Enhancement by polydispersity of the biaxial nematic phase in a mixture of hard rods and plates

The phase diagram of a polydisperse mixture of uniaxial rod-like and plate-like hard parallelepipeds is determined for aspect ratios $\kappa=5$ and 15. All particles have equal volume and polydispersity is introduced in a highly symmetric way. The corresponding binary mixture is known to have a biaxial phase for $\kappa=15$, but to be unstable against demixing into two uniaxial nematics for $\kappa=5$. We find that the phase diagram for $\kappa=15$ is qualitatively similar to that of the binary mixture, regardless the amount of polydispersity, while for $\kappa=5$ a sufficient amount of polydispersity stabilizes the biaxial phase. This provides some clues for the design of an experiment in which this long searched biaxial phase could be observed.Comment: 4 pages, 5 eps figure files, uses RevTeX 4 styl

Molecular theory of elastic constants of liquid crystals. III. Application to smectic phases with tilted orientational order

Using the density functional formalism we derive expression for the distortion free energy for systems with continuous broken symmetry and use it to derive expression for the elastic constants of smectic phases in which director is tilted with respect to the smectic layer normal. As in the previous papers of the series (Phys. Rev. A {\bf 45}, 974 (1992), E {\bf 49}, 501, (1994)) the expressions for the elastic constants are written in terms of order and structural parameters. The structural parameters involve the generalised spherical harmonic coefficients of the direct pair correlation function of an effective isotropic liquid. The density of this effective isotropic liquid depends on the nature and amount of ordering present in the system and is evaluated self- consistently. We estimate the value of elastic constants using reasonable guess for the order and structural- parameters.Comment: 31 pages; 1 Fig. in GIF format, To be appear in Phys. Rev.

Equation of State for Parallel Rigid Spherocylinders

The pair distribution function of monodisperse rigid spherocylinders is calculated by Shinomoto's method, which was originally proposed for hard spheres. The equation of state is derived by two different routes: Shinomoto's original route, in which a hard wall is introduced to estimate the pressure exerted on it, and the virial route. The pressure from Shinomoto's original route is valid only when the length-to-width ratio is less than or equal to 0.25 (i.e., when the spherocylinders are nearly spherical). The virial equation of state is shown to agree very well with the results of numerical simulations of spherocylinders with length-to-width ratio greater than or equal to 2

On the Microscopic Origin of Cholesteric Pitch

We present a microscopic analysis of the instability of the nematic phase to chirality when molecular chirality is introduced perturbatively. We show that previously neglected short-range biaxial correlations play a crucial role in determining the cholesteric pitch. We propose an order parameter which quantifies the chirality of a molecule.Comment: RevTeX 3.0, 4 pages, one included eps figure. Published versio

Enhanced stability of layered phases in parallel hard-spherocylinders due to the addition of hard spheres

There is increasing evidence that entropy can induce microphase separation in binary fluid mixtures interacting through hard particle potentials. One such phase consists of alternating two dimensional liquid-like layers of rods and spheres. We study the transition from a uniform miscible state to this ordered state using computer simulations and compare results to experiments and theory. We conclude that (1) there is stable entropy driven microphase separation in mixtures of parallel rods and spheres, (2) adding spheres smaller then the rod length decreases the total volume fraction needed for the formation of a layered phase, therefore small spheres effectively stabilize the layered phase; the opposite is true for large spheres and (3) the degree of this stabilization increases with increasing rod length.Comment: 11 pages, 9 figures. Submitted to Phys. Rev. E. See related website http://www.elsie.brandeis.ed

Polydispersity and ordered phases in solutions of rodlike macromolecules

We apply density functional theory to study the influence of polydispersity on the stability of columnar, smectic and solid ordering in the solutions of rodlike macromolecules. For sufficiently large length polydispersity (standard deviation $\sigma>0.25$) a direct first-order nematic-columnar transition is found, while for smaller $\sigma$ there is a continuous nematic-smectic and first-order smectic-columnar transition. For increasing polydispersity the columnar structure is stabilized with respect to solid perturbations. The length distribution of macromolecules changes neither at the nematic-smectic nor at the nematic-columnar transition, but it does change at the smectic-columnar phase transition. We also study the phase behaviour of binary mixtures, in which the nematic-smectic transition is again found to be continuous. Demixing according to rod length in the smectic phase is always preempted by transitions to solid or columnar ordering.Comment: 13 pages (TeX), 2 Postscript figures uuencode

