Methyl parathion hydrolysis

Methyl parathion hydrolysis has also been observed to be catalyzed by (Mabey et al., 1984). Additionally, the affected the reaction product distribution. As expected, the neutral (pH<8) hydrolysis of methyl parathion favors cleavage of the C-O bond (soft-soft interaction) (pathway a, 2.106), whereas the base-catalyzed (pH >8) hydrolysis favors cleavage of the P-O bond (hard-hard interaction (pathway b, 2.106). In the presence of Cu +, however, heterolytic cleavage of the P-O bond resulting in the formation of nitrophenol is the dominant process even at low pH (pathway b, 2.106). 

Recently this technique was applied to the evolution of OPH for increased methyl parathion hydrolysis. After two rounds of DNA shuffling, a variant that could hydrolyze methyl parathion 25-foId faster than wild type was isolated. The mutations were not directly located in the active site and could not be otherwise predicted a priori (55), This technique could be used to target other slow degrading pesticides such as chlorpyrifos and diazinon and against chemical warfare agents VX and sarin. 

Noncatalytic Reactions Chemical kinetic methods are not as common for the quantitative analysis of analytes in noncatalytic reactions. Because they lack the enhancement of reaction rate obtained when using a catalyst, noncatalytic methods generally are not used for the determination of analytes at low concentrations. Noncatalytic methods for analyzing inorganic analytes are usually based on a com-plexation reaction. One example was outlined in Example 13.4, in which the concentration of aluminum in serum was determined by the initial rate of formation of its complex with 2-hydroxy-1-naphthaldehyde p-methoxybenzoyl-hydrazone. ° The greatest number of noncatalytic methods, however, are for the quantitative analysis of organic analytes. For example, the insecticide methyl parathion has been determined by measuring its rate of hydrolysis in alkaline solutions. 

In soil and sediments, methyl parathion adsorbs to soil and is expected to display moderate mobility (EPA 1980c). The major degradation process of methyl parathion in soil is biodegradation by microbes (Badway and El-Dib 1984). Degradation by hydrolysis has been observed to occur at higher temperatures... 

Methyl parathion is rapidly degraded in natural water systems. The degradation of methyl parathion occurs much more rapidly in alkaline (pH 8.5) than in neutral (pH 7) or acidic (pH 5) conditions (Badawy and El-Dib 1984). A hydrolysis half-life of 72-89 days was calculated for fresh water at 25° C and pH<8 (EPA 1978c Mabey and Mill 1978) compared with about 4 days at 40° C and pH>8 (EPA 1978c). 

The degradation of methyl parathion by hydrolysis and biodegradation was studied in four types of water (ultrapure water, pH 6.1 river water, pH 7.3 filtered river water, pH 7.3 and seawater, pH 8.1) maintained at 6 and 22° C, in the dark. The half-lives of methyl parathion at 6° C in the four water types were determined to be 237, 95, 173, and 233 days, respectively, and the half-lives at 22° C were... 

Albanis TA, Pomonis PJ, Sdoukos AT. 1988c. The influence of fly ash on hydrolysis, degradation and adsorption of methyl parathion in aqueous soil suspensions. Toxicol Environ Chem 17 351-362. 

Sharmila M, Ramanand K, Sethunanthan N. 1981. Hydrolysis of methyl parathion in a flooded soil. 

FIGURE 2.1 Reaction scheme for OPH hydrolysis of methyl parathion and paraoxon (a) followed by the electrochemical oxidation of p-phenol (b). R and R are ethoxy and methoxy and X is O and S in paraoxon and methyl parathion, respectively. 

A qualitatively similar relationship was observed for the hydrolysis of parathion on Ca- and Al-kaolinite (Figure 4). In order to explain the lower hydrolysis rate of parathion at Al-kaolinite than at Na-kaolinite surfaces, it was suggested (70) that steric hindrance may force the parathion molecule into a position or conformation less favorable to hydrolysis. On the other hand, the hydrolysis rate for methyl parathion on the Al-clay was higher than the one on the Na-clay (71), probably due to the smaller size of the methoxy group as compared to that of the ethoxy group of parathion... 

Organophosphate hydrolysis is frequently observed as the initial reaction for pesticides having organophosphate bonds, such as methyl parathion, chlorpyrifos (9) (eq. 13), diazinon, and coumaphos (19). Several genera of organophosphate-hydrolyzing bacteria have been identified, including... 

Even a rather simple insecticide such as methyl parathion is transformed by insects in a complex manner. The parent insecticide is activated to methyl paraoxon, which is a more potent inhibitor of the target, acetylcholinesterase in the nerve (Figure 1). This activating desulfuration is catalyzed by monooxygenases. Both the parent and the oxon are subject to detoxication by monooxygenase and glutathione transferase, while the oxon is also more labile to hydrolysis. 

Arvlester Hydrolase and Parathion Hydrolase. Arylester hydrolase in mammalian serum (42.431 catalyzes very efficient hydrolysis of the organophosphorus oxons such as paraoxon (Table III). This enzyme does not hydrolyze the parent phosphorothioate insecticides such as methyl parathion, but only the oxon metabolites such as methyl paraoxon (441. This enzyme is has been referred to as phosphotriesterase however, a triester is not required as seen in the rapid hydrolysis of 10 organophosphinates by rabbit serum arylester hydrolase (44). This mechanism appears only rarely in birds... 

Technical fenthion is a yellowish-brown oily liquid, very resistant to climatic factors and sunshine. It is also considerably more stable to hydrolysis than parathion-methyl. 50% hydrolysis of the latter compound proceeds at 80°C in acidic medium in 12.7 hours and in alkaline medium in 5 minutes, and of fenthion in 36 hours and 95 minutes respectively. 

Moisture content was also found to have a significant effect on the hydrolysis kinetics of parathion and methyl parathion on kaolinite. For example, the hydrolysis rate constant for parathion and methyl parathion on kaolinite increased by 2 orders of magnitude when the moisture content was increased to the limit of sorbed water (11% moisture content) (Saltzman et al. 1976). At moisture contents above the limit of sorbed water, however, a significant decrease in the hydrolysis rate constant was observed. Mingelgrin et al. (1977) concluded that the hydrolysis of parathion. 

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