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Arrested Substrate Binding Resolves Catalytic Intermediates in Higher-Plant Water Oxidation.

ABSTRACT: Among the intermediate catalytic steps of the water-oxidizing Mn₄ CaO₅ cluster of photosystem?II (PSII), the final metastable S₃ state is critically important because it binds one substrate and precedes O₂ evolution. Herein, we combine X- and Q-band EPR experiments on native and methanol-treated PSII of Spinacia oleracea and show that methanol-treated PSII preparations of the S₃ state correspond to a previously uncharacterized high-spin (S=6) species. This is confirmed as a major component also in intact photosynthetic membranes, coexisting with the previously known intermediate-spin conformation (S=3). The high-spin intermediate is assigned to a water-unbound form, with a Mn^IV ₃ subunit interacting ferromagnetically via anisotropic exchange with a coordinatively unsaturated Mn^IV ion. These results resolve and define the structural heterogeneity of the S₃ state, providing constraints on the S₃ to S₄ transition, on substrate identity and delivery pathways, and on the mechanism of O-O bond formation.

SUBMITTER: Zahariou G

PROVIDER: S-EPMC7898718 | biostudies-literature | 2021 Feb

REPOSITORIES: biostudies-literature

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Arrested Substrate Binding Resolves Catalytic Intermediates in Higher-Plant Water Oxidation.

Zahariou Georgia G Ioannidis Nikolaos N Sanakis Yiannis Y Pantazis Dimitrios A DA

Angewandte Chemie (International ed. in English) 20201210 6

Among the intermediate catalytic steps of the water-oxidizing Mn4 CaO5 cluster of photosystem II (PSII), the final metastable S3 state is critically important because it binds one substrate and precedes O2 evolution. Herein, we combine X- and Q-band EPR experiments on native and methanol-treated PSII of Spinacia oleracea and show that methanol-treated PSII preparations of the S3 state correspond to a previously uncharacterized high-spin (S=6 ...[more]

PMID: 33030775

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Project description:A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple overlapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt-iron hexacyanoferrate (cobalt-iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (∼0.34 eV/monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O2 evolution onset coincide due to weaker binding and destabilization (∼0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Crucially, the case of CoFe-PB shows that, even where interactions are weaker, adsorption remains non-Nernstian, which strongly influences the observed Tafel slope.

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Arrested Substrate Binding Resolves Catalytic Intermediates in Higher-Plant Water Oxidation.

Publications

Arrested Substrate Binding Resolves Catalytic Intermediates in Higher-Plant Water Oxidation.

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