ABSTRACT
Partial molecular ordering of the spin label, either in an anisotropic medium such as a liquid crystal or lipid membrane or in segmental attachment at a binding site in a macromolecule, restricts the angular amplitude of rotational diffusion of the nitroxide. In the fast-motion regime, motional averaging is incomplete; both residual spectral anisotropy and linewidths then depend on the angle between the director of molecular ordering and the static magnetic field. These are characterized by orientational order parameters. In the slow-motion regime, molecular ordering is introduced into stochastic-Liouvillelineshape simulations via the torque exerted by an orientation potential. A valuable summary is given in the concluding section of this chapter.
