Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 105, Issue 45, Pages 17384-17389Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0802596105
Keywords
manganese complex; photosystem II; X-ray spectroscopy; bioinorganic chemistry; oxygen pressure
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Funding
- Deutsche Forschungsgemeinschaft [Sonderforschungsbereich 498]
- Bundesministerium fur Bildung and Forschung Bio-H2 [035F0318C]
- European Union SOLAR-H2
- UniCat Cluster of Excellence
- Alexander-von-Humboldt Foundation
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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 double right arrow 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.
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