Erfahrungen mit Referenzmethoden

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Experience with reference methods

     

Boundary Integral Equations for Isotropic Linear Elasticity

This articles first investigates boundary integral operators for the three-dimensional isotropic linear elasticity of a biphasic model with piecewise constant Lamé coefficients in the form of a bounded domain of arbitrary shape surrounded by a background material. In the simple case of a spherical inclusion, the vector spherical harmonics consist of eigenfunctions of the single and double layer boundary operators and we provide their spectra. Further, in the case of many spherical inclusions with isotropic materials, each with its own set of Lamé parameters, we propose an integral equation and a subsequent Galerkin discretization using the vector spherical harmonics and apply the discretization to several numerical test cases.     

Cascade of coil-globule conformational transitions of single flexible polyelectrolyte molecules in poor solvent

We show that hydrophobic flexible polyelectrolyte molecules of poly(2-vinylpyridine) and poly(methacryloyloxyethyl dimethylbenzylammonium chloride) are trapped and frozen due to adsorption on the mica surface, and the observed AFM single molecule structures reflect the molecular conformation in solution. An increase of the ionic strength of the solution induces the cascade of abrupt conformational transitions due to the intrachain segregation from elongated coil to compact globule conformations through intermediate pearl necklace-globule conformations with different amounts of beads per chain. The length of the necklaces and the number of beads decrease, while the diameter of beads increases with the increase of ionic strength. Coexistence at the same time of extended coils, necklaces with different amounts of beads, and compact globules indicates the cascade of the first-order-type phase transitions.     

Single flexible hydrophobic polyelectrolyte molecules adsorbed on solid substrate: transition between a stretched chain, necklace-like conformation and a globule

Single flexible polyelectrolyte molecules of poly(2-vinylpyridine) undergo conformational transition from a stretched wormlike coil to a necklace-like globule, and to a compact globule depending on pH and ionic strength in aqueous solution in good agreement with recent theoretical reports. AFM investigations allow the visualization of details of the chain conformation on mica and the extraction of quantitative statistics of molecular dimensions.     

Enzymatic determination of l-lysine by flow-injection techniques

An enzymatic assay that is highly selective for l-lysine, based on flow-injection techniques combined with spectrophotometric detection, is presented. l-Lysine-α-oxidase (E.C. 1.4.3.14) from Trichoderma viride and horseradish peroxidase were used in a coupled enzyme assay. Peroxide produced in the first reaction was converted by peroxidase with phenol and 4-aminoantipyrine to a quinoneimine dye detectable at 500 nm. An analytical enzyme reactor filled with coimmobilized enzymes was incorporated in the flow-injection system. The assay has a measuring frequency of 30 samples h^?1 and a response time of less than 2 min. To adapt the assay to high concentrations of l-Lysine and to minimize interferences, the injected sample volume was reduced to 2 μ-l, resulting in a linearity range of 1–16 mM l-lysine with a sensitivity of 6–7 mV 1 mmol^?1, a limit of detection (3σ) of 1 mM and a reproducibility of 0.5% (repetitive injection of a 10 mM l-lysine sample). The enzyme cartridge is stable for several months and thousands of measurements.     

On-line flow-injection monitoring of the enzyme inductor l-phenylalanine in the continuous cultivation of rhodococcus sp. M4

The o-phthaldialdehyde (OPA) assay for amino acids was adapted for flow injection analysis. With a dual-channel manifold and 2-μl injections, injection rates of 90–100 h^?1 were possible. Depending on the selected fluorimeter sensitivity, linear response ranges of 0.1–1.2 mM or 1–30 mM were obtained for l-phenylalanine with a relative standard deviation (RSD) of 0.9% for the slope of the calibration line. For ten injections of a 0.1 mM standard, the RSD was 1.6%. The detection limit was 0.01 mM. When the flow-injection method was combined with a fermentation sampling device and controlled by a timer, it was possible to monitor the decrease of l-phenylalanine on-line over a period of 87 h in the initial phase of a continuous cultivation of Rhodococcus sp. M4 producing the intracellular enzyme l-phenylalanine dehydrogenase. Correlation of the results with those obtained with an amino acid analyzer was good. Because OPA reacts with all primary amino groups, the applicability of the proposed method is restricted to cases in which only one amino acid and only small amounts of other amino group donators are present.     

Displacement of adsorbed functionalized polystyrene-block-butadiene copolymers by small molecules

This work deals with the displacement of end-anchored copolymers by the addition of solvent displacer. The adsorption behavior of functionalized polystyrene-block-polybutadiene diblock copolymers from dilute solution in toluene using silicon wafers as solid substrates is investigated by means of null-ellipsometry. The desorption phenomena are observed by adding displacers of low molecular weight to the mixture. The displacers used are tetrahydrofuran (THF) and acetone. The critical composition of the binary solvent mixture at which the desorption is complete, is determined experimentally.     

Functionalization of iron oxide magnetic nanoparticles with poly(methyl methacrylate) brushes via grafting‐from atom transfer radical polymerization

A key problem with nanomaterials is the difficulty of controlling the dispersion of nanoparticles inside an organic medium. To overcome this problem, functionalization of the nanoparticle surface is required. Poly(methyl methacrylate) (PMMA) brushes were grown on the surface of iron oxide magnetic nanoparticles with atom transfer radical polymerization and a grafting‐from approach. Modified magnetic nanoparticles with a graft density of 0.1 PMMA chains/nm² were obtained. Cu(II), used as a deactivating complex, allowed good control of the polymerization along with a narrow polydispersity of the polymer chains. The functionalized magnetic nanoparticles were characterized with Fourier transform infrared spectroscopy, thermogravimetric analysis, gel permeation chromatography, and atomic force microscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 925–932, 2007     

Beitrag zur algebra der Logik

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