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Proton position near QB and coupling of electron and proton transfer in photosynthesis

We have calculated the energy levels and wavefunctions of a proton in a histidine (His)–plastoquinone (PQ) system in the reaction centre (RC) of photosystem 2 of higher plants and the RC of purple bacteria for different redox states of PQ QB. For oxidized QB, the proton is located near His. For once-reduced PQ, it is positioned in the middle between the nitrogen of His and the oxygen of PQ. For twofold-reduced PQ, the proton is localized near the oxygen of PQ. Using the values of total energy of the system in these states, we have also estimated the frequency of proton oscillations. On the basis of these results we propose a hypothesis about the coupling of electron–proton transfer.