Hydrolytic probe reactions

Most of the pseudounimolecular hydrolytic probe reactions involve water-catalyzed pH-independent hydrolysis of activated esters, amides, or acid chlorides " " (Scheme 9). The hydrolysis of these probes occurs via a dipolar... 

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. 

The effects of the high ionic strength (3) appear to depend rather much on the details of the hydrolytic probe. For example, aqueous rate constants for 4-nitrophenyl 2,2-dichloropropionate 2 decrease as a function of added electrolyte (as typically happens), but the origin of this rate decrease by added electrolytes was interpreted to be different from the rate-retarding effect exerted by micelles based on activation parameter considerations. For another hydrolytic probe, phenyl chloro-formate lb, increasing electrolyte concentrations also decrease the rate of reaction, but this effect is negligible in comparison to the micellar rate effects. " ... 

The hydrolysis of p-nitrophenyl acetate and bis(p-nitrophenyl phosphate) are frequently used to probe hydrolytic activity. A problem with some other dinuclear systems is that the Zn units are held together by bridging ligands which can be cleaved on reaction with the substrate.440 This is not the case in a ditopic ligand such as those designed by Lippard and co-workers based on Kemp s triacid imide with a xylyl spacer.441,442 Both zinc dimers and mixed metal dimers were formed and a structure characterized with a bridging phosphodiester (Figure 6). 

In the experimental study and interpretation of hydrolytic reaction rates, many kinetic effects provide opportunities for probing the detailed nature of the reaction or have significant practical consequences on drug stability. The topics of temperature dependence, catalysis and solvent effects, were discussed in the first edition of this encyclopedia. 

Transition-state structures computed from multiple kinetic isotope-effect, data have provided important information on the catalytic mechanisms of glycosylases. This is exemplified by the refined transition-state structures derived by Schramm and his associates and used to probe the hydrolytic reactions catalyzed by nucleosidases and the ADPR (adenosine diphosphori-bosyl) transferase toxins of Vibrio cholerae and Corynebacterium diphthe-... 

At this point the author tried a new approach to provide further evidence in the Wacker controversy. This approach involved the study of a very simple reaction whose rate-limiting step is hydroxypalladation. Such a kinetic probe is the exchange reactions of tetrasubstituted allylic alcohols, which cannot undergo oxidation. This would give us a very simple reaction where the rate-determining step must be hydroxypalladation. Thus, consider Scheme 17, where the CFj groups are incorporated for hydrolytic stability in aqueous acid solution. 

In 1988, Walde and coworkers studied the kinetic and structural properties of another serine protease, namely trypsin, in two reverse micellar systems, AOT/ isooctane and CTAB/chloroform/isooctane, employing three different model substrates, an amide and two esters [71], The main aim of this work was to compare the behavior of trypsin in reverse micelles with that of a-chymotrypsin. In the case of trypsin, superactivity was not observed and in general no obvious similarities between the two enzymes were recorded. Some years later, reverse micelles formulated with biocompatible surfactants such as lecithin of variable chain lengths in isooctane/alcohol were studied in relation to their capacity to solubilize a-chymotrypsin and trypsin [72]. The hydrolytic behavior of the same serine proteases, namely a-chymotrypsin and trypsin, in both AOT and CTAB microemulsions was studied and related to the polarity of the reaction medium as expressed by the logP value and measured by the hydrophilic probe 1-methyl-8-oxyquinolinium betaine [39], In this study a remarkable superactivity of trypsin in reverse micelles formed with the cationic surfactant CTAB was reported. 

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