Electron spin label line widths

Selected entries from Methods in Enzymology [vol, page(s)] Electron paramagnetic resonance [effect on line width, 246, 596-598 motional narrowing spin label spectra, 246, 595-598 slow motion spin label spectra, 246, 598-601] helix-forming peptides, 246, 602-605 proteins, 246, 595 Stokes-Einstein relationship, 246, 594-595 temperature dependence, 246, 602, 604. 

Just as chemical exchange can broaden proton NMR spectra (Section 13.5), electron exchange between two radicals can broaden EPR spectra, therefore the distance between two spin probe molecules maybe measured from the line widths of their EPR spectra. The effect can be used in a number of biochemical studies. For example, the kinetics of association of two polypeptides labeled with the synthetic amino acid 2,2,6,6,-tetramethylpiperidine-l-oxyl-4-amino-4-carboxylic acid (6) can be studied by measuring the line width of the EPR spectrum of the label as a function of time. Alternatively, the thermodynamics of association may be studied by examining the temperature dependence of the EPR line width. 

---