Measuring hydrolysis

Hydrolysis rate measurements. Hydrolysis rates were examined by mixing polymer solutions with hydrochloric acid, in apparatus previously described (5). Solutions of polymer and acid are mixed rapidly, and the torque on a rotating PTFE coated fork, attached to a Brookfield LVTD viscometer, recorded as a function of time. Decreases in viscosity were approximated to first-order, and half-lives for viscosity loss calculated. 

Ellington, J. J., Stancil, Ir., F. E., Payne, W. D., Trusty, C. D. (1988) Interim Protocol for Measurement Hydrolysis Rate Constants in Aqueous Solutions. USEPA, EPA/600/3-88/014, Athens, Georgia. 

The hydrolytic susceptibility of the fluoromethylene cyanate ester is greater than that of the aromatic cyanate ester. Direct contact with water results in measurable hydrolysis to the carbamate in a 24-h period for the former while the latter is unaffected.9... 

Gin > Val > Ser > Trp, Pro, Glu, Phe, Nph, Arg, while for HNC the order is Tyr > Leu > He = Met > Trp = Phe > Val = Gin > Pro, Glu, Ser, Nph, Arg. Thus, most substitutions at subsite P l are detrimental relative to He. Interestingly, HNC generally tolerates aromatic amino acids better in this subsite than HFC. For subsite P 2, substitution of Ala by Phe, Trp, Leu, or Arg increases the rates of hydrolysis markedly for both enzymes. The substitution by Trp is most dramatic and causes increases in rate of 7.3- and 8.3-fold for HFC and HNC, respectively. This clearly demonstrates the importance of the interaction of the substrate side-chain in this subsite with these collagenases. Substitution by Glu lowers the rates for both enzymes, while Hyp abolishes measurable hydrolysis. 

Chemical/Physical. The estimated hydrolysis half-life of acetonitrile at 25 °C and pH 7 is >150,000 yr (Ellington et al., 1988). No measurable hydrolysis was observed at 85 °C at pH values 3.26 and 6.99. At 66.0 °C (pH 10.42) and 85.5 °C (pH 10.13), the hydrolysis half-lives based on first-order rate constants were 32.2 and 5.5 d, respectively (Ellington et al., 1987). The presence of hydroxide or hydronium ions facilitates hydrolysis transforming acetonitrile to the intermediate acetamide which undergoes hydrolysis forming acetic acid and ammonia (Kollig, 1993). Acetic acid and ammonia formed react quickly forming ammonium acetate. 

Chemical/Physical. Under laboratory conditions, chloroethane hydrolyzed to ethanol (Smith and Dragun, 1984). An estimated hydrolysis half-life in water at 25 °C and pH 7 is 38 d, with ethanol and HCl being the expected end-products (Mabey and Mill, 1978). Based on a measured hydrolysis rate constant of 5.1 x 10 at 25 °C and pH 7, the half-life is 2.6 yr (Jeffers and Wolfe, 1996). 

EPA. 1988e. Interim protocol for measuring hydrolysis rate constants in aqueous solutions. Washington, DC U.S. Environmental Protection Agency, Office of Research and Development. EPA/600/S3-88/014. 

Under the pH and temperature conditions of natural waters, the various 2-chloro-, 2-methylthio-, and 2-methoxy-.v-triazine herbicides are generally considered to be stable in solution between pH 5 and pH 9, stable in neutral, weakly acidic and weakly alkaline media, and stable to hydrolysis at 20°C in neutral, weakly acidic, and weakly alkaline media (Pesticide Manual, 1997). Metribuzin, metamitron, and hexazinone are also considered to be stable under these conditions (Pesticide Manual, 1997). Such hydrolytic stability of the triazine herbicides is supported by hydrolysis studies (Rhodes, 1980 Widmer et al., 1993 Noblet et al, 1996 Hequet et al, 1997) that indicate either very slow rates of hydrolysis or no measurable hydrolysis (Table 23.3) under these conditions. 

The structure of MAO is complex and has been investigated by cryoscopic measurements hydrolysis reactions infrared, ultraviolet, and NMR spectroscopies and other methods (77). Equilibria are attained between the oligomers. The molecular weight, determined cryoscopically with samples in benzene, is between 1000 and 1500 g/mol. The work of Sinn (77) and Barron et al. (78, 79) has provided details of the structure of MAO and t-butylalumoxane. Among the different oligomers, the units shown in Scheme 1 are important. 

Hydrophobieity parameters for organic substituents have been developed by Hansch et al. using partitioning phenomena [296], and by Menger et al. using kinetic measurements (hydrolysis of long-chain esters) [297] see Section 7.2. Further results connected with the presence of hydrophobic interactions in solutions are discussed in Sections 2.5 and 5.4.8. 

Equation (47) was suggested for the first time by Bredig and Ripley [202]. In order to establish it unambiguously, it is necessary to carry out experiments at a constant ionic strength since feH and kHX are influenced by salt effects. Studies in the presence of halides at a constant ionic strength have never been done. Other approaches have been used instead. Albery and Bell [200] measured hydrolysis rates of ethyl diazoacetate in moderately concentrated perchloric acid and hydrochloric acid solutions. Rates in hydrochloric acid were faster than those in perchloric acid at the same stoichiometric concentration. In order to verify the dependence on the chloride ion concentration, it was assumed that rates of the reaction without participation of chloride (first term in eqn. (47)) are the same in perchloric acid and hydrochloric acid if the H0 values are equal. Activity coefficients were introduced in eqn. (47) as follows ... 

