Spin-label probes, pure nitroxide

Monolayer Studies of Pure Nitroxide Fatty Acid Spin-Label Probes... 

To understand the behavior of the mixed host-probe films, it is essential to understand the behavior of the pure films of both components. We previously investigated in detail the behavior of the pure films of 12-nitrox-ide stearate [2-( 10-carboxydecyl)-2-hexyl-4,4 -dimethyl-3-oxazolidinyloxyl] including its extraordinary temperature dependence (7, 8, 16). In this paper we extend our investigations to the pure films of other nitroxide stearic acid (and methyl ester) probes where the oxazolidine ring is attached to various carbon atoms of the stearic acid (or ester) hydrocarbon chain. This series of spin-label probes is one of those most extensively used to study cell membrane structure. It was used to define, among other things, an order parameter establishing the fluidity (17) and polarity profiles (18) of lipid bilayers. 

The stability of nitroxides will be well known to readers acquainted with the spin-labelling technique (Berliner, 1976), but it must be recognized that nitroxides employed as spin labels or spin probes are almost invariably di-t-alkyl nitroxides. Diaryl and many aryl t-alkyl nitroxides are also sufficiently persistent to be isolated, and it has recently been shown that several acyl t-alkyl nitroxides can also be obtained pure (Perkins and Ward, 1973 Alewood et al., 1978). However, other nitroxides are less persistent. Monosubstituted nitroxides, RN(H)Q-, rapidly disproportionate to nitroso-... 

Hindered nitroxides are a class of free radicals that are unusually long-lived. The presence of methyl groups y to the radical center decreases the rate of self reaction to such an extent that the radical becomes kinetically stable. Hindered nitroxides can be obtained as crystalline solids or pure liquids, which in the absence of other materials are very stable. These materials have found extensive use as spin labels and spin probes. Indeed, almost all spin-label studies have employed nitroxides spin-probe studies have used either nitroxides or paramagnetic metal ions. 

However, in many cases the problem dictates which of the two approaches is more appropriate. Generally, covalently bound labels are suited for characterizing structure and dynamics of individual macromolecules. Examples are dynamics of a polymer chain end or the distribution of end-to-end distances of a polymer chain. Spin probes are suited for characterizing the collective behavior of molecules or, to use a more fashionable term, supramolecular behavior. For example, in a pure polymer, spin probes reside in the voids that make up the free volume. The characteristic tan-perature of nitroxide mobility, q (see Chapter 1) for spin probes thus depeuds ou the size of the probe and the size distribution of free volume voids. As this size distribution in turn depends on the structure of the polymer, the relation of Tsog to the glass transition temperature, T, of the polymer is not linear, even when comparing the same spin probe in different polymers. However, if the same probe is used, q does exhibit some correlation with T. Therefore, it is often (somewhat incorrectly) being referred to as an ESR T. ... 

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