Influence of the axial ligand of P-D1 chlorophyll (D1-His-198) on the E-m of monomer chlorophylls P-D1 and P-D2, and the P-D1(center dot+)/P-D2(center dot+) charge ratio was investigated by theoretical calculations using the PSII crystal structure of Thermosynechococcus vulcanus analyzed at 1.9-angstrom resolution. It was found that the Em(P-D1)/E-m(P-D2) values and P-D1(center dot+)/P-D2(center dot+) ratio remained unchanged upon D1-H198Q mutation. However, Em(PD1) was increased in the D1-H198A mutant, resulting in a more even distribution of the positive charge over P-D1/P-D2. Introduction of a water molecule as an axial ligand resulted in equal E-m values and P-D1(center dot+)/P-D2(center dot+) ratios between the mutant and wild-type, thus confirming the presence of the water ligand in the mutant.