Composition analysis of an ethylene–propylene block copolymer by fractionation and turbidity measurements at the lower critical solubility gap

The composition of an ethylene-propylene block copolymer with a nominal 15% content of ethylene by weight (EP?B15%) has been investigated through fractionation near the lower critical solution temperature (LCST). Observation of the solution heated above the boiling point of the solvent indicated that some polymer was phase separating, apparently continuously, between the LCSTs of polyethylene (PE) an polypropylene (PP) of similar molecular weight. IR and DSC analysis of three fractions obtained by twice separating the concentrated phase from the dilute phase gave the following result: EP?B15% consists of an ethylene-rich block copolymer (93% E), an EP copolymer of intermediate composition, and a propylene-rich copolymer (94% P). The three fractions constitute respectively 12%, 12%, and 76% of the total weights. In order to choose a suitable temperature for fractionation, a turbidity analysis of the solution of the initial polymer is made continuously during phase separation. The trace of turbidity against temperature shows three peaks of turbidity at temperatures T₀, T₁ and T₂, which can be associated with the above fractions. A mixture of PE, PP, and a 33% E random copolymer gives a turbidity trace with characteristic temperatures very similar to that of EP-B15% in the same solvent. Fractionation from several solvents or mixtures of solvents indicated that the composition of the fractions did not depend significantly on the nature of the solvent. Conditions for obtaining a quantitative analysis of a mixture from a thermogram are discussed. Turbidity analysis during phase separation and fractionation at the LCST can be a useful tool in analyzing and separating complex mixtures before use of powerful analytical techniques such as NMR or IR spectroscopy.     

Apparent molar heat capacities and volumes of some alkylated derivatives of uracil and adenine in aqueous solution at 25°C

Densities and apparent molar heat capacities of some alkylated derivatives of uracil and adenine: 1-methyluracil, 1,3-dimethyluracil, 1,3-diethylthymine, 5,6-trimethylene-1,3-dimethyluracil, 5,6-tetramethylene-1,3-dimethyluracil, 5,6-pentamethylene-1,3-dimethyluracil, 2,9-dimethyladenine, 2-ethyl-9-methyladenine, 2-propyl-9-methyladenine, 8-ethyl-9-methyladenine, 6,8,9-trimethyladenine and 8-ethyl-6,9-dimethyladenine were determined using flow calorimetry and flow densimetry at 25°C. It was found that the partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH ₂ -as well as to the number of hydrogen atoms, n _H , belonging to the skeleton of the molecule. In the case of alkylated uracils a difference was observed in the values at infinite dilution V ₂ ^o and C _p2 ^o , depending on the substitution of alkyl and cyclooligomethylene groups.     

Molecular Interactions of Macrocycles with Dipeptides in Aqueous Solutions. Partial Molar Volumes and Heat Capacities of Transfer of a Chiral 18-Crown-6 and of a Calix[4]resorcinarene Derivative from Water to Aqueous Dipeptide Solutions at 25°C

Densities and specific heat capacities of ternary aqueous systems containing a dipeptide (alanyl-alanine, alanyl-glutamic acid, alanyl-serine or L-seryl-L-leucine) and a macrocycle (D--manno-naphtho-18-crown-6-ether or 2,8,14,20-tetrakis[-methyl (aminoformyl)]-4,6,10,12,16,18,22,24-octahydroxycalix[4]arene) were determined at 25°C by flow densimetry and flow calorimetry. The partial molar volume and heat capacity of transfer of a macrocycle from water to the dipeptide solution was determined as a function of the dipeptide concentration. Positive values for transfer volumes and transfer heat capacities are observed with all the solutions studied. With the crown ether, except for alanyl-glutamic acid where a 1:1 complex is clearly evidenced due to specific interactions of the side-chain functional group of the peptide with the crown ether, no stoichiometric complexes are confirmed and the partial molar quantities of transfer increase with the hydrophobic character of the dipeptide. Partial quantities of transfer are smaller with the calixarene than with the crown ether and stoichiometric complexes [calixarene]/[dipeptide] from 2:1 to 1:4 are evidenced, depending on the nature and the concentration of the dipeptide.     

