Structural evidence for intermediates during O₂ formation in photosystem II.

Area of Science:

• Biochemistry
• Photosynthesis Research
• Structural Biology

Background:

• Natural photosynthesis uses photosystem II and the Mn$_{4}$CaO$_{5}$ cluster to split water, generating energy.
• This process involves intermediate states (S$_{0}$ to S$_{4}$) in the Kok cycle, crucial for solar-to-chemical energy conversion.
• Understanding the O-O bond formation chemistry is key to artificial photosynthesis.

Purpose of the Study:

• To provide structural insights into the final reaction step of the photosynthetic water oxidation cycle (S$_{3}$→[S$_{4}$]→S$_{0}$).
• To elucidate the mechanism of oxygen (O$_{2}$) formation and the resetting of the Kok cycle.
• To observe molecular events occurring at the Mn$_{4}$CaO$_{5}$ cluster and associated pathways during water oxidation.

Main Methods:

• Serial femtosecond X-ray crystallography at room temperature.
• Time-resolved structural analysis capturing micro- to millisecond events.
• Analysis of changes in the Mn$_{4}$CaO$_{5}$ cluster, ligands, and water channels.

Main Results:

• Detailed structural changes during the S$_{3}$→[S$_{4}$]→S$_{0}$ transition were observed.
• A specific extra oxygen atom (O$_{x}$) disappears or relocates around 700 μs after the third flash, concurrent with Y$_{z}$ reduction.
• Oxygen evolution begins around 1,200 μs, indicated by a shortened Mn1-Mn4 distance, suggesting a bound peroxide intermediate.

Conclusions:

• The study provides unprecedented structural snapshots of the final water-splitting step in photosynthesis.
• A complex sequence of structural and proton transfer events governs O$_{2}$ formation.
• The findings offer critical insights into the catalytic mechanism of the Mn$_{4}$CaO$_{5}$ cluster for artificial photosynthesis development.