Monte Carlo Study of the Inflation-Deflation Transition in a Fluid Membrane

We study the conformation and scaling properties of a self-avoiding fluid membrane, subject to an osmotic pressure $p$, by means of Monte Carlo simulations. Using finite size scaling methods in combination with a histogram reweighting techniques we find that the surface undergoes an abrupt conformational transition at a critical pressure $p^\ast$, from low pressure deflated configurations with a branched polymer characteristics to a high pressure inflated phase, in agreement with previous findings \cite{gompper,baum}. The transition pressure $p^{\ast}$ scales with the system size as $p^\ast \propto N^{-\alpha}$, with $\alpha = 0.69 \pm 0.01$. Below $p^\ast$ the enclosed volume scales as $V \propto N$, in accordance with the self-avoiding branched polymer structure, and for $p\searrow p^{\ast}$ our data are consistent with the finite size scaling form $V \propto N^{\beta_{+}}$, where $\beta_{+} = 1.43 \pm 0.04$. Also the finite size scaling behavior of the radii of gyration and the compressibility moduli are obtained. Some of the observed exponents and the mechanism behind the conformational collapse are interpreted in terms of a Flory theory.Comment: 20 pages + postscript-file, Latex + Postscript, IFA Report No. 94/1

A note on the Lee-Yang singularity coupled to 2d quantum gravity

We show how to obtain the critical exponent of magnetization in the Lee-Yang edge singularity model coupled to two-dimensional quantum gravity

Self-organized stable pacemakers near the onset of birhythmicity

General amplitude equations for reaction-diffusion systems near to the soft onset of birhythmicity described by a supercritical pitchfork-Hopf bifurcation are derived. Using these equations and applying singular perturbation theory, we show that stable autonomous pacemakers represent a generic kind of spatiotemporal patterns in such systems. This is verified by numerical simulations, which also show the existence of breathing and swinging pacemaker solutions. The drift of self-organized pacemakers in media with spatial parameter gradients is analytically and numerically investigated.Comment: 4 pages, 4 figure

Zone Determinant Expansions for Nuclear Lattice Simulations

We introduce a new approximation to nucleon matrix determinants that is physically motivated by chiral effective theory. The method involves breaking the lattice into spatial zones and expanding the determinant in powers of the boundary hopping parameter.Comment: 20 pages, 6 figures, revtex4 (version to appear in PRC

Semi-Empirical Model for Nano-Scale Device Simulations

We present a new semi-empirical model for calculating electron transport in atomic-scale devices. The model is an extension of the Extended H\"uckel method with a self-consistent Hartree potential. This potential models the effect of an external bias and corresponding charge re-arrangements in the device. It is also possible to include the effect of external gate potentials and continuum dielectric regions in the device. The model is used to study the electron transport through an organic molecule between gold surfaces, and it is demonstrated that the results are in closer agreement with experiments than ab initio approaches provide. In another example, we study the transition from tunneling to thermionic emission in a transistor structure based on graphene nanoribbons.Comment: 8 pages, 8 figures. Submitted to PR

Softening of POPC membranes by magainin

International audienc

Membrane invagination induced by Shiga toxin B-subunit:From molecular structure to tube formation

The bacterial Shiga toxin is composed of an enzymatically active A-subunit, and a receptor-binding homopentameric B-subunit (STxB) that mediates intracellular toxin trafficking. Upon STxB-mediated binding to the glycolipid globotriaosylceramide (Gb(3)) at the plasma membrane of target cells, Shiga toxin is internalized by clathrin-dependent and independent endocytosis. The formation of tubular membrane invaginations is an essential step in the clathrin-independent STxB uptake process. However, the mechanism by which STxB induces these invaginations has remained unclear. Using a combination of all-atom molecular dynamics and Monte Carlo simulations we show that the molecular architecture of STxB enables the following sequence of events: the Gb(3) binding sites on STxB are arranged such that tight avidity-based binding results in a small increment of local curvature. Membrane-mediated clustering of several toxin molecules then creates a tubular membrane invagination that drives toxin entry into the cell. This mechanism requires: (1) a precise molecular architecture of the STxB binding sites; (2) a fluid bilayer in order for the tubular invagination to form. Although, STxB binding to the membrane requires specific interactions with Gb(3) lipids, our study points to a generic molecular design principle for clathrin-independent endocytosis of nanoparticles