3,043 research outputs found
Local orientational ordering in fluids of spherical molecules with dipolar-like anisotropic adhesion
We discuss some interesting physical features stemming from our previous
analytical study of a simple model of a fluid with dipolar-like interactions of
very short range in addition to the usual isotropic Baxter potential for
adhesive spheres. While the isotropic part is found to rule the global
structural and thermodynamical equilibrium properties of the fluid, the weaker
anisotropic part gives rise to an interesting short-range local ordering of
nearly spherical condensation clusters, containing short portions of chains
having nose-to-tail parallel alignment which runs antiparallel to adjacent
similar chains.Comment: 13 pages and 6 figure
Postcritical Behavior of Cables Undergoing Two Simultaneous Galloping Modes
A nonlinear two degree-of-freedom model, describing a flexible elastic suspended cable undergoing galloping oscillations, is analyzed. By using a perturbative approach, the critical conditions occuring for different values of the aerodynamic coefficients are described. Two different type of critical conditions, corresponding to simple or double Hopf bifurcations are found. The nonlinear postcritical behavior of single taut strings in 1:1 primary internal resonance is studied through the multiple scale perturbation method. In the double Hopf bifurcation case the influence of the detuning between the critical eigenvalues on the postcritical behavior is illustrated. It is found that quasi-periodic motions, which are likely to occur in the linear field when the two critical frequencies are incommensurable, are really unstable in the nonlinear range. Therefore, the postcritical behavior of the string consists of stable periodic motions for any detuning values
Gravitational waves from hyperbolic encounters
The emission of gravitational waves from a system of massive objects
interacting on hyperbolic orbits is studied in the quadrupole approximation.
Analytic expressions are derived for the gravitational radiation luminosity,
the total energy output and the gravitational radiation amplitude. An
estimation of the expected number of events towards different targets (i.e.
globular clusters and the center of the Galaxy) is also given. In particular,
for a dense stellar cluster at the galactic center, a rate up to one event per
year is obtained.Comment: 6 pages, 2 figure
Phase diagram and structural properties of a simple model for one-patch particles
We study the thermodynamic and structural properties of a simple, one-patch
fluid model using the reference hypernetted-chain (RHNC) integral equation and
specialized Monte Carlo simulations. In this model, the interacting particles
are hard spheres, each of which carries a single identical,
arbitrarily-oriented, attractive circular patch on its surface; two spheres
attract via a simple square-well potential only if the two patches on the
spheres face each other within a specific angular range dictated by the size of
the patch. For a ratio of attractive to repulsive surface of 0.8, we construct
the RHNC fluid-fluid separation curve and compare with that obtained by Gibbs
ensemble and grand canonical Monte Carlo simulations. We find that RHNC
provides a quick and highly reliable estimate for the position of the
fluid-fluid critical line. In addition, it gives a detailed (though
approximate) description of all structural properties and their dependence on
patch size.Comment: 27 pages, 10 figures, J. Chem. Phys. in pres
Phase diagrams of Janus fluids with up-down constrained orientations
A class of binary mixtures of Janus fluids formed by colloidal spheres with
the hydrophobic hemispheres constrained to point either up or down are studied
by means of Gibbs ensemble Monte Carlo simulations and simple analytical
approximations. These fluids can be experimentally realized by the application
of an external static electrical field. The gas-liquid and demixing phase
transitions in five specific models with different patch-patch affinities are
analyzed. It is found that a gas-liquid transition is present in all the
models, even if only one of the four possible patch-patch interactions is
attractive. Moreover, provided the attraction between like particles is
stronger than between unlike particles, the system demixes into two subsystems
with different composition at sufficiently low temperatures and high densities.Comment: 10 pages, 6 figure
Homozygous mutation in the prokineticin-receptor2 gene (Val274Asp) presenting as reversible Kallmann syndrome and persistent oligozoospermia: case report.
Prokineticin 2 (Prok2) or prokineticin-receptor2 (Prok-R2) gene mutations are associated with Kallmann syndrome
(KS). We describe a new homozygous mutation of Prok-R2 gene in a man displaying KS with an apparent reversal of
hypogonadism. The proband, offspring of consanguineous parents, presented at age 19 years with absent puberty, no
sense of smell, low testosterone and gonadotrophin levels. Magnetic resonance imaging showed olfactory bulb absence.
