The relative permittivity of 1,2-dimethoxyethane and N,N-dimethylformamide mixtures from −10 to 40°C

Binary solutions of N,N-dimethylformamide and 1,2-dimethoxyethane have been investigated by means of dielectric measurements at temperatures ranging from –10 to +40°C, and for nine mixtures covering the whole miscibility field expressed by the mole fraction of one component (0X₁1). The experimental data were used to study the dependence of on T and X₁, of the type = (T), = (X₁), and = (T,X₁). Further, the excess mixing function _E has been evaluated in order to identify particular patterns of interaction between unlike molecules and any other factor that could modify such patterns. The minimum in the _E vs. composition plots suggests the formation of an adduct of stoichiometric ratio DMFDME=11 at all the investigated temperatures.     

Evolutionarily conserved functional mechanics across pepsin-like and retroviral aspartic proteases

The biological function of the aspartic protease from HIV-1 has recently been related to the conformational flexibility of its structural scaffold. Here, we use a multistep strategy to investigate whether the same mechanism affects the functionality in the pepsin-like fold. (i) We identify the set of conserved residues by using sequence-alignment techniques. These residues cluster in three distinct regions: near the cleavage-site cavity, in the four beta-sheets cross-linking the two lobes, and in a solvent-exposed region below the long beta-hairpin in the N-terminal lobe. (ii) We elucidate the role played by the conserved residues for the enzymatic functionality of one representative member of the fold family, the human beta-secretase, by means of classical molecular dynamics (MD). The conserved regions exhibit little overall mobility and yet are involved into the most important modes of structural fluctuations. These modes influence the substrate-catalytic aspartates distance through a relative rotation of the N- and C-terminal lobes. (iii) We investigate the effects of this modulation by estimating the reaction free energy at different representative substrate/enzyme conformations. The activation free energy is strongly affected by large-scale protein motions, similarly to what has been observed in the HIV-1 enzyme. (iv) We extend our findings to all other members of the two eukaryotic and retroviral fold families by recurring to a simple, topology-based, energy functional. This analysis reveals a sophisticated mechanism of enzymatic activity modulation common to all aspartic proteases. We suggest that aspartic proteases have been evolutionarily selected to possess similar functional motions despite the observed fold variations.     

Synthesis of a ditopic cyclophane based on the cyclobutane ring by chalcone photocycloaddition

The intramolecular photocycloaddition of chalcones to give cyclobutanes has proven to be a fast and simple method to shrink a cyclophane ring to a tricyclic system, in order to prepare potential ditopic receptors. In particular, the chalcone 1, having dioxyethylene chains as spacers, is converted in high yield to the cyclobutane 2. NOESY spectroscopy indicates that the formation of 2 occurs by a head-to-head syn ring closure.     

ASV determination of cadmium complexation in seawater. Methodology evaluation

The methodology for using DPASV to study cadmium complexation in seawater is evaluated using EDTA as a model ligand and by analysing natural samples. The results show that the methodology gives an accurate evaluation of metal complexation when inert complexes are studied, both as regards the ligand concentration and the conditional stability constant; the error for both the parameters is lower than 10% at a ligand concentration of about 10(-8) M and a conditional stability constant of 10(9) M-1. Cadmium complexes with ligands present in natural seawater show an evident kinetic lability that may lead to underestimation of the conditional stability constant when a working electrode characterised by a very thick diffusion layer is used. The conditional stability constant in one water sample of the Adriatic coast ranged between 0.14 and 1.4 l/nmol using a rotating disk electrode at rotation rates of 300 and 6000 rpm. The results of cadmium complexation obtained for samples collected in coastal seawater show that the ligands present low specificity for the metal.     

A general NMR method for rapid, efficient, and reliable biochemical screening

High-throughput screening is usually the method of drug-lead discovery. It is now well accepted that, for a functional assay, quality is more important than quantity. The ligand-based or protein-based NMR screening methodologies for detecting compounds binding to the macromolecular target of interest are now well established. A novel and sensitive NMR method for rapid, efficient, and reliable biochemical screening is presented. The method named 3-FABS (three fluorine atoms for biochemical screening) requires the labeling of the substrate with a CF(3) moiety and utilizes (19)F NMR spectroscopy for the detection of the starting and enzymatically modified substrates. The method allows for high-quality screening of large compound or natural product extract collections and for measuring their IC(50) values. Applications of this technique to the screening of inhibitors of the Ser/Thr kinase AKT1 and the protease trypsin are presented. In addition, an interesting application of 3-FABS to functional genomics is also presented.     

