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Numerous membrane importers rely on accessory water-soluble proteins to capture their substrates. These substrate-binding Proteins (SBP) have a strong affinity for their ligands; yet, Substrate release onto the low-affinity membrane transporter must occur for uptake to proceed. It is generally accepted that release is facilitated by the association of SBP and transporter, upon which the SBP adopts a conformation similar to the unliganded state, whose affinity is sufficiently reduced. Despite the appeal of this mechanism, however, direct supporting evidence is lacking. Here, we use experimental and theoretical methods to demonstrate that an allosteric mechanism of enhanced substrate release is indeed plausible.
First, we report the atomic-resolution structure of APO TeaA, the SBP of the Na!-coupled ectoine TRAP transporter TeaBC from Halomonas elongata DSM2581T, and compare it with the substrate-bound structure previously reported. Conformational freeenergy landscape calculations based upon molecular Dynamics simulations are then used to dissect the mechanism that couples ectoine binding to structural change in TeaA. These insights allow us to design a triple mutation that biases TeaA toward apo-like conformations without directly perturbing the binding cleft, thus mimicking the influence of the membrane transporter. Calorimetric measurements demonstrate that the ectoine affinity of the conformationally biased triple mutant is 100-fold weaker than that of the wild type. By contrast, a control mutant predicted to be conformationally unbiased displays wild-type affinity. This work thus demonstrates that substrate release from SBPs onto their Membrane transporters can be facilitated by the latter through a mechanism of allosteric modulation of the former.
The introduction of carbon nanotubes (CNTs) modifies bulk polymer properties, depending on intrinsic quality, dispersion, alignment, interfacial chemistry and mechanical properties of the nanofiller. These effects can be exploited to enhance the matrices of conventional microscale fibre-reinforced polymer composites, by using primary reinforcing fibres grafted with CNTs. This paper presents a methodology that combines atomic force microscopy, polarised Raman spectroscopy, and nanoindentation techniques, to study the distribution, alignment and orientation of CNTs in the vicinity of epoxy-embedded micrometre-scale silica fibres, as well as, the resulting local mechanical properties of the matrix. Raman maps of key features in the CNT spectra clearly show the CNT distribution and orientation, including a parted morphology associated with long grafted CNTs. The hardness and indentation modulus of the epoxy matrix were improved locally by 28% and 24%, respectively, due to the reinforcing effects of CNTs. Moreover, a slower stress relaxation was observed in the epoxy region containing CNTs, which may be due to restricted molecular mobility of the matrix. The proposed methodology is likely to be relevant to further studies of nanocomposites and hierarchical composites.
The isotopic composition of boron is a well established tool in various areas of Science and industries. Boron isotope compositions are typically reported as 6"B values which indicate the isotopic difference of a sample relative to the internationally accepted isotope reference material NIST SRM 951. A significant drawback of all of the available boron isotope reference materials is that non covers a natural boron isotope composition apart from NIST SRM 951.
The variation of isotope abundance ratios is increasingly used to unravell natural and technical questions. In the past the investigation and interpretation of such variations was the field of a limited number of experts. With new upcoming techniques and research topics in the last decades, such as provenance and authenticity of food, the number of published isotope data strongly increased. The development of inductively coupled plasma mass spectrometers (ICPMS) from an instrument for simple quantitative analysis to highly sophisticated isotope abundance ratio machines influenced this process significantly. While in former times only experts in mass spectrometry were able to produce reproducible isotope data, nowadays many laboratories, never been in touch with mass spectrometry before, produce isotope data with an ICPMS. Especially for such user isotope reference materials (IRM) are indispensible to enable a reliable method validation. The fast development and the broad availability of ICPMS also lead to an expansion of the classical research areas and new elements are under investigation. Here all users require IRM to correct for mass fractionation or mass discrimination or at least to enable isotope data related to a common accepted basis. Despite this growing interest suitable IRM are still lacking for a number of isotope systems such as magnesium.
Highly energetic silicon tetraazide 1 is synthesized safely in high yield and high purity. The compound is prepared as a solution in benzene which allows it to be handled safely. Stable Lewis base adducts of Si(N3)4 (e.g. 2) are also prepared by treatment of either Si(N3)4 or the disodium salt of hexaazidosilicate (3) with Lewis bases (2,2-bipyridine or 1,10-phenanthroline). Unlike Si(N3)4 the Lewis base adduct 2 is thermally stable and does not decompose below 265 °C. The decomposition is very energetic (ΔHd = -2.4 kJg-1) which means 2 and related Lewis base adducts could replace environmentally harmful Pb(N3)4 used as primary explosive.
