Well-defined nanofiberous polystyrene nanocomposites with twofold chains by ATRP

Polystyrene nanocomposites, being a combination of nanoclay-attached and free polystyrene chains were prepared using in situ atom transfer radical polymerization. Subsequently, they were electrospun to form fibers with diameter varying from 450?C700 nm according to scanning electron microscopy data; in addition, the transmission electron microscopy and x-ray diffraction analysis revealed that nanoclay layers were oriented along the nanofiber axis during the electrospinning process. Molecular weight of the extracted free polymer chains from the nanocomposites is higher than the attached chains. However, Anchored chains are characterized by higher polydispersity index in comparison with the free ones. Polydispersity index of polymer chains increases by the addition of nanoclay. Thermogravimetric analysis results shows that increasing clay content leads to a decrease in the quantity of polymer chains attached to the clay surface.     

Proper uncertainty bound parameter to robust control of electrical manipulators using nominal model

This paper focuses on the uncertainty bound parameter (UBP) to design the robust control of electrical manipulators. The UBP is commonly obtained by considering the worst case of uncertainties in bounding functions. However, too high estimation of UBP may cause saturation of input, higher frequency of chattering in the switching control laws, and thus a bad behavior of the whole system, while too low estimation of UBP may cause a higher tracking error. A proper UBP is preferred to improve the performance of robust control system. A simple, less dependent and proper UBP is proposed based on the nominal model of electrical manipulator and feedbacks of joint accelerations. This work is motivated by recent experimental results in measuring acceleration by optical encoder. Modeling of an electrical manipulator with presence of uncertainties is presented for control purposes. The proposed robust control is justified by stability analysis.     

Order statistics and linear combination of order statistics arising from a bivariate selection normal distribution

In this paper, we derive distributions of order statistics and a linear combination of order statistics arising from a bivariate selection normal distribution. We show that they are mixtures of univariate selection normal distributions. We then illustrate the usefulness of these results with a real-life data relating to visual acuity analysis. Finally, some concluding remarks are made.     

Dowex as Reusable Acidic Polymeric Catalyst in the Efficient and Regioselective Conversion of Epoxides into β‐Hydroxy Thiocyanates under Solvent Free Conditions

A convenient and efficient procedure for the regioselective ring opening of epoxides in the presence of Dowex (strongly acidic cationic exchange resin), as reusable eco‐friendly catalyst under solvent free conditions is described. Thus, several β‐hydroxy thiocyanates, useful intermediates toward pharmaceutical and biologically active molecules, are easily obtained in high isolated yields.     

The numerical solution of problems in calculus of variation using Chebyshev finite difference method

The Chebyshev finite difference method is used for finding the solution of the ordinary differential equations which arise from problems of calculus of variations. Our approach consists of reducing the problem to a set of algebraic equations. This method can be regarded as a non-uniform finite difference scheme. Some numerical results are also given to demonstrate the validity and applicability of the presented technique. The method is easy to implement and yields very accurate results.     

Mechanisms of amphipathic helical peptide denaturation by guanidinium chloride and urea: a molecular dynamics simulation study

Urea and GdmCl are widely used to denature proteins at high concentrations. Here, we used MD simulations to study the denaturation mechanisms of helical peptide in different concentrations of GdmCl and urea. It was found that the helical structure of the peptide in water simulation is disappeared after 5 ns while the helicity of the peptide is disappeared after 70 ns in 2 M urea and 25 ns in 1 M GdmCl. Surprisingly, this result shows that the helical structure in low concentration of denaturants is remained more with respect to that solvated in water. The present work strongly suggests that urea interact more preferentially to non-polar and aromatic side chains in 2 M urea; therefore, hydrophobic residues are in more favorable environment in 2 M urea. Our results also reveal that the hydrogen bonds between urea and the backbone is the dominant mechanism by which the peptide is destabilized in high concentration of urea. In 1 M and 2 M GdmCl, GdmCl molecules tend to engage in transient stacking interactions with aromatics and hydrophobic planar side chains that lead to displacement of water from the hydration surface, providing more favorable environment for them. This shows that accumulation of GdmCl around hydrophobic surfaces in 1 M and 2 M GdmCl solutions prevents proper solvation of the peptide at the beginning. In high GdmCl concentrations, water solvate the peptide better than 1 M and 2 M GdmCl. Therefore, our results strongly suggest that hydrogen bonds between water and the peptide are important factors in the destabilization of peptide in GdmCl solutions.     

One‐Pot Four‐Component Synthesis of N2‐Alkyl‐N3‐[2‐(1,3,4‐oxadiazol‐2‐yl)aryl]benzofuran‐2,3‐diamines

A simple synthesis of N²‐alkyl‐N³‐[2‐(1,3,4‐oxadiazol‐2‐yl)aryl]benzofuran‐2,3‐diamines 5 via a one‐pot four‐component reaction is described (Scheme 1). A mixture of N‐(isocyanoimino)triphenylphosphorane ( 1 ), a 2‐aminobenzoic acid 2 , a 2‐hydroxybenzaldehyde 3 , and an isocyanide 4 in absolute EtOH at room temperature undergoes a smooth reaction to afford 5 in excellent yields (Table).     

Synthesis of the novel pyrimido[1,6-a]pyrimidine and imidazo[1,2-c]pyrimidine derivatives based on heterocyclic ketene aminals

A concise and efficient route for the synthesis of pyrimido[1,6-a]pyrimidines and imidazo[1,2-c]pyrimidines by simply refluxing a reaction mixture of different heterocyclic ketene aminals and N,N′-bis(arylmethylidene)arylmethane was developed. This protocol provides an alternative method for application in combinatorial and parallel synthesis in drug discovery.