156 research outputs found
Adhesion-induced phase separation of multiple species of membrane junctions
A theory is presented for the membrane junction separation induced by the
adhesion between two biomimetic membranes that contain two different types of
anchored junctions (receptor/ligand complexes). The analysis shows that several
mechanisms contribute to the membrane junction separation. These mechanisms
include (i) the height difference between type-1 and type-2 junctions is the
main factor which drives the junction separation, (ii) when type-1 and type-2
junctions have different rigidities against stretch and compression, the
``softer'' junctions are the ``favored'' species, and the aggregation of the
softer junction can occur, (iii) the elasticity of the membranes mediates a
non-local interaction between the junctions, (iv) the thermally activated shape
fluctuations of the membranes also contribute to the junction separation by
inducing another non-local interaction between the junctions and renormalizing
the binding energy of the junctions. The combined effect of these mechanisms is
that when junction separation occurs, the system separates into two domains
with different relative and total junction densities.Comment: 23 pages, 6 figure
Lateral diffusion of receptor-ligand bonds in membrane adhesion zones: Effect of thermal membrane roughness
The adhesion of cells is mediated by membrane receptors that bind to
complementary ligands in apposing cell membranes. It is generally assumed that
the lateral diffusion of mobile receptor-ligand bonds in membrane-membrane
adhesion zones is slower than the diffusion of unbound receptors and ligands.
We find that this slowing down is not only caused by the larger size of the
bound receptor-ligand complexes, but also by thermal fluctuations of the
membrane shape. We model two adhering membranes as elastic sheets pinned
together by receptor-ligand bonds and study the diffusion of the bonds using
Monte Carlo simulations. In our model, the fluctuations reduce the bond
diffusion constant in planar membranes by a factor close to 2 in the
biologically relevant regime of small bond concentrations.Comment: 6 pages, 5 figures; to appear in Europhysics Letter
Impermeability effects in three-dimensional vesicles
We analyse the effects that the impermeability constraint induces on the
equilibrium shapes of a three-dimensional vesicle hosting a rigid inclusion. A
given alteration of the inclusion and/or vesicle parameters leads to shape
modifications of different orders of magnitude, when applied to permeable or
impermeable vesicles. Moreover, the enclosed-volume constraint wrecks the
uniqueness of stationary equilibrium shapes, and gives rise to pear-shaped or
stomatocyte-like vesicles.Comment: 16 pages, 7 figure
Curvature-coupling dependence of membrane protein diffusion coefficients
We consider the lateral diffusion of a protein interacting with the curvature
of the membrane. The interaction energy is minimized if the particle is at a
membrane position with a certain curvature that agrees with the spontaneous
curvature of the particle. We employ stochastic simulations that take into
account both the thermal fluctuations of the membrane and the diffusive
behavior of the particle. In this study we neglect the influence of the
particle on the membrane dynamics, thus the membrane dynamics agrees with that
of a freely fluctuating membrane. Overall, we find that this curvature-coupling
substantially enhances the diffusion coefficient. We compare the ratio of the
projected or measured diffusion coefficient and the free intramembrane
diffusion coefficient, which is a parameter of the simulations, with analytical
results that rely on several approximations. We find that the simulations
always lead to a somewhat smaller diffusion coefficient than our analytical
approach. A detailed study of the correlations of the forces acting on the
particle indicates that the diffusing inclusion tries to follow favorable
positions on the membrane, such that forces along the trajectory are on average
smaller than they would be for random particle positions.Comment: 16 pages, 8 figure
Mobile CARS - IRS Instrument for Simultaneous Spectroscopic Measurement of Multiple Properties in Gaseous Flows
This paper describes a measurement system based on the dual-pump coherent anti-Stokes Raman spectroscopy (CARS) and interferometric Rayleigh scattering (IRS) methods. The IRS measurement is performed simultaneously with the CARS measurement using a common green laser beam as a narrow-band light source. The mobile CARS-IRS instrument is designed for the use both in laboratories as well as in ground-based combustion test facilities. Furthermore, it is designed to be easily transported between laboratory and test facility. It performs single-point spatially and temporally resolved simultaneous measurements of temperature, species mole fraction of N2, O2, and H2, and two-components of velocity. A mobile laser system can be placed inside or outside the test facility, while a beam receiving and monitoring system is placed near the measurement location. Measurements in a laboratory small-scale Mach 1.6 H2-air combustion-heated supersonic jet were performed to test the capability of the system. Final setup and pretests of a larger scale reacting jet are ongoing at NASA Langley Research Center s Direct Connect Supersonic Combustor Test Facility (DCSCTF)
Effective surface interactions mediated by adhesive particles
In biomimetic and biological systems, interactions between surfaces are often
mediated by adhesive molecules, nanoparticles, or colloids dispersed in the
surrounding solution. We present here a general, statistical-mechanical model
for two surfaces that interact via adhesive particles. The effective,
particle-mediated interaction potential of the surfaces is obtained by
integrating over the particles' degrees of freedom in the partition function.
