Palladium-Catalyzed, Site-Selective C(sp2)8-H Halogenation and Nitration of 4-Quinolone Derivatives.

Selective installation of halo and nitro groups in heterocyclic backbone through a transition-metal-catalyzed C-H bond activation strategy is immensely alluring to access high-value scaffolds. Here in, we disclosed N-pyrimidyl-directed assisted palladium(II)-catalyzed C(sp2)8-H halogenation and nitration of substituted 4-quinolone derivatives in the presence of N-halosuccinimide and tert-butyl nitrite, respectively, offering structurally diversified 8-halo/nitro-embedded 4-quinolone frameworks in high yields. Mechanistic studies indicated that the reaction follows an organometallic pathway with a reversible C-H metalation step. This operationally simple protocol is scalable with a broad substrate scope and excellent functional group compatibility. Moreover, the postdiversifications of the synthesized derivatives are also showcased to ensure the synthetic versatility of the methodology.

Copper(II)-catalyzed, site-selective C(sp)2-H amination using 8-aminoimidazo[1,2-a]pyridine (8-AIP) as a directing group.

An efficient copper(II)-catalyzed regioselective ortho C(sp2)-H amination of arenes/heteroarenes has been developed with the assistance of 8-AIP (8-aminoimidazo[1,2-a]pyridine) as an efficacious 6,5-fused bicyclic removable chelating auxiliary. This operationally simple approach is scalable, has a broad substrate scope, and is highly compatible with functional groups. Furthermore, post-diversification of the synthesized derivatives demonstrates the methodology's synthetic adaptability.

Multienzyme Mimicking Cascade Mn3O4 Catalyst to Augment Reactive Oxygen Species Elimination and Colorimetric Detection: A Study of Phase Variation upon Calcination Temperature.

Over decades, nanozyme has served as a better replacement of bioenzymes and fulfills most of the shortcomings and intrinsic disadvantages of bioenzymes. Recently, manganese-based nanomaterials have been highly noticed for redox-modulated multienzyme mimicking activity and wide applications in biosensing and biomedical science. The redox-modulated multienzyme mimicking activity was highly in tune with their size, surface functionalization, and charge on the surface and phases. On the subject of calcination temperature to Mn3O4 nanoparticles (NPs), its phase has been transformed to Mn2O3 NPs and Mn5O8 NPs upon different calcination temperatures. Assigning precise structure-property connections is made easier by preparing the various manganese oxides in a single step. The present study has focused on the variation of multienzyme mimicking activity with different phases of Mn3O4 NPs, so that they can be equipped for multifunctional activity with greater potential. Herein, spherical Mn3O4 NPs have been synthesized via a one-step coprecipitation method, and other phases are obtained by direct calcination. The calcination temperature varies to 100, 200, 400, and 600 °C and the corresponding manganese oxide NPs are named M-100, M-200, M-400, and M-600, respectively. The phase transformation and crystalline structure are evaluated by powder X-ray diffraction and selected-area electron diffraction analysis. The different surface morphologies are easily navigated by Fourier transform infrared, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy analysis. Fortunately, for the mixed valence state of Mn3O4 NPs, all phases of manganese oxide NPs showed multienzyme mimicking activity including superoxide dismutase (SOD), catalase, oxidase (OD), and peroxidase; therefore, it offers a synergistic antioxidant ability to overexpose reactive oxygen species. Mn3O4 NPs exhibited good SOD-like enzyme activity, which allowed it to effectively remove the active oxygen (O2•-) from cigarette smoke. A sensitive colorimetric sensor with a low detection limit and a promising linear range has been designed to detect two isomeric phenolic pollutants, hydroquinone (H2Q) and catechol (CA), by utilizing optimized OD activity. The current probe has outstanding sensitivity and selectivity as well as the ability to visually detect two isomers with the unaided eye.

A Water-Soluble Wavy Coordination Polymer of Cu(II) as a Turn-On Luminescent Probe for Histidine and Histidine-Rich Proteins/Peptides.

