A high-field EPR study of the anisotropic spin echo decay of QA-· in bacterial photosynthetic reaction centers

Abstract

Electron spin echo experiments were performed on the reduced primary electron acceptor QA-· in Zn2+-reconstituted photosynthetic reaction centers from Rb. sphaeroides R26 using a high-field, high-frequency D-band (130 GHz) pulsed EPR spectrometer. The two-pulse electron spin echo decay was studied as a function of the time separation between the pulses, magnetic field strength and temperature. In the temperature range 80-120 K, the echo decay kinetics of QA-· are exponential. The decay times in this range show a significant magnetic field anisotropy, being the longest at the magnetic field corresponding to the X principal direction of the semiquinione g-tensor, and the shortest at Z principal direction. At 40 K, the decays of QA-· are essentially non-exponential. The possible mechanisms which could explain the obtained experimental data are discussed. The mechanism of small amplitude librational motions of the QA-· molecule, proposed earlier (M. Rohrer et al. (1996) Chem. Phys. Lett. 259, 523-530), is ruled out based on the results of the present work.