The wear behaviour of thin steel wires has been analyzed under oscillating sliding conditions in crossed cylinders contact geometry. The focus of this analysis was the influence of the crossing angle between the wires on the wear. The wires used had 0.45 mm in diameter and the material was cold-drawn eutectoid carbon steel (0.8% C) with a tensile strength higher than 2800 MPa. Two different types of tests were carried out, the first one representing the influence of the crossing angle for a constant load and the second one representing the influence of the crossing angle with constant contact pressure. In the first type of tests it was seen that as the contact angle decreases the contact pressure decreases too and hence less energy specific wear resistance is observed. As a consequence less wear is produced, thus increasing the life of the wires. In the second type of tests it was seen that with constant contact pressure but different crossing angles, nearly the same energy specific wear resistance was observed. This points at an identical wear behaviour in both type of tests but with a running-in and a steady state period as two different wear periods. The tests showed that the running in period may play an important role in the overall wear particle generation and hence the wear occurring in the steady state period is rather mild.
Original equipment manufacturers (OEMs) and end-users perceive 'Zero Wear' differently. The 'Zero Wear' approach will be put into a general relation for different applications and illuminated by individual paths, either based on monolithic materials, thin film coatings or alternative base oils, featuring triboactive materials and lubricious oxides for tribological engine components and dry running foil bearings or specific DLC, ta-C and novel Zirconium-based thin film coatings for concentrated contacts above FZG 14 and alternative engine oils (NoSAP & bio-no-tox). The associated tribometric test equipments for these examples will also be detailed.
Inspired by the globule arc technique a new electrothermal vaporization inductively coupled plasma optical emission spectrometry (ETV-ICP OES) method was developed for the analysis of high-purity copper materials. The performance of the method was investigated for the analytes Ag, Al, As, Bi, Cd, Co, Cr, Fe, Mg, Mn, Ni, P, Pb, S, Sb, Se, Si, Sn, Te, Ti, Zn and Zr. ETV parameters were optimized regarding the release of the analytes, the transport efficiency and the quality of analytical results in terms of precision, trueness and power of detection. The influence of CCl2F2, CHClF2, C2H2F4 and CHF3 as gaseous halogenation modifiers was investigated. A sufficient in situ analyte matrix separation was achieved by using CHF3 as halogenating reagent avoiding a high matrix input from the molten copper sample into the ETV system and the plasma. A complete release from the samples was obtained for all investigated analytes except Se and Te. Acceptable results for the determination of the trace elements Ag, Al, As, Bi, Cd, Co, Cr, Fe, Mg, Mn, Ni, P, Pb, S, Sb, Si, Sn, Ti, Zn and Zr in high-purity copper were achieved. The method includes a preceding sample preparation step of oxidizing the surface of copper samples which results in a significantly enhanced sensitivity. In addition to the calibration with copper samples, the feasibility of the calibration with liquid multi-element solutions was investigated. Except for Ag, Mg and Ni all analytes could be analyzed using aqueous calibration solutions. The trueness of the method was tested by the determination of analyte contents of certified reference materials. Limits of quantification ranging from 0.6 ng g-1 to 29 ng g-1 were achieved. The developed direct solid sampling method is time and cost effective and well suited for the characterization of high-purity copper materials. The method can be automated to a large extent and is applicable for processes accompanying analyses. In contrast to all other investigated trace elements, Se and Te were not released from the matrix at measurable levels under the used conditions. The determination of these elements is still under investigation and will be reported in a succeeding publication.
Coda wave interferometry is a recent analysis method now widely used in seismology. It uses the increased sensitivity of multiply scattered elastic waves with long travel-times for monitoring weak changes in a medium. While its application for structural monitoring has been shown to work under laboratory conditions, the usability on a real structure with known material changes had yet to be proven. This article presents experiments on a concrete bridge during construction. The results show that small velocity perturbations induced by a changing stress state in the structure can be determined even under adverse conditions. Theoretical estimations based on the stress calculations by the structural engineers are in good agreement with the measured velocity variations.
Honeycombs/compaction faults occur in the concrete structures due to improper solidification of the concrete, which may reduce the strength of the concrete and also act as a passage for the water/acids that further corrodes the reinforcements. This paper explores about the acoustic pulse-echo techniques for the detection of honeycomb defects in a laboratory specimen located at the Federal Institute for Materials Research and Testing (BAM), Berlin. Since concrete is an inhomogeneous medium, the defect Signals are masked by the material noise due to large amount of scattering/ reflections of acoustic waves. A filtering method using the discrete wavelet transforms is applied on the ultrasonic time Signals for the better localization of defects.