EXCHANGE OF CHLORIDE BY BROMIDE IN THE MANGANESE PHOTOSYSTEM-II COMPLEX STUDIED BY CW-EPR AND PULSED-EPR

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 chloride and the Mn cluster can be observed. In order to extract only the electron nuclear couplings between the Mn cluster, in the S-2-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.