Specific catalysis kinetic plots

The distinctive kinetic plots for specific-acid and specific-base catalysis. A. The pH dependence of log(k bs) for a specific-acid-catalyzed reaction, B. The pH dependence of log(kobs) for a specific-base-catalyzed reaction. C. The dependence of for a sp>ecific-acid-catalyzed reaction on the concentration of an added acid HA at constant pH. D. The dependence of for a specific-base-catalyzed reaction on the concentration of an added base B at constant pH. The pH values 2 and 6 are just chosen as examples and are not indicative of any particular scenario. 

There are several types of pH-dependent kinetic behavior that can be interpreted in terms of one or more of the various forms of the specific acid-base catalysis relation [equation (7.3.2)]. Skrabal (33) classified the various possibilities that may arise in reactions of this type, and Figure 7.3 is based on this classification. The various forms of the plots of log k versus pH reflect the relative importance of each of the various terms in equation (73.2) as the pH shifts. Curve a represents the most general type of behavior. This curve consists of a region where add catalysis is superimposed on the noncatalytic reaction, a region where neither acid nor base catalysis is significant. 

Purified cottonseed NAPE synthase enzyme exhibited non-Michaelis-Menten biphasic kinetics with respect to the free fatty acid substrates, palmitic and linoleic acids. Kinetic parameters for the two saturable sites were calculated from various transformations e.g., double-reciprocal and Hill plots Cornish-Bowden, 1995) of initial velocity/ substrate concentration data and are summarized in TABLE 1. Preliminary experiments with several group-specific modifiers indicated that NAPE synthase was progressively inactivated by increasing concentrations of 5,5 -dithiobis(2-nitrobenzoic acid) (DTNB), diisopropyl fluorophosphate (DFP), phenylmethylsulfonylfluoride (PMSF), diethylpyrocarbonate (DEPC) (TABLE 2). These results suggest that NAPE synthase may form a thioester- or ester-intermediate through a cysteine or serine residue, respectively, and a histidine residue may participate in catalysis as well. 

The cyclization kinetics of 11 model l-[2-(methoxycarbonyl)phenyl]-3-(2-substitut-ed phenyl)triazenes (177) have been examined in aqueous methanolic buffer solutions at various pH values. 3-(2-Substituted phenyl)benzo[fi ][l,2,3]triazin-4(3 f)-ones (178) were identified as the cyclization products. The log fcobs vs pH plot was linear with a slope of unity. The assumed and confirmed Bac2 mechanism involving specific base catalysis begins by deprotonation of the triazene giving rise to the conjugate base, continues with formation of a tetrahedral intermediate, and ends with elimination of the methanolate ion (Scheme 32). Desilylation with methanolic HCl of the substituted anilide (179), a compound formed by a Ugi reaction, led to a facile intramolecular conversion to an ester (180). This reaction was the key step in a... 

Either hydrolysis or condensation may be controlled using acidic or basic catalysis. Essentially, when examining these two reactions, one would ideally like to be able to freeze them in time in order to obtain solutions with long shelf life. Although this is only possible by, for example, cooling down a solution/mixture, it is preferable to use a solution of monomeric-hydrolyzed silanes as a primer solution and catalysis conveniently allows this. The kinetic constant of the hydrolysis and/or condensation reaction vary as a function of the pH of the solution. If acidic catalysis is examined for hydrolysis, then the constant, plotted as log k as a function of pH, exhibits a V-shaped curve with a minimum specific value of pH. This value corresponds to the slowest kinetics of the reaction, usually around pH 6 for hydrolysis and 10 for condensation. O Figure 11.4 shows an example of such a shape. The position of the minimum of the curve will itself depend on the structure of the molecule. 

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