Collision and escape orbits in a generalized Hénon–Heiles model

The motion of a material point of unit mass in a field determined by a generalized Hénon–Heiles potential $U=A{q}_1^2+B{q}_2^2+C{q}_1^2{q}_2+D{q}_2^3$, with $(q_1,q_2)=$ standard Cartesian coordinates and $(A,B,C,D)\in {(0,\infty )}^2\times R^2$, is addressed for two limit situations: collision and escape. Using McGehee-type transformations, the corresponding collision and infinity boundary manifolds pasted on the phase space are determined. These are fictitious manifolds, but, due to the continuity with respect to initial data, their flow determines the near by orbit behaviour.The dynamics on the collision and infinity manifolds is fully described. The topology of the flow on the collision manifold is independent of the parameters. In the full phase space, while spiraling collision orbits are present, most of the orbits avoid collision. The topology of the flow on the infinity manifold changes as the ratio between $C$ and $D$ varies. More precisely, there are two symmetric pitchfork bifurcations along the line $2C?3D=0$, due to the reshaping of the potential along the bifurcation line. Besides rectilinear and spiraling orbits, the near-escape dynamics includes oscillatory orbits, for which angular momentum alternates sign.     

Functional Law for the Anticipating Ornstein-Uhlenbeck Process

We prove a functional law of simple logarithm for the Ornstein-Uhlenbeck process with anticipating initial data.      

Analytical contributions to the evaluation of painting authenticity from Princely church of Curtea de Arges

The present study contains the analyses performed for pigment samples taken from the Princely church of Curtea de Arges, one of the oldest churches in Romania. The results of our investigations have shown the source of these samples, thus being identified the pigments: natural ultramarine, cinnabar, red earth, and calcium carbonate in the painting from the 14th century, the pigments: lead white, zinc white, and Prussian blue in the repainting from the 19th century and the pigments zinc white, titanium dioxide white, bone white, yellow ochre, red ochre, green earth, artificial ultramarine, and mars red in the interventions carried out in the 20th century. The analyses consisted of light microscopy (LM) and microchemical tests, as well as energy dispersive X-ray (EDX) analysis. This system of analyses allows one to precisely determine the authenticity of certain pigments, thus avoiding the dating errors for different interventions carried out on the original mural painting from the Saint Nicholas Princely church of Curtea de Arges.     

Electrochemical evidence of self-substrate inhibition as functions regulation for cellobiose dehydrogenase from Phanerochaete chrysosporium

The reaction mechanism of cellobiose dehydrogenase (CDH) from Phanerochaete chrysosporium, adsorbed on graphite electrodes, was investigated by following its catalytic reaction with cellobiose registered in both direct and mediated electron transfer modes between the enzyme and the electrode. A wall-jet flow through amperometric cell housing the CDH-modified graphite electrode was connected to a single line flow injection system. In the present study, it is proven that cellobiose, at concentrations higher than 200 μM, competes for the reduced state of the FAD cofactor and it slows down the transfer of electrons to any 2e⁻/H⁺ acceptors or further to the heme cofactor, via the internal electron transfer pathway. Based on and proven by electrochemical results, a kinetic model of substrate inhibition is proposed and supported by the agreement between simulation of plots and experimental data. The implications of this kinetic model, called pseudo-ping-pong mechanism, on the possible functions CDH are also discussed. The enzyme exhibits catalytic activity also for lactose, but in contrast to cellobiose, this sugar does not inhibit the enzyme. This suggests that even if some other substrates are coincidentally oxidized by CDH, however, they do not trigger all the possible natural functions of the enzyme. In this respect, cellobiose is regarded as the natural substrate of CDH.     

Education by Teaching Analytical Chemistry in Romania

A review is given on the history of the development of analytical chemistry in Romania, its present role in University teaching, and on measures that are planned in order to improve teaching and education.Received January 12, 2003; accepted March 7, 2003 Published online July 16, 2003     

