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Activation Process (activation + process)
Selected AbstractsSynthesis of Nanostructured Silicon Carbide through an Integrated Mechanical and Thermal Activation ProcessJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2002Ruiming Ren Changes of crystal structures and microstructures of SiO2 and graphite powder mixtures induced by high-energy milling, the effect of these changes on the reactivity of reactants, and the mechanism of enhanced SiC formation have been studied using a variety of analytical instruments, including X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, solid-state 29Si nuclear magnetic resonance, and nitrogen adsorption (i.e., the BET method). High-energy milling before carbothermic reduction leads to substantial changes in the structural and energy states of the reactants, which in turn increases the reactivity of the reactants and enhances the formation of nanostructured SiC particles. Furthermore, the structural and energy-state changes contribute to the enhanced SiC formation through the increased reaction kinetics as well as the increased reaction driving force. [source] ChemInform Abstract: Water-Controlled Regioselectivity of Pd-Catalyzed Domino Reaction Involving a C,H Activation Process: Rapid Synthesis of Diverse Carbo- and Heterocyclic Skeletons.CHEMINFORM, Issue 26 2010Zhiyao Lu Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Protein tyrosine phosphatases in Chaetopterus egg activationDEVELOPMENT GROWTH & DIFFERENTIATION, Issue 5-6 2003Shantá D. Hinton Changes in protein tyrosine phosphorylation are an essential aspect of egg activation after fertilization. Such changes result from the net contributions of both tyrosine kinases and phosphatases (PTP). This study was conducted to determine what role(s) PTP may have in egg activation. We identified four novel PTP in Chaetopterus pergamentaceus oocytes, cpPTPNT6, cpPTPNT7, cpPTPR2B, and cpPTPR2A, that have significant homology to, respectively, human PTP,, -,, -D2 and -BAS. The first two are cytosolic and the latter two are transmembrane. Several PTP inhibitors were tested to see if they would affect Chaetopterus pergamentaceus fertilization. Eggs treated with ,-bromo-4-hydroxyacetophenone (PTP inhibitor 1) exhibited microvillar elongation, which is a sign of cortical changes resulting from activation. Those treated with Na3VO4 underwent full parthenogenetic activation, including polar body formation and pseudocleavage and did so independently of extracellular Ca2+, which is required for the Ca2+ oscillations that initiate development after fertilization. Fluorescence microscopy identified phosphotyrosine-containing proteins in the cortex and around the nucleus of vanadate-activated eggs, whereas in fertilized eggs they were concentrated only in the cortex. Immunoblots of vanadate-activated and fertilized eggs showed tyrosine hyperphosphorylation of approximately140 kDa protein. These results suggest that PTP most likely maintain the egg in an inactive state by dephosphorylation of proteins independent of the Ca2+ oscillations in the activation process. [source] Acid-Free Synthesis of Carbazoles and Carbazolequinones by Intramolecular Pd-Catalyzed, Microwave-Assisted Oxidative Biaryl Coupling Reactions , Efficient Syntheses of Murrayafoline A, 2-Methoxy-3-methylcarbazole, and GlycozolidineEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 27 2009Vellaisamy Sridharan Abstract A mild and efficient methodology for the synthesis of oxygenated carbazoles from diarylamines under non-acidic conditions was developed, based on a palladium-catalyzed, microwave-assisted double C,H bond activation process. This new protocol was successfully applied to the synthesis of three naturally occurring carbazoles, namely murrayafoline A, 2-methoxy-3-methylcarbazole, and glycozolidine. The scope of the reaction was also expanded to include the synthesis of benzo fused carbazolequinones.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Thin Film Solar Cells: Materials Science at InterfacesADVANCED ENGINEERING MATERIALS, Issue 10 2005J. Fritsche Abstract Interfaces are important for the efficiencies of thin film solar cells. In particular for polycrystalline chalcogenide semiconductors as Cu(In,Ga)(S,Se)2 and CdTe the existing physical concepts, which describe the electronic properties of semiconductor interfaces, are not sufficient. The increased complexity is mostly due to the non-abruptness of the interfaces and the strong tendency for the formation of defects. For the CdTe thin film solar cell a very relevant interface for their operation and efficiency is the CdTe/CdS semiconductor hetero junction. The properties of the semiconductor interfaces have been characterised systematically with photoelectron spectroscopy (XPS/UPS) in integrated ultra high vacuum (UHV) systems for sample preparation and analysis. Withal the key topic is the experimental determination of the band alignment at the semiconductor interfaces. For high efficiency CdTe solar cell production CdCl2 activation is of major importance. The effects of the CdCl2 treatment step on CdTe solar cells had been not completely understood so far. To investigate its influence the activation process has been transferred into the integrated UHV system. We will report about chemical and electronic modifications of the CdTe/CdS hetero interface due to in-situ CdCl2 activation performing sputter depth profiles in combination with X-ray photoelectron spectroscopy (XPS). [source] The crystal structure of pyruvate decarboxylase from Kluyveromyces lactisFEBS JOURNAL, Issue 18 2006Implications for the substrate activation mechanism of this enzyme The crystal structure of pyruvate decarboxylase from Kluyveromyces lactis has been determined to 2.26 Å resolution. Like other yeast enzymes, Kluyveromyces lactis pyruvate decarboxylase is subject to allosteric substrate activation. Binding of substrate at a regulatory site induces catalytic activity. This process is accompanied by conformational changes and subunit rearrangements. In