Orientation Imaging in Scanning Electron and Transmission Electron Microscopy for Characterization of the Shear Banding Phenomenon

The texture evolution during deformation of high purity fcc single crystals with initial (112) [11 ] orientation has been characterised in detail by transmission (TEM) and scanning (SEM–FEG) electron microscopes. The channel-die deformed samples up to reduction of about 1–1.5, first developing strongly anisotropic layers of elongated cells or twin-matrix plates and then compact clusters of SB. Substantial progress in understanding the mechanism of the SB formation was possible thanks to systematic local orientation measurements (orientation mapping) using SEM and TEM. These two techniques of local orientation measurements have been compared with respect to their applicability for the study of shear banding phenomenon and for characterization of the specific nanostructure of SB in metals with fcc lattice. It was shown that well-developed SB exhibit large orientation spreads up to 35–40° with respect to the adjacent areas outside the band. Most of these misorientations occur by rotations about the TD‖〈110〉 axis with significant further rotations about 〈112〉 poles. This ultimately leads to the formation of the texture components whose occurrence cannot be explained by models homogeneous deformation.     

A versatile template for the formation of [2]pseudorotaxanes. 1,2-Bis(pyridinium)ethane axles and 24-crown-8 ether wheels

Linear 1,2-bis(pyridinium)ethane 'axles' and macrocyclic 24-membered crown ether 'wheels' (, and ) combine to form [2]pseudorotaxanes. These interpenetrated adducts are held together by N+...O ion-dipole interactions, a series of C-H...O hydrogen bonds and pi-stacking between electron-poor pyridinium rings of the axle and electron-rich catechol rings of the wheel. 1H NMR spectroscopy was used to identify the structural details of the interaction and to determine the thermodynamics of the binding process in solution. Analysis of nine of these adducts by single crystal X-ray crystallography allowed a detailed study of the non-covalent interactions in the solid state. A wide variety of structural changes could be made to the system. The versatility and potential of the template for the construction of permanently interlocked structures such as rotaxanes and catenanes is discussed.     

Microwave-assisted cyclic amidine synthesis using TiCl4

Microwave-assisted treatment of various heterocyclic amides (benzodiazepinone, phthalazone) with TiCl(4) in the presence of primary or secondary amines provides the corresponding amidines. In addition to the interest of the microwaves for this reaction, our study highlights the higher reactivity of the cyclic acetamide moiety compared to the cyclic benzamide moiety towards this TiCl(4)-mediated reaction.     

Gas permeability in polymer- and surfactant-stabilized bubble films

The gas permeabilities of thin liquid films stabilized by poly(N-isopropylacrylamide) (PNIPAM) and PNIPAM-SDS (sodium dodecyl sulfate) mixtures are studied using the "diminishing bubble" method. The method consists of forming a microbubble on the surface of the polymer solution and measuring the shrinking rates of the bubble and the bubble film as the gas diffuses from the interior to the exterior of the bubble. PNIPAM-stabilized films exhibit variable thicknesses and homogeneities. Interestingly, despite these variable features, the gas permeability of the film is determined principally by the structure of the adsorbed polymer layer that provides an efficient gas barrier with a value of gas permeability coefficient that is comparable to that of an SDS Newton black film. In the presence of SDS, both the film homogeneity and the gas permeability coefficient increase. These changes are related to interactions of PNIPAM with SDS in the solution and at the interface, where coadsorption of the two species forms mixed layers that are stable but that are more porous to gas transfer. The mixed PNIPAM-SDS layers, studied previously for a single water-air interface by neutron reflectivity, are further characterized here in a vertical free-draining film using X-ray reflectivity.     

First evaluation of acyloxymethyl or acylthiomethyl groups as biolabile 2'-O-protections of RNA

[reaction: see text] Short oligo-U sequences containing 2'-O-acyloxymethyl or acylthiomethyl groups as biolabile 2'-O-protections of RNA have been synthesized. These modified homouridylates are deprotected upon cellular esterase activation to release the parent RNA. They exhibit exceptional resistance to nuclease degradation, and the evaluation of their pairing properties shows that the 2'-acyloxymethyl groups do not prevent the duplex dsRNA formation. These biolabile 2'-modifications overcome the first hurdle to turn oligoribonucleotides into candidates for RNA interference drugs.     

An investigation of hemibenzopinacolate radicals used in the synthesis of block-copolymers: ESR measurements and semiempirical calculations

Hemibenzopinacolate radicals may be incorporated in a polymer chain and the latter used as a macromolecular initiator. Kinetics of polymerization are strongly dependent on the substitution of the pinacolate. A large family of substituted (CH₃)₃SiOC(C₆H₅)₂ radicals has been synthesized and studied by electron spin resonance (ESR) and semiempirical molecular orbital calculations. ESR spectra are well resolved and, except for one derivative, allow an unambiguous determination of the hyperfine coupling splitting (HFS) constants. The ground state properties have been calculated within different approximations, namely MNDO, AM1 and PM3 for the geometry optimization and INDO for spin density. Geometries obtained within AM1 and PM3 approximations are satisfactory. The HFS constants evaluated using McConnell type relations and spin densities are in a quite good agreement with the experimental ones. It turns out that the effect of substitution on kinetics of polymerization cannot be attributed to any drastic change in the charge distribution.     

Structure formation in systems involving poly(ethylene oxide) and small molecules

The structure of the intercalates in which poly(ethylene oxide), PEO, is the host component and para-disubstituted benzenes are the guest molecules is completely elucidated by X-ray diffractometry, polarized FTIR spectrography, and calorimetry. It is shown that the association of these molecules results from the adaptation of the shape of their external surfaces. On this basis the possibility of formation of structure involving PEO and small molecules is discussed.     

Glyphosate–Exonuclease Interactions: Reduced Enzymatic Activity as a Route to Glyphosate Biosensing

N-phosphonomethyle-glycine (glyphosate) is the most widely used pesticide worldwide due to its effectiveness in killing weeds at a moderate cost, bringing significant economic benefits. However, owing to its massive use, glyphosate and its residues contaminate surface waters. On site, fast monitoring of contamination is therefore urgently needed to alert local authorities and raise population awareness. Here the hindrance of the activity of two enzymes, the exonuclease I (Exo I) and the T5 exonuclease (T5 Exo) by glyphosate, is reported. These two enzymes digest oligonucleotides into shorter sequences, down to single nucleotides. The presence of glyphosate in the reaction medium hampers the activity of both enzymes, slowing down enzymatic digestion. It is shown by fluorescence spectroscopy that the inhibition of ExoI enzymatic activity is specific to glyphosate, paving the way for the development of a biosensor to detect this pollutant in drinking water at suitable detection limits, i.e., 0.6 nm .