Determination of odour-causing volatile organic compounds in cork stoppers by multiple headspace solid-phase microextraction

Multiple headspace solid-phase microextraction (MHS-SPME) coupled with gas chromatography-mass spectrometry has been applied in order to determine 2,4,6-trichloroanisole (2,4,6-TCA), guaiacol, 1-octen-3-ol and 1-octen-3-one in three samples of cork stoppers. These compounds are responsible for cork taint in wine and can modify the organoleptic properties of bottled wine. Variables such as temperature, addition of water, extraction time, and amount of cork were studied. The extractions were performed with a 50/30 microm divinylbenzene-carboxen-polydimethylsiloxane (DVB-CAR-PDMS) fibre for 45 min at 100 degrees C using 20 mg of cork. For calibration, 50 microL of VOC aqueous solutions were used and the extraction were carried out for 45 min at 75 degrees C. The limits of detection of the method expressed as ng of VOC per g of cork were 0.3 for 2,4,6-TCA, 7.5 for guaiacol, 1.7 for 1-octen-3-one and 1.9 for 1-octen-3-ol. Relative standard deviation of replicate samples was less than 10%. Significant losses of analytes were observed when the samples were ground at room temperature. Finally, a recovery study was performed and the MHS-SPME results were validated using Soxhlet extraction results.     

Formation of palladium silicide by rapid thermal annealing

The formation of palladium silicide Pd₂Si by rapid thermal annealing of Pd layers on silicon has been studied as a function of annealing time (1–60s) in the temperature range 350–500 °C. It is shown that the results found for conventional furnace annealing (long duration, low temperature) can be extrapolated for rapid thermal annealing (shorter time, higher temperature) when taking into account the exact time dependence of the short temperature cycle. The growth rate is essentially diffusion limited and the activation energy is close to 1.1±0.1 eV. Silicide resistivity of about 30–40 cm was obtained for 200–400 nm thick Pd₂Si layers formed at 400 °C for a few seconds.     

Enzymatic kinetic resolution and chemoenzymatic dynamic kinetic resolution of delta-hydroxy esters. An efficient route to chiral delta-lactones

A successful kinetic resolution of a racemic mixture of delta-hydroxy esters 1 was obtained via lipase-catalyzed transesterification (E value up to 360). The combination of the enzymatic kinetic resolution with a ruthenium-catalyzed alcohol racemization led to an efficient dynamic kinetic resolution (ee up to 99% and conversion up to 92%). The synthetic utility of this procedure was illustrated by the practical syntheses of delta-lactones (R)-6-methyl- and (R)-6-ethyl-tetrahydropyran-2-one and (S)-5-(tert-butyldimethylsiloxy)heptanal. The former are important building blocks in the synthesis of natural products and biologically active compounds, and the latter is a key intermediate in the synthesis of widely used commercial insecticide Spinosyn A.     

Dimers of 1,8a-Dihydro-1,8-Naphthyridine Derivatives as Models of Chiral Self-Recognition

A theoretical study of the dimer formation of chiral 1,8a-dihydro-1,8-naphthyridine derivatives has been carried out by means of DFT calculations. In the cases treated, the heterochiral dimers (RS or SR) are always more stable than the homochiral ones (RR or SS). Two possible proton transfer processes have been studied, the concerted and the non-concerted ones. The non-concerted TS corresponds to a true TS while the concerted one presents two imaginary frequencies. The geometrical characteristics of the hydrogen bonds in all the structures calculated have been correlated using the Steiner–Limbach model.     

Gold nanoparticle decoration of DNA on silicon

Electrostatic assembly of cationic nanoparticles onto the negatively charged backbone of double-stranded DNA has been shown to produce one-dimensional chains with potential use as nanoelectronic components. In this paper, micron long DNA templates stretched on aminosilane- and hexamethyldisilazane-modified silicon surfaces are used to assemble 3.5 nm gold nanoparticles passivated with cationic thiocholine. Atomic force microscopy is used to analyze the density and defects along the approximately 5 nm high structures, with comparison between positively charged and neutral surfaces. Low background adsorption of nanoparticles is facilitated by both these surface chemistries, while the neutral surface yields a more densely packed assembly.     

