A Sagittarius-Induced Origin for the Monoceros Ring

The Monoceros ring is a collection of stars in nearly-circular orbits at roughly 18 kpc from the Galactic center. It may have originated (i) as the response of the disc to perturbations excited by satellite companions or (ii) from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros stars differs from that of disc stars at comparable Galactocentric distances, an observation that disfavours the first scenario. On the other hand, circular orbits are difficult to accommodate in the tidal-disruption scenario, since it requires a satellite which at the time of disruption was itself in a nearly circular orbit. Such satellite could not have formed at the location of the ring and, given its low mass, dynamical friction is unlikely to have played a major role in its orbital evolution. We search cosmological simulations for low-mass satellites in nearly-circular orbits and find that they result, almost invariably, from orbital changes induced by collisions with more massive satellites: the radius of the circular orbit thus traces the galactocentric distance of the collision. Interestingly, the Sagittarius dwarf, one of the most luminous satellites of the Milky Way, is in a polar orbit that crosses the Galactic plane at roughly the same Galactocentric distance as Monoceros. We use idealized simulations to demonstrate that an encounter with Sagittarius might well have led to the circularization and subsequent tidal demise of the progenitor of the Monoceros ring.Comment: 6 pages, 4 figures, to match version published in MNRAS Letters (http://onlinelibrary.wiley.com/doi/10.1111/j.1745-3933.2011.01035.x/abstract

Redetermination of MoPt3Si4 from single-crystal data

The crystal structure of molybdenum triplatinum tetrasilicide, MoPt3Si4, determined previously from powder diffraction data [Joubert et al. (2010 ▶). J. Solid State Chem. 183, 173–179], has been redetermined using a single crystal synthesized from the elements by high-frequency melting. The redetermination provides more precise geometrical data and also anisotropic displacement parameters. The crystal structure can be considered to be derived from the PtSi structure type with an ordered substitution of Pt by Mo atoms, but leading to a very distorted Si network compared to the parent structure. Mo and Pt exhibit different coordination polyhedra. These are based on bicapped-square anti­prisms, but with two additional vertices in cis positions for Mo, whereas they are in trans positions for Pt (as in PtSi). The coordination polyhedra for three of the Si atoms can be considered as highly deformed square anti­prisms (as in PtSi), while the fourth Si atom has a bicapped trigonal–prismatic coordination geometry

Design of a Coiled-Coil-based Model Peptide System toExplore the Fundamentals of Amyloid Fibril Formation

Protein deposition as amyloid fibrils underlies more than twenty severely debilitating human disorders. Interestingly, recent studies suggest that all peptides and proteins possess an intrinsic ability to assemble into amyloid fibrils similar to those observed in disease states. The common properties and characteristics of amyloid aggregates thus offer the prospect that simple model systems can be used to systematically assess the factors that predispose a native protein to form amyloid fibrils and understand the origin and progression of fatal disorders associated with amyloid formation. Here, we report the de novo design of a 17-residue peptide model system, referred to as ccβ, which forms a protein-like coiled-coil structure under ambient solution conditions but can be easily converted into amyloid fibrils by raising the temperature. Oxidation of methionine residues at selected hydrophobic positions completely abolished amyloid fibril formation of the peptide while not interfering with its coiled-coil structure. This finding indicates that a small number of site-specific hydrophobic interactions can play a major role in the packing of polypeptide chain segments within amyloid fibrils. The simplicity and characteristics of the ccβ system make it highly suitable for probing molecular details of the assembly of amyloid structure

Protection of cobalt-based refractory alloys by chromium deposition on surface. Part II: Behaviour of the coated alloys in oxidation at hightemperature

International audienceA high chromium content, typically 30wt.%, is generally considered as necessary for cobalt-based alloys to allow them resisting oxidation by hot gases. Cobalt alloys with a bulk poorer in chromium may become resistant against high temperature oxidation if they are enriched in chromium on surface. The aim of this second part of the work is to expose to air at 1200°C three low-chromium cobalt-based alloys (Co-10Cr, Co-10Cr-0.25C-4.4Ta and Co-10Cr-0.5C-8.7Ta in wt.%), previously enriched in chromium on surface by pack-cementation, in order to examine if the applied cementation treatments are enough to ensure a good behavior on long time. The initial surface enrichment in chromium of the Ta-containing alloys, characterized by a maximal content on surface of about 30wt.%Cr and a depth of enrichment of several hundreds of micrometers, gives them a good resistance to oxidation at 1200°C, despite the very low chromium content in the bulk. This is also due to the interdendritic TaC carbides which may facilitate outward diffusion of chromium during oxidation as they facilitated its inward diffusion during cementation. In contrast, the CoCr alloy, enriched in chromium in the same conditions did not display a so good oxidation behavior

Protection of cobalt-based refractory alloys by chromium deposition on surface. Part I: Sub-surface enrichment in chromium by packcementationand diffusion

