Rapid evolution of the Photosystem II electronic structure during water splitting

Area of Science:

• Biochemistry
• Photosynthesis research
• Bioinorganic chemistry

Background:

• Photosynthetic water oxidation is vital for life, producing atmospheric oxygen.
• The manganese-calcium (Mn4Ca) cluster in photosystem II drives this process.
• Understanding the oxygen evolution mechanism is a key scientific challenge.

Purpose of the Study:

• To observe the real-time dynamics of oxygen formation during photosynthesis.
• To investigate the oxidation states of the Mn4Ca cluster during oxygen evolution.
• To elucidate the precise timing of O-O bond formation relative to electron transfer.

Main Methods:

• Utilized time-resolved X-ray emission spectroscopy (XES) to monitor manganese Kβ spectra.
• Applied sequential laser flashes to initiate the oxygen evolution cycle.
• Analyzed spectral changes within microseconds to track rapid reaction steps.

Main Results:

• Observed rapid spectral changes (within 50 μs) in the Mn Kβ XES spectrum during oxygen formation.
• Found no evidence of Mn4Ca cluster oxidation beyond the Mn(IV) state prior to O-O bond formation.
• Attributed the observed spectral shifts to the dynamics of O-O bond formation itself.

Conclusions:

• Propose that O-O bond formation precedes the final electron transfer from the Mn4Ca cluster to tyrosine Yz.
• This model addresses kinetic challenges in oxygen evolution.
• Suggests an evolutionary strategy to prevent the release of reactive peroxide species.