Histidine plays an essential role in most biological systems. Changes in the homeostasis of histidine and histidine-rich proteins are connected to several diseases. Herein, we report a water-soluble Cu(II) coordination polymer, labeled CuCP, for the fluorimetric detection of histidine and histidine-rich proteins and peptides. Single-crystal structure determination of CuCP revealed a two-dimensional wavy network structure in which a carboxylate group connects the individual Cu(II) dimer unit in a syn-anti conformation. The weakly luminescent and water-soluble CuCP shows turn-on blue emission in the presence of histidine and histidine-rich peptides and proteins. The polymer can also stain histidine-rich proteins via gel electrophoresis. The limits of quantifications for histidine, glycine-histidine, serine-histidine, human serum albumin (HSA), bovine serum albumin, pepsin, trypsin, and lysozyme were found to be 300, 160, 600, 300, 600, 800, 120, and 290 nM, respectively. Utilizing the fluorescence turn-on property of CuCP, we measured HSA quantitatively in the urine samples. We also validated the present urinary HSA measurement assay with existing analytical techniques. Job's plot, 1H NMR, high-resolution mass spectrometry (HRMS), electron paramagnetic resonance (EPR), fluorescence, and UV-vis studies confirmed the ligand displacement from CuCP in the presence of histidine.

Label-Free Detection of Epinephrine Using Flower-like Biomimetic CuS Antioxidant Nanozymes.

A biosensor comprising crystalline CuS nanoparticles (NPs) was synthesized via a one-step simple coprecipitation route without involvement of a surfactant. The powder X-ray diffraction method has been used to evaluate the crystalline nature and different phases consist of the formation of CuS NPs. Mainly hexagonal unit cells consist of the formation of CuS NP unit cells. Most of the surfaces are covered with rhombohedral microparticles with a smooth exterior and surface clustering, examined by SEM images, and the shape of NPs was spherical, having an average size of 23 nm, as confirmed by TEM analysis. This study has focused on the peroxidase-mimicking activity, superoxide dismutase (SOD)-mimicking activity, and chemosensor-based colorimetric determination and detection of epinephrine (EP) neurotransmitters with excellent selectivity. The CuS NPs catalyzed the oxidation of the oxidase substrate 3, 3-5, 5 tetramethyl benzidine (TMB) with the help of supplementary H2O2 that followed Michaelis-Menten kinetics with excellent Km and Vmax values calculated by the Lineweaver-Burk plot. Taking advantage of the drop in absorbance upon introduction of EP for the CuS NPs-TMB/H2O2 system, a colorimetric route has been developed for selective and real-time detection of EP. The sensitivity of the new colorimetric probe was vibrant, having a linear range of 0-16 µM, and achieved a low limit of detection of 457 nM. Moreover, the present nanosystem exhibited appreciable SOD-mimicking activity which could effectively remove O2•- from commercial cigarette smoke, along with it acting as a potential radical scavenger as well. The new nanosystem effectively scavenged •OH, O2.-, and metal chelation which were investigated calorimetrically.

Revealing the cholinergic inhibition mechanism of Alzheimer's by galantamine: a metadynamics simulation study.

Galantamine is one of the approved drugs based on the cholinergic hypothesis for the symptomatic treatment of mild to moderate Alzheimer's disease (AD). The etiology of AD is not fully known; however, the reported cholinergic hypothesis suggests the inadequate synthesis of the neurotransmitter acetylcholine (ACh) is responsible for this disease. The crystal structure of galantamine bound human acetylcholinesterase (hAChE) has been reported; however, the inhibition mechanism of hAChE by galantamine is not well understood. A Well-tempered metadynamics (WTMtD) simulation study has been performed with the crystal structure of galantamine bound hAChE. The reported mechanism for the degradation of ACh is suggested through a proton transfer process from a carboxylic group of Glu334 to the hydroxyl group of Ser203, which attacks ACh for the degradation to acetic acid and choline. Such proton transfer process is lowered in the presence of galantamine due to the separation of catalytic triad inside the gorge of AChE as observed with WTMtD. A docking study has been performed to examine the ACh's binding with the catalytic triad of galantamine bound hAChE. The docking results reveal that the approach of ACh to the catalytic triad is interrupted due to the galantamine's presence in the gorge of the enzyme.

