973 research outputs found
Thermodynamics of nano-cluster phases: a unifying theory
We propose a unifying, analytical theory accounting for the self-organization
of colloidal systems in nano- or micro-cluster phases. We predict the
distribution of cluter sizes with respect to interaction parameters and colloid
concentration. In particular, we anticipate a proportionality regime where the
mean cluster size grows proportionally to the concentration, as observed in
several experiments. We emphasize the interest of a predictive theory in soft
matter, nano-technologies and biophysics.Comment: 4 pages, 1 figur
Coarse-Grained Simulations of Membranes under Tension
We investigate the properties of membranes under tension by Monte-Carlo
simulations of a generic coarse-grained model for lipid bilayers. We give a
comprising overview of the behavior of several membrane characteristics, such
as the area per lipid, the monolayer overlap, the nematic order, and pressure
profiles. Both the low-temperature regime, where the membranes are in a gel
phase, and the high-temperature regime, where they are in the fluid phase, are
considered. In the gel state, the membrane is hardly influenced by tension. In
the fluid state, high tensions lead to structural changes in the membrane,
which result in different compressibility regimes. The ripple state, which is
found at tension zero in the transition regime between the fluid and the gel
phase, disappears under tension and gives way to an interdigitated phase. We
also study the membrane fluctuations in the fluid phase. In the low tension
regime the data can be fitted nicely to a suitably extended elastic theory. At
higher tensions the elastic fit consistently underestimates the strength of
long-wavelength fluctuations. Finally, we investigate the influence of tension
on the effective interaction between simple transmembrane inclusions and show
that tension can be used to tune the hydrophobic mismatch interaction between
membrane proteins.Comment: 14 pages, 14 figures, accepted for publication in The Journal of
Chemical Physic
First Order Phase Transition in the 3-dimensional Blume-Capel Model on a Cellular Automaton
The first order phase transition of the three-dimensional Blume Capel are
investigated using cooling algorithm which improved from Creutz Cellular
Automaton for the parameter value in the first order phase transition
region. The analysis of the data using the finite-size effect and the histogram
technique indicate that the magnetic susceptibility maxima and the specific
heat maxima increase with the system volume () at .Comment: 13 pages, 4 figure
Main phase transition in lipid bilayers: phase coexistence and line tension in a soft, solvent-free, coarse-grained model
We devise a soft, solvent-free, coarse-grained model for lipid bilayer
membranes. The non-bonded interactions take the form of a weighted-density
functional which allows us to describe the thermodynamics of self-assembly and
packing effects of the coarse-grained beads in terms of a density expansion of
the equation of state and the weighting functions that regularize the
microscopic bead densities, respectively. Identifying the length and energy
scales via the bilayer thickness and the thermal energy scale, kT, the model
qualitatively reproduces key characteristics (e.g., bending rigidity, area per
lipid molecules, and compressibility) of lipid membranes. We employ this model
to study the main phase transition between the liquid and the gel phase of the
bilayer membrane. We accurately locate the phase coexistence using free energy
calculations and also obtain estimates for the bare and the thermodynamic line
tension.Comment: 21 pages, 12 figures. Submitted to J. Chem. Phy
Cover Article Research Articles, Systems/Circuits
Double cones are the most common photoreceptor cell type in most avian retinas, but their precise functions remain a mystery. Among their suggested functions are luminance detection, polarized light detection, and light-dependent, radical-pair-based magnetoreception. To better understand the function of double cones, it will be crucial to know how they are connected to the neural network in the avian retina. Here we use serial sectioning, multi-beam scanning electron microscopy (ssmSEM) to investigate double cone anatomy and connectivity with a particular focus on their contacts to other photoreceptor and bipolar cells in the chicken retina. We found that double cones are highly connected with neighbouring double cones and with other photoreceptor cells through telodendria-to-terminal and telodendria-to-telodendria contacts. We also identified 15 bipolar cell types based on their axonal stratifications, photoreceptor contact pattern, soma position, and dendritic and axonal field mosaics. Thirteen of these 15 bipolar cell types contacted at least one or both members of the double cone. All bipolar cells were bi- or multistratified. We also identified surprising contacts between other cone types and between rods and cones. Our data indicate a much more complex connectivity network in the outer plexiform layer of the avian retina than originally expected
From supported membranes to tethered vesicles: lipid bilayers destabilisation at the main transition
We report results concerning the destabilisation of supported phospholipid
bilayers in a well-defined geometry. When heating up supported phospholipid
membranes deposited on highly hydrophilic glass slides from room temperature
(i.e. with lipids in the gel phase), unbinding was observed around the main gel
to fluid transition temperature of the lipids. It lead to the formation of
relatively monodisperse vesicles, of which most remained tethered to the
