Spin labels, free radicals

Wien, Morrisett, and McConnell23 have employed nitroxide free-radical spin labels instead of paramagnetic metal ions. They found that lysozyme covalently spin-labeled at histidine-15, which is not at the active site, broadens the proton resonances of bound N-acetyl-a-D-glucosamine, enabling a distance (of the order of 20 A) to be estimated, in agreement with the x-ray structure. 

Griffith OH, Waggnor AS, Nitroxide free radicals spin labels for probing bimolecular structure, Acc. Chem. Res. 1969 2 17-24... 

From the point of view of the biochemist, there are three major classes of samples that might be investigated with ESR spectroscopy (1) samples containing transition or lanthanide elements (2) samples containing free radicals (3) samples that have been extrinsically labeled by the introduction of nitroxide radical spin labels or spin probes. We provide an overview of these three subjects in this chapter. Basic references are given at the beginning of each section. The material described is by no means comprehensive the topics reviewed have become very massive subjects. Rather, we have tried to provide information, often from our own work and experience, that we hope will be of general interest. 

One other aspect of the photolysis of coordinate spin labeled derivatives is of interest. Nitroxides are good free radical scavengers (123). As a result, when methyl-cobalamin is photolyzed in the presence of a nitroxide, the methyl radical generated will react with the free nitroxide and cause disappearance of the ESR spectrum (123). However, once the nitroxide is coordinated it is no longer susceptible to attack by free radicals. Thus the nitroxyl function is quite well protected from approach by other species. 

Subczynski, W. K., J. Widomska, and J. B. Feix. 2009. Physical properties of lipid bilayers from EPR spin labeling and their influence on chemical reactions in a membrane environment. Free Radic. Biol. Med., 46, 707-718. 

A review of the application of ESR to the study of free radical polymerisation is given by Yamada and co-workers [146]. A survey of the application ESR spectroscopy spin label/probe methods in heterogeneous polymer systems is provided by Veksli and co-workers [147]. Spin probe methods allow the study of the MD of the polymer, its free volume, phase separation and phase morphology. 

It is quite common to observe that the lines in a nitroxide spectrum show variations in width. One feature which contributes to this is the anisotropy of the nitroxide grouping, as a consequence of which the high field component of the nitrogen triplet may be perceptibly broadened. This effect is particularly noticeable when free tumbling of the nitroxide molecule is restricted either by the viscosity of the solvent or when the radical is incorporated into a very large (e.g. polymer) molecule. This selective line broadening is, of course, one of the principal sources of information in spin-labelling experiments (Berliner, 1976). 

Sphingomyehn, IR spectrophotometry, 683-4 Spin delocahzation, polar effects in decomposition, 903 Spin labels, free radicals, 665 Spin-lattice relaxation oxidized functional groups, 695 poly (methylstyrene peroxide), 709 Spin trapping artemisinin ESR, 1291 free radicals, 665... 

Because nitroxyl radicals do not react with most organic functional groups, they have found wide application as radical traps, and in living free radical polymerizations. Nitroxyl radicals are also used as spin labels, and are formed in spin trapping by nitroso compounds and nitrones vide infra). 

Since RNA is diamagnetic, EPR studies of RNA require incorporation of unpaired electrons into the biopolymer. Nitroxides in five- or six-membered rings that are flanked by methyl groups are stable organic free radicals that are commonly used for spin-labeling (Fig. 15.1). For a free... 

Many free radicals in their electronic ground states, and also many excited electronic states of molecules with closed shell ground states, have electronic structures in which both electronic orbital and electronic spin angular momentum is present. The precession of electronic angular momentum, L, around the intemuclear axis in a diatomic molecule usually leads to defined components, A, along the axis, and states with A =0, 1, 2, 3, etc., are called , n, A, , etc., states. In most cases there is also spin angular momentum S, and the electronic state is then labelled 2,s+1 Id, 2,s+1 A, etc. 

---