Ab initio investigation of structure and cohesive energy of crystalline urea

The structure and cohesive energy of crystalline urea have been investigated at the ab initio level of calculation. The performance of different Hamiltonians in dealing with a hydrogen-bonded molecular crystal as crystalline urea is assessed. Detailed calculations carried out by adopting both HF and some of the most popular DFT methods in solid-state chemistry are reported. Local, gradient-corrected, and hybrid functionals have been adopted: SVWN, PW91, PBE, B3LYP, and PBE0. First, a 6-31G(d,p) basis set has been adopted, and then the basis set dependence of computed results has been investigated at the B3LYP level. All calculations were carried out by using a development version of the periodic ab initio code CRYSTAL06, which allows full optimization of lattice parameters and atomic coordinates. With the 6-31G(d,p) basis set, structural features are well reproduced by hybrid methods and GGA. LDA gives lattice parameters and hydrogen-bond distances that are too small relative to experiment, while at the HF level the opposite trend is observed. Results show that hybrid methods are more accurate than HF and both LDA and GGA functionals, with a trend in the computed properties similar to that of hydrogen-bonded molecular complexes. When BSSE and ZPE are taken into account, all methods, except LDA, give computed cohesive energies that are underestimated with respect to the experimental sublimation enthalpy. Dispersion energy, not properly taken into account by DFT methods, plays a crucial role. Such a deficiency also affects dramatically the computed crystalline structure, especially when large basis sets are adopted. We show that this is an artifact due to the BSSE. Indeed, with small basis sets the BSSE gives an extra-binding that compensates for the missing dispersion forces, thus yielding structures in fortuitous agreement with experiment.     

Reactivity between CuxV2O5 and AgyV2O5 bronzes studied by spark plasma sintering

A method based on the analysis of reaction layers that form in Cu_xV₂O₅–Ag_yV₂O₅ interdiffusion couples annealed by spark plasma sintering to quickly explore the Cu–Ag–V₂O₅ ternary system at high pressure is presented. Through use of microanalysis profiling, the phases occurring in this system have been obtained much faster than by conventional techniques of solid-state chemistry. Microdiffraction profiling has also been used to properly identify the Cu_0.5Ag_0.5V₂O₅ phase in the reaction layer between CuV₂O₅ and Ag_0.8V₂O₅. The stability domains of the phases have been approximately determined and interpreted. In most cases, reaction kinetics occurs quickly, as expected by the high diffusion coefficient of Cu and Ag in V₂O₅. Though the experiments have been carried out under high pressure (75 MPa), the same phases are obtained than with sealed quartz tubes experiments.     

Determination of free and protein-bound glutathione in HepG2 cells using capillary electrophoresis with laser-induced fluorescence detection

A rapid method using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) was developed to determine free and protein-bound glutathione (GSH) in human HepG2 hepatocarcinoma cells. The samples were derivatized with 5-iodoacetamidofluorescein (5-IAF), and analyzed at 22 kV using sodium phosphate buffer (10mM, pH 11.4) and an uncoated 58 cm x 75 microm I.D. fused silica capillary. The analysis time was less than 10 min and N-acetylcysteine was used as internal standard. The derivatization conditions, such as reaction time, 5-IAF concentration, running buffer and cartridge temperature were optimized. Argon gas was used in the study to prevent the oxidization of GSH during sample preparation. The optimized method required only 30-40 nl sample per analysis and was fast and sensitive. The method was applied to the analyses of HepG2 cells treated with the small metal chelating agent, pyrrolidine dithiocarbamate (PDTC). The results demonstrate that the amount of protein-bound GSH, which reflects the amount of protein S-glutathionylation, increased in a time-dependent manner upon cell treatment with PDTC, reaching a maximum of over 50% increase 2h post-PDTC.     

Fatigue Crack Growth Behaviour in Friction Stir Welded Aluminium–Lithium Alloy Subjected to Biaxial Loads

Experimental Techniques - In this article, biaxial load fatigue crack growth tests are reported. Specimens were made of an advanced aluminium–lithium alloy AA2198-T8 joined by the friction...     

