Hydrophilic scaffolds of oxime as the potent catalytic inactivator of reactive organophosphate

Phosphoester bond hydrolysis by a discrete zirconium-oxo cluster: mechanistic insights into the central role of the binuclear ZrIV-ZrIV active site

Effective degradation of non-natural phosphate triesters (PTs) widely used in pesticides and warfare agents is of paramount relevance for human and environmental safety, particularly under acidic conditions where they are highly stable. Here, we present a detailed reactivity and mechanistic study pioneering discrete {Zr6O8} clusters, which are commonly employed as building blocks for Zr-MOFs and as non-classical soluble coordination compounds for the degradation of PTs using the pesticide ethyl paraoxon as a model. Combined computational studies, mechanistic experiments, and EXAFS analysis show that the reactivity of these clusters arises from their ZrIV-ZrIV bimetallic sites, which hydrolyze ethyl paraoxon under acidic conditions through an intramolecular pathway. Remarkably, the energetics of the reaction is dependent on the protonation state of the active sites, and a weakly acidic medium favors the reaction. Moreover, catalyst stability allowed for its recovery and reuse. Such a mechanism is in close analogy to enzymatic reactions and different from that previously reported for Zr-MOFs. These findings outline the potential of MIV-MIV active sites for PT degradation under challenging aqueous acidic conditions and contribute to the development of bioinspired catalysts and materials.

Caged Oxime Reactivators Designed for the Light Control of Acetylcholinesterase Reactivation†

Despite its promising role in the active control of biological functions by light, photocaging remains untested in acetylcholinesterase (AChE), a key enzyme in the cholinergic family. Here, we describe synthesis, photochemical properties and biochemical activities of two caged oxime compounds applied in the photocontrolled reactivation of the AChE inactivated by reactive organophosphate. Each of these consists of a photocleavable coumarin cage tethered to a known oxime reactivator for AChE that belongs in an either 2-(hydroxyimino)acetamide or pyridiniumaldoxime class. Of these, the first caged compound was able to successfully go through oxime uncaging upon irradiation at long-wavelength ultraviolet light (365 nm) or visible light (420 nm). It was further evaluated in AChE assays in vitro under variable light conditions to define its activity in the photocontrolled reactivation of paraoxon-inactivated AChE. This assay result showed its lack of activity in the dark but its induction of activity under light conditions only. In summary, this article reports a first class of light-activatable modulators for AChE and it offers assay methods and novel insights that help to achieve an effective design of caged compounds in the enzyme control.

Dual acting oximes designed for therapeutic decontamination of reactive organophosphates via catalytic inactivation and acetylcholinesterase reactivation

A conventional approach in the therapeutic decontamination of reactive organophosphate (OP) relies on chemical OP degradation by oxime compounds. However, their efficacy is limited due to their lack of activity in the reactivation of acetylcholinesterase (AChE), the primary target of OP. Here, we describe a set of α-nucleophile oxime derivatives which are newly identified for such dual modes of action. Thus, we prepared a 9-member oxime library, each composed of an OP-reactive oxime core linked to an amine-terminated scaffold, which varied through an N-alkyl functionalization. This library was screened by enzyme assays performed with human and electric eel subtypes of OP-inactivated AChE, which led to identifying three oxime leads that displayed significant enhancements in reactivation activity comparable to 2-PAM. They were able to reactivate both enzymes inactivated by three OP types including paraoxon, chlorpyrifos and malaoxon, suggesting their broad spectrum of OP susceptibility. All compounds in the library were able to retain catalytic reactivity in paraoxon inactivation by rates increased up to 5 or 8-fold relative to diacetylmonoxime (DAM) under controlled conditions at pH (8.0, 10.5) and temperature (17, 37 °C). Finally, selected lead compounds displayed superb efficacy in paraoxon decontamination on porcine skin in vitro. In summary, we addressed an unmet need in therapeutic OP decontamination by designing and validating a series of congeneric oximes that display dual modes of action.

