Time-Resolved Electron Paramagnetic Resonance Study on Cofactor Geometries and Electronic Couplings after Primary Charge Separations in the Photosynthetic Reaction Center

Abstract

To elucidate how cofactor geometries after photoinduced primary charge-separations influence electronic couplings (VCR) for primary charge-recombination (CR) processes in the photosynthetic reaction center, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra both of the primary charge-separated (CS) state (P+•HA–•) and its charge-recombined triplet state (3P*) of the special pair in Rhodobacter sphaeroides R26. To determine the CS-state geometry, quantum mechanical modeling has been performed on the spin polarization of the 3P* generated by the spin dynamics due to the anisotropies of the hyperfine and the spin–spin dipolar interactions in the primary CS state. From transverse magnetizations of the primary CS state, we have also determined the VCR value leading to the singlet excited state (1P*) of the special pair. The above analyses have revealed that while the primary charge separation does not largely modulate the cofactor conformations, it leads to significant enhancement i...