Chloride Maintains a Protonated Internal Water Network in the Photosynthetic Oxygen Evolving Complex.

In photosystem II (PSII), water oxidation occurs at a MnCaO cluster and results in production of molecular oxygen. The MnCaO cluster cycles among five oxidation states, called S states. As a result, protons are released at the metal cluster and transferred through a 35 Å hydrogen-bonding network to the lumen. At 283 K, an infrared band at 2830 cm is assigned to an internal solvated hydronium ion via HO solvent exchange. This result is similar to a previous report at 263 K. Computations on an oxygen evolving complex model predict that chloride can stabilize a hydronium ion on a network of nine water molecules. In this model, a HO stretching mode at 2738 cm is predicted to shift to higher frequency with bromide and to lower frequency with nitrate substitution. The calculated frequencies were compared to S-minus-S reaction-induced Fourier transform infrared spectra acquired from chloride-, bromide-, or nitrate-containing PSII samples, which were active in oxygen evolution. As predicted, the frequency of the 2830 cm band shifted to higher energy with bromide and to lower energy with nitrate substitution. These results support the conclusion that an internal hydronium ion and chloride play a direct role in an internal proton transfer event during the S-to-S transition.