What the multiline signal (MLS) simulation data with average of weighted computations reveal about the Mn hyperfine interactions and oxidation states of the manganese cluster in OEC?

Abstract

Understanding the structure of oxygen evolving complex (OEC) fully still remains a challenge. Lately computational chemistry with the data from more detailed X-ray diffraction (XRD) OEC structure, has been used extensively in exploring the mechanisms of water oxidation in the OEC (Gatt et al., J. Photochem. Photobiol. B 104(1–2), 80–93 2011). Knowledge of the oxidation states is very crucial for understanding the core principles of catalysis by photosystem II (PSII) and catalytic mechanism of OEC. The present study involves simulation studies of the X-band continuous wave electron-magnetic resonance (CW-EPR) generated S 2 state signals, to investigate whether the data is in agreement with the four manganese ions in the OEC, being organised as a ‘3 + 1’ (trimer plus one) model (Gatt et al., Angew. Chem. Int. Ed. 51, 12025–12028 2012; Petrie et al., Chem. A Eur. J. 21, 6780–6792 2015; Terrett et al., Chem. Commun. (Camb.) 50, 8–11 2014) or ‘dimer of dimers’ model (Terrett et al. 2016). The question that still remains is how much does each Mn ion contribute to the “ g2multiline” signal through its hyperfine interactions in OEC also to differentiate between the ‘high oxidation state (HOS)’ and ‘low oxidation state (LOS)’ paradigms? This is revealed in part by the structure of multiline (ML) signal studied in this project. Two possibilities have been proposed for the redox levels of the Mn ions within the catalytic cluster, the so called ‘HOS’ and ‘LOS’ paradigms (Gatt et al., J. Photochem. Photobiol. B 104(1–2), 80–93 2011). The method of data analysis involves numerical simulations of the experimental spectra on relevant models of the OEC cluster. The simulations of the X-band CW-EPR multiline spectra, revealed three manganese ions having hyperfine couplings with large anisotropy. These are most likely Mn III centres and these clearly support the ‘LOS’ OEC paradigm model, with a mean oxidation of 3.25 in the S2 state. This is consistent with the earlier data by Jin et al. (Phys. Chem. Chem. Phys. (PCCP) 16(17), 7799–812 2014), but the present results clearly indicate that heterogeneity in hyperfine couplings exist in samples as typically prepared.