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 14 N 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 ε , 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 pK a of this group is around pH 7.5 in CN − -treated PSII.