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Thermodynamic Instability (thermodynamic + instability)
Selected AbstractsComparison of the atmosphere above the South Pole, Dome C and Dome A: first attemptMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008S. Hagelin ABSTRACT The atmospheric properties above three sites (Dome C, Dome A and the South Pole) on the Internal Antarctic Plateau are investigated for astronomical applications using the monthly median of the analyses from ECMWF (the European Centre for Medium-Range Weather Forecasts). Radiosoundings extended on a yearly time-scale at the South Pole and Dome C are used to quantify the reliability of the ECMWF analyses in the free atmosphere as well as in the boundary and surface layers, and to characterize the median wind speed in the first 100 m above the two sites. Thermodynamic instability properties in the free atmosphere above the three sites are quantified with monthly median values of the Richardson number. We find that the probability to trigger thermodynamic instabilities above 100 m is smaller on the Internal Antarctic Plateau than on mid-latitude sites. In spite of the generally more stable atmospheric conditions of the Antarctic sites compared to mid-latitude sites, Dome C shows worse thermodynamic instability conditions than those predicted above the South Pole and Dome A above 100 m. A rank of the Antarctic sites done with respect to the strength of the wind speed in the free atmosphere (ECMWF analyses) as well as the wind shear in the surface layer (radiosoundings) is presented. [source] Characterization of a nif-regulated flavoprotein (FprA) from Rhodobacter capsulatusFEBS JOURNAL, Issue 3 20002S] ferredoxin, Redox properties, molecular interaction with a [2Fe A flavoprotein from Rhodobacter capsulatus was purified as a recombinant (His)6 -tag fusion from an Escherichia coli clone over-expressing the fprA structural gene. The FprA protein is a homodimer containing one molecule of FMN per 48-kDa monomer. Reduction of the flavoprotein by dithionite showed biphasic kinetics, starting with a fast step of semiquinone (SQ) formation, and followed by a slow reduction of the SQ. This SQ was in the anionic form as shown by EPR and optical spectroscopies. Spectrophotometric titration gave a midpoint redox potential for the oxidized/SQ couple of Em1 = +20 mV (pH 8.0), whereas the SQ/hydroquinone couple could not be titrated due to the thermodynamic instability of SQ associated with its slow reduction process. The inability to detect the intermediate form, SQ, upon oxidative titration confirmed this instability and led to an estimate of Em2 , Em1 of > 80 mV. The reduction of SQ by dithionite was significantly accelerated when the [2Fe,2S] ferredoxin FdIV was used as redox mediator. The midpoint redox potential of this ferredoxin was determined to be ,275 ± 2 mV at pH 7.5, consistent with FdIV serving as electron donor to FprA in vivo. FdIV and FprA were found to cross-react when incubated together with the 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, giving a covalent complex with an Mr of , 60 000. Formation of this complex was unaffected by the redox states of the two proteins. Other [2Fe,2S] ferredoxins, including FdV and FdVI from R. capsulatus, were ineffective as electron carriers to FprA, and cross-reacted poorly with the flavoprotein. The possible function of FprA with regard to nitrogen fixation was investigated using an fprA -deleted mutant. Although nitrogenase activity was significantly reduced in the mutant compared with the wild-type strain, nitrogen fixation was apparently unaffected by the fprA deletion even under iron limitation or microaerobic conditions. [source] Spontaneous Outcropping of Self-Assembled Insulating Nanodots in Solution-Derived Metallic Ferromagnetic La0.7Sr0.3MnO3 FilmsADVANCED FUNCTIONAL MATERIALS, Issue 13 2009César Moreno Abstract A new mechanism is proposed for the generation of self-assembled nanodots at the surface of a film based on spontaneous outcropping of the secondary phase of a nanocomposite epitaxial film. Epitaxial self-assembled Sr,La oxide insulating nanodots are formed through this mechanism at the surface of an epitaxial metallic ferromagnetic La0.7Sr0.3MnO3 (LSMO) film grown on SrTiO3 from chemical solutions. TEM analysis reveals that, underneath the La,Sr oxide (LSO) nanodots, the film switches from the compressive out-of-plane stress component to a tensile one. It is shown that the size and concentration of the nanodots can be tuned by means of growth kinetics and through modification of the La excess in the precursor chemical solution. The driving force for the nanodot formation can be attributed to a cooperative effect involving the minimization of the elastic strain energy and a thermodynamic instability of the LSMO phase against the formation