Modern Applications of Electron Microscopy to Catalysis

The active sites of ultra-dispersed Pt/-Al₂O₃ catalysts are studied using high-resolution electron microscopy, Z-contrast and dark field. In addition we have calculated using a method based in density functional theory the electrostatic potential and charge distribution of the active sites. It is conclude that the most likely Pt clusters that are formed correspond to Pt₁₃ and Pt with icosahedral and decahedral structure. It is shown that this is consistent with the electron microscopy data.     

Analysis of heregulin symmetry by weighted evolutionary tracing

Heregulins are members of the protein family of EGF-like growthand differentiation factors. The primary cell-surface targetsof heregulins are heterodimers of the EGF-receptor homolog HER2with either HER3 or HER4. We used a weighted evolutionary traceanalysis to identify structural features that distinguish theEGF-like domain (hrg) of heregulins from other members of theEGF family. In this analysis, each amino acid sequence is weightedaccording to its uniqueness and the variability in each positionis assigned by an amino acid substitution matrix. Conservedresidues in heregulin that are variable in other EGF-like domainsare considered possible specificity-conferring residues. Thisanalysis identifies two clusters of residues at the foot ofthe boot-shaped hrg domain. The residues in one cluster arerecruited from the N-terminus; those in the other are from the-loop region and show a weak sequence similarity to the N-terminalresidues at the opposite side of the boot. The remaining residueswith high conservation scores distribute themselves into thesetwo distinct surfaces on hrg. This pseudo-twofold symmetry andthe presence of two distinct interfaces may reflect the preferenceof hrg for heterodimeric versus homodimeric HER complexes.     

MCM-41 Heterogenized Chiral Amines as Base Catalysts for Enantioselective Michael Reaction

Cinchonidine and cinchonine have been grafted onto pure silica MCM-41. It has been shown that both supported alkaloids are active catalysts for the Michael addition of ethyl 2-oxocyclopentanecarboxylate and methyl vinyl ketone, anchored cinchonidine being more active and enantioselective than anchored cinchonine. The study of the influence of the polarity of the solvent and reaction temperature on the optical induction shows that there is not a direct correlation between solvent polarity and enantioselectivity, and the maximum optical yield was obtained between 278 and 273 K.     

A new combined computational and NMR-spectroscopical strategy for the identification of additional conformational constraints of the bound ligand in an aprotic solvent

This study documents the feasibility of switching to an aprotic medium in sugar receptor research. The solvent change offers additional insights into mechanistic details of receptor--carbohydrate ligand interactions. If a receptor retained binding capacity in an aprotic medium, solvent-exchangeable protons of the ligand would not undergo transfer and could act as additional sensors, thus improving the level of reliability in conformational analysis. To probe this possibility, we first focused on hevein, the smallest lectin found in nature. The NMR-spectroscopic measurements verified complexation, albeit with progressively reduced affinity by more than 1.5 orders of magnitude, in mixtures of up to 50% dimethyl sulfoxide (DMSO). Since hevein lacks the compact beta-strand arrangement of other sugar receptors, such a structural motif may confer enhanced resistance to solvent exchange. Two settings of solid-phase activity assays proved this assumption for three types of alpha- and/or beta-galactoside-binding proteins, that is, a human immunoglobulin G (IgG) subfraction, the mistletoe lectin, and a member of the galectin family of animal lectins. Computer-assisted calculations and NMR experiments also revealed no conspicuous impact of the solvent on the conformational properties of the tested ligands. To define all possible nuclear Overhauser effect (NOE) contacts in a certain conformation and to predict involvement of exchangeable protons, we established a new screening protocol applicable during a given molecular dynamics (MD) trajectory and calculated population densities of distinct contacts. Experimentally, transferred NOE (tr-NOE) experiments with IgG molecules and the disaccharide Gal'alpha1-3Galbeta1-R in DMSO as solvent disclosed that such an additional crosspeak, that is, Gal'OH2--GalOH4, was even detectable for the bound ligand under conditions in which spin diffusion effects are suppressed. Further measurements with the plant lectin and galectins confirmed line broadening of ligand signals and gave access to characteristic crosspeaks in the aprotic solvent and its mixtures with water. Our combined biochemical, computational, and NMR-spectroscopical strategy is expected to contribute notably to the precise elucidation of the geometry of ligands bound to compactly folded sugar receptors and of the role of water molecules in protein--ligand (carbohydrate) recognition, with relevance to areas beyond the glycosciences.     

Solvent induced crystallization of syndiotactic poly(methyl methacrylate) relation to the formation of polymer aggregates in solution

Polymer Bulletin - Solutions of syndiotactic (s) poly (methyl methacrylate) (PMMA) and samples of solid s-PMMA prepared from these solutions were studied by NMR and infrared (IR) spectroscopy and...     

Parametric study on smoothed particle hydrodynamics for accurate determination of drag coefficient for a circular cylinder

Simulations of two-dimensional (2D) flow past a circular cylinder with the smoothed particle hydrodynamics (SPH) method were conducted in order to accurately determine the drag coefficient. The fluid was modeled as a viscous liquid with weak compressibility. Boundary conditions, such as a no-slip solid wall, inflow and outflow, and periodic boundaries, were employed to resemble the physical problem. A sensitivity analysis, which has been rarely addressed in previous studies, was conducted on several SPH parameters. Hence, the effects of distinct parameters, such as the kernel choices and the domain dimensions, were investigated with the goal of obtaining highly accurate results. A range of Reynolds numbers (1–500) was simulated, and the results were compared with existing experimental data. It was observed that the domain dimensions and the resolution of SPH particles, in comparison to the obstacle size, affected the obtained drag coefficient significantly. Other parameters, such as the background pressure, influenced the transient condition, but did not influence the steady state at which the drag coefficient was determined.     

Multirate control with incomplete information over Profibus-DP network

When a process field bus-decentralized peripherals (Profibus-DP) network is used in an industrial environment, a deterministic behaviour is usually claimed. However, due to some concerns such as bandwidth limitations, lack of synchronisation among different clocks and existence of time-varying delays, a more complex problem must be faced. This problem implies the transmission of irregular and, even, random sequences of incomplete information. The main consequence of this issue is the appearance of different sampling periods at different network devices. In this paper, this aspect is checked by means of a detailed Profibus-DP timescale study. In addition, in order to deal with the different periods, a delay-dependent dual-rate proportional-integral-derivative control is introduced. Stability for the proposed control system is analysed in terms of linear matrix inequalities.     

Preconditioning the solution of the time-dependent neutron diffusion equation by recycling Krylov subspaces

Spectral preconditioners are based on the fact that the convergence rate of the Krylov subspace methods is improved if the eigenvalues of the smallest magnitude of the system matrix are ‘removed’. In this paper, two preconditioning strategies are studied to solve a set of linear systems associated with the numerical integration of the time-dependent neutron diffusion equation. Both strategies can be implemented using the matrix–vector product as the main operation and succeed at reducing the total number of iterations needed to solve the set of systems.