Spin-dynamics of the spin-correlated radical pair in photosystem I. Pulsed time-resolved EPR at high magnetic field

Spin-dynamics of the spin-correlated radical pair (SCRP) P₇₀₀⁺A_1A⁻ in the photosystem I (PSI) reaction center protein have been investigated with high-frequency (HF), time-resolved EPR spectroscopy. The superior spectral resolution of HF EPR enables spin-dynamics for both the donor and acceptor radicals in the pair to be monitored independently. Decay constants of each spin were measured as a function of temperature and compared to data obtained at X-band EPR. Relaxation times, T₁, and decay rates, k_S, are the same at both X- and D-band magnetic fields. The spin-dynamics within the radical pair were determined from theoretical simulation of experimental time-resolved HF EPR spectra. At low temperatures, T < 60 K, the decay of the SCRP from the singlet state, k_S, is the predominant process, while at high temperatures, T > 130 K, the T₁ relaxation is much faster than k_S. The recombination rate k_S was observed to decrease as the temperature is increased. These EPR spectral results are in agreement with previously reported optical measurements of P₇₀₀⁺A₁⁻ radical pair recombination.