The Experts below are selected from a list of 49332 Experts worldwide ranked by ideXlab platform
Timothy P Weihs - One of the best experts on this subject based on the ideXlab platform.
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enhanced Reaction Velocity and diluent homogenization in redox foils using arrested reactive milling thermite powder
Journal of Materials Science, 2017Co-Authors: Alex H Kinsey, Kyle Slusarski, Evan Krumheuer, Timothy P WeihsAbstract:Fully dense thermite foils, commonly referred to as Redox Foils, provide a unique opportunity to enable exothermic brazing of components by utilizing the molten metallic product of the thermite Reaction as a braze. Due to the propensity of thermites to produce gas, however, Redox Foils must be diluted to eliminate metal vapor formation. This dilution can lead to foils that react too slowly to fully propagate in a bonding configuration, and therefore the reactivity of the foils must be improved. To do so, we milled the constituent powders (Al, Cu2O, and Cu) before we mechanically consolidated them into dense Redox Foils. The milling process decreases the reactant spacing, or the distance between the fuel (Al) and oxide (Cu2O), thereby enhancing oxygen transfer between the fuel and the oxide. Two fabrication strategies are employed: milling the thermite reactants (Al and Cu2O) without the Cu diluent, then adding the diluent before consolidation into Redox Foils, and the second is milling all components (Al, Cu2O, and Cu) together before consolidation. Both milling processes results in foils that propagate faster and release heat earlier than foils consolidated from unmilled, single-phase powders of Al, Cu2O and Cu, due to decreased reactant spacings. However, only the second milling strategy is capable of suppressing the production of Cu metal vapor during the Reaction of the foils, as confirmed using emission spectroscopy. The lack of Cu vapor in these foils is attributed to a more uniform distribution of Cu diluent that is verified using digital analysis of foil microstructures.
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effect of varying bilayer spacing distribution on Reaction heat and Velocity in reactive al ni multilayers
Journal of Applied Physics, 2009Co-Authors: Robert Knepper, Murray Snyder, Greg Fritz, Kaitlynn Fisher, Omar M Knio, Timothy P WeihsAbstract:Self-propagating Reactions in Al/Ni nanostructured multilayer foils are examined both experimentally and computationally to determine the impact of variations in reactant spacing on Reaction properties. Heats of Reaction and Reaction velocities have been characterized as a function of average bilayer spacing for sputter-deposited, single-bilayer foils (having a uniform bilayer spacing) and for dual-bilayer foils (having two different bilayer spacings that are labeled thick and thin). In the latter case, the spatial distribution of the thick and thin bilayers is found to have a significant effect on Reaction Velocity, with coarse distributions leading to much higher Reaction velocities than fine distributions. Numerical simulations of Reaction Velocity match experimental data well for most spatial distributions, with the exception of very coarse distributions or distributions containing very small bilayer spacings. A simple model based on thermal diffusivities and Reaction velocities is proposed to predict when the spatial distribution of thick and thin bilayers becomes coarse enough to affect Reaction Velocity. This combination of experiment and simulation will allow for more effective design and prediction of Reaction velocities in both sputter-deposited and mechanically processed reactive materials with variable reactant spacings.
Ramon Grima - One of the best experts on this subject based on the ideXlab platform.
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Noise-induced breakdown of the Michaelis-Menten equation in steady-state conditions
Physical Review Letters, 2009Co-Authors: Ramon GrimaAbstract:The Michaelis-Menten (MM) equation is the basic equation of enzyme kinetics; it is also a basic building block of many models of biological systems. We build a stochastic and microscopic model of enzyme kinetics inside a small subcellular compartment. Using both theory and simulations, we show that intrinsic noise induces a breakdown of the MM equation even if steady-state metabolic conditions are enforced. In particular, we show that (i) given a Reaction Velocity, deterministic rate equations can severely underestimate steady-state intracellular substrate concentrations and (ii) different Reaction schemes which on a macroscopic level are indistinguishable because they are described by the same MM equation obey distinctly different equations in subcellular compartments.
Tongsul Han - One of the best experts on this subject based on the ideXlab platform.
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relationship between enzyme concentration and michaelis constant in enzyme assays
Biochimie, 2020Co-Authors: Kyongil Yun, Tongsul HanAbstract:Abstract The upper bound of enzyme concentration for accurately estimating the parameters in Michaelis-Menten (MM) equation is not completely determined and still under discussion, even though many researchers have investigated the equation’s validity for a long time. In the paper, we broadly investigated the correlation between the system of ordinary differential equations for monosubstrate irreversible enzyme Reaction (HMM-system) and its derivative MM equation focusing on the relationship between initial enzyme concentration [E]0 and Michaelis constant Km by numerical simulation. According to the results, the initial Reaction Velocity v0 is still a function of initial substrate concentration [S]0 at [E]0
Yoo Tanabe - One of the best experts on this subject based on the ideXlab platform.
