Effect of Re addition on the crystallization,heat treatment and structure of the Cu–Ni–Si-Cr alloy

Differential scanning calorimetry (DSC) was used to investigate the thermal behavior and non-isothermal crystallization kinetics of the Fe₆₇Nb₅B₂₈ metallic glasses prepared by melt-spinning method. DSC traces exhibit that the crystallization takes place through a single exothermic reaction, and it processes a good thermal stability in thermodynamics. The activation energies for nucleation and grain growth processes were calculated to be 536 ± 22 and 559 ± 20 kJ mol^?1 by Kissinger equation, respectively, and 551 ± 24 and 574 ± 20 kJ mol^?1 by Ozawa equation, respectively. It means that the grain growth process is more difficult than the nucleation process. The variation of local Avrami exponent n(x) with crystallized fraction x demonstrates that the crystallization mechanism varies at different stages. The n(x) is larger than 2.5 at the initial stage of 0 < x < 0.3, implying a mechanism of diffusion-controlled three-dimensional growth with increasing nucleation rate. The n(x) decreases from 2.5 to 1.5 in the range of 0.3 < x < 0.65, suggesting that the crystallization belongs to three-dimensional nucleation and grain growth with decreasing nucleation rate. And n(x) lies between 1.0 and 1.5 in the range of 0.65 < x < 0.95, indicating that the crystallization corresponds to the growth of particles with an appreciable initial volume. Low-temperature annealing corresponds to the precipitation of α-Fe, Fe₂B, and Fe₂₃B₆ phases, and further annealing leads to the formation of α-Fe, Fe₂B, and FeNbB phases. The magnetic properties in relation to microstructure change of the Fe₆₇Nb₅B₂₈ metallic glasses are discussed.     

Determination of microgram amounts of azide by gas chromatography

Azide ions (5–120 μg) are oxidized by cerium(IV) in an acidic medium. The nitrogen evolved is displaced by helium carrier and quantified by gas chromatography with a thermal conductivity detector. The relative standard deviation is <5%. Many sulphur anions, cyanide and some metal ions which form azide complexes do not interfere.     

Mechanical and thermal properties of green polylactide composites with natural fillers

Green composites of PLA with micropowders derived from agricultural by-products such as oat husks, cocoa shells, and apple solids that remain after pressing have been prepared by melt mixing. The thermal and mechanical properties of the composites, including the effect of matrix crystallization and plasticization with poly(propylene glycol), have been studied. All fillers nucleated PLA crystallization and decreased the cold-crystallization temperature. They also affected the mechanical properties of the compositions, increasing the modulus of elasticity but decreasing the elongation at break and tensile impact strength although with few exceptions. Plasticization of the PLA matrix improved the ductility of the composites.     

Competition between the albumin and three histidine fragment of amyloid precursor protein sites to bind Cu(II)

The aim of this work was to check experimentally the relationship between the five-nitrogen donor system {3 × N_imid, 2 × N⁻} seen e.g. in the peptide fragments of the cysteine-rich amyloid precursor protein (APP) region and the albumin-like {NH₂, 2 × N⁻, N_imid} coordination site. The protected and unprotected octadecapeptides DAHQERMDVSETHLHWHT and Ac-DAHQERMDVSETHLHWHT-NH₂ were synthesized and potentiometric and spectroscopic studies were performed. A comparison of both metal-binding sites that occur in both peptides clearly shows that in the unprotected ligand albumin-like binding is much more efficient than the three His site, although around pH 5 both sites have a comparable ability to bind the Cu(II) ion. However, a comparison of the protected and unprotected peptides with their metal binding sites clearly shows that the three His site is very efficient in binding Cu(II) although less effective than the albumin-like motif.     

Tuning properties of silver particle monolayers via controlled adsorption-desorption processes

Nanoparticle self-assembly at fluid-fluid interfaces has been traditionally exploited in emulsification, encapsulation and oil recovery, and more recently in emerging applications including functional nanomaterials and biphasic catalysis. We provide a review of the literature focusing on the open challenges that still hamper the broader applicability of this potentially transformative technology, and we outline strategies to achieve improved control over interfacial self-assembly of nanoparticles. First, we discuss means to promote spontaneous adsorption by tuning the interfacial energies of the nanoparticles with the fluids using capping ligands, and the occurrence of energy barriers. We then examine the interactions between interfacial nanoparticles and how they affect the formation of equilibrium interfacial suspensions versus non-equilibrium two-dimensional phases, such as weakly attractive glasses and gels. Important differences with colloidal interactions in a bulk suspension arise due to the discontinuity in solvent properties at the interface. For instance, ligand brushes rearrange in asymmetric configurations, and thus play a significant role in determining interparticle interactions. Finally, we briefly discuss the link between interfacial microstructure and the dynamic response of particle-laden interfaces, including interfacial rheology and the fate of nanoparticle monolayers upon out-of-plane deformation.     

