965 research outputs found
Exact results and mean field approximation for a model of molecular aggregation
We present a simple one-dimensional model with molecular interactions
favouring the formation of clusters with a defined optimal size. Increasing the
density, at low temperature, the system goes from a nearly-ideal gas of
independent molecules to a system with most of the molecules in optimal
clusters, in a way that resembles the formation of micelles in a dilution of
amphiphilic molecules, at the critical micellar concentration. Our model is
simple enough to have an exact solution, but it contains some basic features of
more realistic descriptions of amphiphilic systems: molecular excluded volume
and molecular attractions which are saturated at the optimal cluster. The
comparison between the exact results and the mean field density functional
approximation suggests new approaches to study the more complex and realistic
models of micelle formation; in particular it addresses the long-standing
controversy surrounding separation of internal degrees of freedom in the
formulation of cluster association phenomena.Comment: 7 pages, 5 figures, some minor correction
Analysis of nucleotide diversity of NAT2 coding region reveals homogeneity across Native American populations and high intra-population diversity.
N-acetyltransferase 2 (NAT2), an important enzyme in clinical pharmacology, metabolizes antibiotics such as isoniazid and sulfamethoxazole, and catalyzes the transformation of aromatic and heterocyclic amines from the environment and diet into carcinogenic intermediates. Polymorphisms in NAT2 account for variability in the acetylator phenotype and the pharmacokinetics of metabolized drugs. Native Americans, settled in rural areas and large cities of Latin America, are under-represented in pharmacogenetics studies; therefore, we sequenced the coding region of NAT2 in 456 chromosomes from 13 populations from the Americas, and two from Siberia, detecting nine substitutions and 11 haplotypes. Variants *4 (37%), *5B (23%) and *7B (24%) showed high frequencies. Average frequencies of fast, intermediate and slow acetylators across Native Americans were 18, 56 and 25%, respectively. NAT2 intra-population genetic diversity for Native Americans is higher than East Asians and similar to the rest of the world, and NAT2 variants are homogeneously distributed across native populations of the continent
Bulk inhomogeneous phases of anisotropic particles: A fundamental measure functional study of the restricted orientations model
The phase diagram of prolate and oblate particles in the restricted
orientations approximation (Zwanzig model) is calculated. Transitions to
different inhomogeneous phases (smectic, columnar, oriented, or plastic solid)
are studied through minimization of the fundamental measure functional (FMF) of
hard parallelepipeds. The study of parallel hard cubes (PHC's) as a particular
case is also included motivated by recent simulations of this system. As a
result a rich phase behavior is obtained which include, apart from the usual
liquid crystal phases, a very peculiar phase (called here discotic smectic)
which was already found in the only existing simulation of the model, and which
turns out to be stable because of the restrictions imposed on the orientations.
The phase diagram is compared at a qualitative level with simulation results of
other anisotropic particle systems.Comment: 11 pages, 10 figure
Lattice density-functional theory of surface melting: the effect of a square-gradient correction
I use the method of classical density-functional theory in the
weighted-density approximation of Tarazona to investigate the phase diagram and
the interface structure of a two-dimensional lattice-gas model with three
phases -- vapour, liquid, and triangular solid. While a straightforward
mean-field treatment of the interparticle attraction is unable to give a stable
liquid phase, the correct phase diagram is obtained when including a suitably
chosen square-gradient term in the system grand potential. Taken this theory
for granted, I further examine the structure of the solid-vapour interface as
the triple point is approached from low temperature. Surprisingly, a novel
phase (rather than the liquid) is found to grow at the interface, exhibiting an
unusually long modulation along the interface normal. The conventional
surface-melting behaviour is recovered only by artificially restricting the
symmetries being available to the density field.Comment: 16 pages, 6 figure
Condensation of Hard Spheres Under Gravity: Exact Results in One Dimension
We present exact results for the density profile of the one dimensional array
of N hard spheres of diameter D and mass m under gravity g. For a strictly one
dimensional system, the liquid-solid transition occurs at zero temperature,
because the close-pakced density, , is one. However, if we relax this
condition slightly such that , we find a series of critical
temperatures T_c^i=mgD(N+1-i)/\mu_o with \mu_o=const, at which the i-th
particle undergoes the liquid-solid transition. The functional form of the
