SPIN LABEL-THIOUREA ADDUCTS - MODEL FOR SATURATION TRANSFER EPR STUDIES OF SLOW ANISOTROPIC ROTATION

A powdered crystalline adduct of thiourea with a spin label derivative of cholestanone has been chosen as a model of slow molecular rotation in an anisotropic environment. Saturation transfer electron paramagnetic resonance (STEPR) spectra of the adduct at temperatures from 143 to 283 K are compared with those resulting from motion of the same probe in a viscous solvent, diisobutyl phthalate. The range of magnitude of spectral parameters obtained from STEPR spectra is different when the probe moves in a highly anisotropic environment than when it moves in a nearly isotropic one. The temperature dependencies of spectral parameters from the low-field portion of the STEPR spectra, in particular, are diagnostic for anisotropic rotation about a diffusion symmetry axis aligned with the nitroxide y principal molecular axis. The motion of membrane components presents problems requiring this type of analysis. © 1979 American Chemical Society.