TGA–MS degradation studies of some new aliphatic–aromatic polybenzimidazoles

The degradation of a series of polybenzimidazoles containing metylene groups in the chain was studied by TGA and MS. On heating the polymer derived from m-phenylenediacetic acid and 3,3′-diaminobenzidine to about 560°C in argon, partial fast decomposition into a solid sublimate was observed. The sublimate consisted of a mixture of fractions derived from the breaking of the chain at the methylene bonds. These methylene groups can be preoxidized at 250°C to carbonyl groups, eliminating thereby this mode of decomposition. At temperature higher than 560°C, gaseous products similar to those of the corresponding all-aromatic PBI are obtained.     

Activity coefficients for NaCl in ethanol-water mixtures at 25°C

Activity coefficient values for NaCl in ethanol-water mixtures containing 20, 40, 60, 70, 80 and 90 weight % ethanol are calculated from the emf for the galvanic cell $$Na - glass|NaCl(m), H_2 O (100 - Y), EtOH(Y), AgCl|Ag$$ in which the weight % of the corresponding solvent is indicated in parenthesis. The results obtained are analyzed by using different theoretical models. Chemical models which take into account the presence of ion pairs in solutions of high ethanol content are also used. There is good agreement between the results obtained from all the models.     

On the origin of the thermostabilization of proteins induced by sodium phosphate

The B1 domain of protein L shows a linear rise in thermostability with increasing concentrations of sodium phosphate. Equal behavior is observed for a set of mutant proteins where surface lysines are mutated to noncharged residues, but the mutant's thermostabilities show different sensitivities to phosphate, encoded in the varying slopes observed (mi). The melting temperature in the absence of the cosolute also correlates linearly with mi. The stabilizing effect of the phosphate ion reaches a saturation point, which has been experimentally determined for protein L (1610 mM phosphate). These results indicate that the phosphate-induced stabilization is an inherent property of the protein, encoded in the amino acid sequence. Changes in stability upon mutation are attributed to a redistribution of the overall network of solvated surface charges. Stabilization by phosphate is understood in terms of interactions with the protein surface, reducing the unfavorable contacts between like charges, maximizing the number of accessible conformations of the surface-charged side chains, and optimizing solvation.     

Purification, subunit structure and inhibitor profile of cathepsin A.

Cathepsin A (EC 3.4.16.1), a lysosomal carboxypeptidase, has been purified 1374-fold from pig kidney. Purification steps included concanavalin A-Sepharose and phenyl-Sepharose chromatography and chromatofocusing. The specific activity (16.9 U/mg) of the purified enzyme was significantly higher than previously reported values. The enzyme preparation appeared homogeneous when analyzed by non-denaturing polyacrylamide gel electrophoresis and was free of detectable protease contamination. The molecular mass (M(r) = 97,000), isoelectric point (5.0), and sensitivity to inhibitors were consistent with reported properties of cathepsin A. However, the previously reported three-peptide chain structure was not observed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence or absence of 2-mercaptoethanol demonstrated that the enzyme is composed of two M(r) 47,000 subunits, each of which dissociate in the presence of 2-mercaptoethanol into two polypeptide chains of 19,000 and 31,000.     

Large chiral recognition in hydrogen-bonded complexes and proton transfer in pyrrolo[2,3-b]pyrrole dimers as model compounds

The chiral recognition in the formation of hydrogen-bonded (HB) dimers of 1,6a-dihydropyrrolo[2,3-b]pyrrole derivatives as well as in their proton-transfer processes have been studied by means of ab initio calculations. The heterochiral dimers are in general the most stable ones, but amphiprotic substituents that are able to form attactive interactions with twin groups revert this tendency. Energy differences up to 4.0 kcal/mol have been found favoring the homo- or heterochiral complexes. Two possible proton-transfer processes have been studied, the concerted one and the nonconcerted one. The compresion of the systems in the transition structures produce an increase in the energetic differences when compared to the corresponding minima complexes. A Steiner-Limbach relationship has been found for the geometrical properties of the HB in the minima and transition states calculated here. The electron density and its Laplacian at the bond critical point have been found to correlate with the HB distance.