Anomalous dielectric relaxation of water molecules at the surface of an aqueous micelle

Dielectric relaxation of aqueous solutions of micelles, proteins, and many complex systems shows an anomalous dispersion at frequencies intermediate between those corresponding to the rotational motion of bulk water and that of the organized assembly or macromolecule. The precise origin of this anomalous dispersion is not well-understood. In this work we employ large scale atomistic molecular dynamics simulations to investigate the dielectric relaxation (DR) of water molecules in an aqueous micellar solution of cesium pentadecafluorooctanoate. The simulations clearly show the presence of a slow component in the moment-moment time correlation function [PhiMW(t)] of water molecules, with a time constant of about 40 ps, in contrast to only 9 ps for bulk water. Interestingly, the orientational time correlation function [Cmu(t)] of individual water molecules at the surface exhibits a component with a time constant of about 19 ps. We show that these two time constants can be related by the well-known micro-macrorelations of statistical mechanics. In addition, the reorientation of surface water molecules exhibits a very slow component that decays with a time constant of about 500 ps. An analysis of hydrogen bond lifetime and of the rotational relaxation in the coordinate frame fixed on the micellar body seems to suggest that the 500 ps component owes its origin to the existence of an extended hydrogen bond network of water molecules at the surface. However, this ultraslow component is not found in the total moment-moment time correlation function of water molecules in the solution. The slow DR of hydration water is found to be well correlated with the slow solvation dynamics of cesium ions at the water-micelle interface.     

On the polarity of the amide group and its impact on dipeptide conformation

Giving explicit consideration to the lone pairs of the carbonyl oxygen of the amide group it is found that the polarity of the amide group resides essentially in the lone pair. As a confirmation, the lone pairs of the ester group are also found to account for most of its polarity. This localization of amide dipole affects the dipeptide conformation map only in the unallowed regions of the map, but has significant impact on the dipole moment of the dipeptide in different conformations. The experimentally observed dipole moments of dipeptides in solution are in conformity with our assignment of polarity of the amide group.     

FBG Sensor technology to interfacial strain measurement in cfrp-strengthened concrete beam

Experimental Techniques - Fiber-reinforced polymer (FRP) composite materials are very attractive for use in strengthening of civil engineering structures because of their high strength to weight...     

Characterization of electroplated ZnTe coatings

Zinc telluride (ZnTe) layers have been grown on conducting glass substrates using an electrochemical technique and characterized using cyclic voltammetry, X-ray diffractions, UV–vis-near infrared spectroscopy, and scanning electron microscopy (SEM). The reaction kinetics of binary film formation is also discussed. Cyclic voltammetry experiments were performed to elucidate the electrodic processes that occurred when potentials were applied and the optimum potential for electrodeposition were determined. X-ray diffraction studies revealed that the deposited films were polycrystalline in nature with zinc blende cubic structure. The various microstructural parameters were calculated using structural studies. The optical transmission and reflection spectrum were recorded and direct transition band gap energy is estimated about 2.26 eV by Tauc’s plot. The SEM pictures revealed that the nanoneedles protruding from nanorods with bunches of atoms agglomerate each other. The energy dispersive analysis by X-rays spectrum revealed the stoichiometry composition of ZnTe thin film at optimized preparative parameters.     

Modulation of molecular and kinetic properties of enzymes upon immobilization

Pea seedling glutamate dehydrogenase(EC 1.4.1.2) and Jack bean urease (EC 3.5.1.5) were immobilized on aminoethyl cellulose by cross-linking in a two-step reaction with glutaral dehyde. Specific activities of the immobilized dehydrogenase and urease were about 14% and 87%, respectively, of the original material in free solution. Both immobilized enzymes show no appreciable change in their pH dependence, whereas a less efficient binding of the substrates is suggested by the increased apparent Michaelis constants (K_m app.). Sigmoid kinetics were observed for both enzymes when reactions were carried out in a packed bed. Diffusional effects are considered responsible for producing these anomalous kinetics. The implications of these perturbations in terms of the catalytic efficiency of the enzymes, as well as the practical problems involved in the analysis of the kinetic data, are discussed.     

Ortho,Effects in organic molecules on electron impact: X—unusual ortho effects of the methyl group in o-picolinotoluidide

The mass spectrum of o-picolinotoluidide gives rise to three major fragments at m/z 184, m/z 169 and m/z 168, corresponding to the loss of CO from the molecular ion followed by the loss of ?H₂ and ?H₃ by independent pathways. It has been shown that the ortho methyl group and the nitrogen of the pyridine ring in the 2-position are involved in the formation of these three major fragments observed in the mass spectrum of o-picolinotoluidide. The mass spectrum of 2-(o-toluidino) pyridine, the molecular ion of which can resemble the [M? CO]⁺ ion in o-picolinotoluidide, also shows loss of CH₃ and NH₂ radicals from the molecular ions. Based on these observations coupled with the high resolution data, the mass analysed ion kinetic energy spectrometry and high voltage scans of these fragments in both the compounds, two mechanistic pathways have been proposed for the formation of these ions in o-picolinotoluidide.     

Oxidative kinetics of aminoacids by N-bromoacetamide : A study of solvent effect and general base catalysis

The kinetic results of the oxidation of aminoacids by N-bromoacetamide in acid and alkaline media are presented. It is noticed that the order with respect to substrate is dependent on the nature of the medium, 0.30 – 0.80 in perchloric acid (except phenylalanine in which case the order is 1.0) and alkaline media and zero order in aqueous acetic acid. Irrespective of the medium, the reaction is first order in [oxidant]. The rate of oxidation increases with [OH^-] and decreases with [H⁺]. The changes observed in the direction of the pH-rate profile correspond to the ionization constants of the aminoacids. The reaction is inhibited by the addition of acetamide in perchloric acid medium but, is independent of it in alkaline medium. The rate of oxidation is susceptible to changes in the composition of acetic acid and is maximum at 25% aqueous acetic acid. The oxidation of aminoacids by N-bromoacetamide is also catalysed by carboxylate anions. The catalytic constants of propionate, butyrate, acetate, chloropropionate and chloroacetate are measured and Brønsted coefficients (β) so evaluated are in the range 0.38 – 0.46. The proposed mechanism and the derived rate laws are consistent with the observed kinetics.