the nonactivated form of the corresponding enzyme from Saccharomyces cerevisiae, all active sites are solvent accessible due to the high flexibility of loop regions 106,113 and 292,301. The binding of the activator pyruvamide arrests these loops. Consequently, two of four active sites become closed. In Kluyveromyces lactis pyruvate decarboxylase, this half-side closed tetramer is present even without any activator. However, one of the loops (residues 105,113), which are flexible in nonactivated Saccharomyces cerevisiae pyruvate decarboxylase, remains flexible. Even though the tetramer assemblies of both enzyme species are different in the absence of activating agents, their substrate activation kinetics are similar. This implies an equilibrium between the open and the half-side closed state of yeast pyruvate decarboxylase tetramers. The completely open enzyme state is favoured for Saccharomyces cerevisiae pyruvate decarboxylase, whereas the half-side closed form is predominant for Kluyveromyces lactis pyruvate decarboxylase. Consequently, the structuring of the flexible loop region 105,113 seems to be the crucial step during the substrate activation process of Kluyveromyces lactis pyruvate decarboxylase. [source] Carbonyl cyanide m -chlorophenylhydrazone induced calcium signaling and activation of plasma membrane H+ -ATPase in the yeast Saccharomyces cerevisiaeFEMS YEAST RESEARCH, Issue 4 2008Michele B.P. Pereira Abstract The plasma membrane H+ -ATPase from Saccharomyces cerevisiae is an enzyme that plays a very important role in the yeast physiology. The addition of protonophores, such as 2,4-dinitrophenol (DNP) and carbonyl cyanide m -chlorophenylhydrazone (CCCP), also triggers a clear in vivo activation of this enzyme. Here, we demonstrate that CCCP-induced activation of the plasma membrane H+ -ATPase shares some similarities with the sugar-induced activation of the enzyme. Phospholipase C and protein kinase C activities are essential for this activation process while Gpa2p, a G protein involved in the glucose-induced activation of the ATPase, is not required. CCCP also induces a phospholipase C-dependent increase in intracellular calcium. Moreover, we show that the availability of extracellular calcium is required for CCCP stimulation of H+ -ATPase, suggesting a possible connection between calcium signaling and activation of ATPase. [source] (,6 -Arene)ruthenium(N-heterocyclic carbene) Complexes for the Chelation-Assisted Arylation and Deuteration of Arylpyridines: Catalytic Studies and Mechanistic InsightsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7 2010Amparo Prades Abstract A series of (,6 -arene)ruthenium complexes have been tested in the arylation of arylpyridines. One (,6 - p -cymene)ruthenium(N-heterocyclic carbene) complex (labelled as 1 in the text) was found to be the most effective, being capable of arylating a wide set of substantially different arylpyridines. Complex 1 is also able to promote the regioselective deuteration of a series of arylated N-heterocycles, via a nitrogen-directed mechanism. Two of the deuterated amines were used to measure the kinetic isotope effect (KIE) in the arylation process. The detection of an inverse KIE, together with the observation that the CH activation process does not require the addition of a base, suggest that the rate-limiting step in the arylation process may be different to that of previously reported studies. [source] The influence of ovarian fluid on Solea senegalensis sperm motilityJOURNAL OF APPLIED ICHTHYOLOGY, Issue 5 2010P. Diogo Summary The role of ovarian fluid in fertilization has been neglected, particularly in marine species. The aim of this work was therefore to assess the influence of ovarian fluid (OF) as a potential contributor factor to sperm motility in Solea senegalensis. The specificity of interactions between sperm and ovarian fluid was analyzed using homologous and heterelogous ovarian fluid. Additional tests tried to identify the most useful concentration for improving sperm motility throughout the activation process. Ovarian fluid solutions were diluted in artificial seawater (SW) (v:v) 0 : 100, 25 : 75, 50 : 50, 75 : 25 and 100 : 0 (OF:SW). Pure ovarian fluid solutions (100%) did not promote sperm motility by themselves since they lack the osmolarity needed to trigger sperm motility. With 75% of ovarian fluid the activation solution promoted a deficient activation and the best concentrations used were 25 and 50%. The presence of ovarian fluid affected significantly total motility (TM) and progressive motility (PM) in the last seconds post activation. Progressive motility was higher at 45 s for homologous 25% OF (20.4%) than control (9.4%). Homologous 25% OF increased significantly TM and PM at 60 s post activation (32.0 and 10.5%, respectively) when compared to control (15.8 and 1.7%, respectively). Sperm velocity showed significant differences in the presence of ovarian fluid since early seconds post activation. Our data revealed an enhancement of sperm motility with ovarian fluid at low concentrations in the activation solution. There seems to be a high degree of specificity of ovarian fluid-sperm interaction since heterologous fluid had a lower performance enhancing sperm motility than homologous fluid. Our results indicated a possible important female contribution to sperm motility enhancement during the fertilization process in S. senegalensis. [source] Structural Modeling of Car Use on the Way to the University in Different Settings: Interplay of Norms, Habits, Situational Restraints, and Perceived Behavioral Control,JOURNAL OF APPLIED SOCIAL PSYCHOLOGY, Issue 8 2009Christian A. Klöckner This manuscript presents the results of the application of an extended norm activation model to the explanation of car use on the way to the university with a sample of 430 students of 3 German universities. The proposed two-stage structural model is supported by the data. First, a norm activation process starting with awareness of consequences activates subjective and personal norms. Second, behavior