What Can We Learn from a Biomimetic Model of Nature's Oxygen-Evolving Complex?

Area of Science:

• Bioinorganic Chemistry
• Photosynthesis Research
• Quantum Chemistry

Background:

• The oxygen-evolving complex (OEC) in photosystem II is crucial for photosynthesis.
• A synthetic Mn4CaO4 complex structurally mimics the OEC's Mn4CaO5 cluster.
• Observed EPR signals in the synthetic complex resemble those of the OEC S2 state.

Purpose of the Study:

• To computationally characterize the synthetic Mn4CaO4 complex.
• To compute its magnetic and spectroscopic properties.
• To compare its electronic structure with the OEC S2 state and understand EPR signals.

Main Methods:

• Quantum-chemical calculations
• Density matrix renormalization group (DMRG) with full valence active space
• Electron paramagnetic resonance (EPR) simulations

Main Results:

• The synthetic complex's oxidized
• S2 state
• exhibits only one energetically accessible form with S = 5/2.
• No low-spin S = 1/2 valence isomer was found to be energetically accessible.
• One EPR signal (g ≈ 4.9) is assigned to the intact complex; the other (g ≈ 2) likely arises from a rearrangement product.

Conclusions:

• The synthetic model, despite structural similarity, does not replicate the OEC's S2 state valence isomerism, likely due to a lack of a flexible oxo bridge.
• The study provides insights into high-spin EPR signals of OEC S1 and S2 states.
• Findings aid in developing magnetic spectroscopic probes for OEC intermediates.