supported bilayer. We interpret these observations in terms of a sharp decrease
of the bending rigidity modulus in the transition region, combined
with a weak initial adhesion energy. On the basis of scaling arguments, we show
that our experimental findings are consistent with this hypothesis.Comment: 11 pages, 3 figure
Stochastic process behind nonlinear thermodynamic quantum master equation
We propose a piecewise deterministic Markovian jump process in Hilbert space
such that the covariance matrix of this stochastic process solves the
thermodynamic quantum master equation. The proposed stochastic process is
particularly simple because the normalization of the vectors in Hilbert space
is preserved only on average. As a consequence of the nonlinearity of the
thermodynamic master equation, the construction of stochastic trajectories
involves the density matrix as a running ensemble average. We identify a
principle of detailed balance and a fluctuation-dissipation relation for our
Markovian jump process.Comment: 4 page
A Visual Pathway Links Brain Structures Active during Magnetic Compass Orientation in Migratory Birds
The magnetic compass of migratory birds has been suggested to be light-dependent. Retinal cryptochrome-expressing neurons and a forebrain region, “Cluster N”, show high neuronal activity when night-migratory songbirds perform magnetic compass orientation. By combining neuronal tracing with behavioral experiments leading to sensory-driven gene expression of the neuronal activity marker ZENK during magnetic compass orientation, we demonstrate a functional neuronal connection between the retinal neurons and Cluster N via the visual thalamus. Thus, the two areas of the central nervous system being most active during magnetic compass orientation are part of an ascending visual processing stream, the thalamofugal pathway. Furthermore, Cluster N seems to be a specialized part of the visual wulst. These findings strongly support the hypothesis that migratory birds use their visual system to perceive the reference compass direction of the geomagnetic field and that migratory birds “see” the reference compass direction provided by the geomagnetic field
Effects of cholesterol on the binding of the precursor neurotransmitter tryptophan to zwitterionic membranes
The characterization of the microscopical forces between the essential a-amino-acid tryptophan, precursor of the neurotransmitter serotonin and of the hormone melatonin, and the basic components of cell membranes and their environments (phospholipids, cholesterol, ionic species, and water) is of central importance to elucidate their local structure and dynamics as well as the mechanisms responsible for the access of tryptophan to the interior of the cell. We have performed nanosecond molecular dynamics simulations of tryptophan embedded in model zwitterionic bilayer membranes made by di-palmitoyl-phosphatidyl-choline and cholesterol inside aqueous sodium-chloride solution in order to systematically examine tryptophan-lipid, tryptophan-cholesterol, and tryptophan-water interactions under liquid-crystalline phase conditions. Microscopic properties such as the area per lipid, lipid thickness, radial distribution functions, hydrogen-bonding lengths, atomic spectral densities, and self-diffusion coefficients have been evaluated. Our results show that the presence of tryptophan significantly affects the structure and dynamics of the membrane. Tryptophan spends long periods of time at the water-membrane interface, and it plays a central role by bridging a few lipids and cholesterol chains by means of hydrogen-bonds. The computed spectral densities, in excellent agreement with experimental infrared and Raman data, revealed the participation of each atomic site of tryptophan to the complete spectrum of the molecule. Tryptophan self-diffusion coefficients have been found to be in between 10^(-7) and 10^(-6) cm^2/s and strongly depending of the concentration of cholesterol in the system.Postprint (published version
Vesicle shape, molecular tilt, and the suppression of necks
Can the presence of molecular-tilt order significantly affect the shapes of
lipid bilayer membranes, particularly membrane shapes with narrow necks?
Motivated by the propensity for tilt order and the common occurrence of narrow
necks in the intermediate stages of biological processes such as endocytosis
and vesicle trafficking, we examine how tilt order inhibits the formation of
necks in the equilibrium shapes of vesicles. For vesicles with a spherical
topology, point defects in the molecular order with a total strength of
are required. We study axisymmetric shapes and suppose that there is a
unit-strength defect at each pole of the vesicle. The model is further
simplified by the assumption of tilt isotropy: invariance of the energy with
respect to rotations of the molecules about the local membrane normal. This
isotropy condition leads to a minimal coupling of tilt order and curvature,
giving a high energetic cost to regions with Gaussian curvature and tilt order.
Minimizing the elastic free energy with constraints of fixed area and fixed
enclosed volume determines the allowed shapes. Using numerical calculations, we
find several branches of solutions and identify them with the branches
previously known for fluid membranes. We find that tilt order changes the
relative energy of the branches, suppressing thin necks by making them costly,
leading to elongated prolate vesicles as a generic family of tilt-ordered
membrane shapes.Comment: 10 pages, 7 figures, submitted to Phy. Rew.
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