Representation of Hydrogen Atoms in Molecular Dynamics Simulations: The Influence on the Computed Properties of Thin Polystyrene Films

The united atoms (UA) and dummy hydrogen atom (DHA) approaches for molecular dynamics simulations of the interface between oxidized atactic polystyrene (aPS) thin films and water are compared. For both oxidized and non‐oxidized aPS films the polymer density profile decays steepest when using the UA model. The surface roughness of the aPS film and the ordering of the phenyl rings near the surface decrease upon changing from vacuum to water for the UA, but not for the DHA model. This also supports the fact that the non‐oxidized aPS films modeled in DHA representation become less hydrophobic. The water structure close to the interface also suggests that the aPS films modeled using UA are more hydrophobic compared to the aPS films modeled with DHA in the phenyl rings.

     

A New Approach to Cycling in a 2-locus 2-allele Genetic Model

In the reproductive process new genetic types arise due to crossing over and recombination at the meiotic stage. A simplified biological model will be developed which incorporates this effect and the effect of selection. Although a chromosome may contain thousands of genes we will consider a simplified model consisting of two genetic loci, each containing two alleles of some gene.

The model will be then turned into a difference equation or mapping model x* = G(x,r) where x represents the frequency distribution of genotypes in a certain infinite population, x* is this distribution one generation later and r is the recombination parameter. For a certain choice of fitness and recombination parameters the mapping exhibits several fixed points. As r is varied one of the fixed points of the mapping loses its stability due to a conjugate pair of eigenvalues of the linearized mapping leaving the unit disk. It is shown that the required non-resonance conditions and “nonlinear damping” condition are satisfied and thus the fixed point undergoes a Neimark–Sacker bifurcation to a cycling or oscillatory state.

Once a cycling orbit is established one can conclude that genetic variation (over time) of the population can be maintained. This work reformulates and proves earlier observations of Alan Hastings in a way that makes the treatment of chromosomes with more genetic loci more straightforward.     

Stresses in alumina scales on NiCrAlY-alloys determined by XSE and LRS

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated. For this purpose cyclic oxidation experiments on model alloys with different additions of silicon or titanium were carried out at temperatures between 950 and 1100 °C. Selected samples were analysed by X-ray stress evaluation (XSE) after various times of oxidation. It was found that the compressive stresses measured at ambient temperature increase with increasing time and are affected by the presence of silicon. The XSE results were used to calibrate laser Raman spectroscopy (LRS) for strain measurement. Then, the suitability of Raman spectroscopy for stress measurement at higher temperatures was evaluated.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on to occasion of his 65th birthday     

A microfluidic-based enzymatic assay for bioactivity screening combined with capillary liquid chromatography and mass spectrometry

The design and implementation of a continuous-flow microfluidic assay for the screening of (complex) mixtures for bioactive compounds is described. The microfluidic chip featured two microreactors (1.6 and 2.4 microL) in which an enzyme inhibition and a substrate conversion reaction were performed, respectively. Enzyme inhibition was detected by continuously monitoring the products formed in the enzyme-substrate reaction by electrospray ionization mass spectrometry (ESI-MS). In order to enable the screening of mixtures of compounds, the chip-based assay was coupled on-line to capillary reversed-phase high-performance liquid chromatography (HPLC) with the HPLC column being operated either in isocratic or gradient elution mode. In order to improve the detection limits of the current method, sample preconcentration based on a micro on-line solid-phase extraction column was employed. The use of electrospray MS allowed the simultaneous detection of chemical (MS spectra) and biological parameters (enzyme inhibition) of ligands eluting from the HPLC column. The present system was optimized and validated using the protease cathepsin B as enzyme of choice. Inhibition of cathepsin B is detected by monitoring three product traces, obtained by cleavage of the substrate. The two microreactors provided 32 and 36 s reaction time, respectively, which resulted in sufficient assay dynamics to enable the screening of bioactive compounds. The total flow rate was 4 microL min-1, which a 25-fold decrease was compared with a macro-scale system described earlier. Detection limits of 0.17-2.6 micromol L-1 were obtained for the screening of inhibitors, which is comparable to either microtiter plate assays or continuous-flow assays described in the literature.     

Infrared study of anion-water interactions in dichloromethane

The binding energy and number of anion-water interactions were investigated by infrared spectroscopy in aqueous CH₂Cl₂ solutions containing salts and cryptands. In these solutions the frequency of the bound OH vibration and the H-bond energy both correlate linearly (Badger-Bauer rule) and are of the same order of magnitude as those in water-organic H-bond acceptor systems. The frequency shift of the OH vibration and the H-bond energy both show an anion dependence which increases in the order: $$ClO_4^ - {\text{ }}<< {\text{ NO}}_{\text{3}}^ - \lesssim I^ -< {\text{ SCN}}^ - {\text{ }}< {\text{ }}Cl^{ - {\text{ }}} \lesssim CO_3^2 {\text{ }} \lesssim F^ -$$ to 340 cm^?1 and 17.2 kJ-mol^?1 of H-bonds, respectively, when compared to H₂O-CH₂Cl₂ interactions. Anion-water H-bond energies agree with the results of computer simulations and heats of hydration. This indicates that an H-bond may be regarded as an interaction between an acid H-atom and a partial charge-whether the H-bond acceptor is formally charged or not (anion or organic base). The magnitude of water-anion interaction energies show a similar order as is found for water interactions.     

