关键词： hydrogen storage   lithium amide   magnesium hydride   isothermal kinetics   oxide  

摘要： The destabilized complex hydride system composed of LiNH2:MgH2 (1:1 molar ratio) is one of the leading candidates of hydrogen storage with a reversible hydrogen storage capacity of 8.1 wt%. A low sorption enthalpy of similar to 32 kJ/mole H-2 was first predicted by Alapati et al. utilizing first principle density function theory (DFT) calculations and has been subsequently confirmed empirically by Lu et al. through differential thermal analysis (DTA). This enthalpy suggests that favorable sorption kinetics should be obtainable at temperatures in the range of 160 degrees C to 200 degrees C. Preliminary experiments reported in the literature indicate that sorption kinetics are substantially lower than expected in this temperature range despite favorable thermodynamics. Systematic isothermal and isobaric sorption experiments were performed using a Sievert's apparatus to form a baseline data set by which to compare kinetic results over the pressure and temperature range anticipated for use of this material as a hydrogen storage media. Various material preparation methods and compositional modifications were performed in attempts to increase the kinetics while lowering the sorption temperatures. This paper outlines the results of these systematic tests and describes a number of beneficial additions which influence kinetics as well as NH3 formation.

关键词： Zinc   Peptides and proteins   QM/MM   Ions   Inhibitors  

摘要： Angiotensin-converting enzyme (ACE) is an important zinc-dependent hydrolase responsible for converting the inactive angiotensin I to the vasoconstrictor angiotensin II and for inactivating the vasodilator bradykinin. However, the substrate binding mode cif ACE has not been completely understood. In this work, we propose a model for an ACE Michaelis complex based on two known X-ray structures of inhibitor-enzyme complexes. Specifically, the human testis angiotensin-converting enzyme (tACE) complexed with two clinic drugs were first investigated using a combined quantum mechanical and molecular mechanical (QM/MM) approach. The structural parameters obtained from the 550 ps molecular dynamics simulations are in excellent agreement with the X-ray structures, validating the QM/MM approach. Based on these structures, a model for the Michaelis complex was proposed and simulated using the same computational protocol. Implications to ACE catalysis are discussed.

关键词： MOLECULAR dynamics   SIMULATION methods & models   MATERIALS -- Mechanical properties   CARBON nanotubes   FULLERENES   TEMPERATURE effect   ELASTICITY   STRENGTH of materials   DEFORMATIONS (Mechanics)  

摘要： The mechanical characteristics of a single-walled carbon nanotube (SWCNT) filled with C-60 fullerene subject to nanoindentation is studied using molecular dynamics (MD) simulations. The effects of temperature, indentation velocity, adhesion, and tip sizes were evaluated. The simulated results clearly show that the exerted load, Young's modulus, elastic energy, and plastic energy decrease significantly with increasing temperature and decreasing indentation velocity and tip size. C-60 fullerenes can effectively increase the mechanical strength of a SWCNT because they act as a "barrier" to resist the radial deformation, as well as an inner wall in a double-walled carbon nanotube. With the same indentation depth, the ratio of elastic energy to plastic energy for a material gradually increases with the increase in the radius of the tip. This indicates that the elastic recovery of a material is better when the tip has a larger radius. (C) 2011 Elsevier Ltd. All rights reserved.

关键词： Inverse dynamics control   Underactuated systems   Differential flatness  

摘要： Underactuated systems are featured by fewer control inputs than the degrees-of-freedom, m < n. The determination of an input control strategy that forces such a system to complete a set of m specified motion tasks is a challenging task, and the explicit solution existence is conditioned to differential flatness of the problem. The flatness-based solution denotes that all the 2n states and m control inputs can be algebraically expressed in terms of the m specified outputs and their time derivatives up to a certain order, which is in practice attainable only for simple systems. In this contribution the problem is posed in a more practical way as a set of index-three differential-algebraic equations, and the solution is obtained numerically. The formulation is then illustrated by a two-degree-of-freedom underactuated system composed of two rotating discs connected by a torsional spring, in which the pre-specified motion of one of the discs is actuated by the torque applied to the other disc, n = 2 and m = 1. Experimental verification of the inverse simulation control methodology is reported. (C) 2010 Elsevier B.V. All rights reserved.

