Yield stress of nanocrystalline materials: role of grain-boundary dislocations,triple junctions and Coble creep

A theoretical model is suggested which describes the strengthening of nanocrystalline materials due to the effects of triple junctions of grain boundaries as obstacles for grain-boundary sliding. In the framework of the model, a dependence of the yield stress characterizing grain-boundary sliding on grain size and triple-junction angles is revealed. With this dependence we have found that, in as-fabricated nanocrystalline materials, the yield stress depends upon a competition between conventional dislocation slip and grain-boundary sliding. On the other hand, yield stress dependence on grain size in heat-treated nanocrystalline materials is described as that caused by a competition between conventional dislocation slip and Coble creep. Grain-size and triple-junction angle distributions are incorporated into the consideration to account for distributions of grain size and triple-junction angles, occurring in real specimens. The results of the model are compared with experimental data from as-fabricated and heat-treated nanocrystalline materials and shown to be in good agreement.     

Densities,Refractive Indices and Excess Properties of N-<Emphasis Type="Italic">p</Emphasis>-Tolylbenzohydroxamic Acid in Dimethyl Sulfoxide at 298.15 to 313.15 K

Densities and refractive indices have been measured for N-p-tolylbenzohydroxamic acid (p-TBHA) in dimethyl sulfoxide (DMSO) as a function of concentration at (298.15, 303.15, 308.15, 313.15 and 318.15) K. The apparent molar volumes and partial molar volumes were obtained from these density data. The limited partial molar expansivities have been calculated from the temperature dependence of the limiting partial molar volume. The molar refractions were calculated from the experimental refractive index values for p-TBHA in DMSO. The excess volumes, deviations of the refractive indices, and molar refractions were also calculated. The results are discussed in terms of molecular interactions.     

Facile SILAR Processed Bi2S3:PbS Solid Solution on MWCNTs for High‐performance Electrochemical Supercapacitor

Carbon based composite materials have gained much attention because of fulfilling desirable properties for supercapacitor application. In the featured work, the thin film of Bi₂S₃:PbS solid solution has been synthesized on multi‐walled carbon nanotubes (MWCNTs) by simple successive ionic layer adsorption and reaction (SILAR) method. The nanoparticle morphology provides sufficient electroactive channels for electrolyte ions to penetrate during electrochemical activities. The composite exhibits superior specific capacitance 676 F/g at constant specific current density of 5.56 A/g with fast charge‐discharge cycles. In association of energy storage characteristics, the fabricated symmetric cell exhibits excellent energy density of 13.36 Wh/kg by acquiring power density of 0.83 kW/kg. The superior results of the hybrid electrode promise a novel direction for high performance supercapacitor application.     

Cytochrome c: Ascorbate reduction site and possible electron-transfer path

The ascorbate reduction reaction of the native and urea-perturbed forms, 0–8M urea, of horse heart ferricytochrome c is found to be a three-step process: a urea-dependent equilibrium step between a reducible and an irreducible form with a midconcentration of urea of 7.4M, a binding step with a binding constant of 5.9M^–1, and a reduction step with a urea-independent rate constant of 2.9 ± 0.3 s^–1 [J. Biol. Chem. 255 , 9666 (1980)]. The effect of adding urea, in addition to the generation of an irreducible form, is a slight lowering of the ascorbate-protein binding constant, 5.9 to 2.7M^–1, which is limited to the 0–5.5M concentration range. The thermodynamics of the ureadenaturation process also yields a three-step mechanism, N? X₁? X₂? D, with midconcentrations of urea of 2.5–3M, 6.2M, and 7.5M, respectively, where N, D, and the X_s are the native, the 9-M-urea, and the intermediate forms. The three processes are described as the loosening of the heme crevice opening, the solvent exposure of the polypeptide backbone, and the disruption of the tryptophan–porphyrin interactions, respectively [Biochemistry 19 , 199 (1980)]. The reaction of the protein with 2,3-butanedione, a group-specific reagent for the guanidinium groups and an electron donor for this protein, is inhibited in the presence of ascorbate, but only one of the two functional groups is involved [J. Biol. Chem. 255 , 11094 (1980)]. A correlation of kinetic and thermodynamic observations led to the conclusion that the ascorbate reduction of the protein is independent of the state of the heme crevice opening and of the polypeptide organized structures; instead, it is determined by the integrity of the tryptophan indole–porphyrin interactions. This information, when taken in conjunction with the selective inhibition of the reaction of the arginine side chains by ascorbate, establishes the binding site of ascrobate as one of the two arginyl side chains, and not the opening of the crevice or its vicinity. From the three-dimensional structure of the protein, and taking into consideration the variability of the protein sequence, it is suggested that Arg-38 is the ascorbate binding site, and that the electronic interaction between the indole of Trp-59 and the porphyrin moiety must constitute, at least in part, the electron-transfer path to heme iron.     

