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Photosynthetic water oxidation at elevated dioxygen partial pressure monitored by time-resolved X-ray absorption measurements
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Photosynthetic water oxidation at elevated dioxygen partial pressure monitored by time-resolved X-ray absorption measurements
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Journal Article
Photosynthetic water oxidation at elevated dioxygen partial pressure monitored by time-resolved X-ray absorption measurements
2008
Overview
The atmospheric dioxygen (O₂) is produced at a tetramanganese complex bound to the proteins of photosystem II (PSII). To investigate product inhibition at elevated oxygen partial pressure (pO₂ 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 pO₂ 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 O₂ evolution transition itself (S₃[implies]S₀+O₂) was not affected. Additional (nonstandard) oxidation of the Mn complex at high pO₂ explains our experimental findings more readily. Our results suggest that photosynthesis at ambient conditions is not limited by product inhibition of the O₂ formation step.
Publisher
National Academy of Sciences,National Acad Sciences
