title: Structural Changes Correlated with Magnetic Spin State Isomorphism in the S2State of the Mn4CaO5Cluster in the Oxygen-Evolving Complex of Photosystem II.
abstract: The Mn4CaO5cluster in Photosystem II catalyzes the four-electron redox reaction of water oxidation in natural photosynthesis. This catalytic reaction cycles through four intermediate states (Si, i = 0 to 4), involving changes in the redox state of the four Mn atoms in the cluster. Recent studies suggest the presence and importance of isomorphous structures within the same redox/intermediate S-state. It is highly likely that geometric and electronic structural flexibility play a role in the catalytic mechanism. Among the catalytic intermediates that have been identified experimentally thus far, there is clear evidence of such isomorphism in the S2state, with a high-spin (5/2) (HS) and a low spin (1/2) (LS) form, identified and characterized by their distinct electron paramagnetic resonance (EPR spectroscopy) signals. We studied these two S2isomers with Mn extended X-ray absorption fine structure (EXAFS) and absorption and emission spectroscopy (XANES/XES) to characterize the structural and electronic structural properties. The geometric and electronic structure of the HS and LS S2states are different as determined using Mn EXAFS and XANES/XES, respectively. The Mn K-edge XANES and XES for the HS form are different from the LS and indicate a slightly lower positive charge on the Mn atoms compared to the LS form. Based on the EXAFS results which are clearly different, we propose possible structural differences between the two spin states. Such structural and magnetic redox-isomers if present at room temperature, will likely play a role in the mechanism for water-exchange/oxidation in photosynthesis.
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Ruchira
authors.initials: R
authors.lastname: Chatterjee
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Guangye
authors.initials: G
authors.lastname: Han
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA; LCLS, SLAC National Accelerator Laboratory, Menlo Park, CA.
authors.forename: Jan
authors.initials: J
authors.lastname: Kern
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Sheraz
authors.initials: S
authors.lastname: Gul
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Franklin D
authors.initials: FD
authors.lastname: Fuller
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Anna
authors.initials: A
authors.lastname: Garachtchenko
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Iris
authors.initials: I
authors.lastname: Young
authors.affiliation: Center for High Pressure Science & Technology Advanced Research, Shanghai, China.
authors.forename: Tsu-Chien
authors.initials: TC
authors.lastname: Weng
authors.affiliation: SSRL, SLAC National Accelerator Laboratory, Menlo Park, CA.
authors.forename: Dennis
authors.initials: D
authors.lastname: Nordlund
authors.affiliation: LCLS, SLAC National Accelerator Laboratory, Menlo Park, CA.
authors.forename: Roberto
authors.initials: R
authors.lastname: Alonso-Mori
authors.affiliation: PULSE, SLAC National Accelerator Laboratory, Menlo Park, CA.
authors.forename: Uwe
authors.initials: U
authors.lastname: Bergmann
authors.affiliation: SSRL, SLAC National Accelerator Laboratory, Menlo Park, CA.
authors.forename: Dimosthenis
authors.initials: D
authors.lastname: Sokaras
authors.affiliation: RIKEN Research Cluster for Innovation, Wako, Saitama, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Hatakeyama
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Vittal K
authors.initials: VK
authors.lastname: Yachandra
authors.affiliation: Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Junko
authors.initials: J
authors.lastname: Yano
keywords: EPR; EXAFS; Manganese; Oxygen evolving complex; Photosystem II; S states; X-ray absorption spectroscopy; XANES; XES