Chemistry

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When and how do external electric fields (EEFs) lead to catalysis in the presence of a (polar or nonpolar) solvent? This is the question that is addressed here using a combination of molecular dynamics (MD) simulations, quantum mechanical/molecular mechanical calculations with EEF, and quantum mechanical/(local) electric field calculations. The paper focuses on a model reaction, the Menshutkin reaction between CH3I and pyridine in three solvents of varying polarity. Using MD simulations, we find that the EEF causes the solvent to undergo organization; the solvent molecules gradually align with the applied field as the field strength increases. The collective orientation of the solvent molecules modifies the electrostatic environment around the Menshutkin species and induces a global electric field pointing in the opposite direction of the applied EEF. The combination of these two entangled effects leads to partial or complete screening of the EEF, with the extent of screening being proportional to the polarity/polarizability of the solvent. Nevertheless, we find that catalysis of the Menshutkin reaction inevitably emerges once the EEF exceeds the opposing field of the organizing solvent, i.e., once polarization of the Menshutkin complex is observed to set in. Overall, our analysis provides a lucid and pictorial interpretation of the behavior of solutions in the presence of EEFs and indicates that EEF-mediated catalysis should, in principle, be feasible in bulk setups, especially for nonpolar and mildly polar solvents. By application of the charge-transfer paradigm, it is shown that the emergence of OEEF catalysis in solution can be generalized to other reactions as well. Copyright © 2020 American Chemical Society.

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Journal of the American Chemical Society

Solvent Organization and Rate Regulation of a Menshutkin Reaction by Oriented External Electric Fields are Revealed by Combined MD and QM/MM Calculations

Synthesis of phosphorous-doped graphitic materials (P-Gc) using phytic acid as a precursor was done in a microwave oven in a cost- and time-effective green way. The material was used as a solid acid catalyst for microwave (MW)-assisted synthesis of β-ketoenamines and Baeyer-Villiger (BV) oxidation. In the case of BV oxidation, hydrogen peroxide (H2O2) was used as a green oxidant. For β-ketoenamines, in most cases, 100\% conversion with an ∼95\% yield was achieved in ethyl acetate medium. In solventfree conditions, the yield of β-ketoenamines was ∼75\%. A kinetic study suggested that the resonance stabilization of the positive reaction center happens in the transition state for β-ketoenamine synthesis. In BV oxidation, cyclic ketones were converted to their corresponding cyclic esters in good to high yields (∼80\% yield) in a shorter reaction time (6-20 min). As per our knowledge, this is the first report of BV oxidation catalyzed by a heteroatom-doped graphitic material. For BV oxidation, the phosphoric acid functional groups present in P-Gc might increase the electrophilicity of the carbonyl group of the ketones to compensate for the weakness of H2O2 as a nucleophile and a spiro-bisperoxide intermediate has been identified in high-resolution mass spectrometry. © 2020 American Chemical Society.

ACS Omega

Phosphorous-doped graphitic material as a solid acid catalyst for microwave-assisted synthesis of β-ketoenamines and baeyer-villiger oxidation

The pore forming Plasmodium Perforin Like Proteins (PPLP), expressed in all stages of the parasite life cycle are critical for completion of the parasite life cycle. The high sequence similarity in the central Membrane Attack Complex/ Perforin (MACPF) domain among PLPs and their distinct functional overlaps define them as lucrative target for developing multi-stage antimalarial therapeutics. Herein, we evaluated the mechanism of Pan-active MACPF Domain (PMD), a centrally located and highly conserved region of PPLPs, and deciphered the inhibitory potential of specifically designed PMD inhibitors. The E. coli expressed rPMD interacts with erythrocyte membrane and form pores of ~10.5 nm height and ~24.3 nm diameter leading to hemoglobin release and dextran uptake. The treatment with PMD induced erythrocytes senescence which can be hypothesized to account for the physiological effect of disseminated PLPs in loss of circulating erythrocytes inducing malaria anemia. The anti-PMD inhibitors effectively blocked intraerythrocytic growth by suppressing invasion and egress processes and protected erythrocytes against rPMD induced senescence. Moreover, these inhibitors also blocked the hepatic stage and transmission stage parasite development suggesting multi-stage, transmission-blocking potential of these inhibitors. Concievably, our study has introduced a novel set of anti-PMD inhibitors with pan-inhibitory activity against all the PPLPs members which can be developed into potent cross-stage antimalarial therapeutics along with erythrocyte senescence protective potential to occlude PPLPs mediated anemia in severe malaria. © Copyright © 2020 Garg, Shivappagowdar, Hada, Ayana, Bathula, Sen, Kalia, Pati, Singh and Singh.

