Kinetics and speciation of paraoxon hydrolysis by zinc(II)–azamacrocyclic catalysts

Abstract

a b s t r a c tFour Zn 2+ -azamacrocyclic complexes were investigated for their ability to catalyze the hydrolysis of the toxic organophosphate (OP) pesticide diethyl paraoxon. Of the four complexes studied, Zn 5,aneN 3 ) was found to be the most effective catalyst with a pseudo-first order reaction rate of k = 6.08 ± 0.23 Â 10 À4 min À1 . Using 31 P nuclear magnetic resonance (NMR) spectroscopy, the two products diethyl phosphate (DEP) and ethyl (4-nitrophenyl) phosphate (E4NPP) were identified for both catalyzed and background hydrolysis of paraoxon. Reaction rate and selectivity for formation of the non-toxic DEP were observed to correlate with catalyst pK a . The rate of formation of toxic E4NPP, however, was independent of both the presence and nature of the catalyst. The potential roles of buffer concentration and product inhibition were also investigated. Background hydrolysis at elevated reaction temperatures (50°C) displayed no preference for DEP over that of E4NPP despite substantial differences between the characteristics (i.e., pK a values) of the two leaving groups (ethoxide vs. 4-nitrophenoxide anions). As with previous observations of these types of metal-catalyzed hydrolyses, we invoke the formation of a trigonal bipyramidal-like transition state involving a Zn-coordinated phosphate bond, with the leaving group at the apical position and the incoming HO À anion approaching from the opposite end. Kinetic rates for catalytic hydrolysis display an overwhelming propensity for DEP formation, and suggest the importance of steric restrictions on transition state structure, namely a concerted arrangement of the azamacrocycle in opposition to the bulky 4-nitrophenoxy group.

Investigating the breakdown of the nerve agent simulant methyl paraoxon and chemical warfare agents GB and VX using nitrogen containing bases

“…23 This leads to DMP formation and loss of the 4-nitrophenol ( pK a = 7.1), in preference over methanol ( pK a = 15.4), due to the lower pK a of the 4-nitrophenol group. 25 Nucleophilic attack of the phosphotriester is most likely to occur via a weakly associative, pentacoordinate transition state, with the more electronegative and more acidic 4-nitrophenyl group adopting the apical position, as hydroxide attacks in a S N 2 manner. 33,34 A peak shift in the 31 P[ 1 H] NMR, from 2.8 to 1.7 ppm, was observed during the reduction in water, however the splitting pattern in the 31 P NMR, from the phosphorus-proton coupling, remained unchanged ( Fig.…”

“…24 The product distribution, and hence the toxicity of the decomposition products, depends strongly on the conditions of the reaction, with the C-O bond cleavage pathway resulting in the formation of the toxic breakdown product, ethyl 4-nitrophenyl phosphate. 25 The ethyl 4-nitrophenyl phosphate breakdown product is likely to display the same mode of action as paraoxon and CWAs upon the enzyme acetylcholinesterase, 26 due to the continued presence of the reactive 4-nitrophenyl group, which can cause the organophosphate to bind irreversibly to the enzyme. This is in contrast to the diethyl phosphate breakdown product, formed from the P-OAr bond cleavage pathway.…”

Aminoalcohol-Induced Activation of Organophosphorus Hydrolase (OPH) towards Diisopropylfluorophosphate (DFP)

“…In contrast, all the amines investigated, including methylamine, ethylamine, diethylamine, and triethylamine, had slightly activating effects on OPH activity, as observed in the change of k cat and V max at 300 mM concentration (Table 3), indicating the critical role of alkyl-substituted amine (amino nitrogen) in the OPH activation. In recent years, a series of chemically synthetic OP scavengers based on organic nitrogen derivatives, including Zn 2+ -azamacrocyclic complexes [21,34], aminopyridine-substituted polyallylamine [35] and oxime-substituted cyclodextrins [22,36–38], have been established to eliminate pesticides and nerve agents such as parathion, paraoxon, DFP, cyclosarin (GF) and tabun. The mechanism of the reported non-enzyme catalysts exclusively involves essential nitrogen atom(s) with lone pair electrons that function as critical Lewis base to initiate catalysis.…”

“…ALPs have been classified to the metalloenzyme, and the reported activators are nitrogen-containing organic compounds that probably act as phophoacceptor and/or suitable ligands for the critical metal ions. Recently, two sets of synthetic nitrogen-containing organic materials, Zn 2+ -azamacrocyclic complexes and oxime substituted β-cyclodextrin derivatives, were illustrated as the novel non-enzyme catalysts accelerating the hydrolysis of paraoxon and cyclosarin (GF) [21,22]. These reports suggested the activating effects of hydroxyl amine nitrogen derivatives on the particular OP hydrolysis.…”

Fe2O3/Mn2O3 nanoparticles: Preparations and applications in the photocatalytic degradation of phenol and parathion in water

“…In order to alleviate this dilemma, different chemical technologies have been developed with regard to degrading pesticide residues, such as ozonization, advanced oxidation processes, hydrolysis, and by using membranes, to name a few . A noticeable technique among them is the advanced oxidation process, which applies the highly reactive hydroxyl radical.…”