Condensation of free volume in structures of nematic and hexatic liquid crystals

Eight novel liquid crystalline materials were prepared containing highly branched terminal chains, either 2,4,4-trimethylpentyl or 3,5,5-trimethylhexyl. All materials exhibit nematic mesophases, with additional smectic (Sm) C, hexatic B and SmI phases for certain homologues. Analysis by small- and wide-angle X-ray scattering reveals continual build-up of the correlation length within the nematic phases, where we also observe splitting of the small angle peak into four lobes, indicating pretransitional Sm fluctuations. Connoscopy confirms the nematic phase to be uniaxial and optically positive. We observe that in the solid state, the molecules exist as staggered antiparallel pairs as a consequence of the sterically demanding bulky terminal group, and this would also appear to manifest in the hexatic B phase, where the layer spacing was found to be greater than the molecular length. If true, this is an example of pair formation driven by sterics rather than dipole–dipole interactions and suggests that reentrant systems driven purely by steric frustration may be found

Influence of a knot on the strength of a polymer strand

Many experiments have been done to determine the relative strength of different knots, and these show that the break in a knotted rope almost invariably occurs at a point just outside the `entrance' to the knot. The influence of knots on the properties of polymers has become of great interest, in part because of their effect on mechanical properties. Knot theory applied to the topology of macromolecules indicates that the simple trefoil or `overhand' knot is likely to be present with high probability in any long polymer strand. Fragments of DNA have been observed to contain such knots in experiments and computer simulations. Here we use {\it ab initio} computational methods to investigate the effect of a trefoil knot on the breaking strength of a polymer strand. We find that the knot weakens the strand significantly, and that, like a knotted rope, it breaks under tension at the entrance to the knot.Comment: 3 pages, 4 figure

Cd(II) and Pb(II) complexes of the polyether ionophorous antibiotic salinomycin

<p>Abstract</p> <p>Background</p> <p>The natural polyether ionophorous antibiotics are used for the treatment of coccidiosis in poultry and ruminants. They are effective agents against infections caused by Gram-positive microorganisms. On the other hand, it was found that some of these compounds selectively bind lead(II) ions in <it>in vivo </it>experiments, despite so far no Pb(II)-containing compounds of defined composition have been isolated and characterized. To assess the potential of polyether ionophores as possible antidotes in the agriculture, a detailed study on their <it>in vitro </it>complexation with toxic metal ions is required. In the present paper we report for the first time the preparation and the structure elucidation of salinomycin complexes with ions of cadmium(II) and lead(II).</p> <p>Results</p> <p>New metal(II) complexes of the polyether ionophorous antibiotic salinomycin with Cd(II) and Pb(II) ions were prepared and structurally characterized by IR, FAB-MS and NMR techniques. The spectroscopic information and elemental analysis data reveal that sodium salinomycin (SalNa) undergoes a reaction with heavy metal(II) ions to form [Cd(Sal)₂(H₂O)₂] (<b>1</b>) and [Pb(Sal)(NO₃)] (<b>2</b>), respectively. Abstraction of sodium ions from the cavity of the antibiotic is occurring during the complexation reaction. Salinomycin coordinates with cadmium(II) ions as a bidentate monoanionic ligand through the deprotonated carboxylic moiety and one of the hydroxyl groups to yield <b>1</b>. Two salinomycin anions occupy the equatorial plane of the Cd(II) center, while two water molecules take the axial positions of the inner coordination sphere of the metal(II) cation. Complex <b>2 </b>consists of monoanionic salinomycin acting in polydentate coordination mode in a molar ratio of 1: 1 to the metal ion with one nitrate ion for charge compensation.</p> <p>Conclusion</p> <p>The formation of the salinomycin heavy metal(II) complexes indicates a possible antidote activity of the ligand in case of chronic/acute intoxications likely to occur in the stock farming.</p