is determined by car-use habits, perceived behavioral control (PBC), car access, and effort to use public transportation. The influence of personal norms on behavior is mediated by habits. Subgroup analyses of the second stage of the model show a high structural stability, but differences in the regression weights. [source] Porosity and surface characteristics of activated carbons produced from waste tyre rubberJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 1 2002Guillermo San Miguel Abstract Waste tyre rubber has proven to be a suitable precursor for the production of high quality activated carbons. The performance of these carbons in commercial applications such as water treatment or gas purification is highly dependent on their surface characteristics. This paper presents an in-depth investigation on how production conditions may affect the yield and characteristics of activated carbons produced from tyre rubber. For this purpose, three tyre rubbers of different particle sizes were consecutively pyrolysed and then activated in a steam atmosphere at 925,°C using a laboratory-scale rotary furnace. Activation was conducted at different intervals over 80,640,min to achieve different degrees of carbon burn-off. The resulting carbons were analysed for their elemental composition, ash content and nitrogen gas adsorption characteristics. The BET and t -plot models were used to investigate various aspects of their porosity and surface area characteristics. SEM analyses were also conducted for visual examination of the carbon surface. Results show that pyrolytic chars, essentially mesoporous materials, developed a very narrow microporosity during the initial stages of the activation process (up to 15,25,wt% burn-off). Further activation resulted in the progressive enlargement of the average micropore width and a gradual development of the mesoporous structure. Total micropore volumes and BET surface areas increased continuously with the degree of activation to reach values up to 0.498,cm3g,1 and 1070,m2g,1 respectively, while external surface areas developed more rapidly at degrees of activation above 45,wt% burn-off. Results presented in this work also illustrate that carbons produced from powdered rubber developed a narrower and more extensive porosity, both in the micropore and mesopore range, than those produced from rubber of a larger particle size. © 2001 Society of Chemical Industry [source] Kinetic study of the decomposition of 2-butanol on carbon-based acid catalystAICHE JOURNAL, Issue 6 2010J. Bedia Abstract The catalytic conversion of 2-butanol on a carbon-based acid catalyst prepared by chemical activation of olive stone with phosphoric acid was investigated. The carbon catalyst showed a considerable amount of surface phosphorus, presumably in form of phosphate groups, as revealed by XPS, despite a washing step carried out after the activation process. Conversion of 2-butanol yields mainly dehydration products, mostly cis-2-butene and trans-2-butene with lower amounts of 1-butene, and a very small amount of mek as dehydrogenation product. Kinetic interpretation of the experimental data was performed using two elimination mechanisms for the dehydration reaction; an E1-mechanism (two-step mechanism) and an E2-mechanism (one-step mechanism). The rate expressions derived from both models fit properly the experimental results, suggesting that probably the two mechanisms occur simultaneously. This is supported by the similar rate constant obtained for the formation of the carbocation and the olefins in the E1 and E2 mechanisms, respectively. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] The effect of ligand on the rate of propagation of Cu(0)-wire catalyzed SET-LRP of MA in DMSO at 25 °CJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2009Nga H. Nguyen Abstract The effect of initial ligand concentration on the apparent rate constant of propagation of single-electron transfer living radical polymerization (SET-LRP) of MA in DMSO at 25 °C was examined using various lengths of Cu(0) wire as catalyst. It was determined that unlike other parameters such as initiator concentration, solvent concentration, and deactivator concentration, no simple external rate-order for the ligand concentration could be determined. Rather, the response of the rate of SET-LRP to initial ligand concentration is complex and is likely determined by a competition of ligand-dependent extent of disproportionation as well as the role of ligand concentration in the surface mediated activation process. Results suggest that a minimum concentration of ligand is needed to achieve both acceptable reaction rate and reaction control, and therefore, ligand concentration must be considered in designing experimental conditions for SET-LRP. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5629,5638, 2009 [source] Kinetic behavior of ethylene/1-hexene copolymerization in slurry and solution reactorsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 11 2005Long Wu Abstract The copolymerization of ethylene and 1-hexene over a spherical polymer/MgCl2 -supported TiCl4 catalyst was studied as a function of the polymerization temperature from 40 to 100 °C in a slurry reactor and from 120 to 200 °C in a solution reactor with triethylaluminum (TEA) as a cocatalyst (1.0,6.8 mmol). The activities increased from 40 to 80 °C and then declined monotonically with increases in the temperature during the slurry and solution polymerizations. The kinetic behavior in the slurry and solution operations was described by the same rate expression. The modeling results indicated that the catalyst had at least two different types of catalytic sites; one site was responsible for the acceleration,decay nature of the activity profiles, whereas the second site resulted in long-term activity. The apparent activation energy for site activation in the slurry operation was 69.9 kJ/mol; no activation energies for site activation could be estimated for the solution operation because the activation process was essentially instantaneous at the higher