Life Sciences

Frontiers in Cellular and Infection Microbiology

Plasmodium Perforin-Like Protein Pores on the Host Cell Membrane Contribute in Its Multistage Growth and Erythrocyte Senescence

Olefin bonds participate in co-reaction with the benzoxazine functionality of the monomer and are one of the strategies used to affect the crosslink density of a polybenzoxazine network. In general, the double bond incorporation in starting material is usually catalyzed by expensive, rare earth metals affecting the sustainability of the reaction. The natural abundance of feedstocks with inherent double bonds may be a powerful platform for the development of novel greener structures, with potential applications in polymers. Here, we report the design, synthesis, and characterization of a biobased non-halogen flame retardant, consisting of naturally occurring phenols, eugenol (E), and cardanol (C). The presence of a covalently linked phosphazene (P) core allowed the synthesis of hexa-functional flame retardant molecules, abbreviated as EP and CP. The chemical structures of the synthesized EP and CP were confirmed by Fourier transform infrared (FTIR), nuclear magnetic resonance (1H, 13C, 31P NMR), and single crystal XRD (only in the case of EP). Their polymerization with cardanol sourced tri-oxazine benzoxazine monomer, C-trisapm, was followed by FTIR, NMR, and DSC studies. The thermal stability and flame retardant properties of the hybrid phosphazene-benzoxazine copolymers was determined by thermogravimetry analysis (TGA), limiting oxygen index (LOI), vertical burning, and smoke density analyses. SEM images of the char residues of the polymers with or without the addition of reactive phosphazene molecules confirmed the intumescent flame retarding mechanism. Current work highlights the utility of sustainable origin non-halogen flame retardant (FR) molecules and their utility in polybenzoxazine chemistry. © Copyright © 2020 Appavoo, Amarnath and Lochab.

Frontiers in Chemistry

Cardanol and Eugenol Sourced Sustainable Non-halogen Flame Retardants for Enhanced Stability of Renewable Polybenzoxazines

Stereochemical Control of the Triflate Mediated Intramolecular Schmidt Reaction

In proteins, the main-chain N-H⋯OC hydrogen bonds (HBs) play a crucial role in the formation of α-helices and β-sheets. Accurate analysis of such hydrogen bonds and their electrostatic interaction energies is essential for studying binding interactions and for better understanding of the energetics involved in protein folding. Here, we studied 22 high-resolution (0.87 Å to 0.48 Å) secondary protein structures (4.7 kDa to 54.5 kDa) from the RCSB PDB and performed topological analyses of 1443 N-H⋯OC HBs (750 in α-helices and 693 in β-sheets) using the multipole analysis based experimental electron densities as transferred from the ELMAM2 database. This is the first study of its kind involving by far the largest number of high-resolution protein structures and HBs from both α-helices and β-sheets. Further, based on the accurate estimation of the electrostatic interaction energies, the excellent correlations with various topological parameters have been demonstrated. The excellent correlations have also been observed between the topological parameters. Thereby, we identified the limiting values of the topological parameters and the electrostatic interaction energies to establish the presence of the true N-H⋯OC HBs in protein main-chains via quantitative and qualitative analyses of electron densities using quantum crystallographic approaches-the quantum theory of atoms in molecules (QTAIM) and the noncovalent interaction (NCI) index. © The Royal Society of Chemistry.

