Kinetic probe

The most general method for synthesis of cyclic enamines is the oxidation of tertiary amines with mercuric acetate, which has been investigated primarily by Leonard 111-116) and applied in numerous examples of structural investigation and in syntheses of alkaloids 102,117-121). The requirement of a tram-coplanar arrangement of an a proton and mercury complexed on nitrogen, in the optimum transition state, confers valuable selectivity to the reaction. It may thus be used as a kinetic probe for stereochemistry as well as for the formation of specific enamine isomers. 

A new chapter (5) on reaction intermediates develops a number of methods for trapping them and characterizing their reactivity. The use of kinetic probes is also presented. The same chapter presents the Runge-Kutta and Gear methods for simulating concentration-time profiles for complex reaction schemes. Numerical methods now assume greater importance, since useful computer programs are available. The treatment of pH profiles in Chapter 6 is much more detailed. 

Gold(I) thiolates also undergo reactions with disulfides. An example is 2,2-dithio-bis(2-nitrobenzoic acid) (ESSE) used as a kinetic probe of sulfhydryl reactivity in metallothioneins [39-42]. ESSE is readily attacked by metal-bound thiolates. The reaction with the open chain form of AuSTm seems to occur in two stages. The first attack involves the terminal sulfides and the second attack involves the bridging thiolates [19] ... 

Kohler, J.U. and Bradley, J.S., A kinetic probe of the effect of a stabilizing polymer on a colloidal catalyst accelerated enantio selective hydrogenation of ethyl pyruvate catalyzed by poly(vinylpyrroli-done)-stabilized platinum colloids, Langmuir, 14, 2730,1998. 

In acetonitrile 77-isobutyl, NOSI3 had slightly more complex kinetics probing at 436 nm, exhibiting a fast rise within the excitation pulse followed by a decay with a lifetime of 54 psec. This was followed by a longer decay of 1.7 nsec. Similar transient species could be suggested in this case. 

There are three basic methods for carrying out alternative substrate inhibition studies. In the first, the investigator seeks to observe numerical changes in the coefficients of the double-reciprocal form of the enzyme rate expression in the presence and absence of the alternative substrate. For some mechanisms, only certain coefficients will be altered. This method requires extremely accurate estimates of the magnitudes of the coefficients and should always be supplemented with other kinetic probes . 

Stereochemical probes of the specificity of substrates, products, and effectors in enzyme-catalyzed reactions, receptor-ligand interactions, nucleic acid-ligand interactions, etc. Most chirality probe studies attempt to address the stereospecificity of the substrates or ligands or even allosteric effectors. However, upon use of specific kinetic probes, isotopic labeling of achiral centers, chronfium-or cobalt-nucleotide complexes, etc., other stereospecific characteristics can be identified, aU of which will assist in the delineation of the kinetic mechanism as well as the active-site topology. A few examples of chirality probes include ... 

As mentioned in Section 4, the analysis of rate data resulting from unimolecular reactions is considerably easier than the analysis of such data for bimolecular reactions, and the same is true for pseudounimolecular reactions. Kinetic probes currently used to study the micellar pseudophase showing first-order reaction kinetics are almost exclusively compounds undergoing hydrolysis reactions showing in fact pseudofirst-order kinetics. In these cases, water is the second reactant and it is therefore anticipated that these kinetic probes report at least the reduced water concentration (or better water activity in the micellar pseudophase. As for solvatochromic probes, the sensitivity to different aspects of the micellar pseudophase can be different for different hydrolytic probes and as a result, different probes may report different characteristics. Hence, as for solvatochromic probes, the use of a series of hydrolytic probes may provide additional insight. 

For kinetic purposes, acetylperoxyl radicals were generated by laser flash photolysis of an acetylcobalt precursor in the presence of 02, Eq. (26). With use of ABTS - as a kinetic probe, the rate constant was determined for the cross-coupling reaction of Eq. (27), 27 = 1.49 x 108 M 1 s-1. 

Despite its significance, N02 has not received nearly as much attention as NO in kinetic and mechanistic studies in solution (217). The reason probably lies in the short lifetime of N02, which rapidly disproportionates to nitrite and nitrate, see Scheme 12, Eqs. (54)—(55), with an overall combined rate constant k = 6.5 x 107 NT1 s 1, Eq. (57). Direct kinetic studies are thus limited to rapid reactions and require the use of absorbing reactants or kinetic probes. 

