title: Chemical Equilibrium Models for the S3 State of the Oxygen-Evolving Complex of Photosystem II.
abstract: We have performed hybrid density functional theory (DFT) calculations to investigate how chemical equilibria can be described in the S3 state of the oxygen-evolving complex in photosystem II. For a chosen 340-atom model, 1 stable and 11 metastable intermediates have been identified within the range of 13 kcal mol(-1) that differ in protonation, charge, spin, and conformational states. The results imply that reversible interconversion of these intermediates gives rise to dynamic equilibria that involve processes with relocations of protons and electrons residing in the Mn4CaO5 cluster, as well as bound water ligands, with concomitant large changes in the cluster geometry. Such proton tautomerism and redox isomerism are responsible for reversible activation/deactivation processes of substrate oxygen species, through which Mn-O and O-O bonds are transiently ruptured and formed. These results may allow for a tentative interpretation of kinetic data on substrate water exchange on the order of seconds at room temperature, as measured by time-resolved mass spectrometry. The reliability of the hybrid DFT method for the multielectron redox reaction in such an intricate system is also addressed.
authors.affiliation: Photosynthesis Research Center, Graduate School of Natural Science and Technology, Okayama University , Okayama 700-8530, Japan.|The Institute of Scientific and Industrial Research, Osaka University , Ibaraki, Osaka 567-0047, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Isobe
authors.affiliation: Graduate School of Pure and Applied Sciences, University of Tsukuba , Tsukuba, Ibaraki 305-8577, Japan.
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Shoji
authors.affiliation: Photosynthesis Research Center, Graduate School of Natural Science and Technology, Okayama University , Okayama 700-8530, Japan.
authors.forename: Jian-Ren
authors.initials: JR
authors.lastname: Shen
authors.affiliation: The Institute of Scientific and Industrial Research, Osaka University , Ibaraki, Osaka 567-0047, Japan.|Institute for NanoScience Design, Osaka University , Toyonaka, Osaka 560-0043, Japan.
authors.forename: Kizashi
authors.initials: K
authors.lastname: Yamaguchi
mesh_terms: D018360:Crystallography, X-Ray; D007536:Isomerism; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011506:Proteins / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D045332:Photosystem II Protein Complex; D011506:Proteins; D010100:Oxygen