Ag(I) Pyridine–Amidoxime Complex as the Catalysis Activity Domain for the Rapid Hydrolysis of Organothiophosphate-Based Nerve Agents: Mechanistic Evaluation and Application

Abstract

Two novel Ag­(I) complexes containing synergistic pyridine and amidoxime ligands (Ag-DPAAO and Ag-PAAO) were first designed as complex monomers. Taking advantage of the molecular imprinting technique and solvothermal method, molecular imprinted porous cross-linked polymers (MIPCPs) were developed as a robust platform for the first time to incorporate Ag-PAAO into a polymer material as a recyclable catalyst. Advantageously, the observed pseudo first-order rate constant (k obs) of MIPCP-Ag-PAAO-20% for ethyl-parathion (EP) hydrolysis is about 1.2 × 104-fold higher than that of self-hydrolysis (30 °C, pH = 9). Furthermore, the reaction mechanism of the MIPCP-containing Ag-PAAO-catalyzed organothiophosphate was analyzed in detail using density functional theory and experimental spectra, indicating that the amidoxime can display dual roles for both the key coordination with the silver ion and nucleophilic attack to weaken the P-OAr bond in the catalytic active site.

Designed Metal-Containing Peptoid Membranes as Enzyme Mimetics for Catalytic Organophosphate Degradation

“…This is consistent with previously reported OP hydrolysis triggered by Zn 2+ , Cu 2+ , Ni 2+ , and Co 2+ complexes. ,, As mentioned in these studies, the rate-determining step involves the nucleophilic attack of a hydroxide ion on the phosphorus center after the substrate binds with the metal ion. Unlike previous Zn 2+ or Ag + complexes owning hydroxyl groups next to the coordination site, ,,, acting as effective internal nucleophiles, the absence of this nucleophile close to the metal center of the peptoids leads to water being the sole source of external nucleophiles. Accordingly, the pH needs to be high enough to afford an adequate amount of aqueous hydroxide ions and thus to realize rapid OP hydrolysis.…”

“…Many studies have focused on mimicry of the PTE active site, which contains two zinc ions ligated by an aspartic acid (Asp-301), four histidines (His-55, His-57, His-201, and His-230), a carboxylated lysine (Lys-169), and a hydroxide ion (Figure b). , During hydrolysis, the binding between the phosphorus center of the OP and the active site metal ions and the intramolecular nucleophilic attack facilitated by neighboring amino acids are critical for the cleavage of phosphate ester bonds. , For example, various biomimetic catalysts have been developed by mimicking the active site of PTEs using small-molecule metal-binding ligands that preclude the precipitation of metal hydroxide and simultaneously afford active hydroxo–metal complexes. , Direct usage of the soluble monomeric metal complexes is effective , but is difficult to use for many applications, since they do not enable facile separation from the products and reactants. This disadvantage, however, can be conquered by immobilizing the complexes into various supports, such as porous organic polymers , or molecularly imprinted polymers. , However, the embedment of functional sites in the interior area of these materials reduces their accessibility to the substrate, dramatically impacting the overall catalytic efficiency. Thus, it is still a standing challenge to produce materials with optimal active site placement that are effective for catalytic reactions.…”

“…This disadvantage, however, can be conquered by immobilizing the complexes into various supports, such as porous organic polymers 15 , 16 or molecularly imprinted polymers. 16 , 17 However, the embedment of functional sites in the interior area of these materials reduces their accessibility to the substrate, dramatically impacting the overall catalytic efficiency. Thus, it is still a standing challenge to produce materials with optimal active site placement that are effective for catalytic reactions.…”

Recent development in the design of artificial enzymes through molecular imprinting technology

“…79 Guo's group designed two novel Ag( i ) complexes with synergistic pyridine and amidoxime ligands (Ag-PAAO) as functional monomers to mimic the organophosphorus hydrolase (OPH) catalytic site. 80,81 Fig. 5 represents the schematic diagram for the preparation of MIPCP-Ag-PAAO.…”