The structure and activation of substrate water molecules in Sr²⁺-substituted photosystem II

The mechanism of solar water oxidation by photosystem II (PSII) is of fundamental interest and it is the object of extensive studies both in the past and present. The solar water oxidation reaction of PSII occurs in the oxygen-evolving complex (OEC). The OEC consists of a tetranuclear manganese calcium-oxo (Mn₄Ca-oxo) cluster that is surrounded by amino acid residues and inorganic cofactors. The role of the Ca²⁺ ion in the water oxidation reaction is one of the most interesting questions that is yet to be answered. In this study, we probe the structural and functional differences induced by metal ion substitution in the Mn₄Ca-oxo cluster by substituting the Ca²⁺ ion in the OEC by a Sr²⁺ ion. We apply two-dimensional (2D) hyperfine sublevel correlation (HYSCORE) spectroscopy to detect weak magnetic interactions between the paramagnetic Mn₄Sr-oxo cluster and the surrounding protons in the S₂ state of the OEC of Sr²⁺-substituted PSII. We identify three groups of protons that are magnetically interacting with the Mn₄Sr-oxo cluster. Using the recently reported 1.9 Å resolution X-ray structure of the OEC in the S₁ state [Umena et al.] and the high-resolution 2D HYSCORE spectroscopy studies of the S₂ state of the OEC of Ca²⁺-containing PSII [Milikisiyants et al., Energy Environ. Sci., 2012, 5, 7747], we discuss the assignments of the three groups of protons that are magnetically coupled to the Mn₄Sr-oxo cluster. Since hyperfine interactions are highly sensitive to small perturbations in the electronic and geometric structure of paramagnetic centers, a comparison of the 2D HYSCORE spectra of Sr²⁺-substituted and Ca²⁺-containing PSII allows us to draw important conclusions with respect to the structure of the substrate water molecules in the OEC and the role of the Ca²⁺ ion in the water oxidation reaction. In addition, for the first time, we determine the experimental value of the spin projection factor for the Mn(iii) ion of the Mn₄Ca-oxo cluster as ρ₁ = ±1.7 from the assignment of the hyperfine interaction of the paramagnetic cluster with the protons of the D1-His332 residue of PSII.