Determining Manganese Oxidation States in the Oxygen Evolving Complex via S₂ CW-EPR Signal Simulation and Spin Density Analysis

Water oxidation is a biological process that contributes a lot to the ecosystem. Interestingly, it produces abundant hydrogen gas, which can be used as an alternative energy source according to the recent demands of green technology. The mechanism of water oxidation in the photosystem II (PSII), catalysed the oxygen evolving complex (OEC)/ Mn ₄ O ₅ Ca cluster is of great interest. This is because understanding the electronic properties of the Mn ₄ O ₅ Ca cluster can enhance innovation of technologies for the biological catalysis of water oxidation. The oxidation states of each Mn ions in the Mn ₄ O ₅ Ca cluster are still not concluded. Electron paramagnetic resonance (EPR) spectroscopy has been widely used in the study of the OEC in the PSII. Among the 𝑆 _𝑛 states of the Kok cycle, the 𝑆 ₂ state is the most studied due to the multiline (ML) EPR signal it generates at \(\:g=\:2\) and \(\:g=\:4.1\). These ML signals are the hyperfine (HF) structured ⁵⁵ Mn EPR signals with a total spin of ½ in the ground state and can be obtained by trapping PSII prepared samples (either membranes or cores) in the 𝑆 ₂ or 𝑆 ₀ state at cryogenic temperatures, and we used approximately 7 K in this study. This study therefore details an insight to understanding the electronic properties of the Mn ₄ O ₅ Ca cluster using EasySpin simulations and Density Functional Theory (DFT) calculations. Even though research has been done before, the HF contribution of each Mn ion to the fine structure of OEC remains inconclusive. The EPR generated signal has been instrumental in determining the HF contributions of each Mn ion to the fine structure and the oxidation states of each Mn ion through simulation of these ML signals. Simulation of the X-band continuous wave (cw) EPR generated spectra, for low field, wide field and upper field at g = 2 in this study, revealed that three of four Mn ions with large anisotropy which are more certainly Mn ^III centres that support the ‘low oxidation state’ and supported by the spin density of the Mn ions obtained from DFT calculations.