Nitroxide probe films

From N. K. Adam s observation that many bipolar amphiphatic molecules form gaseous expanded films, it is evident that these nitroxide probe films, with the possible exception of 5-NS and 5-NS(Me), behave as typical bipolar amphiphatic films. The shift to lower areas/molecule for 5-NS and 5-NS (Me) shows that, as the oxazolidine ring moves closer to the primary polar group, the bipolar nature of the molecule diminishes. Thus the carboxyl group and oxazolidine ring for these compounds effectively constitute one polar group. 

The molecular motion of redox couples within polymer-coated electrodes has recently been investigated by making use of both nitroxide spin probes and various cationic spin probes [94-97]. Spin probes, such as the nitroxide probe TEMPO (see Sect. 2.1.2) and its derivatives, have well-defined electrochemistry and their ESR spectra in viscous media exhibit effects due to incomplete rotational averaging of the g and hyperfine coupling constant tensors. Analysis of the spectra [98] allows deductions to be made concerning the molecular rotation. Such analysis has been performed for spin probes incorporated into various polymer films. 

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. 

Spin probes at the fast motional limit have also been utilized in studies of heterogeneous molecular assembly by HF EPR. Cramer et al. utilized the spin probe TEMPO (2,2,6,6-tetramethylpiperedine-l-oxyl) to investigate poly(butylacrylate) and poly(butylmethacrylate) films. The same spin probe was also used by Smirnov and co-workers to study rotational diffusion of the nitroxide in two phases of aqueous phospholipid dispersions with W-band EPR. Bakker and co-workers utilized another probe - 4-[N,N-dimethyl-lV-(n-hexadecyl)ammonium]-2,2,6,6-tetramethylpiperidinyl-iV-oxyl (HTAB ) to study absorption and aggregation of cetylpyridinium chloride and cetylpyridin-ium salicylate (CPSa) on silica nanoparticles. From W-band line width analysis it was concluded that CPSa on silica surfaces forms two coexisting aggregate phases, one which excludes HTAB and one in which HTAB is concentrated. 

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