关键词： Dispersion   Simulation   Distinct element method   Aggregates   Drag force  

摘要： The dispersion of bulk powders is important for a number of applications including particle characterisation, and the delivery of therapeutic drugs via the lung using dry powder inhalers (DPIs). In recent years the distinct element method (DEM) coupled with continuum models for a fluid phase to simulate fluid-solids interactions has received much attention. In this paper these computational techniques are used to investigate the aerodynamic dispersion in a uniform fluid flow. As intuitively expected, it is seen that with increasing surface energy it is progressively more difficult to disperse a loose aggregate. However, once the relative particle-fluid velocity goes beyond a certain threshold, dispersion occurs quickly and approaches a completely dispersed state asymptotically. In addition, as loose aggregate diameter is increased the necessary threshold velocity reduces. A relationship between the fluid drag forces acting upon a spherical aggregate and the adhesive force, given by the JKR model, leads to a dimensionless group termed the dispersion index, which includes the Weber number. It is shown that the effect of surface energy and loose aggregate diameter on dispersion behaviour can be described by this dimensionless group. (C) 2010 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.

关键词： Molecular dynamics simulation   mechanical properties   Zn4Sb3   nanofilms  

摘要： This paper reports molecular dynamics simulations performed to study the mechanical properties of Zn4Sb3 nanofilms. In the simulations, interatomic interactions are represented by an enhanced atomic potential, and the crystal structure is based on the core structure of beta-Zn4Sb3. For tensile loading along the [0 1 0] direction, the stability of the crystal structure of the Zn4Sb3 nanofilms is analyzed by the radial distribution function method, and the stress-strain relation of the nanofilms is obtained at room temperature. Our present work indicates that the mechanical properties of Zn4Sb3 nanofilms are quite different from those of bulk Zn4Sb3 due to the impact of surface atoms of the nanostructure. From the atomic configuration, Zn4Sb3 nanofilms exhibit typical brittleness. The size effect and the strain-rate effect on the extension of Zn4Sb3 nanofilms are discussed in detail. Lastly, the mechanical properties of nanofilms based on different Zn4Sb3 crystal structure models are examined.

关键词： myocardial infarction   ventricular remodeling   heart transplantation   inflammation  

摘要： Zhou X, Yun JL, Han ZQ, Gao F, Li H, Jiang TM, Li YM. Postinfarction healing dynamics in the mechanically unloaded rat left ventricle. Am J Physiol Heart Circ Physiol 300: H1863-H1874, 2011. First published March 11, 2011;doi:10.1152/ajpheart.00830.2010.-The healing process is a key determinant for postinfarction left ventricular (LV) remodeling and the development of heart failure, which could be influenced by mechanical (pressure and/or volume) load. So far, limited information exists regarding an indepth characterization of the postinfarct healing process in the mechanically unloaded state. In the present work, we performed isogenic Lewis-to-Lewis rat abdominal heterotopic heart transplantation, which is characterized by hemodynamic unloading in the left ventricle, and simultaneously ligated the left anterior descending coronary artery (T-infarct group). Pathological evolution was dynamically compared with that of in situ infarcted Lewis hearts (I-infarct group) on days 3, 7, 14, and 35. There was a remarkable myocardial salvage in the unloaded heart, as shown by the improvement in infarct size (T-infarct group: 25.47% +/- 4.31% vs. I-infarct group: 38.46% +/- 4.82%, P < 0.01) and the smaller fraction of fibrosis in infarct segments (T-infarct group: 42.12% +/- 8.40% vs. I-infarct group: 75.65% +/- 10.51%, P < 0.01). in addition, there was a progressive disorganization of the two-dimensional collagen :fiber alignment as well as retarded collagen fiber maturation in the T-infarct group. We also observed enhanced angiogenesis, lymphangiogenesis, and inflammatory cell retention in the infarct region during mechanical unloading. Moreover, capillary density and collagen deposition were significantly increased in the noninfarcted area of the unloaded heart compared with the same region in the in situ infarcted heart. In conclusion, ischemic insult in the mechanically unloaded heart elicits an altered inflammatory and healing response, which is characterized by myocardi

