Pulsed ENDOR Studies of Short-Lived Spin-Correlated Radical Pairs in Photosynthetic Reaction Centers

Time-resolved electron−nuclear double resonance (TR-ENDOR) performed on the spin-correlated radical pair in frozen solution samples of PSI shows a superposition of the ENDOR lines expected for the individual radicals and . Presented here are TR-ENDOR spectra taken over a range of EPR field positions. The spectra are unusual because they show both absorptive and emissive contributions and a strong dependence on the field. The nature of the spectra is explored using the theory of spin-correlated radical pairs. It is found that the electron spin polarization in the radical pair is responsible for the absorptive/emissive nature of the TR-ENDOR spectra and that the mixed electronic eigenstates give rise to transitions in addition to those usually found in ENDOR. Computer simulations based on a model containing two coupled electrons and a single spin- 1 / 2 nucleus are able to reproduce the spectra and their field dependence. It is also shown that TR-ENDOR spectra from a spin-correlated radical pair are sensitive to the relative signs of the hyperfine couplings.