Enantioselective total synthesis of iso-cladospolide B, cladospolide C and cladospolide B from tartaric acid

The enantioselective synthesis of the natural products cladospolide B, cladospolide C, and iso-cladospolide B has been accomplished from tartaric acid. Key reactions in the synthetic sequence include the elaboration of a gamma-hydroxy amide derived from tartaric acid via alkene cross metathesis, Yamaguchi lactonization, and ring closing metathesis. (C) 2011 Elsevier Ltd. All rights reserved.

Structural and conformational study of peptides containing glycine and alanine residues by C-13 CPMAS NMR spectroscopy in solid state

The results of the structural and conformational studies carried out using C-13 CPMAS NMR technique on several glycine and alanine containing peptides in the solid state are reported. The study demonstrates the effects of variations in C-13 chemical shifts due to conformation and hydrogen bonding. The possibility of applying this technique to obtain insight into the conformational characteristics of peptides of unknown structures is discussed.

Muon-spin rotation spectra in the mixed phase of high-T-c superconductors: Thermal fluctuations and disorder effects

We study muon-spin rotation (mu SR) spectra in the mixed phase of highly anisotropic layered superconductors, specifically Bi2+xSr2-xCaCu2O8+delta (BSCCO), by modeling the fluid and solid phases of pancake Vortices using liquid-state and density functional methods. The role of thermal fluctuations in causing motional narrowing of mu SR line shapes is quantified in terms of a first-principles theory of the flux-lattice melting transition. The effects of random point pinning are investigated using a replica treatment of liquid-state correlations and a replicated density functional theory. Our results indicate that motional narrowing in the pure system, although substantial, cannot account for the remarkably small linewidths obtained experimentally at relatively high fields and low temperatures. We find that satisfactory agreement with the mu SR data for BSCCO in this regime can be obtained through the ansatz that this ''phase'' is characterized by frozen short-range positional correlations reflecting the structure of the liquid just above the melting transition. This proposal is consistent with recent suggestions of a ''pinned liquid'' or ''glassy'' state of pancake Vortices in the presence of pinning disorder. Our results for the high-temperature liquid phase indicate that measurable linewidths may be obtained in this phase as a consequence of density inhomogeneities induced by the pinning disorder. The results presented here comprise a unified, first-principles theoretical treatment of mu SR spectra in highly anisotropic layered superconductors in terms of a controlled set of approximations. [S0163-1829(99)08033-9].

Phase transitions in triammonium hydrogen disulphate: Crystal structure of an intermediate stable phase at-90 degrees C

Triammonium hydrogen disulphate, (NH4)(3)H(SO4)(2), belongs to the family of crystal structures M3H(XO4)(2) (with M = NH4, K, Rb, Cs, and X = S, Se) which display super protonic phases at elevated temperatures, while at room temperature these are relatively poor proton conductors. The crystal structure of triammonium hydrogen disulphate has been determined by X-ray diffraction at -90 degrees C and the variation in the characteristics of the hydrogen bond is discussed in comparison with that of the structures at -110 degrees C and room temperature. It is concluded that the mechanics involving the proton migration in such systems is realised in terms of the variations in the hydrogen bond features with temperature.

Role of oxygen transients in the facile scission of C-O bonds of alcohols on Zn surfaces

The alkoxy species produced by the interaction of alcohols with Zn surfaces undergoes C-O bond scission at 150 K giving hydrocarbon species, but this transformation occurs even at 80 K when alcohol-oxygen mixtures are coadsorbed, due to the oxygen transients.

A C-H center dot center dot center dot O hydrogen bond stabilized polypeptide chain reversal motif at the C terminus of helices in proteins

The serendipitous observation of a C-H...O hydrogen bond mediated polypeptide chain reversal in synthetic peptide helices has led to a search for the occurrence of a similar motif in protein structures. From a dataset of 634 proteins, 1304 helices terminating in a Schellman motif have been examined. The C-H...O interaction between the T - 4 (CH)-H-alpha and T + 1 C=O group (C...O 3.5 Angstrom) becomes possible only when the T + 1 residue adopts an extended beta conformation (T is defined as the helix terminating residue adopting an alpha(L) conformation). In all, 111 examples of this chain reversal motif have been identified and the compositional and conformational. preferences at positions T - 4, T, and T + 1 determined. A marked preference for residues like Set, Glu and Gln is observed at T - 4 position with the motif being further stabilized by the formation of a side-chain-backbone O...H-N hydrogen bond involving the side-chain of residue T - 4 and the N-H group of residue T + 3. In as many as 57 examples, the segment following the helix was extended with three to four successive residues in beta conformation. In a majority of these cases, the succeeding beta strand lies approximately antiparallel with the helix, suggesting that the backbone C-H...O interactions may provide a means of registering helices and strands in an antiparallel orientation. Two examples were identified in which extended registry was detected with two sets of C-H...O hydrogen bonds between (T - 4) (CH)-H-alpha...C=O (T + 1) and (T - 8) (CH)-H-alpha...C=O (T + 3). 0 2002 Published by Elsevier Science Ltd.

