High-frequency EPR approach to the electron spin-polarization effects observed in the photosynthetic reaction centers

Time-resolved high-frequency electron paramagnetic resonance (EPR) spectroscopy was applied to study the structure and dynamics of the electron transfer pathways in the photosynthetic RC proteins. When the spin-polarized EPR spectra are recorded at the high field, the singlet-triplet mixing in the radical pairs becomes faster due to the increase of Zeeman interaction, and a sequential electron transfer polarization model, which includes both the primary and secondary radical pairs, should be considered. Application of the sequential electron transfer polarization model for the interpretation of the bacterial RC proteins with a “slow” electron transfer rate reveals the importance of the protein dynamics. It was shown that the reorganization energy for the electron transfer process between P ₈₆₅ ⁺ H^?Q_A and P ₈₆₅ ⁺ HQ _A ^? , but not the change in the structure of the donor-acceptor complex, is a dominant factor that alters the electron transfer rate. The relaxation data, obtained in the delay after laser flash experiment, have been used to estimate the magnetic interaction in the weakly coupled radical pair. High-frequency spin-polarized EPR spectra allow the quantitative characterization of isotopically labeled quinone exchange in the PS I reaction center proteins.