Subject spin-probing

Metal- and proton-exchanged zeolites have been recently attracted much attention because of their selective catalytic activity to efficiently reduce nitrogen monoxide (NO) by hydrocarbon in an 02-rich atmosphere [1]. The formation of nitrogen dioxide (NO2) from NO and O2 has been suggested as an important step in the selective reduction [2, 3] NO2 is one of rare stable paramagnetic gaseous molecules and has been subjected to electron spin resonance (ESR) studies [4-7]. The ESR parameters and their relation/to the electronic structure have been well established [4] and NO2 can be used as a "spin probe" for the study of molecular dynamics at the gas-solid interface by ESR. 

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. 

The standard NMR experiment for a dipolar nucleus like P involves subjecting the probe to an external magnetic field with constant field strength and variable radiofrequency v. In such a field, the phosphorus nuclei can either align themselves parallel or antiparallel to the external field. This can be represented as N or N (see Fig. 1.1). The nuclear magnetic moment // is directly proportional to the spin I of the nucleus. 

Nitroxides can be used either as spin probes or as spin labels. Spin probes are subject to non-covalent interactions with the system under study. In many cases, spin probes very similar to one component of the system, e.g., spin-labeled lipids, are introduced. In contrast, spin labels are covalently linked to a complex structure, in many cases to a specific site, e.g., of a protein (SDSL) [10]. An alternative approach includes spin labeling of ligands interacting with the protein under study [11, 12]. 

ESR is a widely used spin probe technique for the study of nitroxide radicals in macromolecular systems. The structure of stable nitroxide radicals is rather diverse, although all of them contain a paramagnetic fragment =N—O as a sttuctural element. Hundreds of these radicals have been synthesised. The following properties make nitroxide radicals ideal subjects in polymer smdies ... 

The spin-label and spin-probe techniques have been used to study a wide variety of polymers, both in bulk and in solution, and several reviews of the subject have been published [1-3]. The object of this article is to provide a brief outline of the theoretical background, with particular reference to spin-labelling, bdbre discussing some recent applications of the technique to polymers in heterogeneous systems. 

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