Structural characterization techniques for the analysis of semiconductor strained heterostructures

A combined method for structural characterization of strained epitaxial heterostructures involving different techniques such as Rutherford backscattering spectrometry (RBS), multiple crystal X-ray diffractometry (MCD) and transmission electron microscopy (TEM) is presented. In order to obtain a complete characterization of the analysed structure, three different quantities are measured independently: the epilayer thickness, the density of misfit dislocations which may appear at the interface, and the significant components of the strain tensor, mainly the tetragonal distortion, affecting the epilayer lattice. In this way the thermodynamic state and the mechanisms of plastic deformation of the structures can be fully investigated. In this contribution we present and discuss the experimental results concerning a set of InP/GaAs samples having different layer thicknesses ranging from 5 to 500 nm. The thickness of the samples has been determined by RBS. Measurements of in-plane strain and tetragonal distortion have been performed by MCD and RBS-channelling respectively, finally TEM has been used for determining the defects densities and distribution.     

Volumetric Properties of Binary Mixtures of Isomeric Butanols and C8 Solvents at 298.15 K

Excess molar volumes, V _m ^E, over the whole composition range for binary mixtures of 1-butanol, 2-butanol, and 2-methyl-2-propanol + 1-octanol, or 2-octanol, or di-n-butyl ether, or n-hexylacetate were determined at 298.15 K from density measurements carried out with a vibrating-tube densimeter. Small V _m ^E values, both positive and negative, are displayed by mixtures containing 1- or 2-octanol, whereas positive and larger values are always found for mixtures containing dibutyl ether and hexylacetate. These results can be justified in terms of H-bonding interactions and/or steric hindrance due to the branched alkyl chains. Partial molar volumes at infinite dilution of the isomeric butanols in the C8 compounds were also calculated from the apparent molar volumes in dilute solution. The solute-solvent interactions and the effects of the local organisation of the solvent around the butanol molecules were discussed using the void and cavity volumes as different estimates of the intrinsic volume of the molecules. The volumetric behavior of butanols seems to be determined by the solute-solvent interactions rather than packaging effects.     

Transition metal-catalysed sequences of nucleophilic addition

Summary In presence of nickel or palladium catalysts, nucleophiles can attack vinylcyclopropanes with concomitant ring cleavage. With palladium catalysts in presence of appropriate substituents the terminal carbon atom of the resulting open chain is able to add to two molecules of a conjugated diene giving rise to long-chain unsaturated compounds.     

Self-assembly of electron donor-acceptor dyads into ordered architectures in two and three dimensions: surface patterning and columnar "double cables"

We report the synthesis and characterization of covalent dyads and multiads of electron acceptors (A) and donors (D), with the purpose of exploiting their nanophase separation behavior toward (a) two-dimensional (2D) surface patterning with well-defined integrated arrays of dissimilar molecular electronic features and (b) bulk self-assembly to noncovalent columnar versions of the so-called "double cable" systems, the likes of which could eventually provide side-by-side percolation pathways for electrons and holes in solar cells. Soluble, alkylated hexa-peri-hexabenzocoronenes (HBCs) bearing tethered anthraquinones (AQs) are shown by scanning tunneling microscopy (STM) to self-assemble at the solution-graphite interface into either defect-rich polycrystalline monolayers or extended 2D crystalline domains, depending on the number of tethered AQs. In the bulk, the thermal stability of the room-temperature HBC columnar phase is increased, which is attributed to the desired nanotriphase separation of HBC columns, insulating alkyl sheaths, and AQ units. Homeotropic alignment (columns normal to surfaces), predicted to be ideal for potential exploitation of such "double cables" in photovoltaic devices, is demonstrated.     

Transition metal-catalysed organic reactions promoted by chelating or metallacycle-forming substrates

Chelating or metallacycle-forming substrates are very useful for directing organometallic reactions. This review covers the more recent research that has been carried out in the authors' laboratory. Rhodium(I)- and (III)-catalysed reactions of C---C coupling of butadiene with N-allylamides or N-alkylbutenamides are described. These reactions are controlled by the size and strength of the chelate ring formed by double-bond insertion into the crotyl-rhodium bond (formed from butadiene) and their regioselectivity can change with the oxidation state of the metal. Rhodium(I)-catalysed reactions of butadiene with enamides are also chelation controlled and lead to different products, depending on the substituents at nitrogen. Cobalt(II) metallacycles have been utilized for promoting some organic reactions. It has been shown that alkenes can be catalytically incorporated into cobaltacyclopentadiene rings, that spirocycles can be obtained from diynes, carbon monoxide and acrylic esters and that a Pauson-Khand-type reaction can be combined with a Michael-type reaction to prepare catalytically new cyclopentenones. The use of palladacycles, derived from norbornene insertion into aryl-palladium bonds, followed by cyclization, has allowed the selective functionalization of either end of the metallacycle and the formation of condensed rings. Conversion of a palladium(II) into a palladium(IV) metallacycle, and catalytic processes involving these intermediates, have been achieved. The formation of alkylaromatic palladacycles has also been exploited for the selective meta functionalization of the aromatic moiety by means of alkyl groups, accompanied by expulsion of the norbornene molecule.