Phosphate nitrophenol ester hydrolysis

Cyclodextrins and their derivatives are already known to catalyse an enormous variety of biochemical and non-biochemical transformations. The basis of the catalysis by native (unmodified) cyciodextrins is the positioning of the reactive secondary hydroxyl groups directly at the entrance to the molecular cavity. One of the most effective reactions catalysed by cyclodextrins is the hydrolysis of aryl and phosphate esters (esterase activity). For example, the rate of hydrolysis of p-nitrophenol esters is increased by factors of up to 750 000 by /TCD. The mechanism of action of the cyclodextrin is shown in Scheme 12.2.1... 

The first observed product is the hydroxo-N-bound phosphoramidate complex, although there are almost certainly other intermediates. Both ester hydrolysis (to nitrophenolate ion) and transfer of a phosphate residue from O to N occur. An acceleration of at least 10 fold can be assessed for both processes, compared with the reaction of the uncoordinated ester with NH3 or 0H ion. The O to N transfer is a general biochemical occurrence, e.g. creatine kinase uses and creatine to transform ATP to ADP and form creatine... 

Hammett (and related sigma) relationships have been applied to aquatic reactions of several classes of aromatic contaminants. For example, alkaline hydrolysis of triaryl phosphate esters fits a Hammett relationship (Table 3) is t he sum of the substituent constants for the aromatic groups and k0 is the hydrolysis rate constant for triphenyl phosphate (0.27 M 1 s-1 t1/2 = 30 days at pH 8). Triaryl esters thus hydrolyze much more rapidly than trialkyl or dialkyl-monoaryl esters under alkaline conditions. Rates of photooxidation of deprotonated substituted phenols by singlet oxygen have been found to be correlated with Hammett a constants (Scully and Hoigne, 1987). The electronic cllects of substituents on pKa values of substituted 2-nitrophenols also fit a I lammett relationship this, of course, is not a kinetic LFER. Two compounds (4-phenyl-2-NP and 3-methyl-2-NP) did not fit the relationship and were not included in the regression. Steric effects may account for the discrepancy for the latter compound. Nitrophenols are used as intermediates in synthesis of dyes and pesticides and also used directly as herbicides and insecticides. 

Many pesticides are esters or amides that can be activated or inactivated by hydrolysis. The enzymes that catalyze the hydrolysis of pesticides that are esters or amides are esterases and amidases. These enzymes have the amino acid serine or cysteine in the active site. The catalytic process involves a transient acylation of the OH or SH group in serin or cystein. The organo-phosphorus and carbamate insecticides acylate OH groups irreversibly and thus inhibit a number of hydrolases, although many phosphorylated or carbamoylated esterases are deacylated very quickly, and so serve as hydrolytic enzymes for these compounds. An enzyme called arylesterase splits paraoxon into 4-nitrophenol and diethyl-phosphate. This enzyme has cysteine in the active site and is inhibited by mercury(ll) salts. Arylesterase is present in human plasma and is important to reduce the toxicity of paraoxon that nevertheless is very toxic. A paraoxon-splitting enzyme is also abundant in earthworms and probably contributes to paraoxon s low earthworm toxicity. Malathion has low mammalian toxicity because a carboxyl esterase that can use malathion as a substrate is abundant in the mammalian liver. It is not present in insects, and this is the reason for the favorable selectivity index of this pesticide. 

Phosphate ester hydrolysis is an important biological process that is catalysed by metal containing en mes and biomimetic systems. An early example from Co(III) chemistry is shown in Scheme 3.6, where R is /7-nitrophenolate and tlw data refer to 1.0 M NaClO at 25°C. ... 

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