Revealing the Inhibition Mechanism of RNA-Dependent RNA Polymerase (RdRp) of SARS-CoV-2 by Remdesivir and Nucleotide Analogues: A Molecular Dynamics Simulation Study.

Antiviral drug therapy against SARS-CoV-2 is not yet established and posing a serious global health issue. Remdesivir is the first antiviral compound approved by the US FDA for the SARS-CoV-2 treatment for emergency use, targeting RNA-dependent RNA polymerase (RdRp) enzyme. In this work, we have examined the action of remdesivir and other two ligands screened from the library of nucleotide analogues using docking and molecular dynamics (MD) simulation studies. The MD simulations have been performed for all the ligand-bound RdRp complexes for the 30 ns time scale. This is one of the earlier reports to perform the MD simulations studies using the SARS-CoV-2 RdRp crystal structure (PDB ID 7BTF). The MD trajectories were analyzed and Molecular Mechanics Poisson-Boltzmann Surface Area (MM-PBSA) calculations were performed to calculate the binding free energy. The binding energy data reveal that compound-17 (-59.6 kcal/mol) binds more strongly as compared to compound-8 (-46.3 kcal/mol) and remdesivir (-29.7 kcal/mol) with RdRp. The detailed analysis of trajectories shows that the remdesivir binds in the catalytic site and forms a hydrogen bond with the catalytic residues from 0 to 0.46 ns. Compound-8 binds in the catalytic site but does not form direct hydrogen bonds with catalytic residues. Compound-17 showed the formation of hydrogen bonds with catalytic residues throughout the simulation process. The MD simulation results such as hydrogen bonding, the center of mass distance analysis, snapshots at a different time interval, and binding energy suggest that compound-17 binds strongly with RdRp of SARS-CoV-2 and has the potential to develop as a new antiviral against COVID-19. Further, the frontier molecular orbital analysis and molecular electrostatic potential (MESP) iso-surface analysis using DFT calculations shed light on the superior binding of compound-17 with RdRp compared to remdesivir and compound-8. The computed as well as the experimentally reported pharmacokinetics and toxicity parameters of compound-17 is encouraging and therefore can be one of the potential candidates for the treatment of COVID-19.

Quantum chemical and well-tempered metadynamics study to design adenine analogs for orthogonal Preq1 riboswitch.

Communicated by Ramaswamy H. Sarma.

Revealing the mechanistic pathway of cholinergic inhibition of Alzheimer's disease by donepezil: a metadynamics simulation study.

Donepezil, an acetylcholinesterase inhibitor, is an approved drug for the symptomatic treatment of Alzheimer's disease (AD). The mechanistic pathway for the inhibition mechanism of acetylcholinesterase (AChE) by donepezil is not well explored. We report for the first time, the inhibition mechanism of AChE by the donepezil drug molecule for the hydrolysis of acetylcholine (ACh) with docking and well-tempered metadynamics (WTMtD) simulations with a human acetylcholinesterase (hAChE) crystal structure (). This study explored the orientation of the donepezil drug molecule inside the gorge of AChE. The 1D free energy surface obtained from WTMtD simulation studies reveals that the orientation of donepezil in the crystal donepezil (-87.25 kJ mol-1) is energetically more favored than the other orientation of donepezil (-74.74 kJ mol-1) for inhibition of AChE. The free energy landscape computation for the two sets of CVs further corroborates the 1D free energy surface. The WTMtD simulation performed with the crystal structure of donepezil bound hAChE gives the conformation of donepezil at Basin-I as similar to the conformation of donepezil observed in the crystal structure (). The WTMtD simulations further reveal that the bridged water molecules are more ordered near the catalytic triad of AChE to deter the nucleophilicity of Ser203 through intermolecular hydrogen bonding when donepezil approaches near to the active site gorge of AChE. The presence of donepezil near the active site of AChE can inhibit its approach for ACh hydrolysis; this is revealed through the docking study, where the drug molecule inside the active gorge of hAChE restricts the approach of ACh to Ser203 for the hydrolysis process.

