Basis of EPR Spectroscopy

The performance of a solid catalyst typically depends on structural properties of the active sites and/or the support. These properties are frequently determined by the conditions of catalyst synthesis and/or the activation procedure. Knowing how properties of final catalysts are influenced by peculiarities of synthesis and conditioning procedures is of interest because, on the basis of such knowledge, synthesis procedures might be optimized to produce desired catalyst properties. The following two examples illustrate the versatility of EPR spectroscopy for monitoring catalyst syntheses in both gas-solid and liquid-solid systems. 

The classic blue copper sites in plastocyanin and azurin exhibit essentially identical EPR spectra, with approximately axial (gj >= gy) EPR signals. This argues that the long 3.0 Cu-0 A carbonyl oxygen makes little contribution to the electronic structure of azurin, consistent with other spectroscopy and the fact that the relatively compact 0 2p orbitals would be expected to have little contribution to bonding at this distance. On the basis of EPR, the perturbed blue copper sites can be divided into 2 classes (1) those which exhibit a rhombic EPR signal (i.e. A gi = g — g, > 0.01, as in cucumber basic protein, nitrite reductase and stellacyanin, Figure 9)159,160 2) those which are perturbed, but still... 

Because some of the forms of heme proteins are intrinsically paramagnetic, electron paramagnetic resonance (EPR) has been used extensively to study the stracture and structure-function relations of these proteins. By far the majority of these investigations are done using continuous-wave (cw) EPR at the conventional X-band microwave frequency ( 9.5 GHz), and these cw-EPR studies have formed the basis of many excellent reviews [4-6]. In the last two decennia, the field of EPR spectroscopy has, however, been revolutionized by many technical developments. Indeed, the construction of pulse-EPR spectrometers, die accompanying developments of the pulse-EPR methodology, and the (ongoing) development of... 

The complexity of the chemical interactions of NO and Hb points to the importance of specific micropopulations of Hb in the chemistry of SNO-formation. Certain reactive micropopulations now clearly appear to be responsible for S-nitrosylation, but flieir identity remains to be fully elucidated [52]. A general model that rationalizes the dependence of SNO-formation and release on the basis of the different micropopulations that exist under different physiological conditions remains to be advanced. Reaching this goal will require the characterization of micropopulations of active species in the presence of a background (majority) of inactive species. This situation is very familiar in EPR spectroscopy, where the paramagnetic species probed are typically a minority species in the sample. The selectivity of EPR for trace nitrosylated or oxidized hemes, especially without interference from deoxy- or oxy-Hbs, continues to be the salient advantage of EPR spectroscopy in this work. 

The spin-Hamiltonian concept, as proposed by Van Vleck [79], was introduced to EPR spectroscopy by Pryce [50, 74] and others [75, 80, 81]. H. H. Wickmann was the first to simulate paramagnetic Mossbauer spectra [82, 83], and E. Miinck and P. Debmnner published the first computer routine for magnetically split Mossbauer spectra [84] which then became the basis of other simulation packages [85]. Concise introductions to the related modem EPR techniques can be found in the book by Schweiger and Jeschke [86]. Magnetic susceptibility is covered in textbooks on molecular magnetism [87-89]. An introduction to MCD spectroscopy is provided by [90-92]. Various aspects of the analysis of applied-field Mossbauer spectra of paramagnetic systems have been covered by a number of articles and reviews in the past [93-100]. 

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