title: Photosynthetic water oxidation at elevated dioxygen partial pressure monitored by time-resolved X-ray absorption measurements.
abstract: The atmospheric dioxygen (O(2)) is produced at a tetramanganese complex bound to the proteins of photosystem II (PSII). To investigate product inhibition at elevated oxygen partial pressure (pO(2) ranging from 0.2 to 16 bar), we monitored specifically the redox reactions of the Mn complex in its catalytic S-state cycle by rapid-scan and time-resolved X-ray absorption near-edge spectroscopy (XANES) at the Mn K-edge. By using a pressure cell for X-ray measurements after laser-flash excitation of PSII particles, we found a clear pO(2) influence on the redox reactions of the Mn complex, with a similar half-effect pressure as determined (2-3 bar). However, XANES spectra and the time courses of the X-ray fluorescence collected with microsecond resolution suggested that the O(2) evolution transition itself (S(3)-->S(0)+O(2)) was not affected. Additional (nonstandard) oxidation of the Mn complex at high pO(2) explains our experimental findings more readily. Our results suggest that photosynthesis at ambient conditions is not limited by product inhibition of the O(2) formation step.
authors.affiliation: Freie Universität Berlin, Institut für Experimentalphysik, Arnimallee 14, 14195 Berlin, Germany. haumann@physik.fu-berlin.de
authors.forename: Michael
authors.initials: M
authors.lastname: Haumann
authors.forename: Alexander
authors.initials: A
authors.lastname: Grundmeier
authors.forename: Ivelina
authors.initials: I
authors.lastname: Zaharieva
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
mesh_terms: D015502:Absorptiometry, Photon; D010100:Oxygen / Q000378:metabolism*; D010313:Partial Pressure; D010788:Photosynthesis / Q000502:physiology*; D045332:Photosystem II Protein Complex / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D010100:Oxygen