Pressure and alloying effects on the metal to insulator transition in NiS2-xSex studied by infrared spectroscopy

The metal to insulator transition in the charge-transfer NiS2-xSex compound has been investigated through infrared reflectivity. Measurements performed by applying pressure to pure NiS2 (lattice contraction) and by Se alloying (lattice expansion) reveal that in both cases an anomalous metallic state is obtained. We find that optical results are not compatible with the linear Se-alloying vs pressure-scaling relation previously established through transport, thus pointing out the substantially different microscopic origin of the two transitions.

Isotope shifts and hyperfine structure in the 555.8-nm S-1(0) -> P-3(1) line of Yb

We apply our technique of using a Rb-stabilized ring-cavity resonator to measure the frequencies of various spectral components in the 555.8-nm 1S0-->3P1 line of Yb. We determine the isotope shifts with 60 kHz precision, which is an order-of-magnitude improvement over the best previous measurement on this line. There are two overlapping transitions, 171Yb(1/2-->3/2) and 173Yb(5/2-->3/2), which we resolve by applying a magnetic field. We thus obtain the hyperfine constants in the 3P1 state of the odd isotopes with a significantly improved precision. Knowledge of isotope shifts and hyperfine structure should prove useful for high-precision calculations in Yb necessary to interpret ongoing experiments testing parity and time-reversal symmetry violation in the laws of physics.

Dynamics of Circulating Concentrations of Gonadotropins and Ovarian Hormones Throughout the Menstrual Cycle in the Bonnet Monkey: Role of Inhibin A in the Regulation of Follicle-Stimulating Hormone Secretion

In higher primates, increased circulating follicle-stimulating hormone (FSH) levels seen during late menstrual cycle and during menstruation has been suggested to be necessary for initiation of follicular growth, recruitment of follicles and eventually culminating in ovulation of a single follicle. With a view to establish the dynamics of circulating FSH secretion with that of inhibin A (INH A) and progesterone (P-4)secretions during the menstrual cycle, blood was collected daily from bonnet monkeys beginning day 1 of the menstrual cycle up to 35 days. Serum INH A levels were low during early follicular phase, increased significantly coinciding with the mid cycle luteinizing hormone (LH) surge to reach maximal levels during the mid luteal phase before declining at the late luteal phase, essentially paralleling the pattern Of P-4 secretion seen throughout the luteal phase. Circulating FSH levels were low during early and mid luteal phases, but progressively increased during the late luteal phase and remained high for few days after the onset of menses. In another experiment, lutectomy performed during the mid luteal phase resulted in significant decrease in INH A concentration within 2 hr (58.3 +/- 2 vs. 27.3 +/- 3 pg/mL), and a 2- to 3-fold rise in circulating FSH levels by 24 hr (0.20 +/- 0.02 vs. 0.53 +/- 0.14 ng/mL) that remained high until 48 hr postlutectomy. Systemic administration of Cetrorelix (150 mu g/kg body weight), a gonadotropin releasing hormone receptor antagonist, at mid luteal phase in monkeys led to suppression of serum INH A and P-4 concentrations 24 hr post treatment, but circulating FSH levels did not change. Administration of exogenous LH, but not FSH, significantly increased INH A concentration. The results taken together suggest a tight coupling between LH and INH A secretion and that INH A is largely responsible for maintenance of low FSH concentration seen during the luteal phase. Am. J. Primatol. 71:817-824, 2009.

Critical Exponent β and Rectilinear Diameter of the Binary-Liquid System Carbon Disulfide + Nitromethane

The system CS2 + CH3NO2 shows β=0.315±0.004 over 10-6<ε=|T-Tc| / Tc<2-10-1 with no indication of a classical value ½ even far away from Tc. The diameter shows a curvature and is of the form - c+b ε+fε7 / 8exp(-gεh).

Simple repetitive sequences in the genome: structure and functional significance

The current explosion of DNA sequence information has generated increasing evidence for the claim that noncoding repetitive DNA sequences present within and around different genes could play an important role in genetic control processes, although the precise role and mechanism by which these sequences function are poorly understood. Several of the simple repetitive sequences which occur in a large number of loci throughout the human and other eukaryotic genomes satisfy the sequence criteria for forming non-B DNA structures in vitro. We have summarized some of the features of three different types of simple repeats that highlight the importance of repetitive DNA in the control of gene expression and chromatin organization. (i) (TG/CA)n repeats are widespread and conserved in many loci. These sequences are associated with nucleosomes of varying linker length and may play a role in chromatin organization. These Z-potential sequences can help absorb superhelical stress during transcription and aid in recombination. (ii) Human telomeric repeat (TTAGGG)n adopts a novel quadruplex structure and exhibits unusual chromatin organization. This unusual structural motif could explain chromosome pairing and stability. (iii) Intragenic amplification of (CTG)n/(CAG)n trinucleotide repeat, which is now known to be associated with several genetic disorders, could down-regulate gene expression in vivo. The overall implications of these findings vis-à-vis repetitive sequences in the genome are summarized.

