Spin Label ESR Spectroscopy and Molecular Mobility in Biological Systems

Abstract

Electron spin resonance (ESR) spectroscopy of spin-labeled biomolecules has proved to be extremely useful in the study of molecular mobility and interactions in biological systems. As will be seen later, the spin label spectrum is sensitive to motions on a time scale determined by the anisotropy of the ¹⁴N hyperfine splitting of the nitroxide free radical group: τ _motion ⪝ h/(A_zz − A_xx) ∼3 × 10⁻⁸ s. If motions are appreciably slower than this they will appear static on the ESR time scale and the spectra will be a superposition of all those corresponding to the different stages in the molecular motion. Gradually increasing rates of motion will give rise to a broadening of these individual spectral components and then a final collapse to the time-average mean value. This principle of motional averaging forms the basis of the utility of the spin label method. It is most conveniently described in terms of the Bloch equations which are introduced in the following section.