¹Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
²Department of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
³SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
⁴iBiTec-S, URA CNRS 2096, CEA Saclay, 91191 Gif sur Yvette, France.

The oxygen-evolving complex (OEC) is a Mn₄CaO₅ cluster located within the Photosystem II (PSII) membrane complex, and catalyzes the light-driven conversion of water to oxygen during photosynthesis.  During the catalytic (Kok cycle, the OEC undergoes transition through five structurally and electronically distinct S_i-states (i = 0-4), coupling the one-electron photochemistry of the reaction center with the four-electron chemistry of water oxidation.  Calcium is an essential cofactor for water oxidation, with previous studies on the photoactivation and assembly of the OEC implicating the requirement of this ion for S-state advancement and enzymatic activity [1,2].  Despite extensive studies on the Mn₄CaO₅ cluster, a more comprehensive understanding of how calcium is involved in photosynthetic water oxidation remains incomplete.  One of the intriguing results from electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS)studies is that depletion of Ca does not significantly perturb the core geometry of the Mn₄ cluster [3-5].  Thus, to investigate the role of calcium in this catalytic reaction, Ca K-edge XAS was used to directly probe the OEC of PSII from Thermosynechococcus elongatus (T. elongatus).  This approach focuses on the native cofactor, avoiding the use of biosynthetically prepared 1-Sr PSII or treatments involving Ca depletion and Sr substitution.  In XAS, X-ray absorption near edge spectroscopy (XANES) can provide insight into the electronic structure and symmetry of the Ca site, whereas Extended X-ray Absorption Fine Structure (EXAFS) presents details on the numbers, types, and distances to ligands and neighboring atoms from the absorbing element.  Additionally, the XANES pre-edge feature may reveal information on the metal-ligand interactions since this formally forbidden transition [from 1s to (n-1)d] can gain intensity when the metal-ligand environment is distorted from a centro-symmetric to a non-centro-symmetric coordination [6].  Ca XANES were measured for these 1-Ca PSII complexes to examine the S-states of the Kok cycle.  To better understand the Ca XANES pre-edge features and elucidate the structural and electronic changes occurring throughout the S-states, a series of inorganic Ca/Mn model complexes were also systematically studied via Ca K-edge XAS.  Ca XANES and EXAFS results from these studies on 1-Ca PSII will be presented, and discussed together with the series of inorganic Ca/Mn model complexes.

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[2] Yachandra VK, Yano J. (2011) J Photochem Photobiol B, 104, 51-59.
[3] Lohmiller T, Cox N, Su JH, Messinger J, Lubitz W. (2012) J Biol Chem, 287, 24721-24733.
[4] Latimer MJ, DeRose VJ, Yachandra VK, Sauer K, Klein MP. (1998) J Phys Chem B, 102, 8257-8265.
[5] Lohmiller T. (2008) Polarized X-ray absorption spectroscopic studies of the Ca^2+-depleted oxygen evolving complex of Photosystem II (Master’s thesis). University of Konstanz, Baden-Württemberg, Germany.
[6] Yano J, Yachandra VK. (2009) Photosynth Res, 102, 241-254.