417 research outputs found
The main transition in the Pink membrane model: finite-size scaling and the influence of surface roughness
We consider the main transition in single-component membranes using computer
simulations of the Pink model [D. Pink {\it et al.}, Biochemistry {\bf 19}, 349
(1980)]. We first show that the accepted parameters of the Pink model yield a
main transition temperature that is systematically below experimental values.
This resolves an issue that was first pointed out by Corvera and co-workers
[Phys. Rev. E {\bf 47}, 696 (1993)]. In order to yield the correct transition
temperature, the strength of the van der Waals coupling in the Pink model must
be increased; by using finite-size scaling, a set of optimal values is
proposed. We also provide finite-size scaling evidence that the Pink model
belongs to the universality class of the two-dimensional Ising model. This
finding holds irrespective of the number of conformational states. Finally, we
address the main transition in the presence of quenched disorder, which may
arise in situations where the membrane is deposited on a rough support. In this
case, we observe a stable multi-domain structure of gel and fluid domains, and
the absence of a sharp transition in the thermodynamic limit.Comment: submitted to PR
Getting More from Pushing Less: Negative Specific Heat and Conductivity in Non-equilibrium Steady States
For students familiar with equilibrium statistical mechanics, the notion of a
positive specific heat, being intimately related to the idea of stability, is
both intuitively reasonable and mathematically provable. However, for system in
non-equilibrium stationary states, coupled to more than one energy reservoir
(e.g., thermal bath), negative specific heat is entirely possible. In this
paper, we present a ``minimal'' system displaying this phenomenon. Being in
contact with two thermal baths at different temperatures, the (internal) energy
of this system may increase when a thermostat is turned down. In another
context, a similar phenomenon is negative conductivity, where a current may
increase by decreasing the drive (e.g., an external electric field). The
counter-intuitive behavior in both processes may be described as `` getting
more from pushing less.'' The crucial ingredients for this phenomenon and the
elements needed for a ``minimal'' system are also presented.Comment: 14 pages, 3 figures, accepted for publication in American Journal of
Physic
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
Cluster phases of membrane proteins
A physical scenario accounting for the existence of size-limited
submicrometric domains in cell membranes is proposed. It is based on the
numerical investigation of the counterpart, in lipidic membranes where proteins
are diffusing, of the recently discovered cluster phases in colloidal
suspensions. I demonstrate that the interactions between proteins, namely
short-range attraction and longer-range repulsion, make possible the existence
of stable small clusters. The consequences are explored in terms of membrane
organization and diffusion properties. The connection with lipid rafts is
discussed and the apparent protein diffusion coefficient as a function of their
concentration is analyzed.Comment: 5 pages - enhanced versio
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
Theoretical investigation of finite size effects at DNA melting
We investigated how the finiteness of the length of the sequence affects the
phase transition that takes place at DNA melting temperature. For this purpose,
we modified the Transfer Integral method to adapt it to the calculation of both
extensive (partition function, entropy, specific heat, etc) and non-extensive
(order parameter and correlation length) thermodynamic quantities of finite
sequences with open boundary conditions, and applied the modified procedure to
two different dynamical models. We showed that rounding of the transition
clearly takes place when the length of the sequence is decreased. We also
performed a finite-size scaling analysis of the two models and showed that the
singular part of the free energy can indeed be expressed in terms of an
homogeneous function. However, both the correlation length and the average
separation between paired bases diverge at the melting transition, so that it
is no longer clear to which of these two quantities the length of the system
should be compared. Moreover, Josephson's identity is satisfied for none of the
investigated models, so that the derivation of the characteristic exponents
which appear, for example, in the expression of the specific heat, requires
some care
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
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
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
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