temperatures. The activation energies for deactivation were 100.3 kJ/mol for the slurry operation and 31.2 kJ/mol for the solution operation. The responses to TEA were similar for the slurry and solution operations; the rates increased with increasing amounts of TEA between 1.0 and 3.4 mmol and then decreased with larger amounts of TEA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2248,2257, 2005 [source] Nickel(II) and palladium(II) complexes with ,-dioxime ligands as catalysts for the vinyl polymerization of norbornene in combination with methylaluminoxane, tris(pentafluorophenyl)borane, or triethylaluminum cocatalyst systems,JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2002Bernd Berchtold Abstract Nickel(II) and palladium(II) complexes with ,-dioxime ligands dimethylglyoxime, diphenylglyoxime, and 1,2-cyclohexanedionedioxime represent six new precatalysts for the polymerization of norbornene that can be activated with methylaluminoxane (MAO), the organo-Lewis acid tris(pentafluorophenyl)borane [B(C6F5)3], and triethylaluminum (TEA) AlEt3. The palladium but not the nickel precatalysts could also be activated by B(C6F5)3 alone, whereas two of the three nickel precatalysts but none of the palladium systems are somewhat active with only TEA as a cocatalyst. It was possible to achieve very high polymerization activities up to 3.2 · 107 gpolymer/molmetal · h. With the system B(C6F5)3/AlEt3, the activation process can be formulated as the following two-step reaction: (1) B(C6F5)3 and TEA lead to an aryl/alkyl group exchange and result in the formation of Al(C6F5)nEt3,n and B(C6F5)3,nEtn; and (2) Al(C6F5)nEt3,n will then react with the precatalysts to form the active species for the polymerization of norbornene. Variation of the B:Al ratio shows that Al(C6F5)Et2 is sufficient for high activation. Gel permeation chromatography indicated that it was possible to control the molar mass of poly(norbornene)s by TEA or 1-dodecene as chain-transfer agents; the molar mass can be varied in the number-average molecular weight range from 2 · 103 to 9 · 105 g · mol,1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3604,3614, 2002 [source] Activation of large lons in FT-ICR mass spectrometryMASS SPECTROMETRY REVIEWS, Issue 2 2005Julia Laskin Abstract The advent of soft ionization techniques, notably electrospray and laser desorption ionization methods, has enabled the extension of mass spectrometric methods to large molecules and molecular complexes. This both greatly extends the applications of mass spectrometry and makes the activation and dissociation of complex ions an integral part of these applications. This review emphasizes the most promising methods for activation and dissociation of complex ions and presents this discussion in the context of general knowledge of reaction kinetics and dynamics largely established for small ions. We then introduce the characteristic differences associated with the higher number of internal degrees of freedom and high density of states associated with molecular complexity. This is reflected primarily in the kinetics of unimolecular dissociation of complex ions, particularly their slow decay and the higher energy content required to induce decomposition,the kinetic shift (KS). The longer trapping time of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) significantly reduces the KS, which presents several advantages over other methods for the investigation of dissociation of complex molecules. After discussing general principles of reaction dynamics related to collisional activation of ions, we describe conventional ways to achieve single- and multiple-collision activation in FT-ICR MS. Sustained off-resonance irradiation (SORI),the simplest and most robust means of introducing the multiple collision activation process,is discussed in greatest detail. Details of implementation of this technique, required control of experimental parameters, limitations, and examples of very successful application of SORI-CID are described. The advantages of high mass resolving power and the ability to carry out several stages of mass selection and activation intrinsic to FT-ICR MS are demonstrated in several examples. Photodissociation of ions from small molecules can be effected using IR or UV/vis lasers and generally requires tuning lasers to specific wavelengths and/or utilizing high flux, multiphoton excitation to match energy levels in the ion. Photodissociation of complex ions is much easier to accomplish from the basic physics perspective. The quasi-continuum of vibrational states at room temperature makes it very easy to pump relatively large amounts of energy into complex ions and infrared multiphoton dissociation (IRMPD) is a powerful technique for characterizing large ions, particularly biologically relevant molecules. Since both SORI-CID and IRMPD are slow activation methods they have many common characteristics. They are also distinctly different because SORI-CID is intrinsically selective (only ions that have a cyclotron frequency close to the frequency of the excitation field are excited), whereas IRMPD is not (all ions that reside on the optical path of the laser are excited). There are advantages and disadvantages to each technique and in many applications they complement each other. In contrast with these slow activation methods, the less widely appreciated activation method of surface induced dissociation (SID) appears to offer unique advantages because excitation in SID occurs on a sub-picosecond time scale, instantaneously relative to the observation time of any mass spectrometer. Internal energy deposition is quite efficient and readily adjusted by altering the kinetic energy of the impacting ion. The shattering transition,instantaneous decomposition of the ion on the surface,observed at high collision energies enables access to dissociation channels that are not accessible using SORI-CID or IRMPD. Finally, we discuss some approaches for tailoring the surface to achieve particular aims in SID. © 2004 Wiley Periodicals, Inc., Mass Spec Rev 