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Reversal Phenomenon of Reaction Velocity in a Mixed Reaction System: Silylations between Simple Alcohols and α- or β-Hydroxyketones Using Anilinosilanes and Catalytic TBAF Agent
The Journal of organic chemistry, 2007Co-Authors: Akira Iida, Chihiro Hashimoto, Tomonori Misaki, Yukiteru Katsumoto, Yukihiro Ozaki, Yoo TanabeAbstract:We encountered a unique phenomenon of Reaction Velocity reversal during the silylation between mixed simple alcohols (A; AOH) and α- or β-hydroxyketones (B; BOH). The present Reaction system using anilinosilanes 1a [PhNH(TMS)] (TMS-A) and 1b [PhNH(TES)] (TES-A) with the catalytic TBAF (0.02 equiv) reagent showed that the order of silylation−Velocity between the independent system and the mixed system was distinctively reversed [νA > νB ≈ νB(mix) > νA(mix)]. A total of seven examples indicate its high generality: AOH = linalool, 3-octanol; BOH = valeroin, benzoin, 5-hydroxy-4-methyl-3-undecanone. Time-dependent IR spectroscopy analysis rationally supported this speculation.
Stefan Neubauer - One of the best experts on this subject based on the ideXlab platform.
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chronic phosphocreatine depletion by the creatine analogue beta guanidinopropionate is associated with increased mortality and loss of atp in rats after myocardial infarction
Circulation, 2001Co-Authors: M Horn, Helga Remkes, Hinrik Stromer, Charlotte Dienesch, Stefan NeubauerAbstract:Background The failing myocardium is characterized by reductions of phosphocreatine (PCr) and free creatine content and by decreases of energy reserve via creatine kinase (CK), ie, CK Reaction Velocity (FluxCK). It has remained unclear whether these changes contribute directly to contractile dysfunction. In the present study, myocardial PCr stores in a heart failure model were further depleted by feeding of the PCr analogue β-guanidinopropionate (GP). Functional and metabolic consequences were studied. Methods and Results Rats were subjected to sham operation or left coronary artery ligation (MI). Surviving rats were assigned to 4 groups and fed with 0% (n=7, Sham; n=5, MI) or 1% (n=7 Sham+GP, n=8 MI+GP) GP. Two additional groups were fed GP for 2 or 4 weeks before MI. After 8 weeks, hearts were isolated and perfused, and left ventricular pressure-volume curves were obtained. High-energy phosphate metabolism was determined with 31P NMR spectroscopy. After GP feeding or MI, left ventricular pressure-volume...
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high energy phosphate metabolism in normal hypertrophied and failing human myocardium
Heart Failure Reviews, 1999Co-Authors: Stefan NeubauerAbstract:This chapter examines the role of cardiac high-energy phosphate metabolism in normal, hypertrophied and failing human myocardium. Myocardial biopsies allow analysis of ATP, total adenine nucleotides, creatine kinase activity and total creatine content, while non-invasive 31P-magnetic resonance spectroscopy can be used to determine phosphocreatine/ATP ratios and, most recently, absolute levels of ATP and phosphocreatine. The failing human myocardium is characterized by reduced phosphocreatine and total creatine levels, normal or slightly reduced ATP levels and reduced creatine kinase activity. These changes are consistent with, but do not prove, a role of high-energy phosphate metabolism as a contributing factor in heart failure. An answer to the precise functional role of high-energy phosphate metabolism necessitates analysis of free ADP levels, free energy change of ATP hydrolysis and creatine kinase Reaction Velocity; these measurements may become feasible in coming years. However, analysis of energy metabolism in a compartmentalized manner, i.e., in the compartments relevant for contractile function such as the perimyofibrillar space, will remain elusive for the foreseeable future.
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effects of chronic dietary creatine feeding on cardiac energy metabolism and on creatine content in heart skeletal muscle brain liver and kidney
Journal of Molecular and Cellular Cardiology, 1998Co-Authors: M Horn, Stefan Frantz, Helga Remkes, Anne Laser, Bert Urban, Andreas Mettenleiter, Klaus D Schnackerz, Stefan NeubauerAbstract:Little is known about the regulation of total creatine concentration in heart, skeletal muscle, brain, liver and kidney in response to increased dietary creatine intake. The phosphorylated fraction of intracellular creatine (phosphocreatine) remain relatively constant, and therefore, higher intracellular creatine levels may increase the energy reserve of the heart [phosphocreatine and phosphoryl transfer via creatine kinase (CK)] and of other organs. To test the effect of supplying exogenous creatine on the myocardial energy reserve and on creatine content of various organs, rats were given chow containing 0 (Untreated), 1, 3, 5, or 7% (of diet weight) creatine for ;40 days. Thereafter, hearts were perfused and left ventricular developed pressure and heart rate were recorded. High-energy phosphate concentrations were determined with 31P-NMR spectroscopy, CK Reaction Velocity by 31P-magnetization transfer. Total creatine was determined in heart, skeletal muscle, brain, liver, kidney and serum by high-performance liquid chromatography (HPLC). Creatine feeding increased serum creatine by 73% (1% creatine), 142% (3%), 166% (5%) and 202% (7%). In the heart, increased serum creatine levels did not affect mechanical function; ATP, phosphocreatine, inorganic phosphate, CK Reaction Velocity and total creatine were all unchanged. Total creatine also remained constant in brain and skeletal muscle, while creatine content increased 4.6-fold in the liver and 1.9-fold in the kidney. We conclude that myocardial energy reserve via CK cannot be increased by exogenous creatine treatment.
