Preparation and Characterization of Chitosan Thin Films on Mixed-Matrix Membranes for Complete Removal of Chromium

Herein we present a new approach for the complete removal of Cr^VI species, through reduction of Cr^VI to Cr^III, followed by adsorption of Cr^III. Reduction of chromium from water is an important challenge, as Cr^IV is one of the most toxic substances emitted from industrial processes. Chitosan (CS) thin films were developed on plain polysulfone (PSf) and PSf/TiO₂ membrane substrates by a temperature-induced technique using polyvinyl alcohol as a binder. Structure property elucidation was carried out by X-ray diffraction, microscopy, spectroscopy, contact angle measurement, and water uptake studies. The increase in hydrophilicity followed the order: PSf < PSf/TiO₂ < PSf/TiO₂/CS membranes. Use of this thin-film composite membrane for chromium removal was investigated with regards to the effects of light and pH. The observations reveal 100 % reduction of Cr^VI to Cr^III through electrons and protons donated from OH and NH₂ groups of the CS layer; the reduced Cr^III species are adsorbed onto the CS layer via complexation to give chromium-free water.     

Application of ethyl chloroformate derivatization for solid-phase microextraction–gas chromatography–mass spectrometric determination of bisphenol-A in water and milk samples

A simple and rapid analytical method based on in-matrix ethyl chloroformate (ECF) derivatization has been developed for the quantitative determination of bisphenol-A (BPA) in milk and water samples. The samples containing BPA were derivatised with ECF in the presence of pyridine for 20 s at room temperature, and the non-polar derivative thus formed was extracted using polydimethylsiloxane solid-phase microextraction (SPME) fibres with thicknesses of 100 μm followed by analysis using gas chromatography–mass spectrometry. Three alkyl chloroformates (methyl, ethyl and isobutyl chloroformate) were tested for optimum derivatisation yields, and ECF has been found to be optimum for the derivatisation of BPA. Several parameters such as amount of ECF, pyridine and reaction time as well as SPME parameters were studied and optimised in the present work. The limit of detection for BPA in milk and water samples was found to be 0.1 and 0.01 μg L⁻¹, respectively, with a signal-to-noise ratio of 3:1. The limit of quantitation for BPA in milk and water was found to be 0.38 and 0.052 μg L⁻¹, respectively, with a signal-to-noise ratio of 10:1. In conclusion, the method developed was found to be rapid, reliable and cost-effective in comparison to silylation and highly suitable for the routine analysis of BPA by various food and environmental laboratories.     

An efficient one pot–three component cyclocondensation in the synthesis of 2-(2-chloroquinolin-3-yl)-2,3-dihydroquinazolin-4(1H)-ones: potential antitumor agents

Montmorillonite K10 efficiently catalyzed a one pot–three component cyclocondensation of isatoic anhydride, NH₄OAc and aromatic/heteroaromatic aldehydes under ambient conditions to produce the corresponding 2-substituted-2,3-dihydroquinazolin-4(1H)-ones in good yields. The 2-(2-chloroquinolin-3-yl)-2,3-dihydroquinazolin-4(1H)-ones 3a–d were screened for their antitumor activity against Ehrlich Ascites Carcinoma tumor cells     

Formation of Periodic γ‐Turns in α/β‐Hybrid Peptides: DFT and NMR Experimental Evidence

Hybrid peptidic oligomers comprising natural and unnatural amino acid residues that can exhibit biomolecular folding and hydrogen‐bonding mimicry have attracted considerable interest in recent years. While a variety of hybrid peptidic helices have been reported in the literature, other secondary structural patterns such as γ‐turns and ribbons have not been well explored so far. The present work reports the design of novel periodic γ‐turns in the oligomers of 1:1 natural‐α/unnatural trans‐β‐norborenene (TNAA) amino acid residues. Through DFT, NMR, and MD studies, it is convincingly shown that, in the mixed conformational pool, the heterogeneous backbone of the hybrid peptides preferentially adopt periodic 8‐membered (pseudo γ‐turn)/7‐membered (inverse γ‐turn) hydrogen bonds in both polar and non‐polar solvent media. It is observed that the stereochemistry and local conformational preference of the β‐amino acid building blocks have a profound influence on accessing the specific secondary fold. These findings may be of significant relevance for the development of molecular scaffolds that facilitate desired positioning of functional side‐chains.     

Study of band offsets in InN/Ge heterojunctions

InN layers were directly grown on Ge substrate by plasma-assisted molecular beam epitaxy (PAMBE). The valence band offset (VBO) of wurtzite InN/Ge heterojunction is determined by X-ray photoemission spectroscopy (XPS). The valence band of Ge is found to be 0.18 ± 0.04 eV above that of InN and a type-II heterojunction with a conduction band offset (CBO) of ~ 0.16 eV is found. The accurate determination of the VBO and CBO is important for the design of InN/Ge based electronic devices.     

The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE

Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si₃N₄ (Si⁴⁺) but also in the intermediate nitridation states with one (Si¹⁺) or three (Si³⁺) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si¹⁺, Si³⁺, and Si⁴⁺ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si⁴⁺ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si₃N₄ by molecular beam epitaxy (MBE).     

Rhodopsin Activation Switches in a Native Membrane Environment†

The elucidation of structure–function relationships of membrane proteins still poses a considerable challenge due to the sometimes profound influence of the lipid bilayer on the functional properties of the protein. The visual pigment rhodopsin is a prototype of the family of G protein‐coupled transmembrane receptors and a considerable part of our knowledge on its activation mechanisms has been derived from studies on detergent‐solubilized proteins. This includes in particular the events associated with the conformational transitions of the receptor from the still inactive Meta I to the Meta II photoproduct states, which are involved in signaling. These events involve disruption of an internal salt bridge of the retinal protonated Schiff base, movement of helices and proton uptake from the solvent by the conserved cytoplasmic E(D)RY network around Glu134. As the equilibria associated with these events are considerably altered by the detergent environment, we set out to investigate these equilibria in the native membrane environment and to develop a coherent thermodynamic model of these activating steps using UV–visible and Fourier‐transform infrared spectroscopy as complementary techniques. Particular emphasis is put on the role of protonation of Glu134 from the solvent, which is a thermodynamic prerequisite for full receptor activation in membranes, but not in detergent. In view of the conservation of this carboxylate group in family A G protein‐coupled receptors, it may also play a similar role in the activation of other family members.     

Kinetics of self-assembled InN quantum dots grown on Si (111) by plasma-assisted MBE

One of the scientific challenges of growing InN quantum dots (QDs), using Molecular beam epitaxy (MBE), is to understand the fundamental processes that control the morphology and distribution of QDs. A systematic manipulation of the morphology, optical emission, and structural properties of InN/Si (111) QDs is demonstrated by changing the growth kinetics parameters such as flux rate and growth time. Due to the large lattice mismatch, between InN and Si (~8%), the dots formed from the Strannski–Krastanow (S–K) growth mode are dislocated. Despite the variations in strain (residual) and the shape, both the dot size and pair separation distribution show the scaling behavior. We observed that the distribution of dot sizes, for samples grown under varying conditions, follow the scaling function.