title: Considerations on the mechanism of photosynthetic water oxidation - dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water.
abstract: Two mechanistic problems of photosynthetic water oxidation at the Mn complex of Photosystem II (PS II) are considered. (I) In the four Mn-oxidizing transitions, any pure Mn oxidation is predicted to cause an increase in redox potential that renders subsequent oxidation steps impossible (redox-potential problem). Formation of unprotonated oxo-bridges may counteract the potential increase. (II) The O-O formation step without any high-pK bases acting as proton acceptors is energetically unfavorable (acceptor-base problem). The pK of oxides in a bridging position between Mn ions may increase drastically upon reduction of Mn in the water-oxidation step (>10 units), thus rendering them favorable proton acceptors. It is proposed that in PS II, in the course of the four oxidizing transitions at least two unprotonated oxo-bridges are formed. Thereby (i) a redox potential increase is prevented and (ii) proton acceptors are prepared for the O-O formation step. Water oxidation in the O-O bond formation step is facilitated by simultaneous Mn reduction and proton transfer to bridging oxides amounting to hydrogen atom or hydride transfer from substrate water to the Mn-oxo core of the Mn complex of PS II.
authors.affiliation: Freie Universität Berlin, FB Physik Arnimallee 14, 14195 Berlin, Germany. holger.dau@physik.fu-berlin.de
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
authors.forename: Michael
authors.initials: M
authors.lastname: Haumann
mesh_terms: D008345:Manganese / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010788:Photosynthesis / Q000502:physiology*; D045332:Photosystem II Protein Complex / Q000378:metabolism; D011522:Protons; D013816:Thermodynamics; D014867:Water / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water; D008345:Manganese