An enantiospecific approach to 8,9-seco-C-aromatic taxanes

An enantiospecific approach to functionalised C-aromatic-8,9-seco-taxanes starting from the readily available monoterpene (R)-carvone is described.

Inhibition of hepatitis C virus replication by herbal extract: Phyllanthus amarus as potent natural source

Hepatitis C virus infection is a major health problem worldwide. Developing effective antiviral therapy for HCV is the need of the hour. The viral enzymes NS3 protease and NS5B RNA dependent RNA polymerase are essential enzymes for polyprotein processing and viral RNA replication and thus can be potential targets for screening anti-HCV compounds. A large number of phytochemicals are present in plants, which are found to be promising antiviral agents. In this study, we have screened inhibitory effect of different plant extracts against the NS3 and NS5B enzymes of hepatitis C virus. Methanolic extracts were prepared from various plant materials and their inhibitory effects on the viral enzymes were determined by in vitro enzyme assays. Effect on viral RNA replication was investigated by using TaqMan Real time RT-PCR. Interestingly, Phyllanthus amarus root (PAR) extract showed significant inhibition of HCV-NS3 protease enzyme; whereas P. amarus leaf (PAL) extract showed considerable inhibition of NS5B in the in vitro assays. Further, the PAR and PAL extracts significantly inhibited replication of HCV monocistronic replicon RNA and HCV H77S viral RNA in HCV cell culture system. However, both PAR and PAL extracts did not show cytotoxicity in Huh7 cells in the MTT assay. Furthermore, addition of PAR together with IFN-alpha showed additive effect in the inhibition of HCV RNA replication. Results suggest the possible molecular basis of the inhibitory activity of PA extract against HCV which would help in optimization and subsequent development of specific antiviral agent using P. amarus as potent natural source. (C) 2011 Elsevier B.V. All rights reserved.

Durable electrocatalytic-activity of Pt-Au/C cathode in PEMFCs

Longevity remains as one of the central issues in the successful commercialization of polymer electrolyte membrane fuel cells (PEMFCs) and primarily hinges on the durability of the cathode. Incorporation of gold (Au) to platinum (Pt) is known to ameliorate both the electrocatalytic activity and stability of cathode in relation to pristine Pt-cathodes that are currently being used in PEMFCs. In this study, an accelerated stress test (AST) is conducted to simulate prolonged fuel-cell operating conditions by potential cycling the carbon-supported Pt-Au (Pt-Au/C) cathode. The loss in performance of PEMFC with Pt-Au/C cathode is found to be similar to 10% after 7000 accelerated potential-cycles as against similar to 60% for Pt/C cathode under similar conditions. These data are in conformity with the electrochemical surface-area values. PEMFC with Pt-Au/C cathode can withstand > 10 000 potential cycles with very little effect on its performance. X-ray diffraction and transmission electron microscopy studies on the catalyst before and after AST suggest that incorporating Au with Pt helps mitigate aggregation of Pt particles during prolonged fuel-cell operations while X-ray photoelectron spectroscopy reflects that the metallic nature of Pt is retained in the Pt-Au catalyst during AST in comparison to Pt/C that shows a major portion of Pt to be present as oxidic platinum. Field-emission scanning electron microscopy conducted on the membrane electrode assembly before and after AST suggests that incorporating Au with Pt helps mitigating deformations in the catalyst layer.

Expression of the receptor guanylyl cyclase C and its ligands in reproductive tissues of the rat: A potential role for a novel signaling pathway in the epididymis

Guanylyl cyclase C (GC-C) is a membrane-associated form of guanylyl cyclase and serves as the receptor for the heat-stable enterotoxin (ST) peptide and endogenous ligands guanylin, uroguanylin, and lymphoguanylin. The major site of expression of GC-C is the intestinal epithelial cell, although GC-C is also expressed in extraintestinal tissue such as the kidney, airway epithelium, perinatal liver, stomach, brain, and adrenal glands. Binding of ligands to GC-C leads to accumulation of intracellular cGMP, the activation of protein kinases G and A, and phosphorylation of the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel that regulates salt and water secretion. We examined the expression of GC-C and its ligands in various tissues of the reproductive tract of the rat. Using reverse transcriptase and the polymerase chain reaction, we demonstrated the presence of GC-C, uroguanylin, and guanylin mRNA in both male and female reproductive organs. Western blot analysis using a monoclonal antibody to GC-C revealed the presence of differentially glycosylated forms of GC-C in the caput and cauda epididymis. Exogenous addition of uroguanylin to minced epididymal tissue resulted in cGMP accumulation, suggesting an autocrine or endocrine activation of GC-C in this tissue. Immunohistochemical analyses demonstrated expression of GC-C in the tubular epithelial cells of both the caput epididymis and cauda epididymis. Our results suggest that the GC-C signaling pathway could converge on CFTR in the epididymis and perhaps control fluid and ion balance for optimal sperm maturation and storage in this tissue.

