A Computational Study of the S<sub>2</sub> State in the Oxygen-Evolving Complex of Photosystem II by Electron Paramagnetic Resonance Spectroscopy
The S<sub>2</sub> state produces two basic electron paramagnetic resonance signal types due to the manganese cluster in oxygen-evolving complex, which are influenced by the solvents, and cryoprotectant added to the photosystem II samples. It is presumed that a single manganese center oxi...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2021-05-01
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| Series: | Molecules |
| Subjects: | |
| Online Access: | https://www.mdpi.com/1420-3049/26/9/2699 |
| Summary: | The S<sub>2</sub> state produces two basic electron paramagnetic resonance signal types due to the manganese cluster in oxygen-evolving complex, which are influenced by the solvents, and cryoprotectant added to the photosystem II samples. It is presumed that a single manganese center oxidation occurs on S<sub>1</sub> → S<sub>2</sub> state transition. The S<sub>2</sub> state has readily visible multiline and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>g</mi><mn>4.1</mn></mrow></semantics></math></inline-formula> electron paramagnetic resonance signals and hence it has been the most studied of all the Kok cycle intermediates due to the ease of experimental preparation and stability. The S<sub>2</sub> state was studied using electron paramagnetic resonance spectroscopy at X-band frequencies. The aim of this study was to determine the spin states of the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>g</mi><mn>4.1</mn></mrow></semantics></math></inline-formula> signal. The multiline signal was observed to arise from a ground state spin ½ centre while the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>g</mi></mrow></semantics></math></inline-formula>4.1 signal generated at ≈140 K NIR illumination was proposed to arise from a spin <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfrac><mn>5</mn><mn>2</mn></mfrac></mrow></semantics></math></inline-formula> center with rhombic distortion. The ‘ground’ state <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>g</mi></semantics></math></inline-formula>4.1 signal was generated solely or by conversion from the multiline. The data analysis methods used involved numerical simulations of the experimental spectra on relevant models of the oxygen-evolving complex cluster. A strong focus in this paper was on the ‘ground’ state <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>g</mi></semantics></math></inline-formula>4.1 signal, whether it is a rhombic <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfrac><mn>5</mn><mn>2</mn></mfrac></mrow></semantics></math></inline-formula> spin state signal or an axial <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfrac><mn>3</mn><mn>2</mn></mfrac></mrow></semantics></math></inline-formula> spin state signal. The data supported an X-band CW-EPR-generated <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>g</mi></semantics></math></inline-formula>4.1 signal as originating from a near rhombic spin 5/2 of the S<sub>2</sub> state of the PSII manganese cluster. |
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| ISSN: | 1420-3049 |