Photoinduced electron transfer between metal-coordinated cyclodextrin assemblies and viologens

Two novel tris(bipyridine)ruthenium(II) complexes bearing two and six beta-cyclodextrin binding sites on their ligands have been synthesised and characterised. Complex 1, bearing two cyclodextrins, adopts a conformation in aqueous solution where parts of the aromatic ligands are self-included into the cyclodextrin moieties. This results in a loss of symmetry of the complex and gives rise to a much more complicated 1H NMR spectrum than expected. Photophysical studies indicate that the appended cyclodextrins protect the luminescent ruthenium core from quenching by oxygen, which results in longer excited state lifetimes and higher emission quantum yields compared with the reference compound, the unsubstituted ruthenium tris(bipyridine). Inclusion of suitable guests such as dialkyl-viologens leads to a quenching of the luminescence of the central unit. In these supramolecular donor-acceptor dyads an efficient photoinduced electron transfer from the excited ruthenium moiety (the donor) to the viologen unit (the acceptor) is observed. The alkyl chain length of the acceptor plays an important role on the binding properties; when it exceeds a certain limit the binding becomes strong enough for electron transfer to occur. Interestingly, a viologen with only one long alkyl tail instead of two shows no efficient quenching; this indicates that cooperative interactions between two cyclodextrins binding one viologen are essential to raise the binding constant of the supramolecular dyad.     

The Use of SIMS, SEM, EPMA, LRS and X-Ray Diffraction Measurements for the Examination of Corrosive Layers and Protective Coatings on Steels and Alloys in Advanced Power Stations

The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. In this paper the following examples are discussed:1) The oxidation behaviour of 9% (w/w) Cr steels in simulated combustion gases: the effects of O₂ and H₂O content on the oxidation behaviour of 9% (w/w) Cr steels in the temperature range of 600–800°C showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion. However, in the presence of water vapour, after an incubation period, the scale became non-protective as a result of a change in the oxidation mechanism.Oxidation experiments on 10% (w/w) Cr steel in simulated steam atmosphere at 650°C have shown that a haematite layer is formed. The investigation suggests that several independent processes, such as oxide dissociation and solid state diffusion of iron cations in the scale, significantly contribute to the overall oxide growth process.2) The development of light-weight intermetallics based on TiAl-basis: TiAl-based intermetallics are promising materials for future turbine components because of their combination of high temperature strength and low density. These alloys, however, exhibits poor oxidation resistance at temperatures above 700°C. The experimental results showed that the oxidation behaviour of TiAl-based intermetallics can be significantly improved by minor additions of 1–2 at-% silver.     

Biosensor immunoassays for the detection of bisphenol A

Bisphenol A (BPA) is a xenoestrogen found in the environment, in consequence, for the biosensor detection of BPA we raised antibodies (polyclonal (PAbs) and monoclonal (MAbs)) against a structural analogue of BPA, 4,4 bis-(4-hydroxyphenyl) valeric acid (BVA). The kinetics of the MAb-BPA interaction were evaluated and the MAb providing the highest affinity was directly immobilized onto the sensor chip surface to evaluate a direct assay. Afterwards, the performance of the MAbs and the PAbs was compared in an inhibition assay using a BVA-coated chip.The highest sensitivity (limit of detection (LOD) of 0.4 μg L⁻¹) was obtained with MAb 12 in the direct assay. However, the inhibition assay was the most robust and the PAbs showed the highest sensitivity (LOD of 0.5-1 μg L⁻¹). The antibodies were specific for BVA and BPA as only minor cross-reactivities were found toward structurally related compounds or other endocrine disruptors. In the inhibition assay (with a run time of 6 min), water samples spiked with BPA at different levels (0.5-50 μg L⁻¹) resulted in recoveries varying between 68% and 121%. The sensitivity of the inhibition assay could be improved 40 times (LOD of 0.03 μg L⁻¹ with the Mab 12-based assay) using solid phase extraction (SPE).     

Mikrogravimetrische Bestimmung von Gold als Dekamethonium-Tetrabromoaurat(III)

Zusammenfassung Die mikrogravimetrische Bestimmung von Gold als Bis(trimethyl)-dekamethylendiammoniumtetrabromoaurat(III) [(CH₃)₃N-(CH₂)₁₀-N(CH₃)₃] [AuBr₄]₂ erlaubt die Erfassung von 700g Au neben 200fachen Überschüssen an Cu²⁺ und Ni²⁺ sowie 50fachen Mengen anderer Übergangsmetalle.

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Summary The microgravimetric determination of gold as bis-(trimethyl) decamethylenediammoniumtetrabromaurate(III) [(CH₃)₃N-(CH₂)₁₀-N(CH₃)₃] [AuBr₄]₂ permits the determination of 700g Au in the presence of 200-fold excesses of Cu²⁺ and Ni²⁺ as well as 50-fold amounts of other transition metals.

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Résumé Le microdosage gravimétrique de l'or à l'état de tétrabromoaurate-III de bis(triméthyl)décaméthylènediammonium [(CH₃)₃N-(CH₂)₁₀-N(CH₃)₃] [AuBr₄]₂ permet de descendre jusqu'à 700g Au en présence d'un excès de 200 fois Cu²⁺ et Ni²⁺ et d'une quantité 50 fois plus grande des autres métaux de transition.