Influence of gauche effect on uncharged oxime reactivators for the reactivation of tabun-inhibited AChE: quantum chemical and steered molecular dynamics studies.

The neutral oxime reactivator RS194B with a seven-membered ring has shown better efficacy towards the tabun-inhibited AChE than that of RS69N with a six-membered ring and RS41A with a five-membered ring. The difference in the efficacy of these reactivators has remained unexplored. We report here the origin of the difference of efficacy of these reactivators based on the conformational analysis, quantum chemical calculations and steered molecular dynamics (SMD) simulations. The conformational analysis using B3LYP/6-31G(d) level of theory revealed that RS41A and RS194B are more stable in gauche conformation due to the gauche effect (-N-C-C-N- bonds) whereas RS69N prefers anti-conformation. The SMD simulations show that RS194B retains in more stable gauche conformation inside the active gorge of AChE during different time intervals that experiences more hydrogen bonding, hydrophobic interactions with the catalytic anionic site (CAS) residues and weaker interactions with the peripheral anionic site (PAS) residues compared to RS41A and RS69N. In an effort to design an even superior reactivator, RS194B-S has been chosen with a subtle change in the geometry of RS194B by replacing the carbonyl oxygen with the sulfur atom. The newly designed reactivator RS194B-S can also be a promising candidate to reactivate tabun-inhibited AChE.

Revealing the importance of linkers in K-series oxime reactivators for tabun-inhibited AChE using quantum chemical, docking and SMD studies.

Inhibition of acetylcholinesterase (AChE) with organophosphorus compounds has a detrimental effect on human life. Oxime K203 seems to be one of the promising reactivators for tabun-inhibited AChE than (K027, K127, and K628). These reactivators differ only in the linker units between the two pyridinium rings. The conformational analyses performed with quantum chemical RHF/6-31G* level for K027, K127, K203 and K628 showed that the minimum energy conformers have different orientations of the active and peripheral pyridinium rings for these reactivator molecules. K203 with (-CH2-CH=CH-CH2-) linker unit possesses more open conformation compared to the other reactivators. Such orientation of K203 experiences favorable interaction with the surrounding residues of catalytic anionic site (CAS) and peripheral anionic site (PAS) of tabun-inhibited AChE. From the steered molecular dynamics simulations, it has been observed that the oxygen atom of the oxime group of K203 reactivator approaches nearest to the P-atom of the SUN203 (3.75 Å) at lower time scales (less than ~1000 ps) as compared to the other reactivators. K203 experiences less number of hydrophobic interaction with the PAS residues which is suggested to be an important factor for the efficient reactivation process. In addition, K203 crates large number of H-bonding with CAS residues SUN203, Phe295, Tyr337, Phe338 and His447. K203 barely changes its conformation during the SMD simulation process and hence the energy penalty to adopt any other conformation is minimal in this case as compared to the other reactivators. The molecular mechanics and Poisson-Boltzmann surface area binding energies obtained for the interaction of K203 inside the gorge of tabun inhibited AChE is substantially higher (-290.2 kcal/mol) than the corresponding K628 reactivator (-260.4 kcal/mol), which also possess unsaturated aromatic linker unit.

The reactivation of tabun-inhibited mutant AChE with Ortho-7: steered molecular dynamics and quantum chemical studies.

