Molecular Dynamics of Q_A^-• and Q_B^-• in Photosynthetic Bacterial Reaction Centers Studied by Pulsed High-Field EPR at 95 GHz

Abstract

The anisotropic transverse relaxation times T2 of the primary and secondary acceptor quinone radical anions QA-• and QB-• in Zn-substituted photosynthetic bacterial reaction centers of Rhodobacter sphaeroides R26 have been studied by means of 2D high-field/high-frequency (3.4 T/95 GHz, W-band) electron spin−echo spectroscopy. The swept magnetic field is the first, and the pulse-separation time is the second variable. Because of the high magnetic field in W-band EPR, the anisotropic Zeeman interactions of quinone radical anions are resolved; therefore, the orientation dependence of the T2 relaxation time can be investigated. For QA-• and QB-•, the monoexponential echo decays at 120 K have different orientation-dependent time constants. The anisotropy of the relaxation times is related to anisotropic stochastic fluctuations of the quinones in their protein-binding pockets, which are temperature-dependent. A model is proposed in which the orientation-dependent relaxation originates in reorientational fluctuations around the quinones' specific hydrogen bonds to surrounding amino acids in the binding sites.