Fluorescence quenching of CdS quantum dots by 4-azetidinyl-7-nitrobenz-2-oxa-1,3-diazole: a mechanistic study

Fluorescence quenching of CdS quantum dots (QDs) by 4‐azetidinyl‐7‐nitrobenz‐2‐oxa‐1,3‐diazole (NBD), where the two quenching partners satisfy the spectral overlap criterion necessary for Förster resonance energy transfer (FRET), is studied by steady‐state and time‐resolved fluorescence techniques. The fluorescence quenching of the QDs is accompanied by an enhancement of the acceptor fluorescence and a reduction of the average fluorescence lifetime of the donor. Even though these observations are suggestive of a dynamic energy transfer process, it is shown that the quenching actually proceeds through a static interaction between the quenching partners and is probably mediated by charge‐transfer interactions. The bimolecular quenching rate constant estimated from the Stern–Volmer plot of the fluorescence intensities, is found to be exceptionally high and unrealistic for the dynamic quenching process. Hence, a kinetic model is employed for the estimation of actual quencher/QD ratio dependent exciton quenching rate constants of the fluorescence quenching of CdS by NBD. The present results point to the need for a deeper analysis of the experimental quenching data to avoid erroneous conclusions.     

Hydride Pinning Pathway in the Hydrogenation of CO2 to Formic Acid on Dimeric Tin Dioxide

Capture of CO₂ and its conversion into organic feedstocks are increasingly needed as society moves towards a renewable energy economy. Here, a hydride-assisted selective reduction pathway is proposed for the conversion of CO₂ to formic acid (FA) over SnO₂ monomers and dimers. Our density functional theory calculations infer a strong chemisorption of CO₂ on SnO₂ clusters forming a carbonate structure, whereas heterolytic cleavage of H₂ provides a new pathway for the selective reduction of CO₂ to formic acid at low overpotential. Among the two investigated pathways for reduction of CO₂ to HCOOH, the hydride pinning pathway is found promising with a unique selectivity for HCOOH. The negatively-charged hydride forms on the cluster during the dissociation of H₂ and facilitates the formation of a formate intermediate, which determines the selectivity for FA over the alternative CO and H₂ evolution reaction. It is confirmed that SnO₂ clusters exhibit a different catalytic behaviour from their surface equivalents, thus offering promise for future work investigating the reduction of CO₂ to FA via a hydride pinning pathway at low overpotential and CO₂ capturing.     

Microwave Dielectric Behaviour of 1,2‐Propanediol‐Water Mixture Studied Using Time Domain Reflectometry Technique

The dielectric behaviour of 1,2‐propanediol was investigated to understand the effect of the hydroxyl group on the dielectric parameters. The measurement of permittivity ?î and ?îî of 1,2‐propanediol was carried out in the frequency range 10 MHz to 20 GHz at 25 °C temperature. Static permittivity and dielectric relaxation time are extracted from a 1,2‐propanediol‐water mixture using the bilinear calibration method and non‐linear least squares fit method. Calculated Kirkwood correlation factor contains information regarding solute‐solvent interaction. The hydrogen bonded model suggested by Luzar is applied to determine the molecular parameters. The excess dielectric parameters and Bruggeman factor show the systematic change in the dielectric parameter of the system with change in concentration.     

Isolation, structure elucidation and biological activity of hederacine A and B, two unique alkaloids from Glechoma hederaceae

Preparative HPLC purification of a methanol extract of the aerial parts of Glechoma hederaceae has yielded two unique alkaloids, hederacine A (1) and hederacine B (2). The structures of these compounds were established unequivocally by UV, IR, MS and a series of 1D and 2D NMR analyses. These alkaloids were inactive against any of the 11 bacterial species at test concentrations, but showed prominent toxicity in the brine shrimp lethality assay (LC₅₀s=3.2, 14.0 and 2.7 μg/mL, respectively, for 1, 2 and the positive control, podophyllotoxin).     

Translational Dynamics of Lipidated Ras Proteins in the Presence of Crowding Agents and Compatible Osmolytes

Ras proteins are small GTPases and are involved in transmitting signals that control cell growth, differentiation, and proliferation. Since the cell cytoplasm is crowded with different macromolecules, understanding the translational dynamics of Ras proteins in crowded environments is crucial to yielding deeper insight into their reactivity and function. Herein, the translational dynamics of lipidated N‐Ras and K‐Ras4B is studied in the bulk and in the presence of a macromolecular crowder (Ficoll) and the compatible osmolyte and microcrowder sucrose by fluorescence correlation spectroscopy. The results reveal that N‐Ras forms dimers due to the presence of its lipid moiety in the hypervariable region, whereas K‐Ras4B remains in its monomeric form in the bulk. Addition of a macromolecular crowding agent gradually favors clustering of the Ras proteins. In 20 wt % Ficoll N‐Ras forms trimers and K‐Ras4B dimers. Concentrations of sucrose up to 10 wt % foster formation of N‐Ras trimers and K‐Ras dimers as well. The results can be rationalized in terms of the excluded‐volume effect, which enhances the association of the proteins, and, for the higher concentrations, by limited‐hydration conditions. The results of this study shed new light on the association state of these proteins in a crowded environment. This is of particular interest for the Ras proteins, because their solution state—monomeric or clustered—influences their membrane‐partitioning behavior and their interplay with cytosolic interaction partners.     

Synthesis of diverse β-(nitrooxy)-substituted amines by regioselective ring-opening of aziridines under neat conditions

An efficient and regioselective method was developed for the synthesis of β-(nitrooxy)-substituted amine derivatives by ring-opening of different aziridines with Zn(NO₃)₂·6H₂O without using additives or catalyst. A library of β-(nitrooxy)-substituted amine derivatives having a variety of substituents has been synthesized. Excellent regioselectivity, high yields, clean reaction, ease of product isolation, easily accessible reactants, and solvent-free as well as environment friendly reaction conditions are the notable advantages of the present methodology. The nitrooxy derivative was successfully transformed into hydroxy derivative by simple reduction. Gram-scale synthesis demonstrates the potential applications of the present method.     

Physicochemical properties and supercapacitor behavior of electrochemically synthesized few layered graphene nanosheets

Journal of Solid State Electrochemistry - The study emphasizes on the scalable production and comparison of few layered graphene nanosheets (FLGNSs). The FLGNSs have been electrochemically...     

Chemoselective Reduction of Some Condensates derived from Chromone-3-carbaldehyde using Sm and Zn

Condensates 4–6 undergo reductive dimerization with Sm in THF containing aqueous NH₄Cl to produce 7–9, whereas with Zn under similar reaction condition 3–5 produce 10–12.     

Enantioselective organocatalytic Biginelli reaction: dependence of the catalyst on sterics, hydrogen bonding, and reinforced chirality

From a systematic investigation involving the synthesis of a series of catalysts and screening studies, the organocatalyst 16, which is sterically hindered, contains a strong hydrogen-bonding site, and is endowed with reinforced chirality, is shown to promote the Biginelli cyclocondensation of aromatic as well as aliphatic aldehydes with ethyl acetoacetate and urea in a remarkably high enantioselectivity (ee ca. 94-99%).     

Yang–Mills on Quantum Heisenberg Manifolds

In the noncommutative geometry program of Connes, there are two variations of the concept of the Yang–Mills action functional. We show that for the quantum Heisenberg manifolds for generic parameter values they agree.