Comparison between Experimental and Broken Symmetry Density Functional Theory (BS-DFT) Calculated Electron Paramagnetic Resonance (EPR) Parameters of the S State of the Oxygen-Evolving Complex of Photosystem II in Its Native (Calcium) and Strontium-Substituted Form.

A comparison between experimental and Broken Symmetry Density Functional theory (BS-DFT) calculated hyperfine couplings for the S state of the oxygen-evolving complex (OEC) has been performed. The effect of Ca substitution by Sr combined with the protonation state of two terminal hydroxo or aqua ligands, W1 and W2, on the calculated hyperfine couplings of Mn, C, N, O, and H nuclei has been investigated. Our findings show best agreement with experiment for OEC models which contain a hydroxide group at the W2 position and a water molecule at W1. For this model the agreement between calculated and experimental data for all hyperfine couplings is excellent. Models with a hydroxide group at W1 are particularly poor models. Sr substitution has a minor influence on calculated hyperfine couplings in agreement with experimental determinations. The sensitivity of the hyperfine couplings to relatively minor changes in the OEC structure demonstrates the power of this methodology in refining the details of its steric and electronic structure which is an essential step in formulating a complete mechanism for water oxidation by the OEC.