24:135,167, 2005 [source] The role of the interdomain B linker in the activation of the XylR protein of Pseudomonas putidaMOLECULAR MICROBIOLOGY, Issue 2 2000Junkal Garmendia In the presence of toluene and other structural analogues, the enhancer binding protein XylR activates the ,54 promoter Pu of the TOL (toluene degradation) plasmid pWW0 of Pseudomonas putida. Introduction of amino acid changes Val-219Asp and Ala-220Pro, which enter a proline kink at the interdomain region (B linker) between the A (signal reception) module and the central portion of XylR, originated a protein with unforeseen properties. These included a minor ability to activate Pu in the absence of aromatic effectors, a much higher responsiveness to m- xylene and a significant response to a large collection of aromatic inducers. Such changes could not be attributed to variations in XylR expression levels or to the fortuitous creation of a novel promoter, but to a genuine change in the properties of the activator. Structural predictions suggested that the mutation entirely disrupted an otherwise probable coiled-coil structure. A second directed mutant within the same region consisting of a major replacement of amino acids A220,N221 by the peptide HHHR produced an even more exacerbated phenotype. These data support a model in which the linker B region influences the effector profile by modifying at a distance the operative shape of the effector pocket and fixing the protein in an intermediate step of the activation process. [source] Mapping of Atrial Activation Patterns After Inducing Contiguous Radiofrequency Lesions: An Experimental StudyPACING AND CLINICAL ELECTROPHYSIOLOGY, Issue 2 2001FRANCISCO J. CHORRO CHORRO, F.J., et al.: Mapping of Atrial Activation Patterns After Inducing Contiguous Radiofrequency Lesions: An Experimental Study. High resolution mapping techniques are used to analyze the changes in atrial activation patterns produced by contiguous RF induced lesions. In 12 Langendorff-perfused rabbit hearts, left atrial activation maps were obtained before and after RF induction of epicardial lesions following a triple-phase sequential protocol: (phase 1) three separate lesions positioned vertically in the central zone of the left atrial wall; (phase 2) the addition of two lesions located between the central lesion and the upper and lower lesions; and (phase 3) the placement of four additional lesions between those induced in the previous phases. In six additional experiments a pathological analysis of the individual RF lesions was performed. In phase 1 (lesion diameter = 2.8 ± 0.2 mm, gap between lesions = 3 ± 0.8 mm), the activation process bordered the lesions line in two (2.0-ms cycles) and four experiments (1.0-ms cycles). In phase 2, activation bordered the lesions line in eight (2.0-ms cycles, P < 0.01 vs control) and nine experiments (1.0-ms cycles, P < 0.001), and in phase 3 this occurred in all experiments except one (both cycles, P < 0.001 vs control). In the experiments with conduction block, the increment of the interval between activation times proximal and distal to the lesions showed a significant correlation to the length of the lesions (r = 0.68, P < 0.05, 100-ms cycle). In two (17%) experiments, sustained regular tachycardias were induced with reentrant activation patterns around the lesions line. In conclusion, in this acute model, atrial RF lesions with intact tissue gaps of 3 mm between them interrupt conduction occasionally, and conduction block may be frequency dependent. Lesion overlap is required to achieve complete conduction block lines. Tachycardias with reentrant activation patterns around a lesions line may be induced. [source] Study of the activation process of Mg dopant in GaN:Mg layersPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2006B. Paszkiewicz Abstract GaN:Mg layers with different concentration of Mg dopant were grown by metalorganic vapour phase epitaxy. The incorporation of Mg was verified by secondary ion mass spectroscopy. In order to dissociate Mg-related complexes and thus electrically activate the acceptor dopant, the as-grown layers were annealed in pure N2 at ,800 °C for 30 minutes. The influence of the post-growth annealing on the layer properties was studied by photoluminescence (PL) and impedance spectroscopy. Impedance spectroscopy measurement showed that the annealed samples reveal higher charge concentrations and lower sheet resistance. Moreover, the relaxation time of hole traps decreased in annealed samples by one order of magnitude compared to as-grown samples. The changes in the electrical properties have been correlated with the changes in the PL spectra. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Characterization and properties of activated nanosilica/polypropylene composites with coupling agentsPOLYMER COMPOSITES, Issue 11 2009Ong Hui Lin In this work, nanosilica/polypropylene composites containing 1 wt% of silica nanoparticles were prepared by melt mixing in a Thermo Haake internal mixer. Prior compounding, nanosilica was subjected to surface activation using sodium hydroxide (NaOH) solution. The effectiveness of the activation process was evaluated by measuring the amount of hydroxyl groups (OH) on the surface of nanosilica via titration method and supported by FTIR analysis. Two coupling agents namely 3-aminopropyl triethoxysilane (APTES) and neopentyl (diallyl)oxy, tri(dioctyl) phosphate titanate (Lica 12) were used for surface treatment after activation process. The mechanical properties of polypropylene matrix reinforced with silica nanoparticles were determined by tensile and impact test. Hydroxyl groups on the nanosilica surface played an important role in enhancing the treatment with silane coupling agents. To increase the amount of hydroxyl groups on the nanosilica surface, the optimum concentration of NaOH is 1 mol%. Tensile strength, tensile modulus, and impact strength of nanosilica/PP composites improved with activation process. As the coupling agent is concerned, APTES coupling agent is more pronounced in enhancing the mechanical properties of the composites when compared with Lica 12 coupling agent. