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

The semiquinone radical Q(A)(-) has been studied by electron spin echo enve lope modulation (ESEEM) spectroscopy in Photosystem II. membranes treated w ith CN- at various pH values. Two protein N-14 nuclei (N-I and N-II) were f ound to be magnetically coupled with the Q(A)(-) spin. N-I is assigned to a n amide nitrogen from the protein backbone while N-II is assigned to the am ino nitrogen, N-e of an imidazole. Above pH 8.5 only the N-I coupling is pr esent while both N-I and N-II couplings are present at lower pH values. The se results are interpreted in terms of a model based on the structure of th e bacterial reaction center and involving two determining factors. First, t he 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-depend ent group on the protein controls the strength of the H-bonds to Q(A)(-). T he pK(a) of this group is around pH 7.5 in CN--treated PSII. (C) 2000 Elsev ier Science Inc. All rights reserved.