Exchange of chloride by bromide in the manganese photosystem-II complex studied by cw- and pulsed-EPR Original Research Article

Cl− is an essential cofactor in Photosystem-II (PS-II) oxygen evolution. By using electron spin echo envelope spectroscopy it is shown that, in some circumstances, magnetic couplings between the Mn cluster, in the S2-state, and those of the observed. In order to extract only the electron nuclear couplings between the Mn cluster, in the S2-state, and those of the halide, a comparison was done between chloride- and bromide-reconstituted samples. In salt-washed, CaCl2- or CaBr2-reconstituted Photosystem-II membranes, frequencies similar to the Larmor frequency of bromide and chloride were detected. When Cl− and Br− reconstituted PS-II membranes are compared to membranes that have also been Ca2+-depleted, and EGTA-treated, these features in the frequency spectra are absent. After chloride depletion at high pH in the presence of sulfate, no difference could be observed in the frequency spectra of bromide- and chloride-reconstituted samples. These results are discussed in view of possible different Cl− binding sites in PS-II.