A Computational Study of the S(2) State in the Oxygen-Evolving Complex of Photosystem II by Electron Paramagnetic Resonance Spectroscopy

The S(2) 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(1) → S(2) state transition. The S(2) state has readily visible multiline and [Formula: see text] 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(2) 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 [Formula: see text] signal. The multiline signal was observed to arise from a ground state spin ½ centre while the [Formula: see text] 4.1 signal generated at ≈140 K NIR illumination was proposed to arise from a spin [Formula: see text] center with rhombic distortion. The ‘ground’ state [Formula: see text] 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 [Formula: see text] 4.1 signal, whether it is a rhombic [Formula: see text] spin state signal or an axial [Formula: see text] spin state signal. The data supported an X-band CW-EPR-generated [Formula: see text] 4.1 signal as originating from a near rhombic spin 5/2 of the S(2) state of the PSII manganese cluster.