Ultrafast spectroscopy of the uranium(IV) and thorium(IV) bis(ketimide) complexes (C5Me5)2An[-N=C(Ph)(CH2Ph)]2 (An = Th, U)

Ultrafast pump-probe spectroscopic studies have been performed on (C 5Me 5) 2U[- N=C(Ph)(CH 2Ph)] 2 and (C 5Me 5) 2Th[- N=C(Ph)(CH 2Ph)] 2 including, for the uranium complex, the first direct measurement of dynamics of electronic deactivation within a 5f-electron manifold. Evidence has been found for strong coupling between the electronic ground state and the f-electron manifold which dominates the dynamics of the excited states of the bis(ketimide) uranium complex. These also demonstrate strong singlet-f manifold coupling, which assists in the deactivation of the photoexcited state of the uranium complex, and provide information on intersystem crossing and internal conversion processes in both complexes.     

Metal-Free Arylation of Benzothiophenes at C4 by Activation as their Benzothiophene S-Oxides

Benzothiophenes, activated by oxidation to the corresponding S-oxides, undergo C−H/C−H-type coupling with phenols to give C4 arylation products. While an electron-withdrawing group at C3 of the benzothiophene is important, the process operates without a directing group and a metal catalyst, thus rendering it compatible with sensitive functionalities—e.g. halides and formyl groups. Quantum chemical calculations suggest a formal stepwise mechanism involving heterolytic cleavage of an aryloxysulfur species to give a π-complex of the corresponding benzothiophene and a phenoxonium cation. Subsequent addition of the phenoxonium cation to the C4 position of the benzothiophene is favored over the addition to C3; Fukui functions predict that the major regioisomer is formed at the more electron-rich position between C3 and C4. Varied selective manipulation of the benzothiophene products showcase the synthetic utility of the metal-free arylation process.     

Role of backbone-solvent interactions in determining conformational equilibria of intrinsically disordered proteins

Intrinsically disordered proteins (IDPs) are functional proteins that do not fold into well-defined three-dimensional structures under physiological conditions. IDP sequences have low hydrophobicity, and hence, recent experiments have focused on quantitative studies of conformational ensembles of archetypal IDP sequences such as polyglutamine and glycine-serine block copolypeptides. Results from these experiments show that, despite the absence of hydrophobic residues, polar IDPs prefer ensembles of collapsed structures in aqueous milieus. Do these preferences originate in interactions that are unique to polar sidechains? The current study addresses this issue by analyzing conformational equilibria for polyglycine and a glycine-serine block copolypeptide in two environments, namely, water and 8 M urea. Polyglycine, a poly secondary-amide, has no sidechains and is a useful model system for generic polypeptide backbones. Results based on large-scale molecular dynamics simulations show that polyglycine forms compact, albeit disordered, globules in water and swollen, disordered coils in 8 M urea. There is minimal overlap between conformational ensembles in the two environments. Analysis of order parameters derived from theories for flexible polymers show that water at ambient temperatures is a poor solvent for generic polypeptide backbones. Therefore, the experimentally observed preferences for polyglutamine and glycine-serine block copolypeptides must originate, at least partially, in polypeptide backbones. A preliminary analysis of the driving forces that lead to distinct conformational preferences for polyglycine in two different environments is presented. Implications for describing conformational ensembles of generic IDP sequences are also discussed.     

Enhancement of the properties and mesophases stability after the electron beam irradiation on a racemic anti-ferroelectric liquid crystalline mixture

The display parameters and the stability of the various mesophases of a newly synthesised racemic mixture of anti-ferroelectric liquid crystals (AFLCs) have been boosted with the help of the electron beam irradiation. The mixture has phase sequence of isotropic–smectic C–smectic C_A crystal. The effective macroscopic polarisation and transition temperatures of the various mesophases of the irradiated mixture have increased as compared with the pure mixture. The macroscopic polarisation of the mixture has increased from 21 to 27 nC/cm² due to the irradiation. From various studies, we have observed that the temperature range of the smectic C_A phase has increased by 20.6°C with marginal decrease in the temperature range of the smectic C phase.     

l-Ascorbic acid in organic synthesis: a facile synthesis of 4-(butenolide-5-methylidenyl)-1,4-dihydropyridines

An efficient synthesis of 4-(butenolide-5-methylidenyl)-1,4-dihydropyridines has been achieved via a three-component reaction of β-keto esters or ketones, ammonium acetate and vinylic aldehydes from ascorbic acid in the presence of tetrabutylammonium hydrogen sulfate in ethylene glycol.     

