Electron spin resonance studies nitroxide free radical

Normally, biological macromolecules which lack unpaired electrons can not be studied by e.s.r. because they do not resonate. These macromolecules can however be spin labelled (spin labelling involves the attachment of a stable and unreactive free radical to the macromolecule) and studied. Thus, lateral diffusion of glycerophosphatides in plasma membranes has been studied by labelling glycerophosphatides with a stable nitroxide free radical. 

Electron spin resonance (ESR) spectroscopy has been widely used to obtain information about the molecular dynamics of polymers. The method requires the introduction of a stable free-radical reporter group, such as a nitroxide, into the system. Nitroxide spin labels can be covalently attached to the polymer of interest, and can therefore serve as probes of the local backbone dynamics of the polymer, providing information on the local orientation, stracture, dynamics, and enviromnent. " A commonly used nitroxide is shown in Eig. 1. Depending on the ESR frequency, motion on time scales between 10 and 10 ° s may be investigated by this method, making it ideal to study the dynamics of macromolecules and macromolecular structures or assemblies. 

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