Hydrolytic rate

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.)... 

A monodentate palladium(II) complex trans-[Pd(py)2(H202)]2+ hydrolyzes Met-Aa amide bonds with a rate comparable with that promoted by [Pd(H20)3(0H)]+. Unlike Pd(H20)3(0H)]+, //chelated complex containing temed (A,A,AAA -tctramcthylcthylenediamine) hydrolyzes Met-Aa amide bonds with hydrolytic rate controlled by temed release. The action of the other two complexes, c -[Pd(ED-TA)C12] (EDTA = ethylene diaminetetraacetic acid) and cis-1,2-bis(2-formylglycinebenzene-sulfenyl)ethane Pd11 chloride differs from the action of similar complexes of U,v-[Pd(en)Cl2] and cw-[Pd(dtco-3-OH)Cl2] (dtco-3-OH = l,5-dithiacycooctan-3-ol).448... 

Hong-xi, C., Mei-ying, Y., Huai-min, G., Jing-hao, G. Studies on the hydrolysis of the organotin polymers. I. Hydrolytic rates of poly(tributyltin... 

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... 

Inadequate understanding of the kinetics of the sorption/desorption process detracts from our ability to completely understand the effects of sorption on hydrolytic rates, and more research is needed in this regard. 

Fig. 4. Plot of the hydrolytic rate coefficients as a function of pAf, of the leaving group for various...

Two major difficulties must be considered in any assay for acid phosphatase. The enzyme is subject to surface inactivation (23, 24). Accordingly, reproducible initial hydrolytic rates are not always obtained, and the kinetic behavior should be checked in any new assay developed. Discrepancies between the amount of inorganic phosphate produced and phenol liberated from phenolic phosphates may be substantial if extensive phosphotransferase activity occurs because of phosphoryl acceptor action on the part of hydroxylic buffers or other constituents of the incubation mixture (25, 26). Fluorogenic assays have been developed with very high sensitivity (27). Reference will be made to particular assays in the discussion of the specific enzymes. 

The reaction of ChE s of different origin towards a homologous scries of choline esters permits a further classification within this group. This is exemplified in Fig. 1 for two representative types, the true ChE from the electric organ of Electrophorus electricus and the pseudo ChE from human plasma, both acting on acetylcholine as substrate. The curves show hydrolytic rates as function of pS = —log (substrate concentration) and demonstrate a fundamental difference between the two enzymes The true type exhibits a bell-shaped pactivity curve, indicative of autoinhibition at high substrate concentrations (18). The pseudo enzyme, on the other hand, possesses a tS-shaped curve i.e., the maximum rate is reached at and beyond an optimal substrate concentration. 

When a substrate like ACh is added, it will compete with both the quaternary ion and the proton for the anionic site. In addition, simultaneous changes in the esteratic site, produced by protons (see VI, 1), will modify the hydrolytic rate. Therefore, a very complicated picture results, which does not allow unequivocal conclusions to be drawn. If, however, hydrolysis is measured under noncompetitive conditions, only the equilibria (1) and (2) have to be considered. This can be done by extrapolating the rates to zero time, i.e., by measuring the amount of active enzyme still available after equilibration with inhibitor or protons. 

The inhibitory effect of these ions increases with increasing length of the fourth substituent, i.e., I6o, the concentration required to reduce the hydrolytic rate to one-half under standard conditions, decreases. A plot of piso (= —log I6o) vs. n, the number of methylene groups in the side chain of the fourth substituent, gives a straight line for either ChE (Fig. 9). This observation can be interpreted as follows ... 

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]. 

Takada, K., et al. 1991. Effect of pH, dietary proteins and trypsin inhibitors on hydrolytic rate of human granulocyte colony-stimulating factor (G-CSF) by rat digestive enzymes. J Pharmacobio-dyn 14 363. 

Hydrolysis. Water, often in combination with light energy or heat, can break chemical bonds. Hydrolytic reactions commonly result in the insertion of an oxygen atom into the molecule with the commensurate loss of some component of the molecule. Ester bonds, such as those found in organophosphate pesticides (i.e., parathion Figure 26.1), are highly susceptible to hydrolysis which dramatically lowers the environmental half-lives of these chemicals. Hydrolytic rates of chemicals are influenced by the temperature and pH of the aqueous media. Rates of hydrolysis increase with increasing temperature and with extremes in pH. 

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. 

Finally, the appreciable hydrolytic rate of all of these provide for the assurance that there will be no soil residues from one season to the next, a decidedly ecological plus. 

Enzyme activity may be enhanced by a chemical pretreatment, as in the example of alkaline hydrogen peroxide on corn fiber, where the hydrolytic rate was increased by a factor of 1.6 (Leathers, 1993). Some enzymes depolymerize polysaccharides by 3-elimination. 

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... 

Table 2.9 Rates of hydrolysis of ethyl glycinate at pH 7.9 (for Cu(ll) pH 7.3) promoted by different divalent ions (Kroll 1952) depending on c- and x-parameters. To compensate for a smaller contration of hydroxide, hydrolytic rates at/with Cu(ll) must be multiplied by four (values in brackets)...

The Taft steric constant Es was proposed as a measure of steric effects that a substituent X exerts on the acid-catalyzed hydrolytic rate of esters of substituted acetic acids XCH2COOR [Taft, 1952], The basic assumption is that the effect of X on acid hydrolysis is purely steric, as the reaction constant q for acid hydrolysis of substituted esters is close to zero. 

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. 

Krokan, H.E., Bjerve, K.S. and Mork, E. (1993) The enteral bioavailability of eicosapen-taenoic acid and docosahexaenoic acids is as good from ethyl ester as from glyceryl ester in spite of lower hydrolytic rates by pancreatic lipase in vitro. Biochim. Biophys. Acta. 1168 59-67. 

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. ... 

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