A generalized variety problem

Semigroup Forum - Let $${\mathscr {C}}\! om $$ denote the variety of all commutative semigroups. For $$n\ge 1$$ let $${{\mathscr {N}}\! il }_n$$ (respectively $${\mathscr {N}}_n$$ ) denote the...     

Ultrasonic welding of CF/PPS composites with integrated triangular energy directors: melting,flow and weld strength development

This paper presents a fully experimental study on melting, flow and weld strength development during ultrasonic welding of CF/PPS composites with integrated triangular energy directors. The main goal of this research was assessing whether the heating time to achieve maximum weld strength could be significantly reduced as compared to ultrasonic welding with flat energy directors. The main conclusion is that, in the specific case under study, the triangular energy directors did heat up, melt and collapse approximately two times faster than the time it took for the flat energy directors to melt and significantly flow. However the heating time needed to achieve maximum weld strength for the integrated triangular energy directors did not differ drastically from that for flat energy directors. This was caused by the fact that a fully welded overlap was not directly achieved right after the collapsing of the triangular energy directors. Instead a solidified resin-rich interface was created which needed to be re-melted as a whole in order to achieve a fully welded overlap and hence maximum weld strength.     

Reduction of Tetrachloroaurate(III) at Boron-Doped Diamond Electrodes: Gold Deposition Versus Gold Colloid Formation

Tetrachloroaurate(III) dissolved in dilute aqueous aqua regia is electrochemically reduced at boron-doped diamond electrodes to form gold metal. The reduction process is studied by voltammetric, SEM, and XPS techniques. Both the deposition of gold and the anodic stripping process are detected. The ratio of cathodic to anodic charge or stripping efficiency, Q_anodic/Q_cathodic, is shown to depend on the concentration of AuCl and on the pretreatment of the boron-doped diamond electrode surface. Cathodic pretreatment of the boron-doped diamond electrode considerably increases the rate for both deposition and stripping. In the presence of power ultrasound emitted from a glass horn system (24 kHz, 8 Wcm⁻²) the current associated with the reduction of AuCl is considerably enhanced and two components in the mass transport controlled limiting current are identified as (i) the deposition of gold onto the boron-doped diamond and (ii) the formation of colloidal gold.     

High-throughput capillary electrophoresis for the identification and differentiation of seven species of Eimeria from chickens

A capillary electrophoretic approach has been evaluated for the identification of seven currently recognised species of Eimeria infecting chickens. The second internal transcribed spacer of ribosomal DNA is PCR-amplified from any of the seven species using a single set of oligonucleotide primers (one of which is fluorescently labelled). The amplicons are heat-denatured and subjected to capillary electrophoresis in a MegaBACE 1000 (Amersham). The chromatograms captured are stored electronically and then analysed using MegaBACE Fragment Profiler software. Using control DNA samples representing monospecific lines of Eimeria, specific peaks in the chromatograms were defined for the unequivocal identification of each of the seven species and their differentiation. Electrophoretic reading and analysis are carried out automatically, thus making it a time- and cost-effective method. This procedure should find applicability as a tool for the quality control of Eimeria vaccines, the monitoring of coccidiosis outbreaks and the high-throughput analysis of oocyst samples for epidemiological surveys.     

