Mechanistic probe application

Trager, W.F. (1988). Isotope effects as mechanistic probes of cytochrome P450-catalysed reactions. In Synthesis and Application of Isotopically Labelled Compounds Proceedings of the Third International Symposium T.A. Baillie and J.R. Jones (Eds.) Amsterdam Elsevier 333-340. 

This book does not deal with glycosyl fluorides. Indeed, they are mostly synthesis reagents that are used as glycosyl donors in both chemical and enzymatic syntheses of polysaccharides. Another application of glycosyl fluorides is as mechanistic probes... 

As with any intermediate, a transient radical can be implicated from products formed in a reaction specific to the radical of interest. Experimentally, this is the basis of so-called mechanistic probe studies. An application of this method might employ, for example, 6-bromo-l-hexene as a probe for a radical intermediate as shown in Figure 4.3. If the 5-hexenyl radical is formed as a transient with an adequate lifetime, then cyclization of this radical to the cyclopentyhnethyl radical could eventually give the cyclic product, and detection of the cyclic product provides evidence that a radical was formed. The mechanistic probe approach is deceptively simple, however. To be useful, one must exclude other possibilities for formation of the rearranged product and demonstrate that the transient was formed in the reaction of interest and not in a side reaction. The latter is especially difficult to demonstrate, and, unfortunately, some mechanistic probe studies that seemingly provided proof of radical intermediates were later found to be complicated by radical-forming side reactions. 

The practical applications of chemiluminescent reactions are numerous. They can function as sources of light, markers, analytical systems, and mechanistic probes, to name only a few. These applications, and the very spectacular nature of the phenomena, will work together to encourage further examinations of these reactions. 

Parahydrogen has been utilized as a mechanistic probe in a number of reactions involving hydrogen. The application of... 

Coenzyme A (CoA) is a cofactor that has been estimated to be used by about 4% of all enzymes, although more recent analysis of the BRENDA database (http //www.brenda.uni-koeln.de/) suggests the number may be closer to 9% (1). The biochemical pathways and processes involving CoA thioesters are diverse and widespread, whereas the kinds of reactions involved primarily follow the inherent reactivity of the thioester functionality. This article provides a brief overview of CoA biosynthesis and a summary of the common types of reactions of CoA thioesters. Also presented is a brief introduction to structural studies and a more extensive description of some types of analogs of natural CoA thioesters that have been employed as mechanistic probes for CoA using enzymes. The application of CoA derivatives and CoA biosynthetic enzymes for the tagging of carrier proteins and carrier protein fusions is also described. 

Mishra PK, Drueckhammer DG. Coenzyme A analogues and derivatives synthesis and applications as mechanistic probes of coenzyme A ester-utilizing enzymes. Chem. Rev. 2000 100 3283-3309. 

Lithium ( Li) (1=1, 312). A review with 117 references was given on the applications of Li NMR spectroscopy and imaging in biology and experimental medicine. The interest derives primarily from the clinical use of Li salts to treat mania and manic-depressive illness. One area of investigation is ionic transport across the cellular membrane and compartmentation, so as to elucidate the mechanisms of therapeutic action and toxicity in clinical practice. The second is the development of a noninvasive, in vivo analytical tool to measure brain Li concentrations in humans, both as an adjunct to treatment and as a mechanistic probe. 

Nitroxides are persistent free radicals [1] which can often be isolated and handled as kinetically stable species. Nitroxides react rapidly with carbon free-radical intermediates [2] with well-characterized rate constants [3], and can thus be used as kinetic and mechanistic probes, as well as to trap carbon radicals in synthetic processes. They are easily oxidized or reduced, and thus have a rich redox chemistry that has been utilized for a variety of oxidations. As nitroxides have an unpaired electron, they are paramagnetic and thus ESR active, making them valuable as spin labels for biomolecules [4] and as spin traps for transient radicals [5]. In addition, nitroxides have been developed as organic ferromagnetic materials [6]. The synthesis of nitroxides has been reviewed in 1994 [7]. This review will focus on the synthetic applications of nitroxides. 

The NMR has provided an invaluable tool in the elucidation of both the static and dynamic stereochemistry of metal complexes. Applications to both diamagnetic and paramagnetic species have been rewarding. In particular, the unique role of NMR as a mechanistic probe for the elucidation of fluxional processes in polynuclear metal carbonyl complexes should be noted. 

Studies on the formation and reactivity of P-centered radicals continue to be a versatile source of mechanistic information and reactions of interest in synthetic chemistry. Various new persistent or stable P-centered radicals have been described and could find applications as paramagnetic probes. The possibility of influencing the properties of organic free radicals bearing an appropriately located phosphorus group should find interesting applications. 

Measuring the pressure dependence of the exchange rate constant leads to activation volumes, AV and this technique has become a major tool for the mechanistic identification of solvent exchange mechanisms (8,16,17). In the last 25 years high-pressure, high-resolution NMR probes were developed which allow the application of all NMR techniques described to pressures up to several hundreds of mega Pascals (18). 

DR. WILLIAM WOODRUFF (University of Texas) I would like to make a comment not so much on your paper as on mechanistic photochemistry in general. I think most of us would agree that if we are going to draw mechanistic conclusions, we really need to know what the structures of the reactants and products are. One of the problems in photochemistry is that we generally do not know the structure of the reactant, which is the excited state. There aren t very many structure-specific probes in solution, in fact, none below about the millisecond time scale where esr and NMR cease to be applicable. In our laboratory, we have been able to obtain the resonant spectra of excited states. In two of the three kinds of systems that we have observed so far, the structures of the excited states are not predictable in a straightforward way, either from the ground state structures or from calculations. 

Arnett and coworkers later examined the reaction of lithium pinacolone enoiate with substituted benzaldehydes in THE at 25 °C. The determination of the heat of reaction indicated that the Hammett p value for the process is 331. Although the aldol reaction was instantaneous in THF at 25 °C, the reaction with o- or p-methylbenzaldehyde could be followed using a rapid injection NMR method in methylcyclohexane solvent at —80 °C. Application of Eberson s criterion based on the Marcus equation, which relates the free energy of ET determined electrochemically and the free energy of activation determined by kinetics, revealed that the barriers for the ET mechanism should be unacceptably high. They concluded that the reaction proceeds via the polar mechanism . Consistent with the polar mechanism, cyclizable probe experiments were negative . The mechanistic discrepancy between the reactions of benzaldehyde and benzophenone was later solved by carbon kinetic isotope effect study vide infraf. ... 

Since it is well established that the P-chiral phosphorothio-ates serve as effective probes in mechanistic studies for the phos-phoryl group transfer enzymes /16/, we turned our attention to the application of diastereomeric 8 (B=Thy, Ar=pN02C6Hi -,/17f) to elucidate the mode of action of spleen phosphodiesterase (SPDE, EC 3.1.1. 18). This enzyme splits the phosphodiester bonds to yield nucleoside 3 -phosphates. In the case of it was expected that its SPDE-catalyzed hydrolysis in 180 H 2O medium leads to P-chiral thymidine 3 - 180 phosphorothioate. On the contrary to our expectation the main product of this reaction was thymidine cyclic 3 ,5 - Rp phosphorothioate (10) /6/. By treatment of 8 under the same conditions, but in the absence of the enzyme, no trace of J 0 was detected. ... 

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