumination. It is shown that in such a supramolecular aggregate where the different molecular blocks form a chain of H bonds, a mobile electron injected on one end of the chain can be carried to the opposite end by switching the positions of the H-bonded hydrogens, in a Bjerrium-type mechanism in which not only protons but also hydrogen atoms are involved. Energy estimates predict that the various steps of the mechanism are all slightly exergonic, so that the proposed mechanism appears to be at least plausible and worthy of further experimental investigations.

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