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Short‐Term Morphology Relaxation of Thermoplastic Polyurethane Elastomers after Fast Strain Steps
No full textStrain steps are applied to elastomers in a pneumatic relaxometer and monitored by small‐angle X‐ray scattering (SAXS). The relaxometer provides a rise time of 13 ms for strain pulses of step height Δε = ±1 in strain. The basic character of the 2D SAXS frames is examined and corresponding invariants Q(t) are analyzed. Three thermoplastic polyurethanes (TPU) of hardness 85 Shore A with different soft segments are studied both unannealed and annealed. The first response of all materials is a fast morphology conversion which finishes within =250 ms. Because it has been untraceable, it is characterized by a settling stroke Q() − Q(0). The second response is a slow morphology adjustment process which complies with logarithmic relaxation. It is characterized by a relaxation rate D = Q(10 t)/Q(t) − 1. Comparison indicates that the nanoscopic morphology relaxation processes appear to have little direct relation to the macroscopic stress relaxation curves. The materials differ with respect to hard‐domain morphology stability and morphology recovery. Most unstable is the morphology of the annealed polyether‐based material. It forms nanofibrillary entities when strained
Understanding electrochemical switchability of perovskite-type exsolution catalysts
No full textExsolution of metal nanoparticles from perovskite-type oxides is a very promising approachto obtain catalysts with superior properties. One particularly interesting property of exsolutioncatalysts is the possibility of electrochemical switching between different activity states. In this work, synchrotron-based in-situ X-ray diffraction experiments on electrochemically polarized LaSrFeO thin film electrodes are performed, in order to simultaneously obtain insights into the phase composition and the catalytic activity of the electrode surface. This shows that reversible electrochemical switching between a high andlow activity state is accompanied by a phase change of exsolved particles between metallic -Fe and Fe-oxides. Reintegration of iron into the perovskite lattice is thus not required for obtaining a switchable catalyst, making this process especially interesting for intermediate temperature applications. These measurements also reveal how metallic particles on LaSrFeO electrodes affect the H oxidation and HO splitting mechanism and why the particle size plays a minor role
Multiscale computation delivers organophosphorus reactivity and stereoselectivity to immunoglobulin scavengers
No full textQuantum mechanics/molecular mechanics (QM/MM) maturation of an immunoglobulin (Ig) powered by supercomputation delivers novel functionality to this catalytic template and facilitates artificial evolution of biocatalysts. We here employ density functional theory-based (DFT-b) tight binding and funnel metadynamics to advance our earlier QM/MM maturation of A17 Ig-paraoxonase (WTIgP) as a reactibody for organophosphorus toxins. It enables regulation of biocatalytic activity for tyrosine nucleophilic attack on phosphorus. The single amino acid substitution l-Leu47Lys results in 340-fold enhanced reactivity for paraoxon. The computed ground-state complex shows substrate-induced ionization of the nucleophilic l-Tyr37, now H-bonded to l-Lys47, resulting from repositioning of l-Lys47. Multiple antibody structural homologs, selected by phenylphosphonate covalent capture, show contrasting enantioselectivities for a P-chiral phenylphosphonate toxin. That is defined by crystallographic analysis of phenylphosphonylated reaction products for antibodies A5 and WTIgP. DFT-b analysis using QM regions based on these structures identifies transition states for the favored and disfavored reactions with surprising results. This stereoselection analysis is extended by funnel metadynamics to a range of WTIgP variants whose predicted stereoselectivity is endorsed by experimental analysis. The algorithms used here offer prospects for tailored design of highly evolved, genetically encoded organophosphorus scavengers and for broader functionalities of members of the Ig superfamily, including cell surface-exposed receptors
Structure and mechanism of CutA, RNA nucleotidyl transferase with an unusual preference for cytosine
No full textTemplate-independent terminal ribonucleotide transferases (TENTs) catalyze the addition of nucleotide monophosphates to the 3′-end of RNA molecules regulating their fate. TENTs include poly(U) polymerases (PUPs) with a subgroup of 3′ CUCU-tagging enzymes, such as CutA in Aspergillus nidulans. CutA preferentially incorporates cytosines, processively polymerizes only adenosines and does not incorporate or extend guanosines. The basis of this peculiar specificity remains to be established. Here, we describe crystal structures of the catalytic core of CutA in complex with an incoming non-hydrolyzable CTP analog and an RNA with three adenosines, along with biochemical characterization of the enzyme. The binding of GTP or a primer with terminal guanosine is predicted to induce clashes between 2-NH of the guanine and protein, which would explain why CutA is unable to use these ligands as substrates. Processive adenosine polymerization likely results from the preferential binding of a primer ending with at least two adenosines. Intriguingly, we found that the affinities of CutA for the CTP and UTP are very similar and the structures did not reveal any apparent elements for specific NTP binding. Thus, the properties of CutA likely result from an interplay between several factors, which may include a conformational dynamic process of NTP recognition
Structure Determination by Continuous Diffraction from Imperfect Crystals
