1,961 research outputs found
Distribution of lipids in non-lamellar phases of their mixtures
We consider a model of lipids in which a head group, characterized by its
volume, is attached to two flexible tails of equal length. The phase diagram of
the anhydrous lipid is obtained within self-consistent field theory, and
displays, as a function of lipid architecture, a progression of phases:
body-centered cubic, hexagonal, gyroid, and lamellar. We then examine mixtures
of an inverted hexagonal forming lipid and a lamellar forming lipid. As the
volume fractions of the two lipids vary, we find that inverted hexagonal,
gyroid, or lamellar phases are formed. We demonstrate that the non-lamellar
forming lipid is found preferentially at locations which are difficult for the
lipid tails to reach. Variations in the volume fraction of each type of lipid
tail are on the order of one to ten per cent within regions dominated by the
tails. We also show that the variation in volume fraction is correlated
qualitatively with the variation in mean curvature of the head-tail interface.Comment: 10 pages, 12 figures (better figures are available upon request), to
appear in J. Chem. Phy
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Meiotic cellular rejuvenation is coupled to nuclear remodeling in budding yeast.
Production of healthy gametes in meiosis relies on the quality control and proper distribution of both nuclear and cytoplasmic contents. Meiotic differentiation naturally eliminates age-induced cellular damage by an unknown mechanism. Using time-lapse fluorescence microscopy in budding yeast, we found that nuclear senescence factors - including protein aggregates, extrachromosomal ribosomal DNA circles, and abnormal nucleolar material - are sequestered away from chromosomes during meiosis II and subsequently eliminated. A similar sequestration and elimination process occurs for the core subunits of the nuclear pore complex in both young and aged cells. Nuclear envelope remodeling drives the formation of a membranous compartment containing the sequestered material. Importantly, de novo generation of plasma membrane is required for the sequestration event, preventing the inheritance of long-lived nucleoporins and senescence factors into the newly formed gametes. Our study uncovers a new mechanism of nuclear quality control and provides insight into its function in meiotic cellular rejuvenation
A microscopic approach to critical phenomena at interfaces: an application to complete wetting in the Ising model
We study how the formalism of the Hierarchical Reference Theory (HRT) can be
extended to inhomogeneous systems. HRT is a liquid state theory which
implements the basic ideas of Wilson momentum shell renormalization group (RG)
to microscopic Hamiltonians. In the case of homogeneous systems, HRT provides
accurate results even in the critical region, where it reproduces scaling and
non-classical critical exponents. We applied the HRT to study wetting critical
phenomena in a planar geometry. Our formalism avoids the explicit definition of
effective surface Hamiltonians but leads, close to the wetting transition, to
the same renormalization group equation already studied by RG techiques.
However, HRT also provides information on the non universal quantities because
it does not require any preliminary coarse graining procedure. A simple
approximation to the infinite HRT set of equations is discussed. The HRT
evolution equation for the surface free energy is numerically integrated in a
semi-infinite three-dimensional Ising model and the complete wetting phase
transition is analyzed. A renormalization of the adsorption critical amplitude
and of the wetting parameter is observed. Our results are compared to available
Monte Carlo simulations.Comment: To be published in Phy. Rev.
