Spin label or probe

The shape and width of the electron spin resonance (ESR) spectrum of a spin label or probe is sensitive to the mode and rate of rotation of the radical. Thus, examination of the ESR spectrum of the labelled or probed polymer can yield information on the dynamics and relaxations of the polymer and is therefore complementary to such techniques as mechanical, NMR and dielectric relaxation measurements. 

The above discussion has considered the spin-label or spin-probe to be in a homogeneous system with a single correlation time. In some cases, however, the observed ESR spectrum is of neither the fast- nor the slow-motion type but is a mixture of the two (Figure 9.3). Such composite spectra arise when the spin-labels or probes simultaneously occupy two motionally distinct environments, one of which is significandy more restricting than the other. This behaviour may occur in situations such as adsorption of a polymer onto a solid [9], phase separations of polymers or polymer blends [25], polymer networks [26] or any other system where more than one phase is present. 

Electron spin resonance (ESR) spectroscopy of labeled polymers can be employed to determine the mobility of a polymer molecule in a blend. The polymer can be covalently bonded with a spin label (nitroxyl radical) or a spin probe can be blended in the polymer blend matrix. This method allows for observation of segmental motions at a length scale smaller than that typically observed with the glass transition. The ESR spectra depends on the environment of the spin labels or probes and can be used to assess phase behavior. A review of ESR spectroscopy applied to polymer blends can be found in [416]. 

Both ESR and fluorescence spectroscopy give an indirect measure of motion in polymers as they make use of either spin label or probe methods. In the case of ESR, nitroxyl radicals dispersed (spin probe) in a polymer matrix or covalently bonded to the polymer chains (spin label) are employed to probe the local environment. Therefore, ESR spectra provide information on molecular motion and microstructure of polymer matrices. Similarly, fluorescent probes are sensitive to the glass structure. This is because photon emission increases when non-radiative processes are hindered by lack of mobUity of the probe. Interestingly, studies on poly (vinyl acetate) (PVAc) have shown that changes in the fluorescence intensities with aging time and temperature follow closely those observed by volumetric relaxation [85]. 

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-... 

The name spin trapping was coined by Janzen, and derives from analogy with the use of stable nitroxyls as spin labels (or spin probes ) that provide spectroscopic information regarding their microscopic environment, a procedure pioneered by McConnell et alP ... 

There have been some attempts to use spin labels to probe for differences in the properties of membranes of normal and tumour cells. Thus mouse embryo fibroblasts have been labelled with C (n = 4, m = 10) and it was found that on transformation with oncogenic DNA or RNA... 

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. 

S. Weber and E. Schleicher on the example of flavoproteins which play a role in both chemically and light-activated electron transfer processes. Chapter 3 on synthetic polymers by D. Hinderberger argues that careful analysis of mundane nitroxide spin label or spin probe CW EPR spectra can reveal a lot of information which is hard to obtain by any other characterization technique. 

Fig. 9.1. Typical triplet spectra of nitroxide radicals in most of spin-label or spin-probe studies (upper) A, rigid state B, rapid motion.

Spin probes are free radicals or paramagnetic transition metal ions that are admixed to the system of interest, while spin labels are stable free radicals that are covalently bound to a macromolecule of interest. Spin labeling thus involves modification of the synthesis of the material and some modification of its structure. Suitable spin probes are often commercially available, so that spin probing typically requires less effort. If a question can be answered either by labeling or probing, probing is thus the technique of choice. 

A stable radical with the desired spectroscopic properties is synthesised and then, allowed to interact covalently or non-covalently with the macromolecule. The radical is often called a spin label or spin probe and acts as a monitor of motion. 

The CuAAc methodology was used to prepare a variety of nucleoside bioconjugates bearing spin labels or fluorescent probes (compounds 27-30) [40 -43] and was also used for the F-labeling of nucleosides (compound 27G) (Figure 10.5) [44]. 

Less frequently used at present is electron spin resonance spectroscopy, which is based on the use of spin probes as model componnds or covalent spin labeling of drugs. Microviscosity and micropolarity of the molecnlar environment of the probe can be derived from electron spin resonance spectra. Moreover, the spectra allow us to differentiate isotropic and anisotropic movements, which result from the incorporation of the probe into liposomal structures. Quantitative distribution of the spin probes between the internal lipid layer, the snrfactant, and the external water phase is to be determined noninvasively. On the basis of the chemical degradation of drugs released from the lipid compartment, agents with reductive features (e.g., ascorbic acid) allow us to measure the exchange rate of the drugs between lipophilic compartments and the water phase [27,28]. 

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