Skin decontamination procedures against potential hazards substances exposure

Decontamination of unprotected skin areas is crucial to prevent excessive penetration of chemical contaminants after criminal or accidental release. A review of literature studies was performed to identify the available decontamination methods adopted to treat skin contamination after chemical, radiological and metal exposures. In this bibliographic review, an overview of the old and recent works on decontamination procedures followed in case of potential hazards substances contaminations with a comparison between these systems are provided. Almost all data from our 95 selected studies conducted in vitro and in vivo revealed that a rapid skin decontamination process is the most efficient way to reduce the risk of intoxication. The commonly-used or recommended conventional procedures are simple rinsing with water only or soapy water. However, this approach has some limitations because an easy removal by flushing may not be sufficient to decontaminate all chemical deposited on the skin, and skin absorption can be enhanced by the wash-in effect. Other liquid solutions or systems as adsorbent powders, mobilizing agents, chelation therapy are also applied as decontaminants, but till nowadays does not exist a decontamination method which can be adopted in all situations. Therefore, there is an urgent need to develop more efficient and successful decontaminating formulations.

Nanomaterial-Enabled Sensors and Therapeutic Platforms for Reactive Organophosphates

Unintended exposure to harmful reactive organophosphates (OP), which comprise a group of nerve agents and agricultural pesticides, continues to pose a serious threat to human health and ecosystems due to their toxicity and prolonged stability. This underscores an unmet need for developing technologies that will allow sensitive OP detection, rapid decontamination and effective treatment of OP intoxication. Here, this article aims to review the status and prospect of emerging nanotechnologies and multifunctional nanomaterials that have shown considerable potential in advancing detection methods and treatment modalities. It begins with a brief introduction to OP types and their biochemical basis of toxicity followed by nanomaterial applications in two topical areas of primary interest. One topic relates to nanomaterial-based sensors which are applicable for OP detection and quantitative analysis by electrochemical, fluorescent, luminescent and spectrophotometric methods. The other topic is directed on nanotherapeutic platforms developed as OP remedies, which comprise nanocarriers for antidote drug delivery and nanoscavengers for OP inactivation and decontamination. In summary, this article addresses OP-responsive nanomaterials, their design concepts and growing impact on advancing our capability in the development of OP sensors, decontaminants and therapies.

Impact of nanoceria shape on degradation of diethyl paraoxon: Synthesis, catalytic mechanism, and water remediation application

A series of nanomaterials have been demonstrated to be powerful for direct degradation of diethyl paraoxon (EP) to diethyl phosphate and 4-nitrophenol in aqueous solution. However, comparison of catalytic activity of different nanomaterials toward EP is rarely explored. In the present study, four different morphological nanoceria (cubes, rods, polyhedral, and spheres) were synthesized, characterized, and evaluated as a catalyst for the degradation of EP in comparison to other commercially available nanomaterials. Among the tested nanoceria, the cerium dioxide (CeO₂) nanopolyhedra possess the best catalytic activity toward the hydrolysis of EP owing to their abundant oxygen vacancy sites, optimal ratio of Ce(III) to Ce(IV), and specific exposed facets. Under the conditions of 0.2 M NH₃/NH₄Cl buffer and 25 °C, the CeO₂ nanopolyhedra catalyzed the reduction of EP to 4-nitrophenol with a >99% conversion at pH 8.0 for 50 h, at pH 10.0 for 12 h, and at pH 12.0 for 2.5 h. The catalytic degradation of nearly 100% EP in NH₃/NH₄Cl buffer (pH 10.0) at 25 °C is in the decreasing order of CeO₂ nanopolyhedra > CeO₂ nanorods > ZnO nanospheres (NSs) > CeO₂ nanocubes > TiO₂ NSs > CeO₂ NSs > Fe₃O₄ NSs ~ Co₃O₄ NSs ~ control experiment. The mechanism for the degradation of EP was confirmed by monitoring catalytic kinetics of the CeO₂ nanopolyhedra in the presence of EP, dimethyl paraoxon, 4-nitrophenyl phosphate, and parathion. The nanocomposites were simply fabricated by electrostatic self-assembly of the CeO₂ nanopolyhedra and poly(diallyldimethylammonium chloride)-capped gold nanoparticles (PDDA-AuNPs). The resultant nanocomposites still efficiently catalyzed NaBH₄-mediated reduction of 4-nitrophenol to 4-aminophenol with a normalized rate constant of 6.68 ± 0.72 s^-1 g^-1 and a chemoselectivity of >99%. In confirmation of the robustness and applicability of the as-prepared nanocomposites, they were further used to catalyze the degradation of EP to 4-amionphenol in river water and seawater.

