ANISOTROPIC MOTION OF SEMIQUINONES IN PHOTOSYNTHETIC REACTION CENTERSOF 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 mic rowave frequency) is performed on the primary electron acceptor ubisem iquinone-10 anion radical (UQ-10(-.)) in Zn-substituted photosynthetic reaction centers (RCs) of Rhodobacter sphaeroides R26. In addition, i n vitro measurements of UQ-10(-.) in a frozen solution of deuterated i sopropanol 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 l eads to a spectral resolution of the anisotropic g-tensor. This allows a sensitive detection of the orientation-dependent echo-decay functio ns. The T-2 relaxation anisotropy at high field is dominated by librat ions. 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 envir onment are considered.