MOLECULAR-MOTION OF THE CHOLESTANE SPIN-LABEL IN A MULTIBILAYER IN THE GEL PHASE STUDIED USING ECHO-DETECTED EPR

Echo-detected EPR spectra of the cholestane spin label in a macroscopically oriented lipid multibilayer have been obtained at temperatures between 50 and 210 K, The drastic temperature dependence which the spectra reveal was attributed to relaxation induced by the orientational oscillations of the spin-label molecule, The scheme of computer simulations of echo-detected spectra for the model of uniaxial orientational oscillations (librations) is described in detail, The simulated spectra show a remarkable dependence on the orientation of the motional axis in the molecular frame, The simulation and experimental results agree when the projection of this axis in the plane of the nitroxide fragment makes an angle of 70 degrees +/- 10 degrees with the direction of the NO bond, This was attributed to librational motion of the label about the long axis of the cholestane moiety. (C) 1995 Academic Press, Inc.