Electron Spin Polarization of the Oxidized Primary Electron Donor in Reaction Centers of Photosynthetic Purple Bacteria

Abstract

Fast time-resolved EPR spectroscopy is used to study electron spin polarization (ESP) in perdeuterated native, Fe2+-containing reaction centers (RCs) of photosynthetic purple bacteria. The spin-correlated radical pair(SCRP) model (previously used to simulate ESP observed in Fe-depleted RCs (Hore, P. J.; Hunter, D. A.; McKie, C. D.; Hoff, A. J. Chem. Phys. Lett. 1987, 137, 495) is extended to include the large anisotropy arising from the magnetic interactions between Fe2+ and the reduced primary electron−acceptor quinone (QA•−), which results in different quantization axes for the P•+ and the (QA•−Fe2+) spins. Using spectral simulations, it is shown that the ESP spectrum is solely due to the P•+ part of the spin-correlated radical pair [P•+(QA•−Fe2+)], whereas the rapid decay of the spin-polarized signal is due to spin-lattice relaxation of the (QA•−Fe2+) complex. The simulations are very sensitive to the relative orientation of the g matrices of P•+ and (QA•−Fe2+). Using orientation II of the g matrix of ...