Application of molecular mechanics to refine and understand the crystal structure of polythiophene and its oligomers

A molecular mechanics computational procedure, previously used for the refinement and the analysis of several crystalline polymers, was applied to investigate the crystal structures of the tetramer (T4) and hexamer (T6) of thiophene, as well as the crystal structure of polythiophene (PT). Simultaneous minimization of intra- and intermolecular energies of the T4 and T6 structures, obtained by Rietveld analysis of powder X-ray diffraction profiles, leads to molecular conformations showing smaller deviations from the ring co-planarity than the original models. For both oligomers the calculations confirm that the molecular centre of inversion is not a crystallographic centre of symmetry, as also revealed by X-ray diffraction of the T6 single crystal. This surprising effect appears to arise from intermolecular interactions between the terminal residues, hence is not relevant with respect to the PT polymer structure. The small energy cost for constraining the molecules at the crystallographic centre of symmetry is in agreement with experimental findings that reveal the existence of polymorphs for both T4 and T6. The calculations on the T6 single crystal were used to upgrade the MM2-like force field, which was then used to determine the minimum-energy model of the monoclinic crystal structure of polythiophene.     

Nanostructured thin films obtained by ultra-short pulse laser deposition of vanadium carbide

In this paper we will report the results obtained by femtosecond Pulsed Laser Deposition on vanadium carbide. These results, compared with those obtained for another group 5 carbide, tantalum carbide, evidence large analogies between the two systems. Optical emission spectroscopy shows in both cases the presence of particles in the secondary plume and in both cases the films are formed by nanoparticles and present a stoichiometry corresponding to the hemicarbide.     

Comparison of Sepharose CL-2B and CL-4B Matrices in Purification of the Hepatitis B Surface Antigen by Affinity Chromatography

Sepharose CL-4B and Sepharose CL-2B with immobilized CB.Hep-1 monoclonal antibody were compared for studying the matrix efficiency in the immunopurification of the recombinant hepatitis B surface antigen (rHBsAg). The elution capacities of both matrices were similar over eight chromatographic cycles, significant differences were only observed in the first purification cycles. A high percentage of the adsorbed rHBsAg was not eluted from both matrices. The rHBsAg purity was not affected by matrix characteristics (pore size and percentage of agarose) and differences between Sepharose CL-4B and Sepharose CL-2B do not provoke differences in the antibody released from the matrices under defined experimental conditions.     

Internal transport barriers in plasmas with reversed plasma flow

Evidences of internal particle transport barriers have been observed in plasma discharges with reversed plasma flow. To investigate the influence of the radial electric field profile on these barriers, we apply a drift wave map that describe the plasma particle transport and allows the integration of particle drift in the presence of a given electrostatic turbulence spectrum. With this procedure we show that transport barriers due to the shearless flow invariant lines are created inside the plasma. Moreover, by varying the radial electric field profile, we observe the formation and destruction of internal transport barriers constituted by shearless invariant lines, as well as its effects on the transport in the map's phase space. Applicability of our results are discussed for the Texas Helimak, a toroidal plasma device in which the radial electric field can be changed by application of bias potential.     

Copolymerization of VDF and HFP in Supercritical Carbon Dioxide: A Robust Approach for Modeling Precipitation and Dispersion Kinetics

A kinetic model is developed for the heterogeneous free‐radical copolymerization of vinylidene fluoride and hexafluoropropylene in supercritical CO₂. The model accounts for polymerization in both the dispersed (polymer‐rich) phase and in the continuous (polymer‐free) supercritical phase, for radical interphase transport, diffusion limitations, and chain‐length‐dependent termination in the polymer‐rich phase. A parameter evaluation strategy is developed and detailed to estimate most of the kinetic parameters a priori while minimizing their evaluation by direct fitting. The resulting model predictions compare favorably with the experimental results of conversion and MWD at varying monomer feed composition, monomer concentration, interphase area, and pressure of the system.

     

Sulfonated RAFT Copolymers as Heparin Mimetics: Synthesis,Reactivity Ratios,and Anticoagulant Activity

The glycosaminoglycan heparin is a clinically important anticoagulant drug, primarily used to reduce the risk of blood clots (thrombosis) during surgery. Despite its importance in medicine and its continuous use over many decades, heparin suffers from several limitations associated with its heterogeneity and its extraction from animal tissues. In order to address these limitations, reversible addition‐fragmentation chain transfer polymerization is utilized to prepare a library of heparin mimetic copolymers from the sulfonated monomers sodium 4‐styrene sulfonate, potassium‐3‐sulfopropyl acrylate, potassium‐3‐sulfopropyl methacrylate, and sodium‐2‐acrylamido‐2‐methyl‐1‐propane sulfonate. Copolymers are prepared using combinations of two different monomers in various ratios. Monomer reactivity ratios are also determined for some representative monomer combinations, and all polymers are characterized by ¹H NMR spectroscopy and gel permeation chromatography. The anticoagulant activities of the copolymers are determined by activated partial thromboplastin time and thrombin clotting time assays and structure–activity relationships are explored.     

Ideals and graphs,Gröbner bases and decision procedures in graphs

The well known correspondence between even cycles of an undirected graph and polynomials in a binomial ideal associated to a graph is extended to odd cycles and polynomials in another binomial ideal. Other binomial ideals associated to an undirected graph are also introduced. The results about them with topics on monomial ideals are used in order to show decision procedures for bipartite graphs, minimal vertex covers, cliques, edge covers and matchings with algebraic tools. All such procedures are implemented in Maple 9.5.     

A theoretical approach to the influence of the macrocycle conformation on the molecular electronic structure in Mg-porphyrins

Nonplanar saddled (sad) ruffled (ruf) and domed (dom) conformations of the Mg-porphyrin (MgP) macrocycle in several degrees of deformation have been computed. These symmetrical distortion modes were induced in unsubstituted macrocycle using molecular definitions for calculations which permits us to achieve a systematical variation of the nonplanarity varying only a convenient geometrical parameter of the molecule. Series of nonplanar macrocycles like those synthesized in previous works employing peripheral substitutions are obtained. The procedure here used to induce deformations gives the possibility of investigating the modulator role of the out-of-plane distortions on the geometry and electronic properties of the porphyrin avoiding additional influences due to the substituents or the surrounding protein scaffolding.     

General form of the Arrow-Barankin-Blackwell theorem in normed spaces and thel ∞-case

We deal with the problem of scalarization in vector optimization. A very general form of the Arrow-Barankin-Blackwell theorem is proved in normed spaces. We study the question also in the spacel of all bounded sequences of real numbers, which is of particular interest in economic problems, and generalize many of the existing results on this subject.