Rotational isomerism about the Ca-CO bond in proline derivatives. 1H and 13C NMR studies of benzyloxycarbonyl-Pro-N-methylamide and pivaloyl-Pro-N-methylamide

The 270 MHz 1H n.m.r. spectrum of benzyloxycarbonyl-Pro-N-methylamide in CDCl3 is exchange broadened at 293° K. Spectral lines due to two species are frozen out at 253° K and a dynamically averaged spectrum is obtained at 323° K. A selective broadening of the Cβ and Cγ resonances in the 13C n.m.r. spectrum is observed at 253° K, with a splitting of the Cβ and Cγ resonances into a pair of lines of unequal intensity. A similar broadening of Cβ and Cγ peaks is also detected in pivaloyl-Pro-N-methylamide where cis-trans interconversion about the imide bond is precluded by the bulky t-butyl group. The rate process is thus attributed to rotation about the Cα-CO bond (ψ) and a barrier (ΔG#) of 14kcal mol-1 is estimated. 13C n.m.r. data for pivaloyl-Pro-N-methylamide in a number of solvents is presented and the differences in the Cβ and Cγ chemical shifts are interpreted in terms of rotational isomerism about the Cα-CO bond.

A Methanol-Tolerant Carbon-Supported Pt-Au Alloy Cathode Catalyst for Direct Methanol Fuel Cells and Its Evaluation by DFT

A Pt-Au alloy catalyst of varying compositions is prepared by codeposition of Pt and Au nanoparticles onto a carbon support to evaluate its electrocatalytic activity toward an oxygen reduction reaction (ORR) with methanol tolerance in direct methanol fuel cells. The optimum atomic weight ratio of Pt to Au in the carbon-supported Pt-Au alloy (Pt-Au/C) as established by cell polarization, linear-sweep voltammetry (LSV), and cyclic voltammetry (CV) studies is determined to be 2:1. A direct methanol fuel cell (DMFC) comprising a carbon-supported Pt-Au (2:1) alloy as the cathode catalyst delivers a peak power density of 120 mW/cm2 at 70 °C in contrast to the peak power density value of 80 mW/cm2 delivered by the DMFC with carbon-supported Pt catalyst operating under identical conditions. Density functional theory (DFT) calculations on a small model cluster reflect electron transfer from Pt to Au within the alloy to be responsible for the synergistic promotion of the oxygen-reduction reaction on a Pt-Au electrode.

Cloning, overexpression, folding and purification of a biosynthetically derived three disulfide scorpion toxin (BTK-2) from Mesobuthus tamulus

BTK-2, a 32 residue scorpion toxin initially identified in the venom of red Indian scorpion Mesobuthus tamulus was cloned, overexpressed and purified using Cytochrome 155 fusion protein system developed in our laboratory. The synthetic gene coding for the peptide was designed taking into account optimal codon usage by Escherichia coli. High expression levels of the fusion protein enabled facile purification of this peptide. The presence of disulfide bonded isomers, occurring as distinctly populated states even in the fusion protein, were separated by gel filtration chromatography. The target peptide was liberated from the host protein by Tev protease cleavage and subsequent purification was achieved using RP-HPLC methods. Reverse phase HPLC clearly showed the presence of at least two isomeric forms of the peptide that were significantly populated. The oxidative folding of BTK-2 was achieved under ambient conditions during the course of purification. Structural characterization of the two forms, by solution homonuclear and heteronuclear NMR methods, has shown that these two forms exhibit significantly different structural properties, and represent the natively folded and a "misfolded" form of the peptide. The formation of properly folded BTK-2 as a major fraction without the use of in vitro oxidative refolding methods clearly indicate the versatility of the Cytochrome b(5) fusion protein system for the efficient production of peptides for high resolution NMR studies.

Influence of Cooling Rate on the Enthalpy Relaxation and Fragility of a Metallic Glass

Structural relaxation behavior of a rapidly quenched (RQ) and a slowly cooled Pd40Cu30Ni10P20 metallic glass was investigated and compared. Differential scanning calorimetry was employed to monitor the relaxation enthalpies at the glass transition temperature, T-g , and the Kolrausch-Williams-Watts (KWW) stretched exponential function was used to describe its variation with annealing time. It was found that the rate of enthalpy recovery is higher in the ribbon, implying that the bulk is more resistant to relaxation at low temperatures of annealing. This was attributed to the possibility of cooling rate affecting the locations where the glasses get trapped within the potential energy landscape. The RQ process traps a larger amount of free volume, resulting in higher fragility, and in turn relaxes at the slightest thermal excitation (annealing). The slowly cooled bulk metallic glass (BMG), on the other hand, entraps lower free volume and has more short-range ordering, hence requiring a large amount of perturbation to access lower energy basins.

