Bromide does not bind to the Mn4Ca complex in its S1 state in Cl--depleted and br--reconstituted oxygen-evolving photosystem II:: Evidence from X-ray absorption spectroscopy at the BrK-edge

Chloride is an important cofactor in photosynthetic water oxidation. It can be replaced by bromide with retention of the oxygen-evolving activity of photosystem II ( PSII). Binding of bromide to the Mn4Ca complex of PSII in its dark-stable S-1 state was studied by X-ray absorption spectroscopy ( XAS) at the Br K-edge in Cl--depleted and Br--substituted PSII membrane particles from spinach. The XAS spectra exclude the presence of metal ions in the first and second coordination spheres of Br-. EXAFS analysis provided tentative evidence of at least one metal ion, which may be manganese or calcium, at a distance of similar to 5 angstrom to Br-. The native Cl- ion may bind at a similar distance. Accordingly, water oxidation may not require binding of a halide directly to the metal ions of the Mn complex in its S-1 state.