1,419 research outputs found
Minimal Bending Energies of Bilayer Polyhedra
Motivated by recent experiments on bilayer polyhedra composed of amphiphilic
molecules, we study the elastic bending energies of bilayer vesicles forming
polyhedral shapes. Allowing for segregation of excess amphiphiles along the
ridges of polyhedra, we find that bilayer polyhedra can indeed have lower
bending energies than spherical bilayer vesicles. However, our analysis also
implies that, contrary to what has been suggested on the basis of experiments,
the snub dodecahedron, rather than the icosahedron, generally represents the
energetically favorable shape of bilayer polyhedra
Non-local fluctuation correlations in active gels
Many active materials and biological systems are driven far from equilibrium
by embedded agents that spontaneously generate forces and distort the
surrounding material. Probing and characterizing these athermal fluctuations is
essential for understanding the properties and behaviors of such systems. Here
we present a mathematical procedure to estimate the local action of
force-generating agents from the observed fluctuating displacement fields. The
active agents are modeled as oriented force dipoles or isotropic compression
foci, and the matrix on which they act is assumed to be either a compressible
elastic continuum or a coupled network-solvent system. Correlations at a single
point and between points separated by an arbitrary distance are obtained,
giving a total of three independent fluctuation modes that can be tested with
microrheology experiments. Since oriented dipoles and isotropic compression
foci give different contributions to these fluctuation modes, ratiometric
analysis allows us characterize the force generators. We also predict and
experimentally find a high-frequency ballistic regime, arising from individual
force generating events in the form of the slow build-up of stress followed by
rapid but finite decay. Finally, we provide a quantitative statistical model to
estimate the mean filament tension from these athermal fluctuations, which
leads to stiffening of active networks.Comment: 12 pages, 7 figures; some clarifications and ammended figure
notation
Imaging of Sources in Heavy-Ion Reactions
Imaging of sources from data within the intensity interferometry is
discussed. In the two-pion case, the relative pion source function may be
determined through the Fourier transformation of the correlation function. In
the proton-proton case, the discretized source function may be fitted to the
correlation data.Comment: 12 pages, 3 postscript figures, accepted Physics Letters
Effect of symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei
Using an isospin-dependent transport model, we study the effects of nuclear
symmetry energy on two-nucleon correlation functions in heavy ion collisions
induced by neutron-rich nuclei. We find that the density dependence of the
nuclear symmetry energy affects significantly the nucleon emission times in
these collisions, leading to larger values of two-nucleon correlation functions
for a symmetry energy that has a stronger density dependence. Two-nucleon
correlation functions are thus useful tools for extracting information about
the nuclear symmetry energy from heavy ion collisions.Comment: Revised version, to appear in Phys. Rev. Let
Intermittency and Exotic Channels
It is pointed out that accurate measurements of short-range two-particle
correlations in like-charge and in channels should be
very helpful in determining the origin of the \lq\lq intermittency\rq\rq\
phenomenon observed recently for the like-charge pion pairs.Comment: 5 p., plain tex, preprint T94/078(Saclay), LPTHE 94/58(Orsay
Protein-DNA charge transport: Redox activation of a DNA repair protein by guanine radical
DNA charge transport (CT) chemistry provides a route to carry out oxidative DNA damage from a distance in a reaction that is sensitive to DNA mismatches and lesions. Here, DNA-mediated CT also leads to oxidation of a DNA-bound base excision repair enzyme, MutY. DNA-bound Ru(III), generated through a flash/quench technique, is found to promote oxidation of the [4Fe-4S](2+) cluster of MutY to [4Fe-4S](3+) and its decomposition product [3Fe-4S](1+). Flash/quench experiments monitored by EPR spectroscopy reveal spectra with g = 2.08, 2.06, and 2.02, characteristic of the oxidized clusters. Transient absorption spectra of poly(dGC) and [Ru(phen)(2)dppz](3+) (dppz = dipyridophenazine), generated in situ, show an absorption characteristic