3,602 research outputs found
Continued study of NAVSTAR/GPS for general aviation
A conceptual approach for examining the full potential of Global Positioning Systems (GPS) for the general aviation community is presented. Aspects of an experimental program to demonstrate these concepts are discussed. The report concludes with the observation that the true potential of GPS can only be exploited by utilization in concert with a data link. The capability afforded by the combination of position location and reporting stimulates the concept of GPS providing the auxiliary functions of collision avoidance, and approach and landing guidance. A series of general recommendations for future NASA and civil community efforts in order to continue to support GPS for general aviation are included
Preliminary study of NAVSTAR/GPS for general aviation
The activities conducted as a planning effort to focus attention on the applicability of the global positioning system for general aviation are described. The description of GPS, its impact on economic and functional aspects of general aviation avionics, as well as a declaration of potential extensions of the basic concept have been studied in detail
Effect of Salt Concentration on the Electrophoretic Speed of a Polyelectrolyte through a Nanopore
In a previous paper [S. Ghosal, Phys. Rev. E 74, 041901 (2006)] a
hydrodynamic model for determining the electrophoretic speed of a
polyelectrolyte through an axially symmetric slowly varying nanopore was
presented in the limit of a vanishingly small Debye length. Here the case of a
finite Debye layer thickness is considered while restricting the pore geometry
to that of a cylinder of length much larger than the diameter. Further, the
possibility of a uniform surface charge on the walls of the nanopore is taken
into account. It is thereby shown that the calculated transit times are
consistent with recent measurements in silicon nanopores.Comment: 4 pages, 2 figure
Equilibrium Chemical Engines
An equilibrium reversible cycle with a certain engine to transduce the energy
of any chemical reaction into mechanical energy is proposed. The efficiency for
chemical energy transduction is also defined so as to be compared with Carnot
efficiency. Relevance to the study of protein motors is discussed. KEYWORDS:
Chemical thermodynamics, Engine, Efficiency, Molecular machine.Comment: 5 pages, late
Molecular Electroporation and the Transduction of Oligoarginines
Certain short polycations, such as TAT and polyarginine, rapidly pass through
the plasma membranes of mammalian cells by an unknown mechanism called
transduction as well as by endocytosis and macropinocytosis. These
cell-penetrating peptides (CPPs) promise to be medically useful when fused to
biologically active peptides. I offer a simple model in which one or more CPPs
and the phosphatidylserines of the inner leaflet form a kind of capacitor with
a voltage in excess of 180 mV, high enough to create a molecular electropore.
The model is consistent with an empirical upper limit on the cargo peptide of
40--60 amino acids and with experimental data on how the transduction of a
polyarginine-fluorophore into mouse C2C12 myoblasts depends on the number of
arginines in the CPP and on the CPP concentration. The model makes three
testable predictions.Comment: 15 pages, 5 figure
Fluctuation-Driven Molecular Transport in an Asymmetric Membrane Channel
Channel proteins, that selectively conduct molecules across cell membranes,
often exhibit an asymmetric structure. By means of a stochastic model, we argue
that channel asymmetry in the presence of non-equilibrium fluctuations, fueled
by the cell's metabolism as observed recently, can dramatically influence the
transport through such channels by a ratchet-like mechanism. For an
aquaglyceroporin that conducts water and glycerol we show that a previously
determined asymmetric glycerol potential leads to enhanced inward transport of
glycerol, but for unfavorably high glycerol concentrations also to enhanced
outward transport that protects a cell against poisoning.Comment: REVTeX4, 4 pages, 3 figures; Accepted for publication in Phys. Rev.
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Undulation instability in a bilayer lipid membrane due to electric field interaction with lipid dipoles
Bilayer lipid membranes [BLMs] are an essential component of all biological
systems, forming a functional barrier for cells and organelles from the
surrounding environment. The lipid molecules that form membranes contain both
permanent and induced dipoles, and an electric field can induce the formation
of pores when the transverse field is sufficiently strong (electroporation).
Here, a phenomenological free energy is constructed to model the response of a
BLM to a transverse static electric field. The model contains a continuum
description of the membrane dipoles and a coupling between the headgroup
dipoles and the membrane tilt. The membrane is found to become unstable through
buckling modes, which are weakly coupled to thickness fluctuations in the
membrane. The thickness fluctuations, along with the increase in interfacial
area produced by membrane buckling, increase the probability of localized
membrane breakdown, which may lead to pore formation. The instability is found
to depend strongly on the strength of the coupling between the dipolar
headgroups and the membrane tilt as well as the degree of dipolar ordering in
the membrane.Comment: 29 pages 8 fig
Orientational Ordering and Dynamics of Rodlike Polyelectrolytes
The interplay between electrostatic interactions and orientational
correlations is studied for a model system of charged rods positioned on a
chain, using Monte Carlo simulation techniques. It is shown that the coupling
brings about the notion of {\em electrostatic frustration}, which in turn
results in: (i) a rich variety of novel orientational orderings such as chiral
phases, and (ii) an inherently slow dynamics characterized by
stretched-exponential behavior in the relaxation functions of the system.Comment: 7 pages, 10 figure
Solid domains in lipid vesicles and scars
The free energy of a crystalline domain coexisting with a liquid phase on a
spherical vesicle may be approximated by an elastic or stretching energy and a
line tension term. The stretching energy generally grows as the area of the
domain, while the line tension term grows with its perimeter. We show that if
the crystalline domain contains defect arrays consisting of finite length grain
boundaries of dislocations (scars) the stretching energy grows linearly with a
characteristic length of the crystalline domain. We show that this result is
critical to understand the existence of solid domains in lipid-bilayers in the
strongly segregated two phase region even for small relative area coverages.
The domains evolve from caps to stripes that become thinner as the line tension
is decreased. We also discuss the implications of the results for other
experimental systems and for the general problem that consists in finding the
ground state of a very large number of particles constrained to move on a fixed
geometry and interacting with an isotropic potential.Comment: 7 pages, 6 eps figure
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