Measurements of Molecular Orientations from Ordered Samples

One of the first examples of using spin-labeling HF EPR to study macroscopic ordering was presented by Poluektov and co-workers. The authors observed interesting macroscopic ordering effects for different spin probes in methoxy-benzylidene-n-butylaniline (MBBA). The ordering of the domains was induced by an external magnetic field and the tube walls. 

More recently, a striking demonstration of the excellent resolution of HF EPR to orientations of spin labeled molecules was provided by a 250 GHz EPR study 

The same cholestane nitroxide probe has been recently utilized by Mangels et al to study alignment of mixed l,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and l,2-dihexanoyl-5n-glycero-3-phosphocholine (DHPC) bicelles at 3.4 T magnetic field of W-band EPR. These studies demonstrated that the order parameter for cholestane in DMPC/DHPC bicelles containing 10% cholesterol is S = 0.64 + 0.04 which is very similar to that determined for mechanically aligned bilayers. 

3 Structural Studies Based on -Factor Measurements. - Accurate measurements of spin-label magnetic parameters provide important data about the structure of nitroxide radicals. These data are also required for the analysis of the molecular dynamics of spin probes. The high resolution of HF EPR makes possible accurate determination of the principal axis components of the -matrix and y4-tensor from powder pattern spectra eliminating the need to prepare and study single crystals. 

Spin-label EPR spectra in the rigid limit are even more informative and can be 

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