Electron spin resonance active sites

In order to investigate the active sites of these proteins, laccases I and III were subjected to ESR (electron spin resonance) spectroscopic analysis. The ESR spectra shown in Figure 5 indicate clear differences in peaks 2 and 6 which support the concept that the copper atoms in laccases I and III have different conformations in each molecule. Furthermore, immunological similarity between laccases I and III was also investigated. Antibody specific for laccase III was prepared from rabbit serum by conventional methods. When applied to Ouchterlony diffusion plates containing laccase I, no precipitation lines developed (Figure 6). This result showed that there were no conserved epitopes on the surfaces laccases I and III. 

The activity of lyophilized HL-ADH in several solvents increased by orders of magnitude upon addition of small amounts of water to solvents of dielectric constant e from 1.9 to 36 (Guinn, 1991). Enzyme flexibility, as measured by electron spin resonance (ESR) spectroscopy with a spin label at the active site, did not depend on water content but on of the pure solvent HL-ADH turns less flexible with decreasing . 

In order to characterize the active site structure of Ca ATPase from sarcoplasmic reticulum, we have employed Gd + as a paramagnetic probe of this system in a series of NMR and EPR investigations. Gadolinium and several other lanthanide ions have been used in recent years to characterize Ca + (and in some cases Mg2+) binding sites on proteins and enzymes using a variety of techniques, including water proton nuclear relaxation rate measurements (35,36,37), fluorescence (38) and electron spin resonance (39). In particular Dwek and Richards (35) as well as Cottam and his coworkers (36,37) have employed a series of nuclear relaxation measurements of both metal-bound water protons and substrate nuclei to characterize the interaction of Gd + with several enzyme systems. 

Other metal ions have been reported. NHase from the Myrrothecium verrucaria contain a Zn + ion in the active site (Marier-Greiner et al, 1991). The electron spin resonance (ESR) spectra of nitrile hydratase from Bacillus pallidas Dac521 have shown no unpaired electrons associated with a metal center suggesting that neither Fe + nor Co + atoms were cofactors. However, becanse ESR cannot detect Zn + or NE+, the possible presence of these metal ions cannot be excluded (Cramp and Cowan, 1999). 

In addition to the structure in the dehydrated state, the structure of supported vanadia catalysts under redox reaction conditions is directly related to the catalytic performance. Vanadia catalysts are usually reduced to some extent during a redox reaction, and the reduced vanadia species have been proposed as the active sites [4, 19-24]. Therefore, information on the valence state and molecular structure of the reduced vanadia catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature programmed reduction (TPR) [25-27], X-ray photoelectron spectroscopy (XPS) [21], electron spin resonance (ESR) [22], UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [18, 28-32], X-ray absorption fine structure spectroscopy (XAFS) [11] and Raman spectroscopy [5, 26, 33-41]. Most of these techniques give information only on the oxidation state of vanadium species. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia [4, 29, 42], it has been very difficult to detect reduced supported... 

This oxidation state is not well known in simple compounds, but electron-spin resonance spectra suggest that many oxide lattices containing chromium may, when suitably oxidized or reduced, contain Cr5 + such ions are believed to be the active sites in polymerization of ethylene to give polyethylene over chromium-containing alumina catalysts (cf. Chapter 24). 

Based on X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), electron spin resonance (ESR), Mbssbauer, and extended X-ray absorption fine structure spectroscopy (EXAFS) , van Veen and collaborators concluded that the thermal treatment at temperatures where catalytic activity is maximum ( 500-600°C) does not lead to complete destruction of the macrocycles, but rather to a ligand modification which preserves the Me-N4 moiety intact. Furthermore, the stability of this catalytic site is improved because the reactive parts of the ligands are bound to the carbon support and thus are no longer susceptible to an oxidative attack. Thermal treatments at higher temperatures (up to 850°C) led to some decomposition of the Me-N4 moiety, and thus to a decrease of the catalytic activity, and to the reduction of some of the ions to their metallic state. 

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