ABSTRACT
EPR spectra are sensitive to the rate of exchange between sites that correspond to distinct spectral positions. Exchange between different angular orientations of a spin label gives the characteristic sensitivity of the spectral lineshapes to rotational diffusion. Fast rotation causes motional narrowing of nitroxide EPR spectra. This can be described by time-dependent perturbation theory, giving analytical results. Spin-label spectra in the slow-motional regime gradually approach the rigid limit of nitroxide EPR spectroscopy, as the rotational correlation time lengthens. These lineshapes are described by solution of the stochasticLiouville equation that incorporates diffusion; this must be performed numerically. A valuable summary is given in the concluding section of this chapter.