Constitutens of high altitude Himalayan herbs. Part XV: A new norditerpenoid alkaloid from the roots of Aconitum balfourii

A new norditerpenoid alkaloid 9-hydroxysenbushine A has been isolated from the roots of high altitude Himalayan herb Aconitum balfourii by means of vacuum liquid chromatography and centrally accelerated radial thin layer chromatographic techniques and identified by means of MS, 1H and 13C NMR spectral methods.     

Automatic discovery of metastable states for the construction of Markov models of macromolecular conformational dynamics

To meet the challenge of modeling the conformational dynamics of biological macromolecules over long time scales, much recent effort has been devoted to constructing stochastic kinetic models, often in the form of discrete-state Markov models, from short molecular dynamics simulations. To construct useful models that faithfully represent dynamics at the time scales of interest, it is necessary to decompose configuration space into a set of kinetically metastable states. Previous attempts to define these states have relied upon either prior knowledge of the slow degrees of freedom or on the application of conformational clustering techniques which assume that conformationally distinct clusters are also kinetically distinct. Here, we present a first version of an automatic algorithm for the discovery of kinetically metastable states that is generally applicable to solvated macromolecules. Given molecular dynamics trajectories initiated from a well-defined starting distribution, the algorithm discovers long lived, kinetically metastable states through successive iterations of partitioning and aggregating conformation space into kinetically related regions. The authors apply this method to three peptides in explicit solvent-terminally blocked alanine, the 21-residue helical F(s) peptide, and the engineered 12-residue beta-hairpin trpzip2-to assess its ability to generate physically meaningful states and faithful kinetic models.     

Calculation of the distribution of eigenvalues and eigenvectors in Markovian state models for molecular dynamics

Markovian state models (MSMs) are a convenient and efficient means to compactly describe the kinetics of a molecular system as well as a formalism for using many short simulations to predict long time scale behavior. Building a MSM consists of grouping the conformations into states and estimating the transition probabilities between these states. In a previous paper, we described an efficient method for calculating the uncertainty due to finite sampling in the mean first passage time between two states. In this paper, we extend the uncertainty analysis to derive similar closed-form solutions for the distributions of the eigenvalues and eigenvectors of the transition matrix, quantities that have numerous applications when using the model. We demonstrate the accuracy of the distributions on a six-state model of the terminally blocked alanine peptide. We also show how to significantly reduce the total number of simulations necessary to build a model with a given precision using these uncertainty estimates for the blocked alanine system and for a 2454-state MSM for the dynamics of the villin headpiece.     

Biomolecule induced nanoparticle aggregation: effect of particle size on interparticle coupling

Gold nanoparticles of variable sizes have been prepared by reducing HAuCl(4) with trisodium citrate by Frens' method. It has been found that the gold particles under consideration produce well-ordered aggregates upon interaction with a biomolecule, glutathione in variable acidic pH condition and exhibit pronounced changes in their optical properties arising due to electromagnetic interaction in the close-packed assembly. The effect of nanoparticle size on the nature of aggregation as well as the variation in the optical response due to variable degree of interparticle coupling effects amongst the gold particles have been investigated. The optical properties of the gold aggregates have been accounted in the light of Maxwell-Garnett effective medium theory considering the changes in the filling factor in different aggregates produced by variable sizes of gold colloids. The aggregates have been characterized by UV-vis spectroscopy, FTIR, Raman, XRD and TEM studies. It has been observed that a new peak appearing at a longer wavelength intensifies and shifts further to the red from the original peak position depends on the particle size, concentration of glutathione and pH of the solution. On the basis of the first appearance of a clearly defined new peak at longer wavelength, a higher sensitivity of glutathione detection has been achieved with gold nanoparticles of larger dimension.     

Optical properties of electroluminescent ZnS doped with Cu+, Mn2+ and Gd3+ ions

ZnS:Gd, ZnS: Cu, Gd and ZnS: Mn, Gd phosphors have been prepared by firing the samples in argon atmosphere. Spectral distributions in these phosphors are discussed with appropriate mechanism. ZnS:Cu, Gd and ZnS:Mn, Gd are found to be examples of multiple band phosphors. Enhancement and quenching of the emission band intensities of all these phosphors have been studied inpel emission. It is observed that Gd³⁺ ions play an important role in transferring their excitation energy to other centres. The voltage and frequency variation ofel brightness are in agreement with collision excitation mechanism in Schottky barrier at the metal semiconductor interface. Studies in phosphorescence and thermoluminescence of these phosphors have also been carried out. It is observed that trap-depth changes slowly with temperature and dopant concentration. The values of trapping parameters have been evaluated. The irregular variation of the life-time of electrons in the traps. with temperature shows the existence of retrapping in these phosphors.