No full textThe coherent diffraction pattern of a non-periodic finite object does not consist of Bragg peaks but is continuously and smoothly varying. Such patterns do not suffer from the well-known phase problem of crystallography. In this case, robust iterative algorithms exist to determine the electron density of the object from the diffraction pattern alone. Continuous diffraction is accessible from ensembles of aligned molecules, including disordered protein crystals. We discuss the application of the concepts of coherent diffractive imaging to such cases and describe the experimental considerations to adequately measure the weak continuous diffraction signals
Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout
No full textThe Cherenkov Telescope Array (CTA) is the major next-generation observatory for ground-based very-high-energy gamma-ray astronomy. It will improve the sensitivity of current ground-based instruments by a factor of five to twenty, depending on the energy, greatly improving both their angular and energy resolutions over four decades in energy (from 20 GeV to 300 TeV). This achievement will be possible by using tens of imaging Cherenkov telescopes of three successive sizes. They will be arranged into two arrays, one per hemisphere, located on the La Palma island (Spain) and in Paranal (Chile). We present here the optimised and final telescope arrays for both CTA sites, as well as their foreseen performance, resulting from the analysis of three different large-scale Monte Carlo productions
Long-term stable supercontinuum generation and watt-level transmission in liquid-core optical fibers
No full textDue to their unique properties such as transparency, tunability, nonlinearity, and dispersion flexibility, liquid-core fibers represent an important approach for future coherent mid-infrared light sources. However, the damage thresholds of these fibers are largely unexplored. Here we report on the generation of soliton-based supercontinua in carbon disulfide (CS) liquid-core fibers at average power levels as high as 0.5 W operating stably for a long term (>70 h) without any kind of degradation or damage. Additionally, we also show stable high-power pulse transmission through liquid-core fibers exceeding 1 W of output average power for both CS and tetrachloroethylene as core materials
On a specific state of C fullerene in N-methyl-2-pyrrolidone solution: Mass spectrometric study
No full textA solution of fullerene C in N-methylpyrrolidone (NMP) presents a suitable system for obtaining fullerene's clusters with the tunable size. However, the mechanism of fullerenes interaction with polar NMP molecules is still elusive. Herein, we present the measured laser desorption/ionization mass spectra (LDI MS) of the precipitates produced from C/NMP solutions of different age in comparison with the typical spectra of C crystallized from toluene and benzene. The distinctive characteristics of the C/NMP mass spectra were identified and carefully examined. The number of characteristic peaks and their relative intensities in the spectra strongly depend on the age of initial C/NMP solutions. This effect was attributed to the specific C-NMP interactions in the solution, namely to the formation of charge-transfer complexes of C with NMP molecules followed by fullerene cluster formation. The results of additional measurements carried out by means of small-angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR), UV–Vis absorption spectroscopy together with the density functional theory (DFT) calculations are in accord with the proposed hypothesis
A GeV–TeV Measurement of the Extragalactic Background Light
No full textThe extragalactic background light (EBL) can be probed via the absorption imprint it leaves in the spectra of gamma-ray sources (). We recently developed a dedicated technique to reconstruct the EBL, and its evolution with redshift, from γ-ray optical depth data using a large sample of blazars detected by the Fermi Large Area Telescope. Here, we extend this data set to the TeV regime using ground-based Cherenkov observations of 38 blazars and report the first homogeneous measurement of the EBL spectral intensity covering the ultraviolet to infrared wavelengths (~0.1–100 μm). A minimal EBL throughout the wavelength range with respect to integrated galaxy light is found, allowing little additional unresolved emission from faint or truly diffuse populations setting an upper limit of lesssim4 nW m−2 sr−1 at 1.4 μm. In particular, the cosmic optical background at z = 0 is found to be . This work lays the foundation for accurate gamma-ray measurements of the EBL across its whole spectral range using a combination of GeV and TeV data
Secondary neutrino and gamma-ray fluxes from SimProp and CRPropa
No full textThe interactions of ultra-high energy cosmic rays (UHECRs) with background photons in extragalactic space generate high-energy neutrinos and photons. Simulating UHECR propagation requires assumptions about physical quantities such as the spectrum of the extragalactic background light (EBL) and photodisintegration cross sections. These assumptions, as well as the approximations used in the codes, may influence the computed predictions both of cosmic-ray spectra and composition, and of cosmogenic neutrino and photon fluxes. Following up on our previous work where we studied the resulting uncertainties on cosmic-ray simulations, here we quantify those on neutrinos and photons, using the Monte Carlo codes CRPropa and SimProp in various source scenarios. We discuss the results in the light of the constraining power of the neutrino and photon spectra on the origin of the UHECRs. We show that cosmogenic neutrino fluxes are more sensitive to the parametrization of the EBL than UHECR spectra, whereas the overall cosmogenic gamma-ray production rates are relatively independent on details of the propagation. We also find large differences between neutrino fluxes predicted by the latest released versions of CRPropa and SimProp, and discuss their causes and possible improvements in future versions of the codes