Influence of the axial ligand of PD1 chlorophyll (D1-His-198) on the Em of monomer chlorophylls PD1 and PD2, and the PD1•+/PD2•+ charge ratio was investigated by theoretical calculations using the PSII crystal structure of Thermosynechococcus vulcanus analyzed at 1.9-Å resolution. It was found that the Em(PD1)/Em(PD2) values and PD1•+/PD2•+ ratio remained unchanged upon D1-H198Q mutation. However, Em(P D1) was increased in the D1-H198A mutant, resulting in a more even distribution of the positive charge over PD1/PD2. Introduction of a water molecule as an axial ligand resulted in equal E m values and PD1•+/PD2 •+ ratios between the mutant and wild-type, thus confirming the presence of the water ligand in the mutant.
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