Electron spin resonance studies nitroxide radical

Electron Spin Resonance.—Nitroxide radicals of varying structiu-e have been employed in studies of micelle structure and mobility. The basic spectrum is a triplet due to N-electron coupling, which may show hyperfine coupling to 3-C-H in appropriate cases. On micelle formation or incorporation of the probe the spectrum normally broadens because of reduction in the rotational correlation time and shows enhanced broadening and change in positions of the high-field line. Cationic micelles incorporate the probe (10) with an association constant of 3 x 10 and at low surfactant concentrations there is... 

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. 

Comparison of equation (9.21) with equations (9.9) and (9.6) reveals the resemblance of nuclear and electron resonance methods. In equation (9.21), P is the so-called Bohr magneton, and the electron g value is a porportionality constant which is dependent upon the magnetic species and its environment. Only unpaired electrons give rise to ESR spectra, and this is why, except in the study of lipid autoxidation, lipids can only be studied through the use of spin labels, which are stable, usually nitroxide, radicals (Fig. 9.26). 

Nitroxide radical spin labels can be used to measure inter-spin distances by pulsed electron paramagnetic resonance (EPR) spectroscopy, and to allow study of structure and folding of DNA and RNA ohgonucleotides [27-30]. Spin-labeling of nucleobases by (TuAAC in solution has first been demonstrated using the radical azide 4-azido-2,2,6,6-tetramethylpiperidine 1-oxyl (4-azido-TEMPO) [31], Alkyne modifications at position 7 of 7-deazapurines and C5-modified pyrimidines allowed attachment of the spin label in the major groove of duplex DNA [31]. In the same year, click chemistry on solid support to introduce spin labels into DNA was reported [32]. 

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. 

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