How much does each manganese contribute to multiline signal through its hyperfine interactions in the oxygen evolving complex?

Document Type : Research Article

Authors

1 Botswana International University of Science and technology

2 Botswana International University of Science and Technology

10.22034/jchemlett.2021.271342.1016

Abstract

Interpretation of the mechanism of biological water oxidation is pinned on the understanding of the manganese oxidation states of the Mn4CaO5 catalytic cluster in photosystem II (PSII). There are five intermediate oxidation states (S-states) in the catalytical cycle of water oxidation used by oxygen evolving photoautotrophs, with four previously characterised by electron paramagnetic resonance and electron nuclear double resonance spectroscopy. The intermediate S2 (Kok) state, Mn oxidation states are still unresolved. However, two alternative paradigms have been proposed: “high oxidation state” (MnIV3MnIII) and “low oxidation state” (MnIII3MnIV). Herein we investigate the electron paramagnetic resonance generated S2 state signal (multiline) using the Monte Carlo simulations. Here we show that Mn2 and Mn1 have large nuclear hyperfine values, consistent with ρ values of -1 and ~ 2 respectively, whereas Mn3 and Mn4 nuclear hyperfine values are less than 100 megahertz. However, the nuclear hyperfine values for Mn4 are very small, the hyperfine values for Mn3 and Mn1 are highly anisotropic and virtually rhombic, signifying that they should have MnIII oxidation states. Nevertheless, the hyperfine values are least anisotropic for Mn2, so the probable oxidation state of this ion is MnIV, which somehow favours the “low oxidation state”.

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