Zur Aktivierung der Xanthindehydrase in Milch

Zusammenfassung Es werden drei grundsätzliche Möglichkeiten zur Aktivierung der Xanthindehydrase in Milch beschrieben. An einer Milchprobe, in der das Enzym durch Zusatz von Cu total gehemmt vorlag, konnte die Hemmung durch Zugabe von KCN wieder aufgehoben werden. Ferner wird über die Ablösung des Enzyms von der Fettkügelchenmembran durch Na₂WO₄ und über die Aufspaltung in Untereinheiten durch Enzyme mit Labwirkung berichtet.     

On-line detection of toxic components using a microbial fuel cell-based biosensor

Safe drinking water without toxic chemicals is crucial for people's health. A recently developed sensor for the detection of toxic components in water is the microbial fuel cell (MFC)-based biosensor. In this biosensor, substrate consumption rate and metabolic activity of bacteria are directly related to the electric current. A reduction in current under otherwise similar conditions is an indication of toxic inhibition. Under steady state conditions, current can be described by the Butler–Volmer–Monod (BVM) model. Knowing which parameters of this model change under toxic contamination can give an indication on the type of toxicity. The model requires that the substrate concentration is known. It is shown in this paper that is not possible to estimate both the substrate concentration as well as the BVM parameters on-line from current data at constant overpotential. However, it appears that substrate concentration and substrate consumption rate can be estimated on-line, and that after a linear reparametrization the BVM parameters can be estimated by ordinary least-squares techniques from a polarization curve that is generated as soon as a suspect change in current occurs. Analysis shows that a weighted least-squares method is necessary to secure a good fit at the overpotentials where current is most sensitive to changes in kinetic parameters. A protocol for on-line detection of toxicity and for detection of the type of kinetic inhibition is provided.     

Online screening of homogeneous catalyst performance using reaction detection mass spectrometry

An integrated online screening system was developed to rapidly screen homogeneous catalysts for activity toward a selected synthesis. The continuous-flow system comprises standard HPLC pumps for the delivery of substrates, an HPLC autosampler for the injection of homogeneous catalysts, a thermostated reactor to mediate synthesis, and a single-stage quadrupole mass spectrometer (MS) equipped with atmospheric pressure chemical ionization for the determination of product formation. MS detection offers sensitivity, specificity, and speed when applied to the analysis of dynamic processes in the condensed phase. By applying the present methodology for the study of substrate conversion mediated by homogeneous catalysts, the concentration of substrates and reaction product could be monitored while information about the catalysts could also be obtained. In an initial screening application, the performance of a selected number of Lewis acids in the multicomponent synthesis of a highly substituted 2-imidazoline was determined. Limit of detection and limit of quantitation were determined by injecting different concentrations of 2-imidazoline standards and proved to be 1.6 and 5.2 nM, respectively. The results obtained with the new screening method were in good agreement with a traditional bench-scale experiment. Moreover, the system was capable of determining catalyst performance with very low catalyst and solvent consumption while the ruggedness of the system was exhibited with a 24-h continuous analysis of 280 successive catalyst injections with a peak area variation within 7% relative standard deviation.     

Electron-beam-induced deposition in ultrahigh vacuum: lithographic fabrication of clean iron nanostructures

The generation of nanostructures with arbitrary shapes and well-defined chemical composition is still a challenge and targets the core of the fast-growing field of nanotechnology. One approach is the maskless nanofabrication technique of electron-beam-induced deposition (EBID). Up to now, the purity of these EBID structures has been rather poor. Here we demonstrate that by performing the EBID process solely under ultrahigh vacuum conditions, the lithographic generation of iron nanostructures on Si(100) with an unprecedented purity of higher than 95% is possible. One particular new aspect is the formation of EBID deposits with reduced size in a strain-induced diffusive process, resulting in deposits significantly smaller than 10 nm.     

A flow injection kinase assay system based on time-resolved fluorescence resonance energy-transfer detection in the millisecond range

A flow injection analysis (FIA) system for biochemical assays using time-resolved fluorescence resonance energy transfer (TR-FRET) in the millisecond time scale was developed. As a model system, we studied a kinase assay, measuring the phosphorylation of poly(GT)-biotin (substrate) by a receptor tyrosine kinase (epidermal growth factor receptor). A streptavidin labeled with XL665 (SA-XL665)-the acceptor-was coupled to the biotin moiety, and an antiphosphotyrosine antibody labeled with europium cryptate (Ab-EuK)-the donor-was coupled to the phosphorylated tyrosine group(s). Long-lived FRET can only occur if the substrate is successfully phosphorylated. For the time-resolved detection of such long-lived luminescence phenomena in a flow system, the repetition rate of the excitation source plays a crucial role. Good results were obtained for a small-sized commercially available quadrupled Nd:YAG laser emitting at 266 nm with a repetition rate of 7.8 kHz and a pulse width of 0.3 ns. The long-lived emissions of the donor at 625 nm and that of the acceptor at 665 nm were monitored simultaneously with two photomultipliers, using a delay time of 50 micros and a gate time of 75 micros to exclude background fluorescence interferences. In the FIA experiments, the Ab-EuK concentration was 6 nM and the substrate concentration and SA-XL665 concentrations were 7 nM. By monitoring the intensity changes at 625 and 665 nm, the inhibition of tyrosine kinase by tyrphostin AG1478 was studied and an IC(50) value of 5.1 +/- 0.4 nM obtained.     