Supramolecular Organization in Tetra Aqua (mu-8-Hydroxyquinoline-5-sulfonate) Barium (II) and Ag center dot center dot center dot I Interactions in a Pseudopolymorphic Form of (7-Iodo-8-hydroxyquinoline-5-sulfonate) Silver (I) Monohydrate

The complexes, Ba (HQS) (H2O)(4) (HQS = 8-hydroxyquinoline-5-sulfonic acid) (1) and Ag (HIQS) (H2O) (Ferron = 7-iodo-8-hydroxyquinoline-5-sulfonic acid) (2) have been synthesized and characterized by X-ray diffraction analysis and spectroscopic studies. In compound 1, Ba2+ ion has a nine-coordinate monocapped antiprismatic geometry. In compound 2, Ag+ has distorted tetrahedral coordination and Ag center dot center dot center dot I interactions generate the supramolecular architectures. The complexes have been characterized by FT-IR and UV-Visible measurements. In both the structures, the inversion-related organic ligands are stacked over one another leading to three-dimensional networks.

PVA-PSSA Membrane with Interpenetrating Networks and its Methanol Crossover Mitigating Effect in DMFCs

A membrane with interpenetrating networks between poly(vinyl alcohol) (PVA) and poly(styrene sulfonic acid) (PSSA) coupled with a high proton conductivity is realized and evaluated as a proton exchange membrane electrolyte for a direct methanol fuel cell (DMFC). Its reduced methanol permeability and improved performance in DMFCs suggest the new blend as an alternative membrane to Nafion membranes. The membrane has been characterized by powder X-ray diffraction, scanning electron microscopy, time-modulated differential scanning calorimetry, and thermogravimetric analysis in conjunction with its mechanical strength. The maximum proton conductivity of 3.3×10−2 S/cm for the PVA–PSSA blend membrane is observed at 373 K. From nuclear magnetic resonance imaging and volume localized spectroscopy experiments, the PVA–PSSA membrane has been found to exhibit a promising methanol impermeability, in DMFCs. On evaluating its utility in a DMFC, it has been found that a peak power density of 90 mW/cm2 at a load current density of 320 mA/cm2 is achieved with the PVA–PSSA membrane compared to a peak power density of 75 mW/cm2 at a load current density of 250 mA/cm2 achievable for a DMFC employing Nafion membrane electrolyte while operating under identical conditions; this is attributed primarily to the methanol crossover mitigating property of the PVA–PSSA membrane.

On the conformation of cyclohexane-1,4-dione and its derivatives—I : Infrared and raman spectra of cyclohexane 1,4-dione and infrared spectrum of its octadeutero analogue

Raman spectra of cyclohexane 1,4-dione (I), in chloroform, benzene and water solutions have been recorded. Temperature effect on the spectrum has been studied. The IR spectra of I and its octadeutero analogue in the solid state have also been studied. The spectra have been found on the basis of selection rules applicable for Raman and IR spectra, to be consistent with a single conformer of C2 symmetry. Plausible causes of conformational preference have been discussed.

A Durable PEFC with Carbon-Supported Pt-TiO2 Cathode: A Cause and Effect Study

Durability is central to the commercialization of polymer electrolyte fuel cells (PEFCs). The incorporation of TiO2 with platinum (Pt) ameliorates both the stability and catalytic activity of cathodes in relation to pristine Pt cathodes currently being used in PEFCs. PEFC cathodes comprising carbon-supported Pt-TiO2 (Pt-TiO2/C) exhibit higher durability in relation to Pt/C cathodes as evidenced by cell polarization, impedance, and cyclic voltammetry data. The degradation in performance of the Pt-TiO2/C cathodes is 10% after 5000 test cycles as against 28% for Pt/C cathodes. These data are in conformity with the electrochemical surface area and impedance values. Pt-TiO2/C cathodes can withstand even 10,000 test cycles with nominal effect on their performance. X-ray diffraction, transmission electron microscope, and cross-sectional field-emission-scanning electron microscope studies on the catalytic electrodes reflect that incorporating TiO2 with Pt helps in mitigating the aggregation of Pt particles and protects the Nafion membrane against peroxide radicals formed during the cathodic reduction of oxygen. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3421970] All rights reserved.

Effect of relative density and confining pressure on Poisson ratio from bender and extender elements tests

By using the bender and extender elements tests, together with measurements of the travel times of shear (S) and primary (P) waves, the variation of Poisson ratio (nu) was determined for dry sands with respect to changes in relative densities and effective confining pressures (sigma(3)). The tests were performed for three different ranges of particle sizes. The magnitude of the Poisson ratio decreases invariably with an increase in both the relative density and the effective confining pressure. The effect of the confining pressure on the Poisson ratio was found to become relatively more significant for fine-grained sand as compared with the coarse-grained sand. For a given material, at a particular value of sigma(3), the magnitude of the Poisson ratio decreases, almost in a linear fashion, with an increase in the value of maximum shear modulus (G(max)). The two widely used correlations in literature, providing the relationships among G(max), void ratio (e) and effective confining pressure (sigma(3)), applicable for angular granular materials, were found to compare reasonably well with the present experimental data for the fine- and medium-grained sands. However, for the coarse-grained sand, these correlations tend to overestimate the values of G(max).