High Temperature Transformation of Iron Minerals in Bauxite

Mössbauer hyperfine and X-ray diffraction studies have been carried on original bauxite sample and thermally treated (between 200 and 1200°C) samples. Mössbauer spectrum of original bauxite shows the dominance of hematite (α-Fe₂O₃) along with Al-substituted hematite and goethite (α-FeOOH) of fine particle size. Mössbauer parameters for thermally treated samples indicate transformation of almost all of iron oxy-hydroxides into hematite up to 350°C. Around 800°C the formation of a new iron phase, assigned as iron substituted mullite (3Al₂O₃ 2SiO₃), occurs and its intensity increases with the rise in temperature and reaches a saturation around 1200°C. Furthermore, our studies confirm that HCl leaching is quite effective for the iron oxide removal.     

Densely functionalized 1,2-dihydrobenzo[b][1,6]naphthyridines: one-pot synthesis via sequential Ugi and Heck reactions

A facile one-pot synthesis of functionalized 1,2-dihydrobenzo[b][1,6]naphthyridines is described via sequential Ugi four components using 2-chloroquinoline-3-carboxaldehydes, allyl amine, acetic acid, isocyanides and ligand free Heck reactions in good yields.     

Divalent NI Compounds: Identifying new Carbocyclic Carbenes to Design Nitreones using Quantum Chemical Methods

Nitreones are compounds with oxidation state 1 at the nitrogen, these compounds carry formal positive charge as well as two lone pairs of electrons at nitrogen center. These compounds are also known as divalent N^I compounds and can be represented with the general formula L → N⁺ ← L, where L is an electron donating ligand. In the recent past, several divalent N^I compounds have been reported with L = N-heterocyclic carbene (NHC), remote N-heterocyclic carbene (rNHC), carbocyclic carbene (CCC) and diaminocarbene. Recently, our group reported that a novel six-membered CCC (cyclohexa-2,5-diene-4-[diaminomethynyl]-1-ylidene) can stabilize N⁺ center in nitreones. As an independent carbene, this species is very unstable. In this work, modulation of this CCC using (a) annulation, (b) heterocyclic ring modification, (c) substitutions adjacent to the carbenic carbon, (d) exocyclic double bond insertion and (e) ring contraction, has been reported. These modulations and quantum chemical analyses helped in the identification of five new six-membered CCCs which carry improved donation and stability properties. Further, these CCCs were employed in the design of new divalent N^I compounds (nitreones) which carry coordination bonds between ligands and N⁺ center. The molecular and electronic structure properties, and the donor→acceptor coordination interactions present in the resultant low oxidation state divalent N^I compounds have been explored.     

Facile catalytic dehydration of fructose to 5-hydroxymethylfurfural by Niobium pentachloride

Different metal chlorides in ionic liquids were examined as catalysts for fructose dehydration to 5-hydroxymethylfurfural (5-HMF). Niobium pentachloride (NbCl₅) exhibited the highest 5-HMF yield. Reaction of 1.0 mmol fructose in 10.0 mmol 1-butyl-3-methylimidozolium chloride ([bmim]Cl) with 0.20 mmol NbCl₅ afforded 79% of 5-HMF at 80 °C after 30 min. No degradation products were formed under these conditions. FTIR analysis indicates the moderate Lewis acidity of NbCl₅, which was found suitable for the selective formation of 5-HMF. Other catalysts with higher Lewis acidities promoted 5-HMF side product formation which ultimately resulted in lower yields.     

Structural Insights into Pixatimod (PG545) Inhibition of Heparanase,a Key Enzyme in Cancer and Viral Infections

Pixatimod (PG545), a heparan sulfate (HS) mimetic and anticancer agent currently in clinical trials, is a potent inhibitor of heparanase. Heparanase is an endo-β-glucuronidase that degrades HS in the extracellular matrix and basement membranes and is implicated in numerous pathological processes such as cancer and viral infections, including SARS−CoV-2. To understand how PG545 interacts with heparanase, we firstly carried out a conformational analysis through a combination of NMR experiments and molecular modelling which showed that the reducing end β-D-glucose residue of PG545 adopts a distorted conformation. This was followed by docking and molecular dynamics simulations to study the interactions of PG545 with heparanase, revealing that PG545 is able to block the active site by binding in different conformations, with the cholestanol side-chain making important hydrophobic interactions. While PG545 blocks its natural substrate HS from binding to the active site, small synthetic heparanase substrates are only partially excluded, and thus pentasaccharide or larger substrates are preferred for assaying this class of inhibitor. This study provides new insights for the design of next-generation heparanase inhibitors and substrates.     

