title: Photosynthetic water oxidation at high O2 backpressure monitored by delayed chlorophyll fluorescence.
abstract: The atmospheric dioxygen is produced by photosynthetic organisms. This light-driven process culminates in what appears as one step: a four-electron abstraction from two water molecules bound to the Mn4Ca complex of photosystem II. Recently, an intermediate of the O2-producing reaction sequence was stabilized by elevated oxygen backpressure and detected by UV flash photometry [Clausen, J., and Junge, W. (2004) Nature 430, 480]. We scrutinized its properties by delayed chlorophyll fluorescence measurements. Half-suppression of oxygen evolution was observed at a similar O2 pressure of 2.3 bar, as previously, now with photosystem II membrane particles from spinach, without artificial electron acceptors, and at a high signal-to-noise ratio. The data are tentatively interpreted as the stabilization of a 2-fold oxidized state of the catalytic center (S2*) with bound peroxide and its slow conversion into the normal S2 state by the release of peroxide.
authors.affiliation: Universität Osnabrück, FB Biologie/Chemie, Abt. Biophysik, Barbarastrasse 11, D-49069 Osnabrück, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Clausen
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Junge
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
authors.forename: Michael
authors.initials: M
authors.lastname: Haumann
mesh_terms: D002734:Chlorophyll / Q000378:metabolism*; D005453:Fluorescence; D007700:Kinetics; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010545:Peroxides / Q000737:chemistry; D010783:Photometry / Q000379:methods; D010788:Photosynthesis; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism*; D011312:Pressure; D014867:Water / Q000737:chemistry*
chemical_list: D010545:Peroxides; D045332:Photosystem II Protein Complex; D014867:Water; D002734:Chlorophyll; D010100:Oxygen