Visualization of Protein‐Specific Glycosylation inside Living Cells

Protein glycosylation is a ubiquitous post‐translational modification that is involved in the regulation of many aspects of protein function. In order to uncover the biological roles of this modification, imaging the glycosylation state of specific proteins within living cells would be of fundamental importance. To date, however, this has not been achieved. Herein, we demonstrate protein‐specific detection of the glycosylation of the intracellular proteins OGT, Foxo1, p53, and Akt1 in living cells. Our generally applicable approach relies on Diels–Alder chemistry to fluorescently label intracellular carbohydrates through metabolic engineering. The target proteins are tagged with enhanced green fluorescent protein (EGFP). Förster resonance energy transfer (FRET) between the EGFP and the glycan‐anchored fluorophore is detected with high contrast even in presence of a large excess of acceptor fluorophores by fluorescence lifetime imaging microscopy (FLIM).     

Sterically Active Electron Pairs in Lead Sulfide? An Investigation of the Electronic and Vibrational Properties of PbS in the Transition Region Between the Rock Salt and the α-GeTe-Type Modifications

Recently, we have investigated the energy landscape of PbS for many different pressures on the ab initio level by using Hartree-Fock and density functional theory to globally search for possible thermodynamically stable and metastable structures. The perhaps most fascinating observation was that besides the experimentally known modification exhibiting the rock salt structure a second minimum exists close-by on the landscape showing the low-temperature α-GeTe-type structure. In the present study, we investigate the possible reasons for the existence of this metastable modification; in particular we address the question, whether the α-GeTe-type modification might be stabilized (and conversely the rock salt modification destabilized) by steric effects of the non-bonding electron pair.     

Turing patterns in the chlorine dioxide-iodine-malonic acid reaction with square spatial periodic forcing

We use the photosensitive chlorine dioxide-iodine-malonic acid reaction-diffusion system to study wavenumber locking of Turing patterns to two-dimensional "square" spatial forcing, implemented as orthogonal sets of bright bands projected onto the reaction medium. Various resonant structures emerge in a broad range of forcing wavelengths and amplitudes, including square lattices and superlattices, one-dimensional stripe patterns and oblique rectangular patterns. Numerical simulations using a model that incorporates additive two-dimensional spatially periodic forcing reproduce well the experimental observations.     

Atomistic modeling of thermodynamic equilibrium and polymorphism of iron

We develop two new modified embedded-atom method (MEAM) potentials for elemental iron, intended to reproduce the experimental phase stability with respect to both temperature and pressure. These simple interatomic potentials are fitted to a wide variety of material properties of bcc iron in close agreement with experiments. Numerous defect properties of bcc iron and bulk properties of the two close-packed structures calculated with these models are in reasonable agreement with the available first-principles calculations and experiments. Performance at finite temperatures of these models has also been examined using Monte Carlo simulations. We attempt to reproduce the experimental iron polymorphism at finite temperature by means of free energy computations, similar to the procedure previously pursued by Müller et al (2007 J. Phys.: Condens. Matter 19 326220), and re-examine the adequacy of the conclusion drawn in the study by addressing two critical aspects missing in their analysis: (i) the stability of the hcp structure relative to the bcc and fcc structures and (ii) the compatibility between the temperature and pressure dependences of the phase stability. Using two MEAM potentials, we are able to represent all of the observed structural phase transitions in iron. We discuss that the correct reproductions of the phase stability among three crystal structures of iron with respect to both temperature and pressure are incompatible with each other due to the lack of magnetic effects in this class of empirical interatomic potential models. The MEAM potentials developed in this study correctly predict, in the bcc structure, the self-interstitial in the (110) orientation to be the most stable configuration, and the screw dislocation to have a non-degenerate core structure, in contrast to many embedded-atom method potentials for bcc iron in the literature.