Mass Spectrometric Analysis of Cyclotides from Clitoria ternatea: Xxx-Pro Bond Fragmentation as Convenient Diagnostic of Pro Residue Positioning

Cyclotides, a class of macrocyclic plant peptides, characterized by a cyclic backbone and three inter-locking disulfide bonds, may be divided into two major structural subfamilies, Möbius and Bracelet, based on the presence or absence of a specific proline residue. The present study describes the suite of cyclotides obtained from Clitoria ternatea, characterized by LC−MS and MS/MS techniques. Notable variations in product ion distributions were observed in cyclotides belonging to different structural subfamilies based on the number and positions of proline residues. For instance, Cter M which is an abundant Möbius cyclotide in this plant containing three proline residues, displayed distinct b- and y- ion characteristics in the MS/MS spectra compared to Cliotide T1, another commonly identified cyclotide but belonging to the Bracelet subfamily having two proline residues. The distinct fragmentation pattern of prototypical cyclotides of each structural subfamily, determined by Xxx-Pro bond fragmentation, was used to rapidly identify and sequence a novel cyclotide ctr pep 30 from this plant.     

Effect of pressure on electrical resistance of WSe<Subscript>2</Subscript> single crystal

The results of electrical resistance measurements under pressure on single crystals of WSe₂ are reported. Measurements up to 8.5 GPa are carried out using Bridgman anvil set up and beyond it using diamond anvil cell (DAC) up to a pressure of 27 GPa. There is no clear indication of any phase transition till the highest pressure is reached in these measurements.     

Scavenging mechanism of curcumin toward the hydroxyl radical: a theoretical study of reactions producing ferulic acid and vanillin

Curcumin is known to be an antioxidant, as it can scavenge free radicals from biological media. A sequence of H-abstraction and addition reactions involving up to eight OH radicals and curcumin or its degradation products leading to the formation of two other antioxidants, namely, ferulic acid and vanillin, was studied. Single electron transfer from curcumin to an OH radical was also studied. All relevant extrema on the potential energy surfaces were located by optimizing geometries of the reactant and product complexes, as well as those of the transition states, at the BHandHLYP/6-31G(d,p) level of density functional theory in the gas phase. Single-point energy calculations were also performed in the gas phase at the BHandHLYP/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ levels of theory. Solvent effects in aqueous media were treated by performing single-point energy calculations at all of the above-mentioned levels of theory employing the polarizable continuum model and the geometries optimized at the BHandHLYP/6-31G(d,p) level in the gas phase. A few reaction steps were also studied by geometry optimization in aqueous media, and the thus-obtained Gibbs free energy barriers were similar to those obtained by corresponding single-point energy calculations. Our calculations show that the hydrogen atom of the OH group attached to the phenol moiety of curcumin would be most efficiently abstracted by an OH radical, in agreement with experimental observations. Further, our study shows that OH addition would be most favored at the C10 site of the heptadiene chain. It was found that curcumin can serve as an effective antioxidant.     

Rayleigh Taylor instability of rotating compressible fluid through porous media

The hydromagnetic instability of a stratified horizontal layer of viscous compressible rotating fluid through porous media in the presence of vertical magnetic field is considered. The solution has been obtained through the use of variational principle. The dispersion relation is derived for a layer having exponential density stratification along the vertical direction. It is found that viscosity has stabilizing influence while permeability of porous medium, magnetic resistivity and coriolis forces have destabilizing influence on the system.     

Entrapment and kinetic resolution of stabilized axial and equatorial conformers of spiro-β-lactams

The facile synthesis of the stabilized axial and equatorial conformers of spiro-β-lactams was achieved via entrapment of cyclohexanone imines (Schiff bases) with acetoxyacetyl chloride in a [2 + 2]-cycloaddition reaction followed by their kinetic resolution. The immobilization of the racemic substrates on an inert solid support significantly reduced the reaction time and improved the enantioselectivity of conformers during kinetic resolution. The mechanism of the formation of the spiro-β-lactams was explored using B3LYP/6-31+G* level quantum chemical calculations.