Shielded α-Nucleophile Nanoreactor for Topical Decontamination of Reactive Organophosphate

Here, we describe a nanoscale reactor strategy with a topical application in the therapeutic decontamination of reactive organophosphates (OPs) as chemical threat agents. It involves functionalization of poly(amidoamine) dendrimer through a combination of its partial PEG shielding and exhaustive conjugation with an OP-reactive α-nucleophile moiety at its peripheral branches. We prepared a 16-member library composed of two α-nucleophile classes (oxime, hydroxamic acid), each varying in its reactor valency (43-176 reactive units per nanoparticle), and linker framework for α-nucleophile tethering. Their mechanism for OP inactivation occurred via nucleophilic catalysis as verified against P-O and P-S bonded OPs including paraoxon-ethyl (POX), malaoxon, and omethoate by ¹H NMR spectroscopy. Screening their reactivity for POX inactivation was performed under pH- and temperature-controlled conditions, which resulted in identifying 13 conjugates, each showing shorter POX half-life up to 2 times as compared to a reference Dekon 139 at pH 10.5, 37 °C. Of these, 10 conjugates were further confirmed for greater efficacy in POX decontamination experiments performed in two skin models, porcine skin and an artificial human microtissue. Finally, a few lead conjugates were selected and demonstrated for their biocompatibility in vitro as evident with lack of skin absorption, no inhibition of acetylcholinesterase (AChE), and no cytotoxicity in human neuroblastoma cells. In summary, this study presents a novel nanoreactor library, its screening methods, and identification of potent lead conjugates with potential for therapeutic OP decontamination.

Reactivity and mechanism of α-nucleophile scaffolds as catalytic organophosphate scavengers

Despite their unique benefits imparted by their structure and reactivity, certain α-nucleophile molecules remain underexplored as chemical inactivators for the topical decontamination of reactive organophosphates (OPs). Here, we present a library of thirty α-nucleophile scaffolds, each designed with either a pyridinium aldoxime (PAM) or hydroxamic acid (HA) α-nucleophile core tethered to a polar or charged scaffold for optimized physicochemical properties and reactivity. These library compounds were screened for their abilities to catalyze the hydrolysis of a model OP, paraoxon (POX), in kinetic assays. These screening experiments led to the identification of multiple lead compounds with the ability to inactivate POX two- to four-times more rapidly than Dekon 139-the active ingredient currently used for skin decontamination of OPs. Our mechanistic studies, performed under variable pH and temperature conditions suggested that the differences in the reactivity and activation energy of these compounds are fundamentally attributable to the core nucleophilicity and pKa. Following their screening and mechanistic studies, select lead compounds were further evaluated and demonstrated greater efficacy than Dekon 139 in the topical decontamination of POX in an ex vivo porcine skin model. In addition to OP reactivity, several compounds in the PAM class displayed a dual mode of activity, as they retained the ability to reactivate POX-inhibited acetylcholine esterase (AChE). In summary, this report describes a rationale for the hydrophilic scaffold design of α-nucleophiles, and it offers advanced insights into their chemical reactivity, mechanism, and practical utility as OP decontaminants.