International audienceThe feasibility of surface chromium enrichment by pack-cementation was assessed for different low chromium-containing cobalt alloys, in order to improve their resistance against high temperature. A binary Co-10Cr alloy, two ternary Co-10Cr-0.5C and Co-10Cr-1.0C alloys and two TaC-containing Co-10Cr-based alloys were elaborated by foundry for the study. 7.5h-long and 15h-long cementations at 1050°C, followed or not by a 75h-long heat treatment at 1200°C were performed on these alloys. Microstructure examinations performed using a Scanning Electron Microscope and concentration profiles using Electron Probe Micro Analysis-Wavelength Dispersion Spectrometry were realized in order to analyze the level of Cr-enrichment of the sub-surface region, with as studied criteria: the nature of the external Cr-enriched zone, the maximal chromium content on surface and the depth of chromium enrichment. The Cr-enrichment of the sub-surface succeeded for the Co-10Cr alloy and for the two tantalum-containing alloys, with the formation of an external metallic zone containing around 30wt.%Cr. In contrast the chromium carbides-containing alloys were effectively enriched in chromium in surface but in the form of a continuous chromium carbide layer which can induce other problems such as spallation and then possible fast oxidation of the denuded alloy. Finally it appeared that only the carbon-free alloys, and the alloys reinforced by carbides more stable than chromium carbides, are potentially able to be successful enriched in chromium in their sub-surface by pack-cementation

Methods of NMR structure refinement: molecular dynamics simulations improve the agreement with measured NMR data of a C-terminal peptide of GCN4-p1

The C-terminal trigger sequence is essential in the coiled-coil formation of GCN4-p1; its conformational properties are thus of importance for understanding this process at the atomic level. A solution NMR model structure of a peptide, GCN4p16-31, encompassing the GCN4-p1 trigger sequence was proposed a few years ago. Derived using a standard single-structure refinement protocol based on 172 nuclear Overhauser effect (NOE) distance restraints, 14 hydrogen-bond and 11 ϕ torsional-angle restraints, the resulting set of 20 NMR model structures exhibits regular α-helical structure. However, the set slightly violates some measured NOE bounds and does not reproduce all 15 measured 3J(HN-HCα)-coupling constants, indicating that different conformers of GCN4p16-31 might be present in solution. With the aim to resolve structures compatible with all NOE upper distance bounds and 3J-coupling constants, we executed several structure refinement protocols employing unrestrained and restrained molecular dynamics (MD) simulations with two force fields. We find that only configurational ensembles obtained by applying simultaneously time-averaged NOE distance and 3J-coupling constant restraining with either force field reproduce all the experimental data. Additionally, analyses of the simulated ensembles show that the conformational variability of GCN4p16-31 in solution admitted by the available set of 187 measured NMR data is larger than represented by the set of the NMR model structures. The conformations of GCN4p16-31 in solution differ in the orientation not only of the side-chains but also of the backbone. The inconsistencies between the NMR model structures and the measured NMR data are due to the neglect of averaging effects and the inclusion of hydrogen-bond and torsional-angle restraints that have little basis in the primary, i.e. measured NMR dat

Production of in vitro amplified DNA pseudolibraries and high-throughput cDNA target amplification

<p>Abstract</p> <p>Background</p> <p>Many structural biology- and high-throughput laboratories experience the acquisition of multiple cDNAs from different sources as a rather time- and resource-consuming procedure. The techniques presented here solve these problems.</p> <p>Results</p> <p>An advanced target cDNA amplification procedure employing RNA- or cDNA-derived pseudolibraries circumvents the usual DNA transfection during library establishment. A small sample of reverse transcribed ss- or ds-cDNA or DNA from a pre-existing library is multiplied by in vitro rolling circle ramification amplification. The resulting cDNA pseudolibrary serves as a template for numerous highly efficient PCR amplifications and permits production and analysis of target cDNAs on an automated liquid handling workstation.</p> <p>Conclusion</p> <p>The overall efficiency of the simple protocol collection approaches 100% for targets from libraries with low complexity such as Drosophila and yields >80% of amplicons up to 3 kb size in the case of human cDNA.</p

13C, 15N Resonance Assignment of Parts of the HET-s Prion Protein in its Amyloid Form

The partial 15N and 13C solid-state NMR resonance assignment of the HET-s prion protein fragment 218-289 in its amyloid form is presented. It is based on experiments measured at MAS frequencies in the range of 20-40kHz using exclusively adiabatic polarization-transfer schemes. The resonance assignment within each residue is based on two-dimensional 13C--13C correlation spectra utilizing the DREAM mixing scheme. The sequential linking of the assigned residues used a set of two- and three-dimensional 15N--13C correlation experiments. Almost all cross peaks visible in the spectra are assigned, but only resonances from 43 of the 78 amino-acid residues could be detected. The missing residues are thought to be highly disordered and/or highly dynamic giving rise to broad resonance lines that escaped detection in the experiments applied. The line widths of the observed resonances are narrow and comparable to line widths observed in micro-crystalline samples. The 43 assigned residues are located in two fragments of about 20 residue