onset temperature, T_c^1=mgDN/\mu_o, is consistent with the previous result
[Physica A 271, 192 (1999)] obtained by the Enskog equation. We also show that
the increase in the center of mass is linear in T before the transition, but it
becomes quadratic in T after the transition because of the formation of solid
near the bottom
Newton black films as wetting systems
Newton black films (NBFs) can appear under a wide range of experimental conditions. NBFs define the adhesive states of foams and emulsions, showing their formation is a very general physical phenomenon. We show that the existence of NBFs and their whole experimental behavior can be understood within the theory of wetting transitions. NBFs are experimental realizations of partial wetting or pre-wetting states. Hence, they provide experimental systems to investigate the pre-wetting transition, and the spreading behavior under conditions that are very difficult to realize in other experimental systems. We also introduce two new computational approaches to obtain the disjoining pressure isotherm from canonical simulations, and to estimate the contact angles of droplets of nanoscopic dimensionsFinancial support for this work was provided by The Royal Society project “Intrinsic Structure of Aqueous Interfaces” and the Dirección General de Investigación, Ministerio
de Ciencia y Tecnología of Spain, under Grant No. FIS2010-22047-C05, and by the Comunidad Autónoma de Madrid under the R&D Program of activities MODELICOCM/
S2009ESP-1691. H.M. would like to thank the Universidad
Autonoma de Madrid for the award of a FPU-UAM
doctoral grant. F.B. would like to thank the EPSRC for the
award of a Leadership Fellowship (EP/J003859/1
Density Functional for Anisotropic Fluids
We propose a density functional for anisotropic fluids of hard body
particles. It interpolates between the well-established geometrically based
Rosenfeld functional for hard spheres and the Onsager functional for elongated
rods. We test the new approach by calculating the location of the the
nematic-isotropic transition in systems of hard spherocylinders and hard
ellipsoids. The results are compared with existing simulation data. Our
functional predicts the location of the transition much more accurately than
the Onsager functional, and almost as good as the theory by Parsons and Lee. We
argue that it might be suited to study inhomogeneous systems.Comment: To appear in J. Physics: Condensed Matte
Schwinger Boson Formulation and Solution of the Crow-Kimura and Eigen Models of Quasispecies Theory
We express the Crow-Kimura and Eigen models of quasispecies theory in a
functional integral representation. We formulate the spin coherent state
functional integrals using the Schwinger Boson method. In this formulation, we
are able to deduce the long-time behavior of these models for arbitrary
replication and degradation functions.
We discuss the phase transitions that occur in these models as a function of
mutation rate. We derive for these models the leading order corrections to the
infinite genome length limit.Comment: 37 pages; 4 figures; to appear in J. Stat. Phy
A computer simulation approach to quantify the true area and true area compressibility modulus of biological membranes
We present a new computational approach to quantify the area per lipid and the area compressibility modulus of biological membranes. Our method relies on the analysis of the membrane fluctuations using our recently introduced coupled undulatory (CU) mode [Tarazona et al., J. Chem. Phys. 139, 094902 (2013)], which provides excellent estimates of the bending modulus of model membranes. Unlike the projected area, widely used in computer simulations of membranes, the CU area is thermodynamically consistent. This new area definition makes it possible to accurately estimate the area of the undulating bilayer, and the area per lipid, by excluding any contributions related to the phospholipid protrusions. We find that the area per phospholipid and the area compressibility modulus features a negligible dependence with system size, making possible their computation using truly small bilayers, involving a few hundred lipids. The area compressibility modulus obtained from the analysis of the CU area fluctuations is fully consistent with the Hooke's law route. Unlike existing methods, our approach relies on a single simulation, and no a priori knowledge of the bending modulus is required. We illustrate our method by analyzing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers using the coarse grained MARTINI force-field. The area per lipid and area compressibility modulus obtained with our method and the MARTINI forcefield are consistent with previous studies of these bilayersWe acknowledge the support of the Spanish Ministry of Science and Innovation (Grant Nos. FIS2010-22047-C05 and FIS2013-47350-C5). We would also like to acknowledge the Imperial College High Performance Computing Service for providing computational resources. F.B. would like to thank the EPSRC (No. EP/J003859/1) for the award of a Leadership Fellowshi
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