of the guanine radical that is depleted in the presence of MutY with formation instead of a long-lived species with an absorption at 405 nm; we attribute this absorption also to formation of the oxidized [4Fe-4S](3+) and [3Fe4S](1+) clusters. In ruthenium-tethered DNA assemblies, oxidative damage to the 5'-G of a 5'-GG-3' doublet is generated from a distance but this irreversible damage is inhibited by MutY and instead EPR experiments reveal cluster oxidation. With ruthenium-tethered assemblies containing duplex versus single-stranded regions, MutY oxidation is found to be mediated by the DNA duplex, with guanine radical as an intermediate oxidant; guanine radical formation facilitates MutY oxidation. A model is proposed for the redox activation of DNA repair proteins through DNA CT, with guanine radicals, the first product under oxidative stress, in oxidizing the DNA-bound repair proteins, providing the signal to stimulate DNA repair
Ductus arteriosus aneurysm presenting as pulmonary artery obstruction: Diagnosis and management
The occurrence of pulmonary artery obstruction in an 8 day old infant as a complication of an aneurysm of a nonpatent ductus arteriosus is reported, together with the echocardiographic and angiographic findings. To relieve the obstruction, the aneurysm and an intrapulmonary thrombus were successfully removed with the use of cardiopulmonary bypass when the infant was 3 months old
C14. Um caso de polimiosite, fibrose pulmonar e cancro do pulmão
The authors present the case of a 67 year-old man with a 50-pack-year history of cigarette smoking. He had been in the navy for 30 years where he was exposed to asbestos fibbers.He had been asymptomatic until the age of 62, when he started to experience increasing exertional dyspnoea. One year later he had an episode of fever, productive cough and myalgias. Digital clubbing was noticed and bibasilar crackles were present on chest auscultation.Thoracic CT scan obtained at that time, were compatible with Usual Interstitial Pneumonia and bronchoalveolar lavage excluded other diseases. Pulmonary function studies: restriction and decreased DLco.Deflazacort, acetylcysteine and azathioprine were administered. Azathioprine was then switched to cyclophosphamide because of clinical and functional deterioration. Two years later he began Interferon Gamma 1b, with clinical and functional improvement lasting one year.He was then diagnosed Polymyositis and received Immunoglobulin.Twelve months later he was admitted to the hospital with intermittent fever, non productive cough, worsening exertional dyspnoea and myalgias. The Thoracic scan showed honeycombing and bronchiectasis. In the left inferior lobe there was a peripheral nodule. The patient was submitted to a Thansthoracic Needle Biopsy, the tissue immunocytochemistry pattern revealed Small Cell Lung Cancer. In the staging procedures there were positive mediastinal and hilar nodes and multiple hepatic metastases.Only one administration of chemotherapy was performed with carboplatinum and etoposide, without response. Unfortunately the patient died a fortnight later.Polymyositis has been associated with a variety of malignancies, in this case, the patient developed pulmonary fibrosis previously to the diagnosis of Polymyositis, and only several years later Small Cell Lung cancer was diagnosed
Spicules and the effect of rigid rods on enclosing membrane tubes
Membrane tubes (spicules) arise in cells, or artificial membranes, in the
nonlinear deformation regime due to, e.g. the growth of microtubules, actin
filaments or sickle hemoglobin fibers towards a membrane. We calculate the
axial force exerted by the cylindrical membrane tube, and its average radius,
by taking into account steric interactions between the fluctuating membrane and
the enclosed rod. The force required to confine a fluctuating membrane near the
surface of the enclosed rod diverges as the separation approaches zero. This
results in a smooth crossover of the axial force between a square root and a
linear dependence on the membrane tension as the tension increases and the tube
radius shrinks. This crossover can occur at the most physiologically relevant
membrane tensions. Our work may be important in (i) interpreting experiments in
which axial force is related to the tube radius or membrane tension (ii)
dynamical theories for biopolymer growth in narrow tubes where these
fluctuation effects control the tube radius.Comment: 10 pages, 1 figur
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