Effect of charge distribution over a chlorophyll dimer on the redox potential of P680 in photosystein II as studied by density functional theory calculations

The effect of charge distribution over a chlorophyll dimer on the redox potential of P680 in photosystem II was studied by density functional theory calculations using the P680 coordinates in the X-ray structure. From the calculated ionization potentials of the dimer and the monomeric constituents, the decrease in the redox potential by charge delocalization over the dimer was estimated to be similar to 140 mV. Such charge delocalization was previously observed in the isolated D1-D2-Cyt b(559) complexes, whereas the charge was primarily localized on P-D1 in the core complexes. The calculated potential decrease of similar to 140 mV can explain the inhibition of Y-Z oxidation in the former complexes and in turn implies that the charge localization on P-D1 upon formation of the core complex increases the P680 potential to the level necessary for water oxidation.