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers [source] Fracture and yielding behaviors of polystyrene/ethylene-propylene rubber blends: Effects of interfacial agentsPOLYMER ENGINEERING & SCIENCE, Issue 12 2001T. Haanh The aim of this work is to investigate the effects of two triblock copolymers, used as coupling agents, on fracture and yielding behaviors of a blend of 80 volume % of polystyrene (PS) and 20 volume %of ethylene-propylene rubber (EPR), over a large range of loading rates and temperatures. For this purpose, blends containing different concentrations of two triiblock copolymers were studied at various test conditions. The focus was put on the time-temperature dependence of fracture performance of the blends. Addition of triblock copolymer makes the PS/EPR blend more ductile. The time-temperature dependence of the brittle-ductile transition in fracture performance of the blend is controlled by an energy activation process. The interfacial agent lowers the temperature at brittle-ductile transition and reduces the energy barrier controlling the fracture process. This effect, however, is much more pronounced for the lower molecular weight interfacial agent. The correlation between temperature, loading rate and yield stress of the blends seems to be controlled by a molecular relaxation process according to the Ree-Eyring theory. This model, based on the assumption of two relaxation processes (, and ,) acting in parallel, allows prediction of yield stress at various loading rates and temperatures. Addition of the interfacial agents results in a reduction of the activation energy and an increase in the activation volume V* for both the , and , processes. Furthermore, the similarity of the value of the activation energy ,H, in the , yielding process and the energy barrier ,H controlling the brittle-ductile transition in fracture seems to suggest that a similar secondary relaxation mechanism controls the yielding and the fracture behavior of the blend. [source] Ion-dependent gating of kainate receptorsTHE JOURNAL OF PHYSIOLOGY, Issue 1 2010Derek Bowie Ligand-gated ion channels are an important class of signalling protein that depend on small chemical neurotransmitters such as acetylcholine, l -glutamate, glycine and ,-aminobutyrate for activation. Although numerous in number, neurotransmitter substances have always been thought to drive the receptor complex into the open state in much the same way and not rely substantially on other factors. However, recent work on kainate-type (KAR) ionotropic glutamate receptors (iGluRs) has identified an exception to this rule. Here, the activation process fails to occur unless external monovalent anions and cations are present. This absolute requirement of ions singles out KARs from all other ligand-gated ion channels, including closely related AMPA- and NMDA-type iGluR family members. The uniqueness of ion-dependent gating has earmarked this feature of KARs as a putative target for the development of selective ligands; a prospect all the more compelling with the recent elucidation of distinct anion and cation binding pockets. Despite these advances, much remains to be resolved. For example, it is still not clear how ion effects on KARs impacts glutamatergic transmission. I conclude by speculating that further analysis of ion-dependent gating may provide clues into how functionally diverse iGluRs families emerged by evolution. Consequently, ion-dependent gating of KARs looks set to continue to be a subject of topical inquiry well into the future. [source] G-protein-coupled receptor oligomers: two or more for what?THE JOURNAL OF PHYSIOLOGY, Issue 22 2009GABAB receptors, Lessons from mGlu G-protein-coupled receptors (GPCRs) are key players in the precise tuning of intercellullar communication. In the brain, both major neurotransmitters, glutamate and GABA, act on specific GPCRs [the metabotropic glutamate (mGlu) and GABAB receptors] to modulate synaptic transmission. These receptors are encoded by the largest gene family, and have been found to associate into both homo- and hetero-oligomers, which increases the complexity of this cell communication system. Here we show that dimerization is required for mGlu and GABAB receptors to function, since the activation process requires a relative movement between the subunits to occur. We will also show that, in contrast to the mGlu receptors, which form strict dimers, the GABAB receptors assemble into larger complexes, both in transfected cells and in the brain, resulting in a decreased G-protein coupling efficacy. We propose that GABAB receptor oligomerization offers a way to increase the possibility of modulating receptor signalling and activity, allowing the same receptor protein to have specific properties in neurons at different locations. [source] Delay of the egg activation process in the Black Tiger Shrimp Penaeus monodon by manipulation of magnesium levels in spawning waterAQUACULTURE RESEARCH, Issue 2 2010Pattira Pongtippatee Abstract The aim of this study was to determine whether magnesium (Mg2+) in seawater is required for egg activation of the black tiger shrimp Penaeus monodon and whether manipulation of Mg2+ levels can be used to delay the process and thereby synchronize egg activation. Female P. monodon broodstock were allowed to spawn in artificial seawater containing Mg2+ at varying levels with respect to the normal (100%) level: 100%, 50%, 20% and 0%. Egg activation occurred normally at 100% Mg2+, incompletely at 50% and 20% Mg2+ levels and did not occur at all with 0% Mg2+. The fertilization rate with 100% Mg2+ was observed to be 83%, but fertilization failed to take place in all the other groups. The fertilization rate was restored from 0% to 76% following the 20% Mg2+ level treatment when Mg2+ levels returned to normal (100%) as soon as spawning was completed. This study suggests that the level of Mg2+ in seawater plays a vital role in P. monodon egg activation, and that commencement of this process could