Chloride facilitates Mn(III) formation during photoassembly of the Photosystem II oxygen-evolving complex.

The MnCa oxygen-evolving complex (OEC) in Photosystem II (PSII) is assembled in situ from free Mn, Ca, and water. In an early light-driven step, Mn in a protein high-affinity site is oxidized to Mn. Using dual-mode electron paramagnetic resonance spectroscopy, we observed that Mn accumulation increases as chloride concentration increases in spinach PSII membranes depleted of all extrinsic subunits. At physiologically relevant pH values, this effect requires the presence of calcium. When combined with pH studies, we conclude that the first Mn oxidation event in OEC assembly requires a deprotonation that is facilitated by chloride.