Pulsed field studies of the magnetization reversal in molecular nanomagnets

We report experimental studies of crystals of Mn12 molecular magnetic clusters in pulsed magnetic fields with sweep rates up to 4x10^3 T/s. The steps in the magnetization curve are observed at fields that are shifted with respect to the resonant field values. The shift systematically increases as the rate of the field sweep goes up. These data are consistent with the theory of the collective dipolar relaxation in molecular magnets.Comment: 4 pages, 4 figure

Amyloid-like Fibrils from an α-Helical Transmembrane Protein

The propensity to misfold and self-assemble into stable aggregates is increasingly being recognized as a common feature of protein molecules. Our understanding of this phenomenon and of its links with human disease has improved substantially over the past two decades. Studies thus far, however, have been almost exclusively focused on cytosolic proteins, resulting in a lack of detailed information about the misfolding and aggregation of membrane proteins. As a consequence, although such proteins make up approximately 30% of the human proteome and have high propensities to aggregate, relatively little is known about the biophysical nature of their assemblies. To shed light on this issue, we have studied as a model system an archetypical representative of the ubiquitous major facilitator superfamily, the Escherichia coli lactose permease (LacY). By using a combination of established indicators of cross-β structure and morphology, including the amyloid diagnostic dye thioflavin-T, circular dichroism spectroscopy, Fourier transform infrared spectroscopy, X-ray fiber diffraction, and transmission electron microscopy, we show that LacY can form amyloid-like fibrils under destabilizing conditions. These results indicate that transmembrane α-helical proteins, similarly to cytosolic proteins, have the ability to adopt this generic state.We are grateful for the award of the Marie Curie Career Development Fellowship (K.S.) and for support of this work by a Wellcome Trust Programme Grant 094425/Z/10/Z (C.M.D. and M.V.) and by an ERC Advanced Grant (294342) (N.J.H., P.J.B.)

Diagnostic accuracy of four different D-dimer assays: a post-hoc analysis of the YEARS study

Introduction: For exclusion of pulmonary embolism (PE) clinical decision rules in combination with a D-dimer assay are applied. Currently available D-dimer assays are not standardized and it is unknown whether these differences have an impact on diagnostic management of suspected PE. Therefore, the aim is to explore differences between D-dimer assays and their impact on diagnostic outcome. Methods: Data from all patients included in the YEARS study were collected. The YEARS study is a prospective, multicentre, cohort outcome study evaluating 3462 patients with suspected PE in which four different D-dimer assays were applied (Liatest, Innovance, Tinaquant, Vidas). Median D-dimer concentrations were calculated for each D-dimer assay. Sensitivity, specificity, PPV and NPV for detection of PE of all four assays were determined in patients without YEARS items and in those with >1 YEARS items (i.e. symptomatic deep vein thrombosis, haemoptysis, and whether PE is the most likely diagnosis). Results: A total of 1323, 1100, 768 and 271 D-dimer concentrations were collected using the Liatest Innovance, Tinaquant and Vidas assay, respectively. Median D-dimer concentrations differed significantly between assays, with lowest values in the Tinaquant assay. In patients without YEARS items using a cutoff level of 1000 ng/mL, the NPV varied from 99,5 to 100%. In patients with >1 YEARS items using a 500 ng/mL cutoff, the NPV varied from 97,0 to 100% depending on the assay. Conclusions: The overall high NPV for all assays demonstrates the clinical value of the D-dimer assay. However, these results confirm differences between D-dimer assays, which have an impact on follow-up imaging. This emphasizes the need for standardization of D-dimer assays.Experimentele farmacotherapi