ABSTRACT
Spin labels are stable nitroxide free radicals. They are used to label (macro) molecules of interest or as probes of their host environment. The nitroxide EPR spectrum is sensitive to the dynamic molecular environment, specifically via its rotational mobility. This chapter describes the basic features of spin-label EPR: g-values which determine the spectral position; nitrogen hyperfine couplings which determine the spectral splittings; spin–lattice relaxation which determines the saturation behaviour of the spectrum at high radiation powers; transverse relaxation which determines homogeneous linewidths and Lorentzian lineshapes; and Gaussian lineshapes that characterize inhomogeneous broadening from unresolved proton hyperfine splittings. Sensitivity of the spectral lineshapes to rotational diffusion for correlation times spanning both sides of the nanosecond regime is illustrated, as is the restriction of rotational amplitude that is characteristic of a confined or ordered environment. Valuable basic background to this introductory chapter, and to the subsequent chapters, can be found in the appendices at the end of the book.
