Evidence of O–O Bond Formation in the Final Metastable S₃ State of Nature’s Water Oxidizing Complex Implying a Novel Mechanism of Water Oxidation

A novel mechanism for the final stages of Nature’s photosynthetic water oxidation to molecular oxygen is proposed. This is based on a comparison of experimental and broken symmetry density functional theory (BS-DFT) calculated geometries and magnetic resonance properties of water oxidizing complex models in the final metastable oxidation state, S3. We show that peroxo models of the S3 state are in vastly superior agreement with the current experimental structural determinations compared with oxo–hydroxo models. Comparison of experimental and BS-DFT calculated 55Mn hyperfine couplings for the electron paramagnetic resonance (EPR) visible form shows better agreement for the oxo–hydroxo model. An equilibrium between oxo–hydroxo and peroxo models is proposed for the S3 state and the major implications for the final steps in the water oxidation mechanism are analyzed and discussed.