be delayed by manipulation of the Mg2+ level during and immediately after spawning. [source] Aerosol growth and activation in polluted air masses over a tropical metropolis in the Indian sub-continentATMOSPHERIC SCIENCE LETTERS, Issue 2 2009S. Varun Raj Abstract Air pollution can affect cloud formation in more than one way. When the pollutant gases are condensable (e.g. oxides of sulphur), then the process of aerosol activation is eased to a certain extent aiding cloud formation. However, polluted days are often characterised by low updraught speeds which inhibit aerosol growth. In this study, we have critically examined the aerosol activation process in a polluted coastal environment where both effects are present. We have concentrated on the Chennai region (one of the largest cities in the world) of the Indian sub-continent because its pace of industrialisation is increasing rapidly, adding to increasing SO2 pollution over the years. Air masses over Chennai contain a mixture of aerosol particles including NaCl, because of its proximity to the Bay of Bengal, along with ammonium sulphate. We have used observational data along with a detailed microphysical chemical parcel model to study cloud activation effects. We find that over Chennai, often the presence of the condensable pollutant vapour (SO2) more than compensates for the low updraught speeds by lowering the level of maximum super saturation significantly. This latter effect favours the activation of ammonium sulphate as well as NaCl aerosol particles. We have undertaken a systematic analysis to quantify the relative strengths of these two competing effects and find that even with low updraught speeds, oxides of sulphur can perturb the activation domain comprising a mixture of aerosol particles to such an extent that aerosol particles in polluted environments often grow efficiently. This effect is non-intuitive in the sense that one associates smaller cloud droplet sizes with polluted air masses. This is the first microphysical modelling study for the Indian sub-continent where National Environmental Engineering Research Institute (NEERI) observations have been applied to cloud microphysical processes. Copyright © 2009 Royal Meteorological Society [source] Proposal for molecular mechanism of thionins deduced from physico-chemical studies of plant toxinsCHEMICAL BIOLOGY & DRUG DESIGN, Issue 6 2004B. Stec Abstract:, We propose a molecular model for phospholipid membrane lysis by the ubiquitous plant toxins called thionins. Membrane lysis constitutes the first major effect exerted by these toxins that initiates a cascade of cytoplasmic events leading to cell death. X-ray crystallography, solution nuclear magnetic resonance (NMR) studies, small angle X-ray scattering and fluorescence spectroscopy provide evidence for the mechanism of membrane lysis. In the crystal structures of two thionins in the family, ,1 - and , -purothionins (MW: approximately 4.8 kDa), a phosphate ion and a glycerol molecule are modeled bound to the protein. 31P NMR experiments on the desalted toxins confirm phosphate-ion binding in solution. Evidence also comes from phospholipid partition experiments with radiolabeled toxins and with fluorescent phospholipids. This data permit a model of the phospholipid,protein complex to be built. Further, NMR experiments, one-dimensional (1D)- and two-dimensional (2D)-total correlation spectroscopy (TOCSY), carried out on the model compounds glycerol-3-phosphate (G3P) and short chain phospholipids, supported the predicted mode of phospholipid binding. The toxins' high positive charge, which renders them extremely soluble (>300 mg/mL), and the phospholipid-binding specificity suggest the toxin,membrane interaction is mediated by binding to patches of negatively charged phospholipids [phosphatidic acid (PA) or phosphatidyl serine (PS)] and their subsequent withdrawal. The formation of proteolipid complexes causes solubilization of the membrane and its lysis. The model suggests that the oligomerization may play a role in toxin's activation process and provides insight into the structural principles of protein,membrane interactions. [source] Understanding Sulfone Behavior in Palladium-Catalyzed Domino Reactions with Aryl IodidesCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2006Inés Alonso Dr. Abstract Unlike traditionally used acyclic 1,2-disubstituted alkenes, the reaction of ,,,-unsaturated phenyl sulfones with aryl iodides under Heck reaction conditions takes place mainly by means of a four-component domino process, involving one unit of the alkene and three units of the aryl iodide, affording substituted 9-phenylsulfonyl-9,10-dihydrophenanthrenes. We report here the results of a computational study on the mechanism of this domino arylation reaction. Based on these results we can explain why vinyl sulfones, unlike other electron-deficient alkenes such as enones, preferentially follow this domino pathway instead of the usual Heck pathway. The key step is a CH activation process in which a five-membered palladacycle is formed. The greater ability of vinyl sulfones, relative to enones, to reach the transition state that leads to the formation of the initial palladacycle makes the difference. Al contrario que los alquenos acíclicos 1,2-disustituidos tradicionalmente utilizados, la reacción de fenil sulfonas ,,, -insaturadas con yoduros de arilo bajo condiciones de reacción de Heck tiene lugar mayoritariamente a través de un proceso dominó de cuatro componentes, en el que participan una unidad de alqueno y tres unidades de yoduro de arilo, dando lugar a 9-fenilsulfonil-9,10-dihidrofenantrenos sustituidos. Aquí se presentan los resultados de un estudio computacional sobre el mecanismo de esta reacción de arilación dominó. De acuerdo con estos resultados se puede explicar por qué en el caso de las vinil sulfonas este camino de reacción predomina sobre la reacción de Heck, al contrario que en el caso de otros alquenos pobres en electrones como las enonas. La etapa clave es un proceso de activación CH en el que se forma un paladaciclo