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 S-2 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 angstrom resolution X-ray structure of the OEC in the S-1 state [Umena et al.] and the high-resolution 2D HYSCORE spectroscopy studies of the S-2 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 rho(1) = +/- 1.7 from the assignment of the hyperfine interaction of the paramagnetic cluster with the protons of the D1-His332 residue of PSII.