Kinetics inhibited reactions

Eig. 3. The effect on kinetic parameters of adding a competitive inhibitor. Reaction velocity as a function of [3] is shown. (—x —) Uninhibited reaction (---) inhibited reaction. As indicated on the figure, the parameter is increased by adding the competitive inhibitor both curves eventually reach the... 

In the first group of studies, involving kinetic inhibition studies, comparisons of the uilibrium (K ), phosphorylation (IC), and inhibition constant (K.) for the inhibition of electric eel and human erythrocyte AChE by ANTX-A(S) and DFP were done (Table II). From Table II it is seen that ANTX- A(S) has a higher affinity for human erythrocyte AChE (K =0.253 fiM) than electric eel AChE (K j=3.67 aM). AN DC-A(S) also shows greater affinity for AChE than DFP (K =300 fiM). And finally the bimolecular rate constant, Kj, which indicates the overall rate of reaction, shows AChE is more sensitive toward inhibition by ANTX-A(S) (Kj=1.36 pM- min- ) than DFP (K, = 0.033 /iM- min ). These studies add information to the comparative activity of ANTX-A(S) and other irreversible AChE inhibitors but do not show the site of inhibition. 

The permanganate oxidation of phenols is complicated by the intervention of lower oxidation states of manganese, (c/. the oxidation of toluene, p. 298). For example, the oxidation of 2,6-dinitrophenol in weakly acidic solution displays an induction period, following second-order kinetics thereafter. However, addition of potassium fluoride inhibits reaction almost completely, but manganous ions strongly accelerate it. 

Table 17.11 Inhibition Kinetics Initial Reaction Rates in the Presence of Inhibitors A or B at Different Substrate Concentrations...

The application of the calculated reaction enthalpy allows us to estimate the kinetic chain length (approximately 30) and other kinetic data (reaction rate, final conversion, inhibition time) of the crosslinking reaction. The reaction rate (dx/dt) of this process is a function of the light intensity, the exposure time, of the thiol content of the system (see Fig. 1) and also of the photoinitiator used. The final degree of conversion of the double bonds is generally high (80 - 100 %). 

This problem was first approached in the work of Denisov [59] dealing with the autoxidation of hydrocarbon in the presence of an inhibitor, which was able to break chains in reactions with peroxyl radicals, while the radicals produced failed to contribute to chain propagation (see Chapter 5). The kinetics of inhibitor consumption and hydroperoxide accumulation were elucidated by a computer-aided numerical solution of a set of differential equations. In full agreement with the experiment, the induction period increased with the efficiency of the inhibitor characterized by the ratio of rate constants [59], An initiated inhibited reaction (vi = vi0 = const.) transforms into the autoinitiated chain reaction (vi = vio + k3[ROOH] > vi0) if the following condition is satisfied. 

It is easy to understand the lower reactivity of non-ionic nucleophiles in micelles as compared with water. Micelles have a lower polarity than water and reactions of non-ionic nucleophiles are typically inhibited by solvents of low polarity. Thus, micelles behave as a submicroscopic solvent which has less ability than water, or a polar organic solvent, to interact with a polar transition state. Micellar medium effects on reaction rate, like kinetic solvent effects, depend on differences in free energy between initial and transition states, and a favorable distribution of reactants from water into a micellar pseudophase means that reactants have a lower free energy in micelles than in water. This factor, of itself, will inhibit reaction, but it may be offset by favorable interactions with the transition state and, for bimolecular reactions, by the concentration of reactants into the small volume of the micellar pseudophase. 

One of the most important characteristics of micelles is their ability to enclose all kinds of substances. Capture of these compounds in micelles is generally driven by hydrophobic, electrostatic and hydrogen-bonding interactions. The dynamics of solubilization into micelles are similar to those observed for entrance and exit of individual surfactant molecules, but the micelle-bound substrate will experience a reaction environment different from bulk water, leading to kinetic medium effects308. Hence, micelles are able to catalyse or inhibit reactions. The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentrations of the reacting species in or at the micelle. 

O DormeU, V. B., Chumley, P. H., Hogg, N., Bloodsworth, A., Darley-Usmar, V. M., and Freeman, B. A., 1997, Nitric oxide inhibition of Upid peroxidation kinetics of reaction with lipid peroxyl radicals and comparison with alpha-tocopherol, Biochemistry 36 15216-15223. 

The kinetics of retarded or inhibited polymerization can be analyzed using a scheme consisting of the usual initiation (Eq. 3-14), propagation (Eq. 3-15), and termination (Eq. 3-16) reactions in addition to the inhibition reaction... 

The rate of transfer is accelerated by electron-releasing substituents on the aromatic ring of the antioxidant and retarded by steric protection of the labile hydrogen or its replacement by deuterium. The subsequent fate of the radical A determines the over-all kinetics of the inhibited reaction and the practical usefulness of the antioxidant. If A is a fairly stable phenoxy radical, it will probably add a peroxy radical or dimerize. 

There are two reasons for reactions not going to completion. The first is that they may be kinetically inhibited, as illustrated by a mixture of H2 and 02 gas, which is stable at ambient conditions. We know that such a mixture has a tendency to form H20, as can be observed by igniting the system with a spark. Without the spark, however, the system reacts at negligible rate. Rates of chemical reactions are the subject of chemical kinetics, which will not be dealt with in this book. 

Feedback to the pituitary is modeled following Michaelis-Menten kinetics for an allosteric inhibited reaction which gives ... 

Other reactions may be taken into consideration, with an effect on polymer structure, namely the formation of short- and long-chain branches. A complete list of reactions in S-PVC polymerization may be found in Kiparissides et al. [5]. On the above basis kinetic equations may be written. To keep it simple the chain transfer, back-biting and inhibition reactions are disregarded, while termination is considered to occur only by disproportionation. The elementary reaction rates for initiator decomposition and free radicals generation are as follows ... 

Gas phase oxidation of sulphur dioxide is kinetically inhibited and virtually impossible without a catalyst at any temperature. At ordinary temperatures the reaction is so slow that, in practical terms, it does not occur at all. Increasing the temperature increases the rate of reaction, but simultaneously the position of the equilibrium shifts unfavourably -away from sulphur trioxide and towards sulfur dioxide and oxygen. Without a catalyst, the temperature needed to make the system react at a practical speed is so high that a very poor conversion [i.e. very little SO3 production] is obtained."... 

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