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Radical pair state in photosystem II.
A W Rutherford1, M C Thurnauer
1Department of Physiology and Biophysics, University of Illinois, Urbana, Illinois 61801.
Summary
A stable light-induced EPR signal in photosystem II particles and chloroplasts indicates a radical pair triplet state. This signal is formed when the reaction center is in the D(+)P(680)Ph(-) state, with radical pair distances of 6-7 Å.
Area of Science:
- Photosynthesis research
- Biophysics
- Electron paramagnetic resonance (EPR) spectroscopy
Background:
- Photosystem II (PSII) is crucial for oxygenic photosynthesis.
- Understanding electron transfer in PSII is key to photosynthesis research.
- Light-induced EPR signals provide insights into radical pair states.
Purpose of the Study:
- To characterize a stable light-induced EPR signal in PSII.
- To determine the origin and properties of this EPR signal.
- To elucidate the low-temperature photochemical sequence in PSII.
Main Methods:
- Electron paramagnetic resonance (EPR) spectroscopy at 5 K.
- Studies on isolated PSII particles and chloroplasts.
- Varying redox potential and temperature conditions.
Main Results:
- A stable light-induced EPR signal was observed in PSII particles and chloroplasts at 5 K.
- The signal originates from a radical pair triplet state, likely D(+)P(680)Ph(-).
- Radical pair distances were calculated to be 6-7 Å, with at least two distinct pairs.
Conclusions:
- The EPR signal arises from dipole-dipole interactions within a radical pair triplet state.
- The D(+)P(680)Ph(-) state is identified as the source of the signal in specific conditions.
- Low-temperature electron donation pathways in PSII were clarified.