Surface water ty, = 0.0108-0.033 h, based on measured hydrolysis rate constant for bis(chloromethyl) ether (Tou et al. 1974 quoted, Howard et al. 1991) and chloromethyl methyl ether (Ellington et al. 1987 quoted, Ellington 1989 Howard et al. 1991). 

Groundwater c, = 48-336 h, based on measured hydrolysis data and estimated aqueous aerobic biodegradation half-life (Howard et al. 1991). 

It should be noted that the inductive effect is not the only factor affecting the rate of hydrolysis. The substituent may also have a steric effect. For example, a bulky substituent may shield the ester from attack and lower the rate of hydrolysis. It is therefore necessary to separate out these two effects. This can be done by measuring hydrolysis rates under basic conditions and also under acidic conditions. Under basic conditions, steric and electronic factors are important, whereas under acidic conditions only steric factors are important. By comparing the rates, values for the electronic effect (ar), and for the steric effect (Es) (see below) can be determined. 

The data for PMDA-ODA shown in Figure 4 indicate a much lower susceptibility to imide hydrolysis. Here, the measured hydrolysis after 400 h is only 1% based on the initial ratios. The sample cured to 4000 C appears to... 

The same criticisms i.e. absence of pH measurement, hydrolysis scheme) apply to dissolution experiments in Na2S04 and H2SO4 using various SOVZr ratios. The proposed formation of Zr0(S04)3 in Na2S04 solutions and of Zr(S04)4 and Zr203 (804)3 ll H2SO4 solutions therefore cannot be considered as confirmed. 

A comparison of the results of Biro and Szent-Gyorgyi (1949) with those of Korey (1950) reveals a similar parallehsm between breakdown and structural changes in the protein. In these experiments, it is true, the measured hydrolysis rate does not represent an initial velocity, but the second of the conditions under (ii) above is fulfilled, for the preparations were as free to contract in the contraction experiments as in the enzymatic. The optimum ATP concentration found for shortening (Korey, 1950) and for breakdown (Biro and Szent-Gyorgyi, 1949) are both 10 for extracted fiber bundles at 20°C. The fact that this value is higher than for the isolated model fiber may be only an apparent discrepancy, for the effective concentration of ATP in the interior of the fiber bundle is appreciably lower than that in the bath. 

Effect of pH on Hydrolysis. Hydrolysis of the four insecticides was also studied in buffered solutions at different pH values. Pyrolan and dimetilan were stable towards hydrolysis in the pH range of 6.0-10.0 used here. Thus at a concentration of 4.8 mg/liter and at pH 10 only 4% of dimetilan and 8% of pyrolan were hydrolyzed at the end of 100 days. Baygon was also stable towards hydrolysis over pH range of 3.0-7.0. However, at pH 8.0 measurable hydrolysis was observed which increased... 

Below pH 8.0, however, this compound resisted hydrolysis. Sevin was the most susceptible insecticide to hydrolysis. Measurable hydrolysis started at pH 7.0 and increased with increase in pH. In acidic pH medium, however, the compound resisted hydrolysis. 

These half-lives are significantly shorter than the measured hydrolysis half-life of 20 weeks at pH 7 at 25 °C (Smith et al., 1978). 

The directly measured hydrolysis rate of Val-tRNA is only 10 s M This is to be compared with the required (by the Hopfield scheme) rate of 125 s Note also that the rate constant for the hydrolysis of Ile-tRNA catalyzed by IRS at saturating concentrations is 0.014 s at 25°C and pH 7.78. That is, the rate of hydrolysis of valine is 70 times higher than that of He. 

Lipase is active at very low temperatures even in systems frozen solid at —15° (286, 287). Lipases of Pseudomonas fragi. Staphylococcus, and Penicillium caused measurable hydrolysis of emulsions of com oil, coconut oil, and lard in 2-4 days at —7°, in 7 days at —18°, and in 21 days at — 29° (288). 

Next to trypsin chymotrypsin is the most preferred proteolytic enzyme in sequencing. Its specificity is less absolute than that of trypsin. Primarily the bonds that follow phenylalanine, tyrosine and tryptophan are cleaved, but measurable hydrolysis takes place next to leucine and methionine residues as well. It is advisable, therefore, to determine in preliminary experiments the conditions (enzyme-substrate ratio, time, temperature) best suited for the formation of a few and well separable fragments. Occasionally also less specific enzymes, such as pepsin, papain or thermolysin find application in structure elucidation. For the hydrolysis of specific bonds new microbial proteases can be isolated. There are known prolidases and also enzymes which hydrolyze solely the bond which follows a pyroglutamyl residue and so on. 

The extent of hydrolysis, and thus the actual amount of hydrolyzed species, An(OH)jj ", is a function of pH as well as temperature. At higher pH," An(OH)3" precipitates and is the solubility-controlling solid phase in the system An(in)/H20/0H". However, tiie measured solubility quotient varies as a function of time and depends on the crystallinity of the sample. Many hydrolysis studies for trivalent actinides have focused on Am " because Am(ni) is rather stable toward oxidation under various experimental conditions and has an isotope — Am — with a relatively long half-life (tj -- 7380 y). The measured hydrolysis constants and solubility products found for Am in the recent literature are summarized in Table I. 

This is often included. A solution of 1-g sample in 25ml of xylene should remain clear. This measures hydrolysis to the acid. 

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