Benchmark ab initio characterization of the complex potential energy surfaces of the HOO- + CH3Y [Y = F, Cl, Br, I] reactions

The α-effect is a well-known phenomenon in organic chemistry, and is related to the enhanced reactivity of nucleophiles involving one or more lone-pair electrons adjacent to the nucleophilic center. The gas-phase bimolecular nucleophilic substitution (SN2) reactions of α-nucleophile HOO- with methyl halides have been thoroughly investigated experimentally and theoretically; however, these investigations have mainly focused on identifying and characterizing the α-effect of HOO-. Here, we perform the first comprehensive high-level ab initio mapping for the HOO- + CH3Y [Y = F, Cl, Br and I] reactions utilizing the modern explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ [n = 2-4] basis sets. The present ab initio characterization considers five distinct product channels of SN2: (CH3OOH + Y-), proton abstraction (CH2Y- + H2O2), peroxide ion substitution (CH3OO- + HY), SN2-induced elimination (CH2O + HY + HO-) and SN2-induced rearrangement (CH2(OH)O- + HY). Moreover, besides the traditional back-side attack Walden inversion, the pathways of front-side attack, double inversion and halogen-bond complex formation have also been explored for SN2. With regard to the Walden inversion of HOO- + CH3Cl, the previously unaddressed discrepancies concerning the geometry of the corresponding transition state are clarified. For the HOO- + CH3F reaction, the recently identified SN2-induced elimination is found to be more exothermic than the SN2 channel, submerged by ∼36 kcal mol-1. The accuracy of our high-level ab initio calculations performed in the present study is validated by the fact that our new benchmark 0 K reaction enthalpies show excellent agreement with the experimental data in nearly all cases.

Reduction and Degradation of Paraoxon in Water Using Zero-Valent Iron Nanoparticles

Paraoxon is an emerging organophosphate pollutant that is commonly used as a pesticide and a drug, hence increasing the risk of contamination of water supplies. Its intensive use for vector control has led to pollutions in soil and water. Paraoxon is very toxic, with an LD50 of 2 to 30 mg/kg in rats. It can be metabolized in the body from parathion; thus, exposure can lead to serious health effects. In this study, zero valent iron (Fe°/ZVI NPs) nanoparticles were synthesized and investigated for the degradation of Paraoxon, a chemical warfare agent and insecticide, in an aqueous solution. The effects of solution pH, initial pollutant concentration, ZVI NPs dosage and contact time on mineralization efficiency were examined. Batch experiments demonstrated that 15 mg L−1 of Paraoxon was mineralized at degradation efficiencies of 75.9%, 63.9% and 48.9% after three-hour treatment with 6.0, 4.0 and 2.0% w/v Fe°, respectively. The calculated kinetic rate constant kobs was 0.4791 h−1, 0.4519 h−1 and 0.4175 h−1 after treating 10, 15 and 20 mg L−1 of Paraoxon solution with 6.0% w/v Fe, respectively. The degradation dynamics were described by the first-order kinetic law as evidenced by rate constants independent of the initial Paraoxon concentration. The degradation efficiency was strongly dependent on pH, increasing with a decrease in pH, with maximum removal at pH 4. p-nitrophenol was detected as a degradation product, suggesting cleavage of the O-P bond and hydrolysis as possible reaction processes. This study showed that Fe° particles have the potential for degrading Paraoxon.

Origin of the α-Effect in SN 2 Reactions

The α-effect is a term used to explain the dramatically enhanced reactivity of α-nucleophiles (R-Y-X:- ) compared to their parent normal nucleophile (R-X:- ) by deviating from the classical Brønsted-type reactivity-basicity relationship. The exact origin of this effect is, however, still heavily under debate. In this work, we have quantum chemically analyzed the α-effect of a set of anionic nucleophiles, including O-, N- and S-based normal and α-nucleophiles, participating in an SN 2 reaction with ethyl chloride using relativistic density functional theory at ZORA-OLYP/QZ4P. Our activation strain and Kohn-Sham molecular orbital analyses identified two criteria an α-nucleophile needs to fulfill in order to show α-effect: (i) a small HOMO lobe on the nucleophilic center, pointing towards the substrate, to reduce the repulsive occupied-occupied orbital overlap and hence (steric) Pauli repulsion with the substrate; and (ii) a sufficiently high energy HOMO to overcome the loss of favorable HOMO-LUMO orbital overlap with the substrate, as a consequence of the first criterion, by reducing the HOMO-LUMO orbital energy gap. If one of these two criteria is not fulfilled, one can expect no α-effect or inverse α-effect.

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.

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.

Protein-Metal-Ion Interactions Studied by Mass Spectrometry-Based Footprinting with Isotope-Encoded Benzhydrazide

Metal ions, usually bound by various amino-acid side chains in proteins, play multiple roles in protein folding, conformational change, cellular communication, and catalysis. Ca(II) and Mg(II), abundant among biologically relevant cations, execute their cellular functions associated with the conformational change of bound proteins. They bind with proteins where carboxylic acid residues are dominant ligands. To develop mass spectrometry for mapping protein-binding sites, we implemented a new carboxyl group footprinter, benzhydrazide, and refined it with isotope encoding. The method uses carbodiimide chemistry to footprint carboxylic residues, whereby 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide activates a carboxyl group followed by nucleophilic attack by benzhydrazide forming a stable labeled product. We tested the effectiveness of isotope-encoded benzhydrazide by studying Ca²⁺ and Mg²⁺ binding of calmodulin, an EF-hand protein. The footprinting results indicate that the four active sites for metal-ion binding (EF hands I, II, III, and IV) and the linker region (peptide 78-86) undergo conformational changes upon Ca(II) and Mg(II) binding, respectively. The outcome is consistent with previously reported results and 3-D structures, thereby validating a new reagent that is more reactive and discriminating for specific amino-acid protein footprinting. This reagent should be important for locating metal-binding sites of other metalloproteins.

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

Despite its efficacy as a skin decontaminant of reactive organophosphates (OP), Dekon 139-a potassium salt of 2,3-butanedione monooxime (DAM)-is associated with adverse events related to percutaneous absorption largely due to its small size and lipophilicity. In order to address this physicochemical issue, we synthesized and evaluated the activity of a focused library of 14 hydrophilic oxime compounds, each designed with either a DAM or monoisonitrosoacetone (MINA) oxime tethered to a polar or charged scaffold in order to optimize the size, hydrophilicity, and oxime acidity. High-throughput colorimetric assays were performed with paraoxon (POX) as a model OP to determine the kinetics of POX inactivation by these compounds under various pH and temperature conditions. This primary screening led to the identification of 6 lead compounds, predominantly in the MINA series, which displayed superb catalytic activity by reducing the POX half-life (t_1/2) by 2-3 fold relative to Dekon 139. Our mechanistic studies show that POX inactivation by the oxime compounds occurred faster at a higher temperature and in a pH-dependent manner in which the negatively charged oximate species is ≥ 10-fold more effective than the neutral oxime species. Lastly, using one of the lead compounds, we demonstrated its promising efficacy for POX decontamination in porcine skin ex vivo, and showed its potent ability to protect acetylcholine esterase (AChE) through POX inactivation. In summary, we report the rational design and chemical biological validation of novel hydrophilic oximes which address an unmet need in therapeutic OP decontamination.