Cleavage of silicon- and germanium-transition metal bonds. Dependences of the stereochemistry on the nature of the ligands and the geometry of complexes

The cleavages of some new optically active complexes containing CoSi (orGe), MnSi (orGe), ReGe and WGe bonds are described. Electrophiles cleave the CoSi bond with good retention of configuration at silicon, while the MnSi bond is not cleaved under the same conditions. The M′Si and M′Ge bonds (where M'  transition metal) are cleaved by nucleophiles with retention or inversion of configuration. In the case of triginal bipyramidal geometry (cobalt complexes) the stereochemical outcome of the reaction is strongly dependent upon electronic effects, the size of the ligand trans to the CoSi (orGe) bond, and the nature of the nucleophilic reagant, in accord with the general rules for nucleophilic substitution at silicon. In contrast the transition metalsilicon orgermanium bonds in the octahedral complexes of manganese, rhenium and tungsten are always cleaved with poor retention of configuration regardless of the nature of the ligands or the nucleophilic reagent. The results provide the first cases in which the stereochemistry of nucleophilic displacement at silicon is independent of the electronic features of both the leaving group and the nucleophile.     

From simple to complex oscillatory behavior in metabolic and genetic control networks

We present an overview of mechanisms responsible for simple or complex oscillatory behavior in metabolic and genetic control networks. Besides simple periodic behavior corresponding to the evolution toward a limit cycle we consider complex modes of oscillatory behavior such as complex periodic oscillations of the bursting type and chaos. Multiple attractors are also discussed, e.g., the coexistence between a stable steady state and a stable limit cycle (hard excitation), or the coexistence between two simultaneously stable limit cycles (birhythmicity). We discuss mechanisms responsible for the transition from simple to complex oscillatory behavior by means of a number of models serving as selected examples. The models were originally proposed to account for simple periodic oscillations observed experimentally at the cellular level in a variety of biological systems. In a second stage, these models were modified to allow for complex oscillatory phenomena such as bursting, birhythmicity, or chaos. We consider successively (1) models based on enzyme regulation, proposed for glycolytic oscillations and for the control of successive phases of the cell cycle, respectively; (2) a model for intracellular Ca(2+) oscillations based on transport regulation; (3) a model for oscillations of cyclic AMP based on receptor desensitization in Dictyostelium cells; and (4) a model based on genetic regulation for circadian rhythms in Drosophila. Two main classes of mechanism leading from simple to complex oscillatory behavior are identified, namely (i) the interplay between two endogenous oscillatory mechanisms, which can take multiple forms, overt or more subtle, depending on whether the two oscillators each involve their own regulatory feedback loop or share a common feedback loop while differing by some related process, and (ii) self-modulation of the oscillator through feedback from the system's output on one of the parameters controlling oscillatory behavior. However, the latter mechanism may also be viewed as involving the interplay between two feedback processes, each of which might be capable of producing oscillations. Although our discussion primarily focuses on the case of autonomous oscillatory behavior, we also consider the case of nonautonomous complex oscillations in a model for circadian oscillations subjected to periodic forcing by a light-dark cycle and show that the occurrence of entrainment versus chaos in these conditions markedly depends on the wave form of periodic forcing. (c) 2001 American Institute of Physics.     

Two Distinct Structures of Membrane-Associated Homodimers of GTP- and GDP-Bound KRAS4B Revealed by Paramagnetic Relaxation Enhancement

KRAS homo-dimerization has been implicated in the activation of RAF kinases, however, the mechanism and structural basis remain elusive. We developed a system to study KRAS dimerization on nanodiscs using paramagnetic relaxation enhancement (PRE) NMR spectroscopy, and determined distinct structures of membrane-anchored KRAS dimers in the active GTP- and inactive GDP-loaded states. Both dimerize through an α4–α5 interface, but the relative orientation of the protomers and their contacts differ substantially. Dimerization of KRAS-GTP, stabilized by electrostatic interactions between R135 and E168, favors an orientation on the membrane that promotes accessibility of the effector-binding site. Remarkably, “cross”-dimerization between GTP- and GDP-bound KRAS molecules is unfavorable. These models provide a platform to elucidate the structural basis of RAF activation by RAS and to develop inhibitors that can disrupt the KRAS dimerization. The methodology is applicable to many other farnesylated small GTPases.