The same value was obtained by use of 10-methyl-9,10-dihydroacri-dine (AcrH2) as a kinetic probe. Independent kinetic determination... 

In my talk I surveyed recent advances in the methodology and selected 2D-IR spectra of secondary structures. The results promise to provide structurally based kinetic probes for conformational dynamics, sharp tests of anharmonic potential surfaces and novel information regarding the transient and equilibrium vibrational dynamics of peptides. The heterodyned 2D-IR approach has proven useful in determining structures of peptides in solution and the anharmonic nature of the potential surfaces of peptides and secondary structures [1-10], as have polarized photon echo [2,6,10-12] or pump-probe techniques [4,13-16]. 

Kinetic Probes An extremely useful application of parallel reactions is the kinetic probe method for determination of rate constants in cases where spectroscopic (e.g., absorbance) changes accompanying the reaction of interest are too small to be useful for direct determination. In those cases, an extra reagent (kinetic probe) is added to provide a parallel path with a large absorbance change and thus allow the determination of the rate constant of interest. 

A large volume of kinetic data has been obtained using kinetic probes and competition kinetics in general. As is obvious from the equations and principles involved, this approach is useful only when the rate of the probe reaction is comparable to the rate of the reaction under investigation. If one or the other term in Equation 8.50 dominates to the point that the competition is heavily one-sided (>90% in either direction), then the effects of the smaller term on the kinetics and product distribution will be minimal, and results highly uncertain. 

Fig. 8 Typical kinetic probe reactions indicating the occurrence of radical processes...

Kim, C. H., Balny, C., and King, T. E., 1987, Role of the hinge protein in the electron transfer between cardiac cytochrome c i and c. Equilibrium constants and kinetic probes, J. Biol. Chem. 262 810388108. 

Depending on the timescales involved and the nature of the chemical reaction, the kinetic information can be obtained from product distribution, luminescence quenching, or kinetic measurements in the presence of a kinetic probe or a radical precursor. Inevitably, however, the result is obtained as a relative rate constant. The conversion to the absolute value requires that the rate constant for the competing process(es) be known. 

Fenter P., Park C., Zhang Z., Krekeler M. P. S., and Sturchio N. C. (2003) Orthoclase dissolution kinetics probed by in situ X-ray reflectivity effects of temperature, pH and crystal orientation. Geochim. Cosmochim. Acta 67, 197-211. 

Kinetic Probing of Pore-Archileclure and Acidic Properties M. SchulZt 0. Barlti and Z. Siwei... 

In this study, the reaction was characterized using a combination of in-situ kinetic probes, heat flow from reaction calorimetry and measurement of the hydrogen uptake rate, in addition to the commonly-used method of analysis of samples taken from the reactor. Heat flow from the reaction calorimetry and measurement of rate of hydrogen uptake are intrinsically superior kinetic tools in that they both provide rate data directly and in a quasi-continuous fashion. Consequently, they are capable of producing clear and detailed kinetic pictures which offers hints on reaction pathways and mechanism (4,5). It will be shown that thermodynamic information regarding each step in the hydrogenation reaction network may also be obtained directly from a combination of the heat flow and the hydrogen uptake data. 

Kinetic Probing of Pore-Architecture and Acidic Properties 7 83... 

As a method, TRXRD is still in its infancy. While it has already proven to be a powerful, information-dense structural and kinetic probe there is much room for improvement. In what follows I draw upon my own experience as to the limitations of the method and will address these at four levels facility, sample, instrumentation, and experimental design. The view held here is that deficits in the technique will be corrected more expeditiously if attention is drawn to them at an early stage. Alas, in some cases, a solution must await the development of new technologies. This exercise also serves to emphasize that TRXRD is not the panacea but rather another tool in an arsenal of physicochemical techniques with which to tackle critical issues in condensed matter phase science. 

Game, S., Holman, G. D. and Eisenthal, R. S. (1989) Sugar transport in Trypanosoma brucei a suitable kinetic probe. FEBS Lett. 194 126-130. 

Hydrated electrons have also been exploited as kinetic probes in model biological systems for many years. In the first tunable laser study of ionization potentials (7 ) of aromatic molecules in model membranes, we observed a significant reduction in the threshold for pyrene in aqueous... 

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