关键词： alpha-Amino acids   Oxidation   Chloramine-T   Kinetics   Mechanism  

摘要： The kinetics of oxidation of alpha-amino acids (AAs) by chloramine-T (CAT) using P-cyclodextrin (BCD) as catalyst was studied in aqueous sodium hydroxide medium at 313 K. The kinetics of reactions was fractional-order with respect to [amino acids] and [beta-cyclodextrin]. First-order with respect to [chloramine-T] and inverse fractional-order with respect to [OH-] have been found. Effect of ionic strength, added salt and reaction product (PTS) had no effect on reaction rate. The dependence of the reaction rate on temperature was studied and activation parameters were computed from Arrhenius-Eyring plots. The reaction mechanism and the derived rate law are consistent with the observed experimental results. (C) 2011 Elsevier B.V. All rights reserved.

关键词： Nanoimprinting   Alloy   Composition   Elastic recovery   Molecular dynamics  

摘要： The nanoimprinting process of nickel-aluminum (Ni-Al) alloys is studied using molecular dynamics (MD) simulations based on the many-body tight-binding potential. The effects of the temperature, loading and unloading velocities, holding/dwelling time, and composition of Ni-Al alloys are evaluated in terms of molecular trajectories, imprinting force, potential energy, stress, slip vector, and elastic recovery ratio. Simulation results show that the imprinting force increases with decreasing temperature and increasing loading velocity and Ni content. The average potential energy of a specimen decreases and its stress increases with increasing loading velocity. Slip planes of (110) and (110) form during Ni-Al alloy imprinting. During unloading, the adhesion force increases with increasing unloading velocity. The formability of a Ni-Al alloy can be enhanced by increasing the Ni content. Elastic recovery for a pattern can be avoided by decreasing the imprinting temperature and increasing the holding time. At a critical line width of 6.5 nm, elastic recovery ratios can be maintained in a range of 8-11%. (C) 2011 Elsevier B. V. All rights reserved.

关键词： beta-lactamase   pKa calculation   MD simulation   Quantum mechanical calculation   Enzymatic reaction mechanism  

摘要： beta-Lactamases are bacterial enzymes that act as a bacterial defense system against beta-lactam antibiotics. beta-Lactamase cleaves the beta-lactam ring of the antibiotic by a two step mechanism involving acylation and deacylation steps. Although class C beta-lactamases have been investigated extensively, the details of their mechanism of action are not well understood at the molecular level. In this study, we investigated the mechanism of the acylation step of class C beta-lactamase using pKa calculations, molecular dynamics (MD) simulations and quantum mechanical (QM) calculations. Serine64 (Ser64) is an active site residue that attacks the beta-lactam ring. In this study, we considered three possible scenarios for activation of the nucleophile Ser64, where the activation base is (1) Tyrosine150 (Tyr150), (2) Lysine67 (Lys67), or (3) substrate. From the pKa calculation, we found that Tyr150 and Lys67 are likely to remain in their protonated states in the pre-covalent complex between the enzyme and substrate, although their role as activator would require them to be in the deprotonated state. It was found that the carboxylate group of the substrate remained close to Ser64 for most of the simulation. The energy barrier for hydrogen abstraction from Ser64 by the substrate was calculated quantum mechanically using a large truncated model of the enzyme active site and found to be close to the experimental energy barrier, which suggests that the substrate can initiate the acylation mechanism in class C beta-lactamase.

关键词： single-molecule force spectroscopy   force-clamp spectroscopy   silyl ester   acyloxysilane   hydrolysis   mechanochemistry  

摘要： We have investigated the strength of silyl ester bonds formed between carboxymethylated amylose (CMA) molecules and silane-functionalized silicon oxide surfaces using AFM-based single-molecule force spectroscopy in the force-clamp mode. Single tethered (MA molecules were picked up, and bond lifetimes were determined at constant clamp forces of 0.8, 1.0, and 1.2 nN at seven temperatures between 295 and 320 K at pH 2.0. The results reveal biexponential rupture kinetics. To obtain the reaction rate constants for each force and temperature individually, the results were analyzed with a biexponential kinetic model using the maximum likelihood estimation (MLE) method. The force-independent kinetic and structural parameters of the underlying bond rupture mechanisms were extracted by fitting the entire data set with a parallel MLE fit procedure using the Zhurkov/Bell model and, alternatively, an Arrhenius kinetics model combined with a Morse potential as an analytic representation of the binding potential. With activation energies between 37 and 40 kJ mol(-1), and with Arrhenius prefactors between 5 x 10(4) and 2 x 10(6) s(-1), the results point to the hydrolysis of the silyl ester bond.

关键词： Graphene sheets   Molecular dynamics   Fracture mechanism   Stone-Wales defect   Single vacancy defect  

摘要： In this paper, the effects of two main types of structural defects, i.e. Stone-Wales and single vacancy, on the mechanical properties of single-layered graphene sheets (SLGSs) are investigated. To this end, molecular dynamics simulations based on the Tersoff-Brenner potential function and Nose-Hoover thermostat technique are implemented. The results obtained have revealed that the presence of defects significantly reduces the failure strain and the intrinsic strength of SLGSs, while it has a slight effect on Young's modulus. Furthermore, the examination of loading in both armchair and zig-zag directions demonstrated that SLGSs are slightly stronger in the armchair direction and defects have lower effect in this direction. Considering the fracture mechanism, the failure process of defective and perfect graphene sheets is also presented. (C) 2011 Elsevier Ltd. All rights reserved.

标准号: BS EN ISO 11148-1-2011

标准号: BS EN ISO 11680-1-2011

标准号: BS EN ISO 11680-2-2011