New lamellar structure formed by an adamantyl derivative of cholic acid

The self-aggregation of the sodium salt of a new adamantyl amide of the 3beta-amino derivative of cholic acid (Na-AdC) in aqueous solution has been investigated by surface tension, dynamic light scattering, fluorescence, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) measurements. These last two techniques suggest that a lamellar phase, consisting of charged bilayers of Na-AdC separated by solvent and periodically stacked, is formed in aqueous solution. The structure of the bilayer is inferred from the resolution of the crystal of the compound in its acid form. The adamantyl moieties, which are mutually interlocked, reside in the central region of the bilayer, and the carboxylic groups are directed toward the hydrophilic region. The structure is open enough to allow water molecules to interact with a fluorescence probe located at the central hydrophobic region.     

Dual column capillary gas chromatographic system for the in situ analysis of volatile organic compounds on a cometary nucleus

Two Wall Coated Open Tubular capillary columns, coated with poly(cyanopropylphenyl-dimethyl)siloxane and poly(diphenyl-dimethyl)siloxane stationary phases, have been selected for use in the COmetary SAmpling and Composition space experiment for the separation and identification of the wide range of volatile organic compounds which could be present in cometary nuclei. This article presents the main characteristics of the tandem column system for the analysis of solutes of cometary interest within the constraints of space instrumental operating conditions. The high efficiency of the columns is demonstrated and the influence of the operating conditions on their separation properties are investigated. The studied columns exhibit complementary retention pattern: their use in a dual column system makes it possible to achieve the separation and the identification of the compounds of interest. Finally, the good analytical behavior of the columns when analyzing samples which include large amounts of water, the main presumed volatile in comets, is demonstrated. The presented results thus show the suitability of the selected tandem columns system for the desired analyses, and their performance on adaptation to in-situ cometary chemical investigation.     

Early stages of formation of branched host-guest supramolecular polymers

A structural characterization of host-guest supramolecular copolymers, formed by an adamantane dimer and two beta-cyclodextrin trimers in aqueous solution, has been carried out by combining small angle X-ray scattering and light scattering experiments. A shape-reconstruction method was applied to the SAXS data to obtain relatively high-resolution conformation information, and a correlation with the experimental dynamic light scattering results was performed, by estimating the hydrodynamic radii of the reconstructed shape through a shell model method. When applied on the solutions of the trimers, the analysis provides a globular reconstructed shape with a hydrodynamic radius in agreement with the experimental one. For the polymers, elongated structures were inferred which grow both in length and in cross section by increasing the concentration. Depending on the beta-cyclodextrin trimer employed in the polymer preparation, polymerization degrees ranging between roughly 7 and 14 or 9 and 22 were obtained in the concentration range 4.00-10.0 or 3.10-6.60 mM of the trimer (6.00-15.0 or 4.65-9.90 mM of the dimer). Aggregation schemes were proposed accounting for the formation of hyperbranched, linear, and network like polymers. The experimental results are not far from those expected on the basis of the aggregation in hyperbranched structure, for which the growth of elongated aggregates can be predicted in the early stages of the polymerization. However, the coexistence of the other structures, in particular of the linear one, cannot be ruled out.     

Modeling excitable media by a one variable cellular automaton: Application to the cardiac case

The dynamics of an assembly of cardiac cells is modeled by a simple cellular automaton that reduces to a single variable the two variable competition of the standard models of excitable media. Furthermore, a short superexcitability period is introduced, as suggested by the dynamics of the single cardiac miocyte. The model reproduces several pathological cardiac behaviors as, e.g., the fast transition from normal behavior to fibrillation, showing how this latter one can either occur over the whole spatial domain or can be confined within a limited region.     

MHD plasma physics in rail accelerators for hydrogen-pelletinjection in fusion reactors

The behavior of the electromagnetic and thermal quantities in a plasma arc placed between two conducting rails is analyzed. The plasma hydrogen drives the hydrogen pellets for the refueling of magnetic fusion reactors. Considering the general equations of electromagnetism and of plasma fluid dynamics and assuming steady-state conditions in a frame which is moving at the same rate as the plasma arc armature, a one-dimensional model is deduced. The effects of an applied magnetic field on the behavior of all flow variables are investigated. Results indicate that the adverse effects of plasma arc heating can be reduced by the application of a magnetic-induction field normal to the current path in the armature. At the maximum acceleration pressure (30 bar) applicable to the hydrogen pellet in the proposed one-dimensional model, the arc temperature at the pellet backend falls from 20000 to 14000 K when a magnetic induction of about 5 T is applied     

Decoding of complex isothermal chromatograms recovered from space missions. Identification of molecular structure

A chemometric approach, based on the study of the autocovariance function, is described to study isothermal GC chromatograms of multicomponent mixtures: isothermal GC analysis is the method of choice in space missions since it is, to date, the only method compatible with flight constraints. Isothermal GC chromatograms look inhomogeneous and disordered with peak density decreasing at higher retention times: a time axis transformation is proposed to make retention an homogeneous process so that CH2 addition in terms of an homologous series yields a constant retention increment. The time axis is transformed into a new scale based on the retention times of n-alkanes, as they are the basis of the universal Kovats indices procedure. The order introduced into the chromatogram by retention time linearization can be simply singled out by the experimental autocorrelation function (EACF) plot: if constant inter-distances are repeated in different regions of the chromatogram, well-shaped peaks are evident in the EACF plot. By comparison, with a standard mixture it is possible to identify peaks diagnostic of specific molecular structures: study of the EACF plot provides information on sample chemical composition. The procedure was applied to standard mixtures containing compounds representative of the planetary atmospheres that will be investigated in the near future: in particular, those related to Titan's atmosphere (Cassini-Huygens mission) and cometary's nucleus (Rosetta mission). The employed experimental conditions simulated those applied to GC instruments installed on space probes and landers in space missions. The method was applied to two specific investigations related to space research, i.e., a comparison of retention selectivity of different GC columns and identification of the chemical composition of an unknown mixture.     

Thermodynamics of formation of host-guest supramolecular polymers

The interactions between three beta-cyclodextrin hosts (having 1-3 binding sites) and two adamantyl guests (having 1-2 binding sites) have been studied by ITC, ROESY, static and dynamic light scattering (SLS and DLS), and AFM and TEM techniques. The enthalpy and free energy values (determined from ITC experiments) evidence that the single interaction between one binding site of the guest and one binding site of the host is independent of the number of binding sites of the interacting species. The average values are deltaH degrees = -26.6 +/- 2.3 kJ mol(-1) and deltaG degrees = -30.4 +/- 3.2 kJ mol(-1), indicating that the process is mainly enthalpy driven. In all cases, the experimental molar ratio (from ITC experiments) agrees with the expected one from the number of binding sites of both the host and guest. The formation of polymer-like entities was demonstrated by SLS, DLS, AFM, and TEM measurements. The structure of polymers is linear when both the host and the guest are ditopic entities and dendritic (or Cayley tree type) when the host and the guest have three and two binding sites, respectively.