Kinetics of hydrogen evolution on submicron size Co, Ni, Pd and CoNi alloy powder electrodes by d.c. polarization and a.c. impedance studies

Submicron size Co, Ni and Co-Ni alloy powders have been synthesized by the polyol method using the corresponding metal malonates and Pd powder by reduction of PdOx in methanol. The kinetics of the hydrogen evolution reaction ( HER) in 6 M KOH electrolyte have been studied on electrodes made from the pressed powders. The d.c. polarization measurements have resulted in a value close to 120 mV decade(-1) for the Tafel slope, suggesting that the HER follows the Volmer-Heyrovsky mechanism. The values of exchange current density (i(o)) are in the range 1-10 mA cm(-2) for electrodes fabricated in the study. The a.c. impedance spectra measured at several potentials in the HER region showed a single semicircle in the Nyquist plots. Exchange current density (i(o)) and energy transfer coefficient (alpha) have been calculated by employing a nonlinear least square-fitting program.

Reactivity switching and selective activation of C-1 or C-3 in 2,3-unsaturated thioglycosides

Reactivity switching and selective activation of C-1 or C-3 in 2,3-unsaturated thioglycosides, namely, 2,3-dideoxy-1-thio-D-hex-2-enopyranosides are reported. The reactivity switching allowed activation of either C-1 or C-3, with the use of either N-iodosuccinimide (NIS)/triflic acid (TfOH) or TfOH alone. C-1 glycosylation with alcohol acceptors occurred in the presence of NIS/TfOH, without the acceptors reacting at C-3. On the other hand, reaction of 2,3-unsaturated thioglycosides with alcohols mediated by triflic acid led to transposition of C-1 ethylthio-moiety to C-3 intramolecularly, to form 3-ethylthio-glycals. Resulting glycals underwent glycosylation with alcohols to afford 3-ethylthio-2-deoxy glycosides. However, when thiol was used as an acceptor, only a stereoselective addition at C-3 resulted, so as to form C-1, C-3 dithio-substituted 2-deoxypyranosides. (C) 2011 Elsevier Ltd. All rights reserved.

A study of Al2O3:C films on Si(100) grown by low pressure MOCVD

We report the characterization of carbonaceous aluminium oxide, Al2O3:C, films grown on Si(100) by metalorganic chemical vapor deposition. The focus is on the study of the effects of carbon on the dielectric properties of aluminium oxide in a qualitative manner. The carbon present in the aluminium oxide film derives from aluminium acetylacetonate used as the source of aluminium. As-grown films comprise nanometer-sized grains of alumina (∼ 20–50 nm) in an amorphous carbonaceous matrix, as examined by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The films are shiny; they are smooth as observed by scanning electron microscopy (SEM). An attempt has been made to explore the defects (viz., oxide charge density) in the aluminium oxide films using room temperature high frequency capacitance – voltage (C-V) and current–voltage (I-V) measurements. The hysteresis and stretch-out in the high frequency C-V plots is indicative of charge trapping. The role of heteroatoms, as characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy, in the transport of charge in Al2O3:C films is discussed.

Effect of Post-Metallization Hydrogen Annealing on C-V Characteristic of Zirconia Grown Using Atomic Layer Deposition

Substantial amount of fixed charge present in most of the alternative gate dielectrics gives rise to large shifts in the flat-band voltage (VFB) and charge trapping and de-trapping causes hysterectic changes on voltage cycling. Both phenomena affect stable and reliable transistor operation. In this paper we have studied for the first time the effect of post-metallization hydrogen annealing on the C-V curve of MOS capacitors employing zirconia, one of the most promising gate dielectric. Samples were annealed in hydrogen ambient for up to 30 minutes at different temperatures ranging from room temperature to 400°C. C-V measurements were done after annealing at each temperature and the hysteresis width was calculated from the C-V curves. A minimum hysteresis width of ∼35 mV was observed on annealing the sample at 200°C confirming the excellent suitability of this dielectric