One Electron Multiple Proton Transfer in Model Organic Donor-Acceptor Systems: Implications for High Frequency EPR

Area of Science:

• Biophysics
• Spectroscopy
• Computational Chemistry

Background:

• Electron paramagnetic resonance (EPR) spectroscopy is crucial for identifying radical species in biological reactions.
• Tyrosyl radicals, often with histidine, mediate key electron and proton transfer processes, like in photosystem II.
• Bio-mimetic model complexes aid in understanding complex biological mechanisms.

Purpose of the Study:

• To investigate the sensitivity of magnetic resonance parameters to proton-coupled electron transfer (PCET) events.
• To analyze conformational substates in molecular constructs mimicking tyrosine-histidine pairs.
• To differentiate between one-electron one-proton transfer (EPT) and one-electron two-proton transfer (E2PT) products.

Main Methods:

• Theoretical calculations of magnetic resonance parameters.
• Simulation of proton-coupled electron transfer (PCET) events in bio-mimetic models.
• Analysis of electronic g-tensor components and 14N hyperfine values.

Main Results:

• The gX-component of the electronic g-tensor is highly sensitive to the first proton transfer (EPT), showing a significant increase.
• The gX-value shows minimal sensitivity to the second proton transfer (E2PT), making differentiation difficult.
• Rotation around single bonds, particularly of the phenoxyl group, causes substantial changes in gX-values due to hydrogen bonding.

Conclusions:

• EPR spectroscopy's gX-component can effectively detect initial proton transfer events in tyrosine-histidine mimics.
• Distinguishing between EPT and E2PT products using EPR parameters like gX and 14N hyperfine values is challenging.
• Conformational flexibility, specifically phenoxyl group rotation, significantly impacts magnetic resonance parameters.