Photo-Induced Radical Pairs Investigated Using Out-Of-Phase Electron Spin Echo
Electron spin echo experiment on photo-induced radical pairs in photosynthetic reaction centers produces the echo signal which is out-of-phase due to spin coherence. This signal is strongly modulated because of the electron spin-spin dipolar and exchange couplings in the pair. Fourier transformation of the time-domain modulation produces a “Pake doublet” from the two coupled spins, which allows the accurate determination of the distance between the two radicals. The precision with which the distance can be measured is 0.3 – 0.4 Å, for radicals separated in the pair by ~ 30 Å. This high precision provides a very sensitive tool for structural investigations in photosynthetic reaction centers. The method is applicable to spin-correlated radical pairs separated by 20 – 40 Å and possessing an EPR linewidth accessible for the full excitation by the microwave pulses.
KeywordsElectron Paramagnetic Resonance Radical Pair Microwave Pulse Photosynthetic Reaction Center Electron Spin Echo Envelope Modulation
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