Synthesis of betulin derivatives and the determination of their relative lipophilicities using reversed‐phase thin‐layer chromatography

A series of superlipophilic or highly lipophilic semisynthetic betulin derivatives was prepared and their relative lipophilicity was measured by reversed‐phase thin‐layer chromatography (RP‐TLC) at different pH values using 1,4‐dioxane–acetate buffer mixtures as mobile phases. Cholesterol, 17β‐estradiol and pure betulin were used as the reference compounds. Linear relationships were found between R_M values and 1,4‐dioxane concentrations in the mobile phases. LogP values were also calculated with computer programs ACD/LogP (ChemSketch 11.0, Advanced Chemistry Development Inc.) and ClogP (Daylight Chemical Information Systems Inc.). The empirical and theoretical data were compared, and the R_M0 values correlated well with logP. Two of the synthesized betulin derivatives are reported for the first time. Copyright © 2009 John Wiley & Sons, Ltd.     

Selective cleavage of the linear ether bond in benzyl glycidyl ether and triphenylmethyl glycidyl ether by potassium alkalide as two-electron-transfer reagent

The linear ether bond was exclusively cleaved in benzyl glycidyl ether and triphenylmethyl glycidyl ether under the influence of K⁻, K⁺(15-crown-5)₂ (1), whereas the strongly strained three-membered oxacyclic ring remained undisturbed. Potassium glycidoxide and benzylpotassium were found as the primary reaction products of benzyl glycidyl ether with 1. Subsequently, benzylpotassium reacted with benzyl glycidyl ether giving the next potassium glycidoxide molecule and bibenzyl. Benzyl phenyl ether was used as a model compound to explain the mechanism of bibenzyl formation. The reaction of triphenylmethyl glycidyl ether with 1 resulted in potassium glycidoxide and stable triphenylmethylpotassium. After treating with a quenching agent a new glycidyl ether or glycidyl ester was obtained from potassium glycidoxide. These results were found when the reaction occurred at the excess of glycidyl ether. In another case, i.e. at the excess of 1 further reactions took place with the participation of potassium anions and various new compounds were observed in the reaction mixture after benzylation or methylation. Thus, the method of substrates delivery influences the course of studied processes in a decisive way.     

A molecular mechanics study of the effect of substitution in position 1 on the conformational space of the oxytocin/vasopressin ring

Summary The effect of the substitution in position 1 on the low-energy conformations of the oxytocin/vasopressin 20-membered ring was investigated by means of molecular mechanics. Three representative substitutions were considered: -mercapto-,-dimethyl)propionic acid (Dmp), (-mercapto-,-cyclopentamethylene)propionic acid (Cpp), both forming strong antagonists, and (,-dimethyl--mercapto)propionic acid (-Dmp), forming analogs of strongly reduced biological activity, with the -mercaptopropionic (Mpa) residue taken as reference. Both ECEPP/2 (rigid valence geometry) and AMBER (flexible valence geometry) force fields were employed in the calculations. Three basic types of backbone conformations were taken into account which are distinguished by the type of -turn at residues 3 and 4: 1/III, II, and I/III, all types containing one or two intra-annular hydrogen bonds. The allowed (ring-closed) disulfide-bridge conformations were searched by an algorithm formulated in terms of scanning the disulfide-bridge torsional angle C-S-S-C. The ECEPP/2 and AMBER energies of the obtained conformations were found to be in reasonable agreement. Two of the low-energy conformers of the [Mpa¹]-compound agreed very well with the cyclic part of the two conformers found in the crystal structure of [Mpa¹]-oxytocin. An analysis of the effect of -substitution on relative energies showed that the conformations with the N-C-CH₂-CH₂ (₁) and C-CH₂-CH₂-S (₁) angles of the first residue around (–100°, 60°) and (100°, –60°) are not affected; this in most cases implies a left-handed disulfide bridge. In the case of -substitution the allowed values of ₁ are close to ± 60°. This requirement, being in contradiction to the one concerning -substitution, could explain the very low biological activity of the -substituted analogs. The conformational preferences of substituted compounds can largely be explained by the analysis of local interactions within the first residue. Based on the selection of the conformations which are low in energy for both the reference and -substituted compounds, two distinct types of possible binding conformations were proposed, the first one being similar to the crystal conformer with a left-handed disulfide bridge, the second one having a right-handed bridge, but a geometry different from that of the crystal conformer with the right-handed bridge. The first type of disulfide-bridge arrangement is equally favorable for both I/III and II types of backbone structure, while the second one is allowed only for the II type of backbone. No conformation of the I/III type has a low enough energy to be considered as a possible binding conformation for all of the active compounds studied in this work.