Electron paramagnetic resonance structural information from

Electron paramagnetic resonance (EPR) and NMR spectroscopy are quite similar in their basic principles and in experimental techniques. They detect different phenomena and thus yield different information. The major use of EPR spectroscopy is in the detection of free radicals which are uniquely characterised by their magnetic moment that arises from the presence of an unpaired electron. Measurement of a magnetic property of a material containing free radicals, like its magnetic susceptibility, provides the concentration of free radicals, but it lacks sensitivity and cannot reveal the structure of the radicals. Electron paramagnetic resonance spectroscopy is essentially free from these defects. 

The discussion of activity from X-ray data in conjunction with kinetic data is also difficult because, apart from considerations of dynamics, these techniques do not provide the essential knowledge about the energy states of given atoms or groups. It is necessary to inspect the electronic structure of at least certain regions of the protein. Methods exist for this inspection, and these include electron paramagnetic resonance, ultraviolet, circular dichroism, Raman and Mossbauer spectroscopies. The full understanding of activity can only come when the information derived from all available methods is assimilated and rationalized. 

Valuable spectroscopic studies on the dithiolene chelated to Mo in various enzymes have been enhanced by the knowledge of the structure from X-ray diffraction. Plagued by interference of prosthetic groups—heme, flavin, iron-sulfur clusters—the majority of information has been gleaned from the DMSO reductase system. The spectroscopic tools of X-ray absorption spectroscopy (XAS), electronic ultraviolet/visible (UV/vis) spectroscopy, resonance Raman (RR), MCD, and various electron paramagnetic resonance techniques [EPR, electron spin echo envelope modulation (ESEEM), and electron nuclear double resonance (ENDOR)] have been particularly effective probes of the metal site. Of these, only MCD and RR have detected features attributable to the dithiolene unit. Selected results from a variety of studies are presented below, chosen because their focus is the Mo-dithiolene unit and organized according to method rather than to enzyme or type of active site. 

It is clear from the above considerations that the most often used and widely reported XRD analysis becomes insufficient to provide detailed information about amorphous phases. The benefits of in-depth studies of mechanochemical reaction mechanisms by selection of appropriate analytical tools, able to provide data on a short-range (local) structural scale, such as nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy, infrared spectroscopy (IR), Raman spectroscopy, etc., were demonstrated by the pioneering work of Senna, Watanabe and coworkers (Watanabe et al., 19%, 1997 Senna, 1997). In those cases, the synthesis of selected complex oxide systems have been studied from starting mixtures comprising typacaUy hydroxide and oxide compounds extensive data on these studies can be foimd in Avvakumov et al. (2001). 

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