Silicone composites containing stabilized silver clusters or nanoparticles

Silver nanoparticles are of high importance due to their electrical, magnetic, and optical properties, as well as catalytic and biocidal activity that are superior to the bulk silver and other metals. To prepare certain devices, generally, silver is incorporated into a matrix either as preformed or in situ‐generated particles. Silver nanoparticles were generated in situ into a silicone matrix formed by cohydrolysis of the mixture of silanes, each of them having a certain role: dimethyldiethoxysilane (DMDES) as a precursor for highly flexible polydimethylsiloxane, methyltriethoxysilane (MTES) as a cross‐linker highly compatible with polydimethylsiloxane, and 3‐aminopropyltriethoxysilane as a stabilizer, since it can readily complex to silver atoms through its amine functionality. Dimethylformamide (DMF) was used as a solvent for the silver nitrate and reducing agent. The samples were investigated both in sol state and as aged coating films deposited on glass substrate. The complexation of the silver and the matrix formation were emphasized by FTIR. The size of the formed silicone particles encapsulating silver was estimated by dynamic light scattering (DLS) (about 100 nm) in sol and by AFM in film (about 90 nm). The formation of the clusters or nanoparticles depending on the ratio between the reducing and complexing agents was evidenced by UV–Vis absorption spectra. Thus, it would create conditions to stop and isolate clusters at the desired size by precise control of the experimental conditions. The composites could be used alone as antibacterial‐coating materials but also, porous silica having incorporated silver clusters with potential applicability in catalysis may result after their calcination. Copyright © 2010 John Wiley & Sons, Ltd.     

Gold layers on untreated and plasma-treated substrates of quaternized polysulfones

Thin gold layers were sputtered on the quaternized polysulfones (containing different tertiary amines—N,N-dimethylethylamine and N,N-dimethyloctylamine, respectively) surfaces unmodified and modified by low-pressure and high-frequency plasma treatment. Adhesion and morphological aspects of complex structures were studied for different gold sputtering and plasma treatment times. Water contact angle, atomic force microscopy, and surface properties reveal that adhesion increases with gold sputtering and plasma treatment times. Values of the mean adhesion force between cantilever and the studied surfaces, measured from AFM investigation, were correlated with quaternized polysulfone structures, modification of hydrophobicity after plasma treatment, and gold deposition on polymer surfaces.     

Rapid and accurate prediction of binding free energies for saquinavir-bound HIV-1 proteases

To explain drug resistance by computer simulations at the molecular level, we first have to assess the accuracy of theoretical predictions. Herein we report an application of the molecular mechanics Poisson-Boltzmann surface area (MM/PBSA) technique to the ranking of binding affinities of the inhibitor saquinavir with the wild type (WT) and three resistant mutants of HIV-1 protease: L90M, G48V, and G48V/L90M. For each ligand-protein complex we report 10 ns of fully unrestrained molecular dynamics (MD) simulations with explicit solvent. We investigate convergence, internal consistency, and model dependency of MM/PBSA ligand binding energies. Converged enthalpy and entropy estimates produce ligand binding affinities within 1.5 kcal/mol of experimental values, with a remarkable level of correlation to the experimentally observed ranking of resistance levels. A detailed analysis of the enthalpic/entropic balance of drug-protease interactions explains resistance in L90M in terms of a higher vibrational entropy than in the WT complex, while G48V disrupts critical hydrogen bonds at the inhibitor's binding site and produces an altered, more unfavorable balance of Coulomb and polar desolvation energies.     

Activation/Inhibition Effects during the Coelectrodeposition of PtAg Nanoparticles: Application for ORR in Alkaline Media

PtAg bimetallic nanoparticles for oxygen reduction reaction (ORR) in alkaline media were prepared by pulse electrodeposition (PED). During PED the reduction of Ag⁺ ions predominates, thus an increased Ag content in the co‐deposit is accomplished. The mechanism for this anomalous co‐deposition was elucidated by potential pulse experiments, which revealed that nuclei formation mainly occurs via the reduction of Pt²⁺ ions. The growth of the particles is diffusion controlled leading to the formation of a Ag shell covering a PtAg alloyed region. However, the shell is not growing homogeneously on the PtAg alloy. Hence, regions of the PtAg alloy are exposed, which exhibit an enhanced ORR activity compared to a pure Ag surface.     

A simple method for the preparation of colloidal polymer-supported silver nanoparticles

Polysulfone?silver composite nanoparticles have been prepared by combining polymer nanoprecipitation and redox synthesis of silver, in the presence of a glucose-modified cyclosiloxane as stabilizing agent. Based on previous kinetic investigations and on model reactions, we concluded that the reducing agent in this case is the tetrahydrofuran (THF) used as solvent for polysulfone. Dynamic light scattering measurements on the obtained polymer-silver composite particles indicated particle average diameter of 176 nm with a polydispersity index of 0.25. The UV–vis spectrum exhibited the silver plasmon resonance. By different microscopic methods (atomic force microscopy—AFM, high resolution transmission electron microscopy—HRTEM, and scanning electron microscopy—SEM), larger polymer particles coated with silver nanoprticles were observed. The Energy Dispersive X-Ray analysis—EDX; confirmed the presence of Ag on the surface of the particles, while the selected area electron diffraction showed single crystalline silver nanospheres with face-centered cubic structure.