Transport properties of a side-chain liquid-crystalline polymer. Poly(4-acryloyloxy biphenyl)

The transport properties of a side-chain liquid-crystalline polymer forming a smectic A phase have been investigated using dichloromethane as a permeant. Samples differing in the isotropization enthaoly were analyzed. A. Correlation between this thermal parameter and sorption has been found, while the diffusion coefficient is substantially the same in all samples. The results obtained can be explained in terms of a biphasis model in which a disordered permeable phase is present within the liquid-crystalline phase, which is impermeable to the diffusant molecules.     

Primate beta-defensins--structure, function and evolution

Host defense peptides (HDPs) are endogenous antibiotics that play a multifunctional role in the innate immunity of mammals. Among these, beta-defensins contribute to mucosal and epithelial defense, also acting as signal molecules for cellular components of innate and adaptive immunity. Numerous members of this family have been identified in mammalian and avian species, and genomic studies in human and mouse indicate a considerable complexity in their gene organization. Recent reports on the evolution of primate and rodent members of this family indicate quite a complex pattern of variation. In this review we briefly discuss the evolution of mammalian beta-defensins in relation to other types of defensins, and then concentrate on the evolution of beta-defensins 1 to 4 in primates. In particular, the surprisingly varied patterns of evolution, which range from neutral or weakly purifying, to positive selection to a high level of conservation are analyzed in terms of possible genetics, structural or functional implications, as well as to observed variations on the antimicrobial activity in vitro. The role of polymorphisms in the genes encoding for these host defense peptides in determining susceptibility to human diseases are also briefly considered.     

Single electron traps at the surface of polycrystalline MgO: assignment of the main trapping sites

Paramagnetic centers at the surface of ionic oxides in the form of trapped electrons can be generated by exposure of the material to alkali metal or hydrogen atoms or of molecular hydrogen under UV irradiation. For many years, it has been assumed that the resulting paramagnetic centers consist of oxygen vacancies filled by one electron. High-resolution electron spin resonance spectra and ab initio quantum chemical calculations show that the paramagnetic centers consist of (H(+))(e(-)) electron pairs formed at morphological irregularities of the surface. At least three different kinds of (H(+))(e(-)) centers, [A], [B], and [C], have been identified with abundances of 80%, 10%, and 8%, respectively. In this work, we compare a wide set of measured and computed g-factors and hyperfine coupling constants of the unpaired electron with the surrounding (25)Mg, (17)O, and (1)H nuclei and we propose a general assignment of the centers. (H(+))(e(-)) pairs formed at Mg(4c) ions at steps and edges account for species [A], centers formed at Mg(4c) ions at reverse corners correspond to species [B], and species [C] originates from (H(+))(e(-)) pairs formed at Mg(3c) ions at corners and kinks.     

Mechanical properties and molecular mobility in oriented polyolefines

The static modulus of elasticity (E) and the correlation time of rotation ( _c ) of 2,-2,6,6-tetramethylpiperidine-1-oxyl are studied as a function of the temperature (210<T <350°K) for oriented films of isotactic polypropylene and polyethylene of high and low density.E and _c change both upon heating and polymer orientation; this result indicates that sample properties are influenced by the microstructure of the amorphous phase where probes are localized.     

Aza-Diels-Alder reaction in fluorinated alcohols. A one-pot synthesis of tetrahydroquinolines

Hexafluoroisopropanol and trifluoroethanol are found to promote imino-Diels-Alder reactions of the N-aryl aldimine 1 with alkyl vinyl ethers to afford the corresponding tetrahydroquinolines in good yields without Lewis acid under mild and neutral conditions. The reaction is also efficient in a three component process from aldehyde, amine and vinyl ethers.     

Effects of variable selection on CoMFA coefficient contour maps in a set of triazines inhibiting DHFR

Summary An example of a CoMFA study is described with the aim to discuss one of the major problems of this 3D QSAR method: lack of variable selection. It is shown that the use of nonrelevant energy parameters might produce CoMFA contour maps which poorly reflect the actual nature of the binding site and are in part statistical artefacts. The data set employed in our analysis comparises triazine inhibitors of dihydrofolate reductase (DHFR), isolated from chicken liver, which have already been the object of a QSAR study by other authors. Since three-dimensional structures of triazine-DHFR complexes are known, it was possible not only to reduce ambiguities in the superimposition of the ligands, but also to compare the resulting CoMFA contour maps with the enzyme active site.Supplementary material available: The Cartesian coordinates and the atomic charges of the PM3-optimized structures used in the CoMFA study are available as MOL2 files upon request.To whose memory this paper is dedicated.     

Electron traps on oxide surfaces: (H+)(e-) pairs stabilized on the surface of 17O enriched CaO.

(H+)(e-) pairs generated at the surface of polycrystalline CaO are analyzed for the first time in terms of the interaction of the unpaired electron spin with the nuclear spin of the 17O anions of the surface. CaO crystals enriched in the 17O isotope are prepared and the corresponding hyperfine coupling constants are measured in electron paramagentic resonance (EPR) spectra. The results are analyzed on the basis of cluster model density functional theory calculations. The computed hyperfine coupling constants for (H+)(e-) pairs formed on the edge, corner, and reverse corner sites of the CaO surface allow a tentative assignment of two observed spectral features to specific morphological surface sites.     

Phase organization of solvent-crystallized syndiotactic polystyrene films

Sorption and diffusion of dichloromethane vapor in the helical forms—that is, δ and γ—of syndiotactic polystyrene were investigated. The thermal transition from the δ to the γ form occurs through an intermediate mesomorphic form, also characterized by chains in helical conformation but lacking true crystalline order. The mesomorphic phase was found impermeable to the vapor at low activity, becoming permeable at higher activities. The presence of a mesomorphic phase, along with the crystalline and the amorphous forms, was suggested in the crystallized samples, too, and its fraction increases greatly going from the δ to the γ form.