关键词： SULFUR dioxide   WATER -- Composition   MOLECULAR dynamics   QUANTUM theory   DISTRIBUTION (Probability theory)  

摘要： An ab initio Quantum Mechanical Charge Field Molecular Dynamics Simulation (QMCF MD) was performed to investigate structure and dynamics behavior of hydrated sulfur dioxide (SO2) at the Hartree-Fock level of theory employing Dunning DZP basis sets for solute and solvent molecules. The intramolecular structural characteristics of SO2, such as S = O bond lengths and O = S = O bond angle, are in good agreement with the data available from a number of different experiments. The structural features of the hydrated SO2 were primarily evaluated in the form of S-O-wat and O-SO2-H-wat radial distribution functions (RDFs) which gave mean distances of 2.9 and 2.2 angstrom, respectively. The dynamical behavior characterizes the solute molecule to have structure making properties in aqueous solution or water aerosols, where the hydrated SO2 can easily get oxidized to form a number of sulfur(VI) species, which are believed to play an important role in the atmospheric processes.

关键词： QMCF-MD   hydrophobic   hydophilic   cage-like structure   H-bond donor   H-bond acceptor  

摘要： An ab initio quantum mechanical charge field molecular dynamics simulation was carried out for one methanol molecule in water to analyze the structure and dynamics of hydrophobic and hydrophilic groups. It is found that water molecules around the methyl group form a cage-like structure whereas the hydroxyl group acts as both hydrogen bond donor and acceptor, thus forming several hydrogen bonds with water molecules. The dynamic analyses correlate well with the structural data, evaluated by means of radial distribution functions, angular distribution functions, and coordination number distributions. The overall ligand mean residence time, tau identifies the methanol molecule as structure maker. The relative dynamics data of hydrogen bonds between hydroxyl of methanol and water molecules prove the existence of both strong and weak hydrogen bonds. The results obtained from the simulation are in excellent agreement with the experimental results for dilute solution of CH(3)OH in water. The overall hydration shell of methanol consists in average of 18 water molecules out of which three are hydrogen bonded. (C) 2010 Wiley Periodicals, Inc. J Comput Chem 32: 886-892, 2011

关键词： uroporphyrinogen decarboxylase III   uroporphyrinogen III   porphyrin biosynthesis   quantum mechanics/molecular mechanics and density functional theory  

摘要： In humans, uroporphyrinogen decarboxylase is intimately involved in the synthesis of heme, where the decarboxylation of the uroporphyrinogen-III occurs in a single catalytic site. Several variants of the mechanistic proposal exist;however, the exact mechanism is still debated. Thus, using an ONIOM quantum mechanical/molecular mechanical approach, the mechanism by which uroporphyrinogen decarboxylase decarboxylates ring D of uroporphyrinogen-III has been investigated. From the study performed, it was found that both Arg37 and Arg50 are essential in the decarboxylation of ring D, where experimentally both have been shown to be critical to the catalytic behavior of the enzyme. Overall, the reaction was found to have a barrier of 10.3 kcal mol(-1) at 298.15 K. The rate-limiting step was found to be the initial proton transfer from Arg37 to the substrate before the decarboxylation. In addition, it has been found that several key interactions exist between the substrate carboxylate groups and backbone amides of various active site residues as well as several other functional groups. (C) 2010 Wiley Periodicals, Inc. J Comput Chem 32: 822-834, 2011