Experimental and computational studies of the recognition of amino acids by galactosyl-imine and -amine derivatives: an attempt to understand the lectin-carbohydrate interactions

A galactosyl-naphthyl-imine-based derivative, 1-(beta-D-galactopyranosyl-1'-deoxy-1'-iminomethyl)-2-hydroxynaphthalene (GNI), and a galactosyl-naphthyl-amine-based derivative, 1-(galactopyranosyl-1'-deoxy-1'-aminomethyl)-2-hydroxynaphthalene (GNA), possessing an ONO binding core were studied for their recognition of naturally occurring amino acids using fluorescence and absorption spectroscopy, and the corresponding association constants were derived for the complexes formed. The complexes formed between GNI/GNA and amino acids were supported by electrospray ionization mass spectrometry (ESI/MS). The structures of the complexes were optimized by computational studies using density functional theory, and stabilization energies were computed for the complexes to substantiate the interactions present between GNI/GNA and amino acid. The interactions were found to be primarily hydrogen bonding in nature. These interactions are reminiscent of those present in the lectin-carbohydrate and glycosidase substrate. Thus, the carbohydrate moiety present in GNI shows high specificity toward the -COOH group of the amino acid, which may be relevant to such interactions present between the carbohydrates and the polypeptides.     

Quantitative characterization of ion pairing and cluster formation in strong 1:1 electrolytes

Aqueous solutions of 1:1 strong electrolytes are considered to be the prototype for complete ionic dissociation. Nonetheless, clustering of strong 1:1 electrolytes has been widely reported in all atom molecular dynamics simulations, and their presence is indirectly implicated in a diverse range of experimental results. Is there a physical basis for nonidealities such as ion pairing and cluster formation in aqueous solutions of strong 1:1 electrolytes? We attempt to answer this question by direct comparison of results from detailed molecular dynamics simulations to experimentally observed properties of 1:1 electrolytes. We report the analysis of a series of lengthy molecular dynamics simulations of alkali-halide solutions carried out over a wide range of physiologically relevant concentrations using explicit representations of water molecules. We find evidence for pronounced nonideal behavior of ions at all concentrations in the form of ion pairs and clusters which are in rapid equilibrium with dissociated ions. The phenomenology for ion pairing seen in these simulations is congruent with the multistep scheme proposed by Eigen and Tamm based on data from ultrasonic absorption experiments. For a given electrolyte, we show that the dependence of cluster populations on concentration can be described through a single set of equilibrium constants. We assess the accuracy of calculated ion pairing constants by favorable comparison to estimates obtained by Fuoss and co-workers and based on conductometric experiments. Ion pairs and clusters form on length scales where the size of individual water molecules is as important as the hard core radius of ions. Ion pairing results as a balance between the favorable Coulomb interactions and the unfavorable partial desolvation of ions needed to form a pair.     

Quantitative determination of macroelement and microelement content of fresh and processed bamboo shoots by wavelength dispersive X-ray fluorescence spectrometry

Bamboo shoots are considered as healthy and nutritional food as they supply proteins, dietary fiber, phenols, phytosterols, vitamins, and minerals in considerable quantities and capture a novel place in the spectrum of plant foods. Besides nutrients and bioactive compounds, shoots also contain antinutrient (cyanogen) that needs to be removed by adequate processing prior to consumption. This study was undertaken to investigate the effects of fermentation, brine preservation, and boiling on levels of mineral elements in shoots of Dendrocalamus hamiltonii using wavelength dispersive X-ray fluorescence (WD-XRF) spectrometry. WD-XRF spectrometry is a method that causes the least destruction to the raw plant material as no harsh chemicals are used and gives the most accurate results. Fermentation process resulted in significant increase in the levels of Rb (9%), S (11%), Mg (12%), Zn (46%), Ca (56%), and Fe (87%); however, Na (18.79 g/100 g, dry weight), Cl (24.73 g/100 g, dry weight), and Br (20.0 mg/100 g, dry weight) content increased drastically after brine preservation. Boiling increased the levels of Fe by 139% but did not affect the Rb (20.0 mg/100 g, dry weight) and Zn content (10.2 mg/100 g, dry weight) of the shoots. In general, fermentation, which led to the lowest loss of mineral elements, appears the best method in enhancement and retention of the mineral content in bamboo shoots and holds the prospect at industrial and society levels.     

ABSINTH: a new continuum solvation model for simulations of polypeptides in aqueous solutions

A new implicit solvation model for use in Monte Carlo simulations of polypeptides is introduced. The model is termed ABSINTH for self-Assembly of Biomolecules Studied by an Implicit, Novel, and Tunable Hamiltonian. It is designed primarily for simulating conformational equilibria and oligomerization reactions of intrinsically disordered proteins in aqueous solutions. The paradigm for ABSINTH is conceptually similar to the EEF1 model of Lazaridis and Karplus (Proteins 1999, 35, 133). In ABSINTH, the transfer of a polypeptide solute from the gas phase into a continuum solvent is the sum of a direct mean field interaction (DMFI), and a term to model the screening of polar interactions. Polypeptide solutes are decomposed into a set of distinct solvation groups. The DMFI is a sum of contributions from each of the solvation groups, which are analogs of model compounds. Continuum-mediated screening of electrostatic interactions is achieved using a framework similar to the one used for the DMFI. Promising results are shown for a set of test cases. These include the calculation of NMR coupling constants for short peptides, the assessment of the thermal stability of two small proteins, reversible folding of both an alpha-helix and a beta-hairpin forming peptide, and the polymeric properties of intrinsically disordered polyglutamine peptides of varying lengths. The tests reveal that the computational expense for simulations with the ABSINTH implicit solvation model increase by a factor that is in the range of 2.5-5.0 with respect to gas-phase calculations.