A Comparison of Experimental and Broken Symmetry Density Functional Theory (BS-DFT) Calculated Electron Paramagnetic Resonance (EPR) Parameters for Intermediates Involved in the S₂ to S₃ State Transition of Nature’s Oxygen Evolving Complex

A broken symmetry density functional theory (BS-DFT) magnetic analysis of the S ₂, S ₂Y _Z ^•, and S ₃ states of Nature's oxygen evolving complex is performed for both the native Ca and Sr substituted forms. Good agreement with experiment is observed between the tyrosyl calculated g-tensor and ¹H hyperfine couplings for the native Ca form. Changes in the hydrogen bonding environment of the tyrosyl radical in S ₂Y _Z ^• caused by Sr substitution lead to notable changes in the calculated g-tensor of the tyrosyl radical. Comparison of calculated and experimental ⁵⁵Mn hyperfine couplings for the S ₃ state presently favors an open cubane form of the complex with an additional OH ligand coordinating to Mn _D. In Ca models, this additional ligation can arise by closed-cubane form deprotonation of the Ca ligand W3 in the S ₂Y _Z ^• state accompanied by spontaneous movement to the vacant Mn coordination site or by addition of an external OH group. For the Sr form, no spontaneous movement of W3 to the vacant Mn coordination site is observed in contrast to the native Ca form, a difference which may lead to the reduced catalytic activity of the Sr substituted form. BS-DFT studies on peroxo models of S ₃ as indicated by a recent X-ray free electron laser (XFEL) crystallography study give rise to a structural model compatible with experimental data and an S = 3 ground state compatible with EPR studies.