CrystEngComm

Electron density based analysis of N-H⋯OC hydrogen bonds and electrostatic interaction energies in high-resolution secondary protein structures: Insights from quantum crystallographic approaches

Coumarin and its derivatives have potential applications in sensing, laser dyes, fluorescent probes, and dye sensitized solar cells due to their unique photophysical properties. However, synthesis and studies of linear pyridine-fused-π-extended coumarins remained unexplored. Here, we synthesised two π-expanded coumarin derivatives (PC1, PC2) in which molecular backbone is linearly extended with pyridyl ring where 5,7-positions are substituted with aryl groups. Single crystal X-ray structure analysis confirmed the molecular geometry. We found in spectroscopic studies that both coumarins in solutions show fluorescence via predominant radiative decay of charge transfer (CT) state due to rotational motion associated with the molecules. In addition, detailed spectroscopic analysis of both coumarins in presence of acid and base revealed the acidochromic property in solutions. Based on the acid-base sensing property of PC1, we could show its use as solid-state acid-base vapor sensor and anti-counterfeiting agent. © 2020

Journal of Luminescence

Synthesis, optical properties, acid-base vapochromism and anti-counterfeiting of novel π-extended pyridine fused coumarins

Development of materials for selective recovery of uranium (U) from contaminated water and sea water is necessary to solve the issues related to sources of U(VI) for nuclear energy and alleviating the environmental pollution. Phosphorous-doped Graphitic (P-Gc) material, synthesized under microwave irradiation (450 W, 40 s) has been utilized as potential adsorbent material for U removal in ppm level from contaminated water selectively at room temperature (RT). The influence of various environmental conditions (e.g. pH, contact time, ionic strength, and temperature) on U(VI) adsorption was investigated by batch method. Maximum Adsorption capacity (QM) was found 1186 mg g-1 from Langmuir fit and this could be due to high specific surface area of P-Gc (1635 m2/g) and the presence of phosphate functionalities (3.18 atomic wt\% P according to XPS) on the adsorbent surface. The results of kinetic and thermodynamic studies revealed that the adsorption was a spontaneous and exothermic chemical process and the adsorption process could be better fitted by a pseudo-second order model, suggesting a chemisorption over physisorption. The adsorption of U(VI) on P-Gc was a multilayer uptake in lower concentration range of U(VI) and monolayer in higher concentration range of U(VI). The material has been reused for five cycles with almost no change in the efficiency which is very important for the wastewater treatment. 90 \% of the adsorbed uranium was regenerated from P-Gc by simple acid treatment. Hence, P-Gc has a great promise in the quest to development of metal free adsorbent for uranium removal at RT. © 2020 Elsevier Ltd.

Journal of Environmental Chemical Engineering

Selective U(VI) removal using phosphorous-doped graphitic carbon

Efficient fluorophores with easy synthetic routes and fast responses are of great importance in clinical diagnostics. Herein, we report a new, rigid pentacyclic pyrylium fluorophore, PS-OMe, synthesised in a single step by a modified Vilsmeier-Haack reaction. Insights into the reaction mechanism facilitated a new reaction protocol for the efficient synthesis of PS-OMe which upon demethylation resulted in a "turn-on"pH sensor, PS-OH. This new fluorescent probe has been successfully used to monitor intracellular acidification at physiological pH. From the fluorescence image analysis, we were able to quantify the intracellular dynamic pH change during apoptosis. This new pH probe is a potential chemical tool for screening, drug discovery and dose determination in cancer therapy. © 2020 The Royal Society of Chemistry.

Chemical Science

A new pentacyclic pyrylium fluorescent probe that responds to pH imbalance during apoptosis

Single-component purely organic molecular crystals exhibiting negative thermal expansion (NTE) are extremely rare. Systematic studies on bromophenyl-substituted dihydro-imidazole, a novel single-component organic lightweight material, have resulted in dimorphs (centrosymmetric and noncentrosymmetric) at room temperature and two more crystal forms at low temperature (LT) upon reversible isosymmetric single-crystal-to-single-crystal transformation. While the noncentrosymmetric forms with a notable second harmonic generation activity exhibit colossal NTE and positive thermal expansion (PTE) effects with axial switching, the centrosymmetric LT phase experiences a moderate NTE but a colossal PTE. The interplay of the hydrogen bonds and weak interactions leading to the scissor jack-like molecular motion resulted in the unusual anisotropic thermal expansions over a large temperature range with record-wide (∼40 to ∼110 K) thermal hysteresis. These environmentally benign thermoresponsive materials experiencing wide thermal hysteresis, minimal volumetric thermal expansions, and reversible axial (biaxial and uniaxial) switching may find important technological applications in nonexpansive organic electronics and thermomechanical devices. © 2020 American Chemical Society

Journal of Physical Chemistry C

Unusual anisotropic thermal expansions with reversible axial switching and record-wide thermal hysteresis in single-component purely organic molecular crystals