A highly toxic nerve agent, tabun, can inhibit acetylcholinesterase (AChE) at cholinergic sites, which leads to serious cardiovascular complications, respiratory compromise and death. We have examined the structural features of the tabun-conjugated AChE complex with an oxime reactivator, Ortho-7, to provide a strategy for designing new and efficient reactivators. Mutation of mAChE within the choline binding site by Y337A and F338A and its interaction with Ortho-7 has been investigated using steered molecular dynamics (SMD) and quantum chemical methods. The overall study shows that after mutagenesis (Y337A), the reactivator can approach more freely towards the phosphorylated active site of serine without any significant steric hindrance in the presence of tabun compared to the wild type and double mutant. Furthermore, the poor binding of Ortho-7 with the peripheral residues of mAChE in the case of the single mutant compared to that of the wild-type and double mutant (Y337A/F338A) can contribute to better efficacy in the former case. Ortho-7 has formed a greater number of hydrogen bonds with the active site surrounding residues His447 and Phe295 in the case of the single mutant (Y337A), and that stabilizes the drug molecule for an effective reactivation process. The DFT M05-2X/6-31+G(d) level of theory shows that the binding energy of Ortho-7 with the single mutant (Y337A) is energetically more preferred (-19.8 kcal mol(-1)) than the wild-type (-8.1 kcal mol(-1)) and double mutant (Y337A/F338A) (-16.0 kcal mol(-1)). The study reveals that both the orientation of the oxime reactivator for nucleophilic attack and the stabilization of the reactivator at the active site would be crucial for the design of an efficient reactivator.

In silico studies on the role of mutant Y337A to reactivate tabun inhibited mAChE with K048.

Organophosphorus compound (OP) tabun is resistant to reactivate by many oxime drugs after the formation of OP-conjugate with AChE. The reactivation of tabun-inhibited mAChE and site-directed mutants by bispyridinium oxime, K048 (N-[4-(4-hydroxyiminomethylpyridinio)butyl]-4-carbamoylpyridinium dibromide) showed that the mutations significantly poor the overall reactivation efficacy of K048. We have unravelled the lowered efficacy of K048 with the tabun-mutant mAChE(Y337A) using docking and steered molecular dynamics (SMD) simulations. The computed results showed some interesting features for the interaction of drug molecule K048 with tabun-mAChE(wild-type) and tabun-mutant mAChE(Y337A). The SMD simulations showed that the active pyridinium ring of K048 is directed towards the phosphorus atom conjugated to the active serine (SUN203) of tabun-mAChE(wild-type). The cradle shaped residues Tyr337-Phe338 present in the choline binding site stabilize the active pyridinium ring of K048 with π-π interaction and the residue Trp86 involved in T-shaped cation-π interaction. However, in the case of tabun-mutant mAChE(Y337A).K048 conjugate, the replacement of aromatic Tyr337 with the aliphatic alanine unit in the choline binding site, however, loses one of the π-π interaction between the active pyridinium ring of K048 and the Tyr337. The placement of aliphatic alanine unit resulted in the displacement of the side chain of Phe338 towards the His447. Such displacement is causing the inaccessibility of the drug towards the phosphorus atom conjugated to the active serine (SUN203) of tabun-mutant mAChE(Y337A). Furthermore, the unbinding of the K048 with SMD studies showed that the active pyridinium ring of the drug undergoes a complete turn along the gorge axis and is directed away from the phosphorus atom conjugated to the active serine of the tabun-mutant mAChE(Y337A). Such effects inside the gorge of tabun-mutant mAChE(Y337A) would lower the efficacy of the drug molecule (K048) for the reactivation process. The binding free energy computed for the tabun-mAChE(wild-type) and tabun-mutant mAChE(Y337A) with K048 showed that the drug molecule prefers to bind strongly with the former enzyme (∼30 kJ/mol) than the later one.

Absolute configuration and total synthesis of a novel antimalarial lipopeptide by the de novo preparation of chiral nonproteinogenic amino acids.

The absolute configuration (via degradation and Marfey's derivatization studies) and the total synthesis of a novel antimalarial lipid-peptide isolated from Streptomyces sp. (IC(50) = 0.8 µM, Plasmodium falciparum 3D7) is disclosed. To this end, versatile stereocontrolled routes to nonproteinogenic amino acids (via catalytic Mannich, Sharpless methods) and enantiomeric trans fatty acids (via Evans alkylation, Kocienski-Julia olefination) have been developed.