Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

During photosynthetic water oxidation, the Mn₄CaO₅ cluster in the oxygen-evolving complex (OEC) cycles through five intermediate oxidation states, collectively called the S states, labeled S₀-S₄ in the Kok cycle. In the S₂ state, two types of low-temperature electron paramagnetic resonance (EPR) signals, g = 2 (spin 1/2) and g ≥ 4.1(spin ≥ 5/2) have been detected and assigned to the Mn₄CaO₅ cluster. In this study, the structure of the Mn₄CaO₅ cluster in the S₂ state with the g ≈ 4 EPR signal (S₂-g4 state) generated from native and fluoride-inhibited photosystem II membranes prepared from spinach is investigated by using X-ray absorption spectroscopy (XAS). Extended X-ray absorption fine structure (EXAFS) studies of the Mn K-edge show that the structure of the Mn₄CaO₅ cluster in S₂-g4 state is different from that in the S₂ state with the g = 2 state (S₂-g2). The structural changes related to the Mn-ligand and Mn-Mn distances of the Mn₄CaO₅ cluster during the S₂-g2 state to S₂-g4 conversion were interpreted based on the geometry of the Mn₄CaO₅ cluster obtained from the polarized XAS model and the 1.9 Å high-resolution crystal structure.

https://conf-slac.stanford.edu/ssrl-lcls-2013/sites/conf-slac.stanford.e...