58 research outputs found
Impact of Cholesterol on Voids in Phospholipid Membranes
Free volume pockets or voids are important to many biological processes in
cell membranes. Free volume fluctuations are a prerequisite for diffusion of
lipids and other macromolecules in lipid bilayers. Permeation of small solutes
across a membrane, as well as diffusion of solutes in the membrane interior are
further examples of phenomena where voids and their properties play a central
role. Cholesterol has been suggested to change the structure and function of
membranes by altering their free volume properties. We study the effect of
cholesterol on the properties of voids in dipalmitoylphosphatidylcholine (DPPC)
bilayers by means of atomistic molecular dynamics simulations. We find that an
increasing cholesterol concentration reduces the total amount of free volume in
a bilayer. The effect of cholesterol on individual voids is most prominent in
the region where the steroid ring structures of cholesterol molecules are
located. Here a growing cholesterol content reduces the number of voids,
completely removing voids of the size of a cholesterol molecule. The voids also
become more elongated. The broad orientational distribution of voids observed
in pure DPPC is, with a 30% molar concentration of cholesterol, replaced by a
distribution where orientation along the bilayer normal is favored. Our results
suggest that instead of being uniformly distributed to the whole bilayer, these
effects are localized to the close vicinity of cholesterol molecules
Surface Scaling Analysis of a Frustrated Spring-network Model for Surfactant-templated Hydrogels
We propose and study a simplified model for the surface and bulk structures
of crosslinked polymer gels, into which voids are introduced through templating
by surfactant micelles. Such systems were recently studied by Atomic Force
Microscopy [M. Chakrapani et al., e-print cond-mat/0112255]. The gel is
represented by a frustrated, triangular network of nodes connected by springs
of random equilibrium lengths. The nodes represent crosslinkers, and the
springs correspond to polymer chains. The boundaries are fixed at the bottom,
free at the top, and periodic in the lateral direction. Voids are introduced by
deleting a proportion of the nodes and their associated springs. The model is
numerically relaxed to a representative local energy minimum, resulting in an
inhomogeneous, ``clumpy'' bulk structure. The free top surface is defined at
evenly spaced points in the lateral (x) direction by the height of the topmost
spring, measured from the bottom layer, h(x). Its scaling properties are
studied by calculating the root-mean-square surface width and the generalized
increment correlation functions C_q(x)= . The surface is
found to have a nontrivial scaling behavior on small length scales, with a
crossover to scale-independent behavior on large scales. As the vacancy
concentration approaches the site-percolation limit, both the crossover length
and the saturation value of the surface width diverge in a manner that appears
to be proportional to the bulk connectivity length. This suggests that a
percolation transition in the bulk also drives a similar divergence observed in
surfactant templated polyacrylamide gels at high surfactant concentrations.Comment: 17 pages RevTex4, 10 imbedded eps figures. Expanded discussion of
multi-affinit
A fast Monte Carlo algorithm for site or bond percolation
We describe in detail a new and highly efficient algorithm for studying site
or bond percolation on any lattice. The algorithm can measure an observable
quantity in a percolation system for all values of the site or bond occupation
probability from zero to one in an amount of time which scales linearly with
the size of the system. We demonstrate our algorithm by using it to investigate
a number of issues in percolation theory, including the position of the
percolation transition for site percolation on the square lattice, the
stretched exponential behavior of spanning probabilities away from the critical
point, and the size of the giant component for site percolation on random
graphs.Comment: 17 pages, 13 figures. Corrections and some additional material in
this version. Accompanying material can be found on the web at
http://www.santafe.edu/~mark/percolation
Percolation and cluster distribution. III. Algorithms for the site-bond problem
Algorithms for estimating the percolation probabilities and cluster size distribution are given in the framework of a Monte Carlo simulation for disordered lattices for the generalized site-bond problem. The site-bond approach is useful when a percolation process cannot be exclusively described in the context of pure site or pure bond percolation. An extended multiple labeling technique (ECMLT) is introduced for the generalized problem. The ECMLT is applied to the site-bond percolation problem for square and triangular lattices. Numerical data are given for lattices containing up to 16 million sites. An application to polymer gelation is suggested.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45140/1/10955_2005_Article_BF01011170.pd
Exploring the impact of targeted distribution of free bed nets on households bed net ownership, socio-economic disparities and childhood malaria infection rates: analysis of national malaria survey data from three sub-Saharan Africa countries
Acceptance Rates of COVID-19 Vaccine Highlight the Need for Targeted Public Health Interventions
We aimed to examine rates of COVID-19 vaccination to elucidate the need for targeted public health interventions. We retrospectively reviewed the electronic medical files of all adults registered in a central district in Israel from 1 January 2021 to 31 March 2022. The population was characterized by vaccination status against COVID-19 and the number of doses received. Univariate and multivariable analyses were used to identify predictors of low vaccination rates that required targeted interventions. Of the 246,543 subjects included in the study, 207,911 (84.3%) were vaccinated. The minority groups of ultra-Orthodox Jews and Arabs had lower vaccination rates than the non-ultra-Orthodox Jews (68.7%, 80.5% and 87.7%, respectively, p p p p < 0.001) associated with low vaccination rates: minority groups of the ultra-Orthodox sector and Arab population, and underlying conditions of asthma, smoking and diabetes mellitus (odds ratios: 0.484, 0.453, 0.843, 0.901 and 0.929, respectively). Specific targeted public health interventions towards these subpopulations with significantly lower rates of vaccination are suggested
- ā¦