Hydrolytic rate constant

FIGURE 7.16 (A) Photocurrents of salamander rod cells following light flashes giving between 10 and 2000 rhodopsin molecule isomerizations. (B) Calculated increments in phosphodiesterase hydrolytic rate constant. (From Lamb, T. D. and Pugh, Jr., E. N., Trends Neurosci., 15, 291-299, 1992. With permission.)... 

Morawetz and Oreskes (1958) postulated that intramolecular general acid catalysis occurs in concert with nucleophilic attack by carboxylate anion in the hydrolysis of the monoanion of succinyl salicylate. At pH 4 the observed hydrolytic rate constant was found to be 10 times greater than that for hydrolysis of acetylsalicylate. A... 

Under experimental conditions of 50 °C and pH 1.5, 5, 7, 9, or 11, no hydrolytic loss of PFOS was observed in a 49-day study [42]. Based on mean values and precision measures, the half-life of PFOS was estimated as > 41 years at 25 °C. However, it is important to note that this estimate was influenced by the analytical limit of quantitation and that no loss of PFOS was detected in the study. Likewise, under experimental conditions of 50 °C and pH 1.5, 5, 7, 9, or 11, the hydrolytic loss of PFOA was observed for 109 days. Results showed that the degradation rate of PFOA was not dependent upon pH levels. The hydrolytic rate constant for PFOA at 50 °C was determined to be 8.1E-5/d, and the minimum calculated half-life was estimated as 92 years [43]. 

Methods to predict the hydrolysis rates of organic compounds for use in the environmental assessment of pollutants have not advanced significantly since the first edition of the Lyman Handbook (Lyman et al., 1982). Two approaches have been used extensively to obtain estimates of hydrolytic rate constants for use in environmental systems. The first and potentially more precise method is to apply quantitative structure/activity relationships (QSARs). To develop such predictive methods, one needs a set of rate constants for a series of compounds that have systematic variations in structure and a database of molecular descriptors related to the substituents on the reactant molecule. The second and more widely used method is to compare the target compound with an analogous compound or compounds containing similar functional groups and structure, to obtain a less quantitative estimate of the rate constant. 

Although S-cinnamoyl-thiosubtilisin and Se-cinnamoyl-selenosubtilisin are cleaved by water at comparable rates, their hydrolysis is slower than that of native 0-cinnamoyl-subtililsin by more than two orders of magnitude. The decrease in hydrolytic rate constant may reflect subtle structural changes within the relevant active site geometries the van der Waals radii of sulfur and selenium are similar (1.85 and 2.0 A, respectively) but that of oxygen is much smaller... 

Studying the influence of pH on degradation rate is not as simple as might at first be imagined. If the hydrolysis rate of the dmg in a series of solutions buffered to the required pH is measured and the hydrolytic rate constant is then plotted as a function of pH, a pH-rate profile will be produced, but this will almost certainly be influenced by the buffers used to... 

In this equation, is the experimentally determined hydrolytic rate constant, /Cq h the uncatalysed or solvent catalysed rate constant, and /CgH- te the specific acid- and base-catalysis rate constants respectively, ttd ky - are the general acid- and base-catalysis rate constants respectively, and [HX] and [X ] denote the concentrations of protonated and unprotonated forms of the buffer. 

Hgure 4.8 Effect of buffer concentration on the hydrolytic rate constant for ciclosidomine at 60°C as a function of pH. 

The following data were obtained for the hydrolytic rate constant, of codeine sulfate in aqueous buffer-free solution at 80°C... 

Figure 4.14 The effect of ionic strength, pi, on the hydrolytic rate constant, k, for phentolamine hydrochloride in buffer solutions of pH 3.1 and 7.2 at 90°C.

Temperature increase usually causes a pronounced increase of hydrolytic degradation. We have seen how to calculate the hydrolytic rate constant at room temperature from data at elevated temperatures using the Arrhenius equation. 

Hydrolytic rate constants for MeHgOAc and Hg(OAc)2 have also been determined. Formation constants for the species [MeHgX2] (X = Cl or Br) in ethanol have been determined. Mass spectroscopic investigations of the species MeZnOR (R = Me, Et, or Bu ) show that all form oligomers in the gaseous state having either six or seven monomer units. ... 

Breslow and co-workers have studied the mechanism of imidazole-catalyzed hydrolysis of both cyclic phosphate esters and of RNA (67-72). These studies are directed toward a more detailed understanding of the mechanism of the hydrolysis of RNA catalyzed by ribonuclease A (RNase A). In particular, recent studies by Breslow and co-workers have addressed the interesting and enzymologically pertinent question of the origin of the bell-shaped dependence of hydrolytic rate constant on pH in both enzymic and nonenzymic reactions. 

Stout SA and Riley CM, The hydrolysis of L-phenylalanine mustard (melphalan), Int. f. Pharm., 24,193-208 (1985). NB Estimated from hydrolytic rate constants in file pH range 1-13. Taken from Stout SA, Chemical stability and in vitro cytotoxicity of melphalan, Ph.D. Thesis, University of Florida, 1987,36-42. 

In contrast to hydrolytic cleavage in a neutral medium, acid hydrolysis of 1-organoxysilatranes is described by a kinetic equation of the second order. The hydrolysis was studied in diluted aqueous solutions at 25 °C in the presence of HCl and KQ. No salting effect was manifest at KCl concentrations from 0.05 to 0.25 mol/L. The hydrolytic rate constants (k) obtained, their mean square deviations, o or values and average lethal doses (LD50) of 1-aroxysilatranes are presented in Table 1. Quantitative evaluation of k as a function of the nature of the substituent was carried out in a similar way by applying the Taft equation. 

The Use of Hydrolytic Rate Constants to Study Glutethimide (Doriden) and Metabolite Concentrations in Human Blood... 

Following the determination of the hydrolytic rate constants of an epimeric pair of tricyclic nitriles (78) by a non-linear, least-squares fitting method, a novel eigenvalue-eigenvector analysis of the sensitivity coefficients permitted maximization of the kinetic dataJ ... 

Because of their high vapor pressure and low boiling points, chloromethanes will have a brief residence time in water resulting in transport to the atmosphere. Hydrolytic rate constants and half-lives of chloromethanes are therefore relatively long (Table 3.5). 

Table 3.5. Hydrolytic rate constants and half-life of some haloaliphatics in water.

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