Active sites catalytic, spectroscopic studies

Spectroscopic studies have been instrumental in elucidating the catalytic mechanism of Ni-Fe hydrogenases. A great deal of controversy concerning this mechanism arises from the fact that, as the as the X-ray crystallographic analysis has shown, there are at least three potential redox-active species at the enzyme s active site the thiolate ligands (75) and the Fe (65) and Ni (9) ions. 

The [NiFe] hydrogenase from D. gigas has been used as a prototype of the [NiFe] hydrogenases. The enzyme is a heterodimer (62 and 26 kDa subunits) and contains four redox active centers one nickel site, one [3Fe-4S], and two [4Fe-4S] clusters, as proven by electron paramagnetic resonance (EPR) and Mosshauer spectroscopic studies (174). The enzyme has been isolated with different isotopic enrichments [6 Ni (I = I), = Ni (I = 0), Fe (I = 0), and Fe (I = )] and studied after reaction with H and D. Isotopic substitutions are valuable tools for spectroscopic assignments and catalytic studies (165, 166, 175). 

The discovery of a new heterodinuclear active site in [NiFe] hydro-genases opens the way for the proposal of catalytic cycles based on the available spectroscopic data on the different active site redox states, namely EXAFS studies that reveal that the Ni-edge energy upon reduction of the enzyme supports an increase in the charge density of the nickel (191). 

Despite several decades of studies devoted to the characterization of Fe-ZSM-5 zeolite materials, the nature of the active sites in N20 direct decomposition (Fe species nuclearity, coordination, etc.) is still a matter of debate [1], The difficulty in understanding the Fe-ZSM-5 reactivity justifies a quantum chemical approach. Apart from mononuclear models which have been extensively investigated [2-5], there are very few results on binuclear iron sites in Fe-ZSM-5 [6-8], These DFT studies are essentially devoted to the investigation of oxygen-bridged binuclear iron structures [Fe-0-Fe]2+, while [FeII(p-0)(p-0H)FeII]+ di-iron core species have been proposed to be the active species from spectroscopic results [9]. We thus performed DFT based calculations to study the reactivity of these species exchanged in ZSM-5 zeolite and considered the whole nitrous oxide catalytic decomposition cycle [10],... 

The microenvironment of the micellar core inferred from fluorescent probes is said to be similar to some organic media (Turner and Brand, 1968 Cordes and Gitler, 1973). Similar conclusions have been obtained by other spectroscopic means (see previous sections). The active site of an enzyme is usually quite hydrophobic and the number of water molecules at the active site is limited. Therefore, it is very useful to study the behavior of the catalytic groups in organic media in relation to micellar and enzymatic catalysis. 

There are several recent reviews of the molybdenum and tungsten enzymes [4-6,23,26-36], In this chapter, we first define the metallocofactors and offer a compilation of the enzymes and their diverse activities. We then focus on the active-site structures, highlighting the confluence of crystallographic and spectroscopic studies. This is followed by a discussion of pertainent spectroscopic, structural, reactivity, and theoretical model studies. We then turn our attention to the mechanisms of catalytic activity of the molybdenum and tungsten enzymes. 

In any gas/solid catalytic system, the reactant must first be adsorbed on the catalyst surface. This is why surface characterization is so important. Studying the adsorption of various molecules under controlled conditions yields information regarding the catalyst surface area, pore volume, and pore size distribution [80]. The key factor here is accessibility. Sophisticated spectroscopic analysis of single-crystal models can tell us a lot about what goes on at the active site, but the molecules must get there first. 

Davydov, A. A., Kantcheva, M. and Chepotko, M. L. FTIR spectroscopic study on nickel(II)-exchanged sulfated alumina nature of the active sites in the catalytic oligomerization of ethene. Catal. Lett., 2002, 83, 97-108. 

Deng H, Lewandowicz A, Schramm VL, Callender R (2004) Activating the phosphate nucleophile at the catalytic site of purine nucleoside phosphorylase a vibrational spectroscopic study. J. Am. Chem. Soc. 126 9516-9517... 

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