Anisotropic motion of semiquinones in photosynthetic reaction centers of Rhodobacter sphaeroides R26 and in frozen isopropanol solution as measured by pulsed high-field EPR at 95 GHz

Pulsed high-field/high-frequency EPR (3.4 T magnetic field, 95 GHz microwave frequency) is performed on the primary electron acceptor ubisemiquinone-10 anion radical (UQ-10−.) in Zn-substituted photosynthetic reaction centers (RCs) of Rhodobacter sphaeroides R26. In addition, in vitro measurements of UQ-10−. in a frozen solution of deuterated isopropanol are presented. The method is two-dimensional EPR, with the swept magnetic field as the first and the pulse separation time as the second variable of the echo-detected spectra. The high Zeeman field leads to a spectral resolution of the anisotropic g-tensor. This allows a sensitive detection of the orientation-dependent echo-decay functions. The T2 relaxation anisotropy at high field is dominated by librations. Comparison of in vivo and in vitro measurements shows different anisotropies of the echo-decay functions. The results are interpreted by calculations considering angular motions around the principal axes of the g-tensor. Hydrogen bonds between UQ-10−. and its protic environment are considered.