Critical Point Field Mixing in an Asymmetric Lattice Gas Model

The field mixing that manifests broken particle-hole symmetry is studied for a 2-D asymmetric lattice gas model having tunable field mixing properties. Monte Carlo simulations within the grand canonical ensemble are used to obtain the critical density distribution for different degrees of particle-hole asymmetry. Except in the special case when this asymmetry vanishes, the density distributions exhibit an antisymmetric correction to the limiting scale-invariant form. The presence of this correction reflects the mixing of the critical energy density into the ordering operator. Its functional form is found to be in excellent agreement with that predicted by the mixed-field finite-size-scaling theory of Bruce and Wilding. A computational procedure for measuring the significant field mixing parameter is also described, and its accuracy gauged by comparing the results with exact values obtained analytically.Comment: 10 Pages, LaTeX + 8 figures available from author on request, To appear in Z. Phys.

Competition or facilitation: Examination of interactions between endangered Sida hermaphrodita and invasive Phragmites australis

Virginia Mallow (Sida hermaphrodita) is a perennial herb of the Malvaceae family that is native to riparian habitats in northeastern North America. Throughout most of its geographical distribution however, it is considered threatened and only two populations are known from Canada. The biology and ecology of S. hermaphrodita are still poorly understood and although few studies have been performed to determine the factors that contribute to the species rarity, it is considered threatened potentially due to the loss of habitat caused by exotic European Common reed (Phragmites australis subsp. australis) invasion. Allelopathic and phytotoxic conditioning of soils to inhibit native species are mechanisms that have been proposed to explain the invasion success of P. australis. To quantify the interaction between the two species and assess the capacity for P. australis to inhibit S. hermaphrodita performance through belowground soil modifications, a series of field vegetation surveys were conducted at the Taquanyah Conservation Area during the growing seasons of 2016, 2017, and 2018. Field performance findings suggested that proximity to P. australis had no significant effect on S. hermaphrodita seedling mortality or seedling root arbuscular mycorrhizal colonization. A supplementary greenhouse study was also conducted to examine plant performance and mycorrhizal colonization of both species in soils that correspond to different soil-vegetation levels ranging between pure stands of S. hermaphrodita to pure stands of P. australis in order to determine the potential for P. australis to allelopathically modify soils making them inhospitable for native species. The results provided no evidence to support previous soil conditioning reports since performance and arbuscular mycorrhizal colonization of both species were inversely promoted in their competitor’s soil. Soil nutrient analysis coupled with the plant performance findings suggested that P. australis may not be as strictly competitive as previously believed since evidence of a belowground facilitative interaction between S. hermaphrodita and P. australis has been observed. Based on the results concluding that belowground conditions did not exclude native species, we believe aboveground competition for light is not only the main factor contributing to S. hermaphrodita’s limited distribution where it occurs with P. australis, but also key to the invasion success of P. australis. Future research and management treatments focussed on disrupting P. australis’ competitive exclusion of light would be beneficial to the recovery of endangered species like S. hermaphrodita

Quasar Outflow Deceleration or Acceleration: Predictions and a Search

Quasar winds can shock and sweep up ambient interstellar medium (ISM) gas, contributing to galactic quenching. We combine and extend past models of energy-conserving shock bubbles around quasars, investigate model implications from an observational standpoint, and test model predictions using new high-resolution spectroscopic observations of the broad absorption line quasar SDSS J030000.56+004828.0 (J0300). Even with constant energy input from the wind, a bubble's expansion decelerates over time as more ISM gas is swept up. Our new observations enable a direct search for this deceleration. We obtain the tightest reported 3-sigma limit on the average rest-frame deceleration (or acceleration) of a quasar outflow: |a|$<$0.1 km s$^{-1}$ yr$^{-1}$ ($<3 \times 10^{-4}$ cm s$^{-2}$) in the relatively low-velocity Ca II outflow of J0300 over 9.65 rest-frame years. We can satisfy these limits with certain parameter choices in our model, but the large velocity range of the Ca II absorption in J0300 rules out the hypothesis that such gas shares the velocity of the swept-up ISM gas in a self-similar shock bubble. We investigate the possibility of ram-pressure acceleration of preexisting ISM clouds and conclude that the velocity range seen in Ca II in J0300 is potentially consistent with such an explanation. The Ca II-absorbing gas clouds in J0300 have been inferred to have high densities by Choi et al., in which case they can only have been accelerated to their current speeds if they were originally at least an order of magnitude less dense than they are today.Comment: MNRAS, in pres

Generalized Born Implicit Solvent Models Do Not Reproduce Secondary Structures of De Novo Designed Glu/Lys Peptides

[Image: see text] We test a range of standard generalized Born (GB) models and protein force fields for a set of five experimentally characterized, designed peptides comprising alternating blocks of glutamate and lysine, which have been shown to differ significantly in α-helical content. Sixty-five combinations of force fields and GB models are evaluated in >800 μs of molecular dynamics simulations. GB models generally do not reproduce the experimentally observed α-helical content, and none perform well for all five peptides. These results illustrate that these models are not usefully predictive in this context. These peptides provide a useful test set for simulation methods

Computational Assay of H7N9 Influenza Neuraminidase Reveals R292K Mutation Reduces Drug Binding Affinity

The emergence of a novel H7N9 avian influenza that infects humans is a serious cause for concern. Of the genome sequences of H7N9 neuraminidase available, one contains a substitution of arginine to lysine at position 292, suggesting a potential for reduced drug binding efficacy. We have performed molecular dynamics simulations of oseltamivir, zanamivir and peramivir bound to H7N9, H7N9-R292K, and a structurally related H11N9 neuraminidase. They show that H7N9 neuraminidase is structurally homologous to H11N9, binding the drugs in identical modes. The simulations reveal that the R292K mutation disrupts drug binding in H7N9 in a comparable manner to that observed experimentally for H11N9-R292K. Absolute binding free energy calculations with the WaterSwap method confirm a reduction in binding affinity. This indicates that the efficacy of antiviral drugs against H7N9-R292K will be reduced. Simulations can assist in predicting disruption of binding caused by mutations in neuraminidase, thereby providing a computational ‘assay.