Multivariate optimization of a headspace solid‐phase microextraction method followed by gas chromatography with mass spectrometry for the determination of terpenes in Nicotiana langsdorffii

A simple and sensitive procedure based on headspace solid‐phase microextraction and gas chromatography with mass spectrometry was developed for the determination of five terpenes (α‐pinene, limonene, linalool, α‐terpineol, and geraniol) in the leaves of Nicotiana langsdorffii. The microextraction conditions (extraction temperature, equilibration time, and extraction time) were optimized by means of a Doehlert design. The experimental design showed that, for α‐pinene and limonene, a low temperature and a long extraction time were needed for optimal extraction, while linalool, α‐terpineol, and geraniol required a high temperature and a long extraction time. The chosen compromise conditions were temperature 60°C, equilibration time 15 min and extraction time 50 min. The main analytical figures of the optimized method were evaluated; LODs ranged from 0.07 ng/g (α‐pinene) to 8.0 ng/g (geraniol), while intraday and interday repeatability were in the range 10–17% and 9–13%, respectively. Finally, the procedure was applied to in vitro wild‐type and transgenic specimens of N. langsdorffii subjected to abiotic stresses (chemical and heat stress). With the exception of geraniol (75–374 ng/g), low concentration levels of terpenes were measured (ng/g level or lower); some interesting variations in terpene concentration induced by abiotic stress were observed.     

The Sound of Silence: Minorities,Abstention and Democracy

Despite the existence of an extensive literature, no definitive conclusion seems to emerge on the extent to which minorities are guaranteed by democratic rules in political systems. This paper contributes to this debate by proposing a modified Heigselmann and Krauss two-dimensional model of preferences in order to capture the role of abstention on minority representativeness. Regardless of the typology of abstention, simulation results show that voter abstention always benefits minorities.     

UV spectra and excitation delocalization in DNA: influence of the spectral width.

The singlet excited states of the model DNA duplex (dA)10.(dT)10 are studied. Calculations are performed in the exciton theory framework. Molecular dynamics calculations provide the duplex geometry. The dipolar coupling is determined using atomic transition charges. The monomer transition energies are simulated by Gaussian functions resembling the absorption bands of nucleosides in aqueous solutions. Most of the excited states are found to be delocalized over at least two bases and result from the mixing of different monomer states. Their properties are only weakly affected by conformational changes of the double helix. On average, the highest oscillator strength is carried by the upper eigenstates. The duplex absorption spectra are shifted a few nanometers to higher energies with respect to the spectra of noninteracting monomers. The states with larger spatial extent are located close to the maximum of the absorption spectrum.     

Interaction of Candida parapsilosis isolates with human hair and nail surfaces revealed by scanning electron microscopy analysis

Candida parapsilosis is found frequently as commensal organism on epithelial tissues, and is also an increasing cause of nosocomial infection. Scanning electron microscope (SEM) observations were used to analyse the capability of C. parapsilosis cells to adhere and grow as biofilm on human natural substrates and to compare the adherence pattern of isolates exhibiting distinct phenotypes. Cells from the crepe phenotype are predominantly elongated and form pseudohyphae whereas cells from the smooth phenotype are yeast-shaped, either in liquid cultures or on human nail and hair surfaces. The electron micrographs revealed that C. parapsilosis cells from the smooth phenotype adhered in higher number to both surfaces compared to the observed for the crepe phenotype. SEM analysis of human hair surface revealed that cells from the smooth phenotype appear as clumped blastoconidia of uniform morphology embedded in a flocculent extracellular material forming biofilm. The extracellular material and biofilm were seeing in a less extension in the crepe phenotype. A distinct adherence pattern was observed when human nail was used as substrate. Here C. parapsilosis cells seem to be linked to surface structures of human nail plate. Fibrillar extracellular material was observed connecting neighbouring cells as well as nail surface.     

Liquid-liquid phase separation of a surfactant-solubilized membrane protein

Solubilization of membrane proteins requires surfactants, whose structural properties play a crucial role in determining the protein phase behavior. We show that ionization of a pH-sensitive surfactant, lauryldymethylamino-N-oxide, bound to the bacterial photosynthetic Reaction Center, induces protein phase segregation in micrometric "droplets." Liquid-liquid phase separation takes place in a narrow pH range, is promoted by increasing temperature, and vanishes by adding salt. After a fast initial droplet growth, the nearly arrested kinetics at a later stage leaves the system in a finely divided, long-lasting emulsified state.     

Rare fluctuations of native proteins sampled by equilibrium hydrogen exchange

We present a method for determining the ensembles of native protein structures that result from the large fluctuations of low probability revealed by hydrogen-exchange experiments. The measured protection factors are used to bias Monte Carlo simulations to sample the structures of the exchange competent species. The approach is illustrated by its application to the case of alpha-lactalbumin.     

Questioni di realità sulle curve trigonali reali

Sunto. In relazione a curve trigonali reali (di genere e specie assegnati) vengono svolte alcune questioni di realità, ed in particolare quella concernente il numero dei circuiti della parte reale ed il loro comportamento rispetto alla g ₃ ⁴ il che dà luogo ad una classificazione di tali curve. Sono perciò riprese od introdotte svariate considerazioni che potrebbero anche venire più largamente utilizzate.     

Effects of nonionic,cationic, and anionic micelles on the kinetics of the complexation reaction of nickel (II) with pyridine-2-azo-p-dimethylaniline

The stopped-flow technique has been used to study the effect of cationic (CTAN), nonionic (Triton X-100), andanionic (SDS) micelles on the rate of the reaction between nickel(II) ion and the ligand pyridine-2-azo-p-dimethylaniline (PADA) at 20.0°C and ionic strength 0.03 mol dm^?3. The complex formation reaction is markedly inhibited by both CTAN and Triton X-100 micelles. The kinetic dataare found to conform to a reaction mechanism which implies only partitioning of the ligand between water and the micellar phase, the estimated bindingconstant of PADA being significantly larger in the presence of CTAN aggregates. Anionic micelles strongly speed the complexation reaction, Which occurs in the micellar phase with the same rate and the same mechanism as in water. The extent of binding of PADA to anionic micelles is similar to that found for the cationic micellar aggregates.     

A scattering polarimeter for X-ray astronomy

Summary We describe a high sensitivity X-ray Thomson polarimeter for astrophysical research, particularly designed for the XMM mission. The efficiency, the modulation factor and the performances of this instrument have been estimated by means of a Monte Carlo simulation. The predicted sensitivity for the high eccentricity Orbit of XMM should allow the detection of the Crab Nebula polarization (3σ, (5÷11)keV) in 15 seconds only. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.