The patient achieved virilization and spermatogenesis with gonadotrophin administration. Two years after discontinuing
hormonal therapy, he maintained moderate oligozoospermia and normal testosterone levels. Prok2 and Prok-
R2 gene sequence analyses were performed. The proband had a homozygous mutation in Prok-R2 exon 2 that harbours
the c.T820>A base substitution, causing the introduction of an aspartic acid in place of valine at position 274
(Val274Asp). His mother had the same mutation in heterozygous state. This report describes a novel homozygous
mutation of Prok-R2 gene in a man with variant KS, underlying the role of Prok-R2 gene in the olfactory and reproductive
system development in humans. Present findings indicate that markedly delayed activation of gonadotrophin
secretion may occur in some KS cases with definite gene defects, and that oligozoospermia might result from a variant
form of reversible hypogonadotrophic hypogonadism
From rods to helices: evidence of a screw-like nematic phase
Evidence of a special chiral nematic phase is provided using numerical
simulation and Onsager theory for systems of hard helical particles. This phase
appears at the high density end of the nematic phase, when helices are well
aligned, and is characterized by the C symmetry axes of the helices
spiraling around the nematic director with periodicity equal to the particle
pitch. This coupling between translational and rotational degrees of freedom
allows a more efficient packing and hence an increase of translational entropy.
Suitable order parameters and correlation functions are introduced to identify
this screw-like phase, whose main features are then studied as a function of
radius and pitch of the helical particles. Our study highlights the physical
mechanism underlying a similar ordering observed in colloidal helical flagella
[E. Barry et al. \textit{Phys. Rev. Lett.} \textbf{96}, 018305 (2006)] and
raises the question of whether it could be observed in other helical particle
systems, such as DNA, at sufficiently high densities.Comment: List of authors correcte
Effects of patch size and number within a simple model of patchy colloids
We report on a computer simulation and integral equation study of a simple
model of patchy spheres, each of whose surfaces is decorated with two opposite
attractive caps, as a function of the fraction of covered attractive
surface. The simple model explored --- the two-patch Kern-Frenkel model ---
interpolates between a square-well and a hard-sphere potential on changing the
coverage . We show that integral equation theory provides quantitative
predictions in the entire explored region of temperatures and densities from
the square-well limit down to . For smaller
, good numerical convergence of the equations is achieved only at
temperatures larger than the gas-liquid critical point, where however integral
equation theory provides a complete description of the angular dependence.
These results are contrasted with those for the one-patch case. We investigate
the remaining region of coverage via numerical simulation and show how the
gas-liquid critical point moves to smaller densities and temperatures on
decreasing . Below , crystallization prevents the
possibility of observing the evolution of the line of critical points,
providing the angular analog of the disappearance of the liquid as an
equilibrium phase on decreasing the range for spherical potentials. Finally, we
show that the stable ordered phase evolves on decreasing from a
three-dimensional crystal of interconnected planes to a two-dimensional
independent-planes structure to a one-dimensional fluid of chains when the
one-bond-per-patch limit is eventually reached.Comment: 26 pages, 11 figures, J. Chem. Phys. in pres
Chemical defence by sterols in the freshwater ciliate Stentor polymorphus
Heterotrich ciliates typically retain toxic substances in specialized ejectable organelles, called extrusomes, which are used in predator-prey interactions. In this study, we analysed the chemical defence strategy of the freshwater heterotrich ciliate Stentor polymorphus against the predatory ciliate Coleps hirtus, and the microturbellarian flatworm Stenostomum sphagnetorum. The results showed that S. polymorphus is able to defend itself against these two predators by deploying a mix of bioactive sterols contained in its extrusomes. Sterols were isolated in vivo and characterized by liquid chromatography-mass spectrometry (LC-MS), and nuclear magnetic resonance (NMR), as ergosterol, 7-dehydroporiferasterol, and their two peroxidized analogues. The assessment of the toxicity of ergosterol and ergosterol peroxide against various organisms, indicated that these sterols are essential for the effectiveness of the chemical defence in S. polymorphus
Chromosome 5 allelic losses are early events in tumours of the papilla of Vater and occur at sites similar to those of gastric cancer.
During our studies of DNA fingerprinting of tumours of the pancreas and papilla (ampulla) of Vater, using arbitrarily primed polymerase chain reaction (AP-PCR), we noticed two bands showing a decreased intensity in six of ten ampullary tumours with respect to matched normal tissues. Those bands were both assigned to chromosome 5. Such a finding was somewhat in contrast with the reportedly low frequency of APC gene mutations in ampullary cancers, located at chromosome 5q21, and suggested that loci different from that of APC might be the target of chromosome 5 allelic losses (LOH) in these tumours. Therefore, we analysed chromosome 5 LOH in a panel of 27 ampullary tumours, including eight adenomas, four early- and 15 advanced-stage cancers, using 16 PCR-amplified CA microsatellite polymorphic markers spanning the entire chromosome. Nineteen cases (70%) showed LOH, and the interstitial deletions found in these tumours described two smallest common deleted regions, in which putative suppressor genes might reside. They were at 5q13.3-q14 and at 5q23-q31 respectively, which correspond to those found in gastric tumours. In addition, the presence of 5q LOH in six of eight adenomas and in three of four early-stage cancers suggests that such phenomena occur at early stages of neoplastic progression of the ampullary epithelium
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