Interestingly, the effective adhesion energy of the surfaces exhibits a maximum
at intermediate particle concentrations, and is considerably smaller both at
low and high concentrations. The effective adhesion energy corresponds to a
minimum in the interaction potential at surface separations slightly larger
than the particle diameter, while a secondary minimum at surface contact
reflects depletion interactions. Our results can be generalized to surfaces
with specific receptors for solute particles, and have direct implications for
the adhesion of biomembranes and for phase transitions in colloidal systems.Comment: 6 pages, 5 figures; to appear in Europhys. Let
Industry strategies for the promotion of E-mobility under alternative policy and economic scenarios
Purpose: In this study, we consider the European electro-mobility market from an industrial perspective, and focus on effects of market conditions and manufacturer strategies, with the objective to gain insight on what could inhibit the successful market penetration of electric powertrain vehicles.
Methods: We use the EC-JRC Powertrain Technology Transition Market Agent Model (PTTMAM), a system dynamics model based around the interactions of conceptual market agent groups in the EU. We assess strategies employed by automobile manufacturers towards the development and market penetration of electric vehicles.
Results: Impacts on electric powertrain sales shares (up to 2050) related to industrial strategies, represented by learning effect, marketing effort and R&D funding, are presented under different scenarios related to policy, regulation and market conditions.
Conclusion: It is concluded from the results presented here that competition between electrical powertrain options may be more inhibitive than competition against conventional counterparts, with both monetary and non-monetary industry support for immature powertrains key to their long-term success when supportive policies are designed to be technology neutral
Combining Optimal Control Theory and Molecular Dynamics for Protein Folding
A new method to develop low-energy folding routes for proteins is presented. The novel aspect of the proposed approach is the synergistic use of optimal control theory with Molecular Dynamics (MD). In the first step of the method, optimal control theory is employed to compute the force field and the optimal folding trajectory for the atoms of a Coarse-Grained (CG) protein model. The solution of this CG optimization provides an harmonic approximation of the true potential energy surface around the native state. In the next step CG optimization guides the MD simulation by specifying the optimal target positions for the atoms. In turn, MD simulation provides an all-atom conformation whose positions match closely the reference target positions determined by CG optimization. This is accomplished by Targeted Molecular Dynamics (TMD) which uses a bias potential or harmonic restraint in addition to the usual MD potential. Folding is a dynamical process and as such residues make different contacts during the course of folding. Therefore CG optimization has to be reinitialized and repeated over time to accomodate these important changes. At each sampled folding time, the active contacts among the residues are recalculated based on the all-atom conformation obtained from MD. Using the new set of contacts, the CG potential is updated and the CG optimal trajectory for the atoms is recomputed. This is followed by MD. Implementation of this repetitive CG optimization - MD simulation cycle generates the folding trajectory. Simulations on a model protein Villin demonstrate the utility of the method. Since the method is founded on the general tools of optimal control theory and MD without any restrictions, it is widely applicable to other systems. It can be easily implemented with available MD software packages
Effects of Intracellular Calcium and Actin Cytoskeleton on TCR Mobility Measured by Fluorescence Recovery
Background: The activation of T lymphocytes by specific antigen is accompanied by the formation of a specialized signaling region termed the immunological synapse, characterized by the clustering and segregation of surface molecules and, in particular, by T cell receptor (TCR) clustering. Methodology/Principal Findings: To better understand TCR motion during cellular activation, we used confocal microscopy and photo-bleaching recovery techniques to investigate the lateral mobility of TCR on the surface of human T lymphocytes under various pharmacological treatments. Using drugs that cause an increase in intracellular calcium, we observed a decrease in TCR mobility that was dependent on a functional actin cytoskeleton. In parallel experiments measurement of filamentous actin by FACS analysis showed that raising intracellular calcium also causes increased polymerization of the actin cytoskeleton. These in vitro results were analyzed using a mathematical model that revealed effective binding parameters between TCR and the actin cytoskeleton. Conclusion/Significance: We propose, based on our results, that increase in intracellular calcium levels leads to actin polymerization and increases TCR/cytoskeleton interactions that reduce the overall mobility of the TCR. In a physiological setting, this may contribute to TCR re-positioning at the immunological synapse
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