of a Ruddelsden,Popper phase Sr3Mn4O7 embedded in the film, and LSO surface nanodots. The mechanism can be described as a generalization of the classical Stranski,Krastanov growth mode involving phase separation. LSO islands induce an isotropic strain to the LSMO film underneath the island which decreases the magnetoelastic contribution to the magnetic anisotropy. [source] Critical opalescence points to thermodynamic instability: relevance to small-angle X-ray scattering of resorcinol,formaldehyde gel formation at low pHJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2008Cedric J. Gommes During the formation at low pH of resorcinol,formaldehyde gels with a structure in the micrometre range, small-angle X-ray scattering exhibits a non-monotonic intensity variation as a function of reaction time. The data are analyzed in terms of scattering by statistical fluctuations of polymer concentration, the amplitude of which is maximal close to the critical point for phase separation between polymer and solvent. The data do not carry any morphological information, but they unambiguously show that the driving force of the gel formation is a thermodynamic instability of the polymerizing solution. [source] Review: physical chemistry of solid dispersionsJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 12 2009Sandrien Janssens Abstract Objectives With poorly soluble drug candidates emerging in the drug discovery pipeline, the importance of the solid dispersion formulation approach is increasing. This strategy includes complete removal of drug crystallinity, and molecular dispersion of the poorly soluble compound in a hydrophilic polymeric carrier. The potential of this technique to increase oral absorption and hence bioavailability is enormous. Nevertheless, some issues have to be considered regarding thermodynamic instability, as well in supersaturated solutions that are formed upon dissolution as in the solid state. Key findings After a brief discussion on the historical background of solid dispersions and their current role in formulation, an overview will be given on the physical chemistry and stability of glass solutions as they form supersaturated solutions, and during their shelf life. Conclusions Thorough understanding of these aspects will elicit conscious evaluation of carrier properties and eventually facilitate rational excipient selection. Thus, full exploitation of the solid dispersion strategy may provide an appropriate answer to drug attrition due to low aqueous solubility in later stages of development. [source] Sintering Diamond/Cemented Carbides by the Pulse Plasma Sintering MethodJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2008Andrzej Michalski Under the conditions of thermodynamic instability, we produced dense sintered composites built of diamond particles (30 vol%) and a cemented carbide matrix. The composites were consolidated by high-current electric pulses at a temperature of about 1100°C and load of 75 MPa for 5 min. The diamond particles are strongly bound with the cemented carbide matrix, by a transition layer composed of a solid solution of carbon and tungsten in cobalt. No graphite precipitates were found in the sintered composites, as examined by microstructure observations, examinations of the phase composition, and an analysis of the Raman scattering spectra. The hardness of the diamond/cemented carbide was 23 GPa. [source] Loss of Ammine from Platinum(II) Complexes: Implications for Cisplatin Inactivation, Storage, and Resistance,CHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2005Justin Kai-Chi Lau Abstract Potential consequences of the binding of the anticancer drug cisplatin to various biomolecules in the cell have been investigated by using a combined density functional theory and continuum dielectric model approach. Since the ammine ligands remain coordinated at the metal upon formation of the most frequent DNA adducts, whereas they were found to be displaced from the metal upon formation of drug metabolites, we have analyzed the factors governing ammine loss from platinum(II) complexes as a possible pathway of cisplatin inactivation. The calculations systematically show the effect of 1) the trans ligand, 2) the charge of complex, 3) the nucleophile, and 4) the environment on the thermodynamic instability and kinetic lability of the platinum,ammine bonds. After initial binding of cisplatin hydrolysis products to thioethers or thiols, loss of the ammine trans to this sulfur ligand rather than replacement of the sulfur ligand itself by other nucleophiles like guanine-N7 is predicted to be the predominant reaction. The results of this study contribute to an understanding of the modes of cisplatin inactivation prior to DNA binding, for example, by elevated glutathione levels in cisplatin-resistant cancer cells. [source] | |||||||||||||