Influence of phosphate and iron on the extent of calcium carbonate precipitation during anaerobic digestion

The extent to which calcium carbonate deposition in an anaerobic reactor can be reduced by adding inhibitors (phosphate and iron) of calcium carbonate crystal growth was investigated. At several concentrations of the additive, the extent of precipitation was assessed in continuous experiments with laboratory‐scale reactors. In the reactor, phosphate concentrations as low as 0.5–5 mg total‐P dm⁻³ were found to severely inhibit CaCO₃ precipitation. However, iron did not inhibit the deposition of CaCO₃, which was found to be due to the fact that iron, in contrast to phosphate, only inhibits the growth of calcite and not the formation of aragonite. The results led to the conclusion that only additives which inhibit the formation of both aragonite and calcite can be used as effective inhibitors during anaerobic digestion. A chemical equilibrium model was developed and shown to be a useful tool to calculate the extent of calcium carbonate deposition during anaerobic digestion provided the proper apparent solubility product of calcium carbonate can be estimated. © 1999 Society of Chemical Industry     

Improved Blue‐Emitting AlN:Eu2+ Phosphors by Alloying with GaN

A method is designed to improve the luminescence of AlN‐based phosphors by tuning the band structure and crystal structure due to alloying with GaN. The pure (Al,Ga)N:Eu phosphors were initially prepared by gas‐phase reaction in an NH₃ atmosphere. GaN alloying was used to expand the crystal lattice of AlN due to Ga³⁺ substituting for smaller Al³⁺ ions, making dissolution of Eu²⁺ easier. The dissolution of Ga in the AlN lattice was proven by the result of the Rietveld refinement and the increase in lattice parameters with increasing Ga content. To introduce other energy states mixing with the 5d states of Eu²⁺, Ga doping was also used to tune the band structure of AlN by acting on Eu²⁺ ions. The theoretical result was analyzed using the Cambridge Sequential Total Energy Package (CASTEP). According to the calculated total and atom resolved partial density of states, it was observed that the Ga 5p states contribute a large portion to the corresponding Eu²⁺ absorption band in (Al,Ga)N:Eu phosphors. As a consequence, an enhanced emission intensity at 470 nm and a high quantum efficiency for excitation at 330 nm was obtained despite of stronger thermal quenching of the (Al,Ga)N:Eu phosphors compared with AlN:Eu.     

Encapsulation of nitride phosphors into sintered phosphate glass by pressureless firing and hot isostatic pressing

The encapsulation conditions of Eu²⁺-doped calcium silicon nitride ((Ca_0.90Eu_0.10)₂Si₅N₈) and strontium silicon nitride ((Sr_0.98Eu_0.02)₂Si₅N₈) phosphors into phosphate glass ((50-x)Na₂O–xZnO–5B₂O₃–45P₂O₅; x = 10-40) were examined by pressureless sintering (normal or infrared firing) and subsequent oxygen-assisted hot isostatic pressing. The evaluation of water resistivity, as well as glass transition temperature (T_g), indicated that the excellent water resistivity and low T_g (313°C) could be achieved at x = 30. The colorless and clear glass body could be fabricated by infrared firing at 370°C for 20 minutes in air, followed by the hot isostatic pressing at 370°C for 24 hours (80%Ar + 20%O₂, pressure: 130 MPa). The encapsulations of 3.0 mass% (Ca_0.90Eu_0.10)₂Si₅N₈ and (Sr_0.98Eu_0.02)₂Si₅N₈ into the glass body produced reddish orange light emission (peak wavelength: 604 nm under excitation wavelength of 423 nm) and red-light emission (617 nm under 383 nm), respectively, showing higher chemical and thermal stability of nitride phosphors in the glass matrix.     

Microbial fuel cell operation with continuous biological ferrous iron oxidation of the catholyte

The oxygen reduction rate at the cathode is a limiting factor in microbial fuel cell (MFC) performance. In our previous study, we showed the performance of an MFC with ferric iron (Fe3+) reduction at the cathode. Instead of oxygen, ferric iron was reduced to ferrous iron (Fe2+) at the cathode with a bipolar membrane between the anode and cathode compartment. This resulted in a higher cathode potential than is usually obtained with oxygen on metal-based chemical catalysts in MFCs. In this study, we investigated the operation of the same MFC with ferric iron reduction at the cathode and simultaneous biological ferrous iron oxidation of the catholyte. We show that the immobilized microorganism Acidithiobacillus ferrooxidans is capable of oxidizing ferrous iron to ferric iron at a rate high enough to ensure an MFC power output of 1.2 W/m2 and a current of 4.4 A/m2. This power output was 38% higher than in our previous study at a similar current density without ferrous iron oxidation. The bipolar membrane is shown to split water into 65-76% of the needed protons and hydroxides. The other part of the protons was supplied as H2SO4 to the cathode compartment. The remaining charge was transported by K+ and HSO4-/SO4(2-) from the one compartment to the other. This resulted in increased salt concentrations in the cathode. The increased salt concentrations reduced the ohmic losses and enabled the improved MFC power output. Iron could be reversibly removed from the bipolar membrane by exchange with protons.