Effect of pH on the semiquinone radical Q(A)- in CN-treated photosystem II: study by hyperfine sublevel correlation...

The semiquinone radical Q(A)- has been studied by electron spin echo envelope modulation (ESEEM) spectroscopy in Photosystem II membranes treated with CN- at various pH values. Two protein 14N nuclei (N(I) and N(II)) were found to be magnetically coupled with the Q(A)- spin. N(I) is assigned to an amide nitrogen from the protein backbone while N(II) is assigned to the amino nitrogen, N(epsilon), of an imidazole. Above pH 8.5 only the N(I) coupling is present while both N(I) and N(II) couplings are present at lower pH values. These results are interpreted in terms of a model based on the structure of the bacterial reaction center and involving two determining factors. First, the non-heme iron, when present, is ligated to the imidazole that H-bonds to one of the Q(A)- carbonyls. This physical attachment of the imidazole to the iron limits the strength of the H-bond to Q(A)-. Second, a pH-dependent group on the protein controls the strength of the H-bonds to Q(A)-. The pKa of this group is around pH 7.5 in CN(-)-treated PSII.