de cinco miembros. La mayor capacidad de las vinil sulfonas en comparación con las enonas para alcanzar el estado de transición que conduce al paladaciclo es la causa fundamental de este comportamiento diferencial. [source] Neutral Group-IV Metal Catalysts for the Intramolecular Hydroamination of AlkenesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 16 2008Carsten Müller Abstract A detailed comparison of the group-IV metal catalysts Ti(NMe2)4, Ind2TiMe2, Ind2ZrMe2 and Ind2HfMe2 in the intramolecular hydroamination of amino alkenes is presented. Among these catalysts, the benchmark catalyst Ti(NMe2)4 is the most active in the formation of pyrrolidines. A comparison between Ind2TiMe2, Ind2ZrMe2 and Ind2HfMe2 suggests that in the synthesis of pyrrolidines, Zr complexes show the highest catalytic activity of the group-IV metal catalysts. Although Ind2TiMe2 - and the Ind2ZrMe2 -catalyzed formation of a pyrrolidine is first-order in the concentration of the substrate, the corresponding Ti(NMe2)4 -catalyzed cyclization is second-order in the concentration of the substrate. The results obtained for the formation of piperidines catalyzed by Ti(NMe2)4, Ind2TiMe2, Ind2ZrMe2 and Ind2HfMe2 suggest that for these reactions, Ti catalysts show increased catalytic activity compared with the corresponding Zr catalysts. Unfortunately, the formation of aminocyclopentane side-products by C,H activation processes is a severe drawback of the Ti catalysts. The corresponding side-products are not formed in Ind2ZrMe2 - and Ind2HfMe2 -catalyzed reactions. However, the former catalyst gives better yields of the desired piperidine products. In contrast to the results obtained for the synthesis of pyrrolidines, the formation of a piperidine is zero-order in the concentration of the substrate for the indenyl catalysts Ind2TiMe2 and Ind2ZrMe2, and first-order for the homoleptic catalyst Ti(NMe2)4. Interestingly, Ind2TiMe2 is able to catalyze a slow hydroamination of an N -methylated amino alkene, whereas the homoleptic complex Ti(NMe2)4 as well as Ind2ZrMe2 and Ind2HfMe2 do not catalyze the same reaction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Interaction between genioglossus and diaphragm responses to transcranial magnetic stimulation in awake humansEXPERIMENTAL PHYSIOLOGY, Issue 4 2007Wei Wang The modulation of activity of the upper airway dilator and respiratory muscles plays a key role in the regulation of ventilation, but little is known about the link between their neuromuscular activation processes in vivo. This study investigated genioglossus and diaphragm responses to transcranial magnetic stimulation applied in different facilitatory conditions. The amplitude and latency of motor-evoked potential responses and the stimulation intensity threshold leading to a motor response (motor threshold) were recorded with stimulation applied at the vertex and anterolateral area in 13 awake normal subjects. Stimuli were applied during inspiration with and without resistance, during expiration with and without maximal tongue protrusion and during deep inspiration. In each stimulation location and condition, no diaphragmatic response was obtained without previous genioglossus activity (diaphragmatic and genioglossus responses latencies during expiration: 18.1 ± 2.9 and 6.3 ± 2.6 ms, respectively, mean ±s.d., P < 0.01). Genioglossus motor-evoked potential amplitude, latency and motor threshold were significantly modified with tongue protrusion with a maximal effect observed for stimulation in the anterolateral area. Deep inspiration was associated with a significant facilitatory effect on both genioglossus and diaphragm motor responses. The facilitatory effects of respiratory and non-respiratory manoeuvres were also observed during focal stimulation where isolated genioglossus responses were observed. Genioglossus and diaphragm differed in their motor threshold both at baseline and following facilitatory manoeuvres. Conclusions: (1) transcranial magnetic stimulation-induced genioglossus response systematically precedes that of diaphragm; (2) this sequence of activation is not modified by respiratory and non-respiratory manoeuvres; and (3) the genioglossus and diaphragm are differently influenced by these manoeuvres in terms of latency of the motor response and of motor threshold. [source] A novel functional role for the highly conserved , -subunit KVGFFKR motif distinct from integrin ,IIb,3 activation processesJOURNAL OF THROMBOSIS AND HAEMOSTASIS, Issue 8 2006K. AYLWARD Summary.,Background: The highly conserved integrin , -subunit membrane-proximal motif KVGFFKR plays a decisive role in modulating the activation of integrin ,IIb,3. Previously, we have shown that a platelet permeable palmityl (pal)-peptide with this seven amino acid sequence can directly activate ,IIb,3 leading to platelet aggregation. Objectives: To investigate further the role of the KVGFFKR motif in integrin ,IIb,3 function. Methods: We used two sequence-specific complementary model systems, palmityl pal-peptides in platelets, and mutant ,IIb,3 -expressing Chinese Hamster Ovary (CHO) cell lines. Results: In platelets we show that the two phenylalanine amino acids in pal-KVGFFKR (pal-FF) peptide are critical for stimulating platelet aggregation. Pal-FF peptide treatment of platelets also gives rise to a tyrosine phosphorylation signal despite the presence of inhibitors of fibrinogen binding. In CHO cells, a double alanine substitution, ,IIb(F992A, F993A),3, induces constitutive integrin activation but prevents actin stress fiber formation upon adhesion to fibrinogen, suggesting that ,IIb,3 -mediated cytoskeletal reorganization is also dependent on F992 and F993. This further highlights a critical role for the two phenylalanine residues in both of these ,IIb,3 -mediated processes. Conclusion: In addition to regulating integrin ,IIb,3 activation state, the KVGFFKR motif also influences cytoskeletal reorganization. This activity is critically determined by F992 and F993 within the seven amino acid sequence. [source] | |||||||||||||||||||||||||