Segmental relaxation in semicrystalline polymers: a mean field model for the distribution of relaxation times in confined regimes
The effect of confinement in the segmental relaxation of polymers is
considered. On the basis of a thermodynamic model we discuss the emerging
relevance of the fast degrees of freedom in stimulating the much slower
segmental relaxation, as an effect of the constraints at the walls of the
amorphous regions. In the case that confinement is due to the presence of
crystalline domains, a quasi-poissonian distribution of local constraining
conditions is derived as a result of thermodynamic equilibrium. This implies
that the average free energy barrier for conformational
rearrangement is of the same order of the dispersion of the barrier heights,
, around . As an example, we apply the results to
the analysis of the -relaxation as observed by dielectric broad band
spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized
from either an isotropic or an oriented glass. It is found that in the latter
case the regions of cooperative rearrangement are significantly larger than in
the former.Comment: 10 pages, 4 figures .ep
Monte Carlo Study of an Extended 3-State Potts Model on the Triangular Lattice
By introducing a chiral term into the Hamiltonian of the 3-state Potts model
on a triangular lattice additional symmetries are achieved between the
clockwise and anticlockwise states and the ferromagnetic state. This model is
investigated using Monte Carlo methods. We investigate the full phase diagram
and find evidence for a line tricritical points separating the ferromagnetic
and antiferromagnetic phases.Comment: 6 pages, 10 figure
Hodge Theory on Metric Spaces
Hodge theory is a beautiful synthesis of geometry, topology, and analysis,
which has been developed in the setting of Riemannian manifolds. On the other
hand, spaces of images, which are important in the mathematical foundations of
vision and pattern recognition, do not fit this framework. This motivates us to
develop a version of Hodge theory on metric spaces with a probability measure.
We believe that this constitutes a step towards understanding the geometry of
vision.
The appendix by Anthony Baker provides a separable, compact metric space with
infinite dimensional \alpha-scale homology.Comment: appendix by Anthony W. Baker, 48 pages, AMS-LaTeX. v2: final version,
to appear in Foundations of Computational Mathematics. Minor changes and
addition
Three-Body approach to the K^- d Scattering Length in Particle Basis
We report on the first calculation of the scattering length A_{K^-d} based on
a relativistic three-body approach where the two-body input amplitudes coupled
to the Kbar N channels have been obtained with the chiral SU(3) constraint, but
with isospin symmetry breaking effects taken into account. Results are compared
with a recent calculation applying a similar set of two-body amplitudes,based
on the fixed center approximation, considered as a good approximation for a
loosely bound target, and for which we find significant deviations from the
exact three-body results. Effects of the hyperon-nucleon interaction, and
deuteron -wave component are also evaluated.Comment: 5 pages, Submitted to Phys. Rev.
Derivation of a Non-Local Interfacial Hamiltonian for Short-Ranged Wetting II: General Diagrammatic Structure
In our first paper, we showed how a non-local effective Hamiltionian for
short-ranged wetting may be derived from an underlying Landau-Ginzburg-Wilson
model. Here, we combine the Green's function method with standard perturbation
theory to determine the general diagrammatic form of the binding potential
functional beyond the double-parabola approximation for the
Landau-Ginzburg-Wilson bulk potential. The main influence of cubic and quartic
interactions is simply to alter the coefficients of the double parabola-like
zig-zag diagrams and also to introduce curvature and tube-interaction
corrections (also represented diagrammatically), which are of minor importance.
Non-locality generates effective long-ranged many-body interfacial interactions
due to the reflection of tube-like fluctuations from the wall. Alternative wall
boundary conditions (with a surface field and enhancement) and the diagrammatic
description of tricritical wetting are also discussed.Comment: (14 pages, 2 figures) Submitted J. Phys. Condens. Matte
Serum S100B is increased during early treatment with antipsychotics and in deficit schizophrenia
Previous studies reported controversial results concerning alterations of astrocytes in schizophrenia. Because S100B may be regarded as a marker for astrocytes, the objective of this study was to examine S100B serum concentrations in 30 patients with schizophrenia with a monoclonal two-site immunoluminometric assay that specifically detects S100B. An ANOVA revealed medication (p0.05). Patients with deficit (250.6±154.9 ng/l) had higher S100B levels than patients with nondeficit schizophrenia (146.7±107.2 ng/l, p<0.05) or controls (p<0.005). S100B was positively correlated with the subscore âthought disturbanceâ of the Brief Psychiatric Rating Scale (p<0.05). In summary, increased serum levels of S100B may indicate alterations of astrocytes during early treatment with antipsychotics and in deficit schizophrenia. Whether S100B is elevated due to injured astrocytes and a disrupted bloodâbrain barrier, or by active secretion of S100B by astrocytes, has to be clarified by further studies
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