A Computational Study of the S2 State in the Oxygen-Evolving Complex of Photosystem II by Electron Paramagnetic Resonance Spectroscopy.

Bernard Baituti,Sebusi Odisitse

doi:10.3390/molecules26092699

Abstract

The S2 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 S1 → S2 state transition. The S2 state has readily visible multiline and 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 S2 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 signal. The multiline signal was observed to arise from a ground state spin ½ centre while the 4.1 signal generated at ≈140 K NIR illumination was proposed to arise from a spin center with rhombic distortion. The ‘ground’ state 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 4.1 signal, whether it is a rhombic spin state signal or an axial spin state signal. The data supported an X-band CW-EPR-generated 4.1 signal as originating from a near rhombic spin 5/2 of the S2 state of the PSII manganese cluster.

Highlights

The invention of photosynthetic oxygen evolution in 4.6 billion years of the history of the earth allowed for the development of oxygenic atmosphere that forms the basis of life activities
The difference spectra for the ML signals, and g4.1 and ‘g2’ near infrared (NIR) signals were obtained by subtracting the appropriate background, or pre-NIR illumination spectra, from the
Difference spectrum from ≈240 K ML difference spectrum, which represents the amount of ML signal lost (≈35%) when re-illuminating the sample at ≈140 K with NIR light to photo-induce the g4.1 signal

Summary

Introduction

The invention of photosynthetic oxygen evolution in 4.6 billion years of the history of the earth allowed for the development of oxygenic atmosphere that forms the basis of life activities. Algae, and cyanobacteria powered by solar energy convert carbon dioxide and water into molecular oxygen and organic matter. This process is catalyzed by a huge membrane pigment–protein complex enzyme in photosystem II (PSII) [1]. The core machinery for oxygen evolution is Mn4 CaO5 cluster (catalytic site) located in photosystem. The oxygen evolution proceeds, removing four electrons and four protons (H+ ) from the two substrate water molecules at the Mn4 CaO5 cluster of the oxygen-evolving complex (OEC) of PSII [3,4]. The structure of PSII from different photosynthetic organisms has been revealed by both cryoelectron microscopy and X-ray crystallography [2,7,12,13,14,15]

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