Electron paramagnetic resonance spectroscopic methods

Munck, E. Surerus, K. K. Hendrich, M. P. Combining Mossbauer Spectroscopy with Integer Spin Electron Paramagnetic Resonance. In Methods in Enzymology, Vol. 227 Physical and Spectroscopic Methods for Probing Metal Ion Environments in Metalloproteins, Riordan, I. F. Vallee, B. L., Eds. Academic Press New York, 1993 Chapter 17, pp 463-479. 

Electron paramagnetic resonance (epr) spectroscopic methods are used for the detection and identification of species that have a nett electronic spin radicals, radical ions, etc. It is extremely sensitive, capable of detecting species down to concentration levels of 1 x 10 12 moles dm "3, and produces spectra that are distinctive and generally easily interpreted. Consequently, the technique has found extensive application in electrochemistry since the late 1950s. In order to understand epr, it may be helpful to review some fundamental concepts. 

Due to the complexity of DOM fractionation has revealed more detailed information on the structural subunits prior to the application of advanced analytical methods. Most effective is the combination of different spectroscopic methods using UV-vis absorbance, fluorescence, 1H- and 13C-nuclear magnetic resonance, and Fourier transform-infrared (FT-IR) spectroscopy. In some studies, also electron paramagnetic resonance spectroscopy (EPR) is used (e.g., Chen et al., 2002). 

The conventional method for determining cation ordering and site populations within a crystal structure is by diffraction techniques using X-ray, electron and neutron sources. For determining site occupancies of transition metal ions, these methods have been supplemented by a variety of spectroscopic techniques involving measurements of Mossbauer, electron paramagnetic resonance (EPR or ESR), X-ray absorption (EXAFS and XANES), X-ray photoelectron (XPS), infrared and optical absorption spectra. 

Calas, G. (1988) Electron paramagnetic resonance. In Spectroscopic Methods in... 

In the case of surface sites incorporating atoms, the valence states and their eventual variation in the course of the reaction can be studied by spectroscopic methods such as electron paramagnetic resonance (EP.R), XPS, ultra-violet photoelectron (UPS), IR, and near-edge X-ray (XANES) spectroscopies (Near-edge X-ray spectroscopy is abbreviated in some parts of the world, particularly in USA, as NEXASS). 

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. 

To understand the mechanism of water oxidation, it is necessary to characterize each of the S states. A variety of spectroscopic methods have been brought to bear on this problem. Electron paramagnetic resonance (EPR) and X-ray spectroscopies have been especially useful because these techniques allow the Mn complex to be probed directly. EPR spectroscopy has the restriction that the Mn complex must be paramagnetic to be studied. The S2 state is an odd-electron state, and EPR spectroscopy has been used extensively to study the Mn complex in the S2 state. X-ray spectroscopy has the advantage that any state of the Mn complex is observable. However, the successful application of EPR and X-ray spectroscopies requires that a specific S state be prepared in high yield in highly concentrated samples. 

We have used two spectroscopic methods to investigate the triplet state of localized diradicals. On the one hand, matrix electron paramagnetic resonance (EPR) spectroscopy was employed, which affords the zero-field splitting (zfs) parameters D and E and gives valuable information on the electronic structure [5-7]. On the other hand, time-resolved transient... 

Electron spin resonance (ESR), which is also termed as electron paramagnetic resonance (EPR), is another spectroscopic method to provide valuable information about the electronic structure of carbon nanotubes. 

As discussed further in the following sections, there are other variations of rapid mixing/quench methods in which the enzymatic reaction is terminated by freezing the reaction mixture with liquid isopropane. The frozen sample is then analyzed hy electron paramagnetic resonance (EPR), solid-state NMR, or other spectroscopic techniques such as resonance Raman spectroscopy that can accommodate a solid sample. Perhaps the major limitation for implementation of this methodology is the sensitivity of the spectroscopic method and the requirement for large amounts of enzyme. ... 

---