709 research outputs found
Extended corresponding-states behavior for particles with variable range attractions
We propose an extension of the law of corresponding states that can be
applied to systems - such as colloidal suspensions - that have widely different
ranges of attractive interactions. We argue that, for such systems, the
``reduced'' second virial coefficient is a convenient parameter to quantify the
effective range of attraction. This procedure allows us to give a simple
definition of the effective range of attraction of potentials with different
functional forms. The advantage of the present approach is that it allows us to
estimate the relative location of the liquid-vapor and solid-fluid coexistence
curves exclusively on basis of the knowledge of the pair-potential.Comment: REVTeX, 5 pages, 2 figure
The role of long-range forces in the phase behavior of colloids and proteins
The phase behavior of colloid-polymer mixtures, and of solutions of globular
proteins, is often interpreted in terms of a simple model of hard spheres with
short-ranged attraction. While such a model yields a qualitative understanding
of the generic phase diagrams of both colloids and proteins, it fails to
capture one important difference: the model predicts fluid-fluid phase
separation in the metastable regime below the freezing curve. Such demixing has
been observed for globular proteins, but for colloids it appears to be
pre-empted by the appearance of a gel. In this paper, we study the effect of
additional long-range attractions on the phase behavior of spheres with
short-ranged attraction. We find that such attractions can shift the
(metastable) fluid-fluid critical point out of the gel region. As this
metastable critical point may be important for crystal nucleation, our results
suggest that long-ranged attractive forces may play an important role in the
crystallization of globular proteins. However, in colloids, where refractive
index matching is often used to switch off long-ranged dispersion forces,
gelation is likely to inhibit phase separation.Comment: EURO-LATEX, 6 pages, 2 figure
Lamellar and inverse micellar structures of skin lipids: Effect of templating
The outermost layer of skin, the stratum corneum (SC), comprises rigid
corneocytes in a layered lipid matrix. Using atomistic simulations we find that
the equilibrium phase of the SC lipids is inverse micellar. A model of the
corneocyte is used to demonstrate that lamellar layering is induced by the
patterned corneocyte wall. The inverse micellar phase is consistent with in
vivo observations in the lacunar spaces and at the stratum granulosum - SC
boundary region, and suggests a functional role in the lipid synthesis pathway
in vivo.Comment: pdflatex 5 pages, 10 page supplementary material. Published Physical
Review Letters. Added link to website with animation
Complete structure of an epithelial keratin dimer: implications for intermediate filament assembly
Keratins are cytoskeletal proteins that hierarchically arrange into filaments, starting with the dimer sub-unit. They are integral to the structural support of cells, in skin, hair and nails. In skin, keratin is thought to play a critical role in conferring the barrier properties and elasticity of skin. In general, the keratin dimer is broadly described by a tri-domain structure: a head, a central rod and a tail. As yet, no atomistic-scale picture of the entire dimer structure exists; this information is pivotal for establishing molecular-level connections between structure and function in intermediate filament proteins. The roles of the head and tail domains in facilitating keratin filament assembly and function remain as open questions. To address these, we report results of molecular dynamics simulations of the entire epithelial human K1/K10 keratin dimer. Our findings comprise: (1) the first three-dimensional structural models of the complete dimer unit, comprising of the head, rod and tail domains; (2) new insights into the chirality of the rod-domain twist gained from analysis of the full domain structure; (3) evidence for tri-subdomain partitioning in the head and tail domains; and, (4) identification of the residue characteristics that mediate non-covalent contact between the chains in the dimer. Our findings are immediately applicable to other epithelial keratins, such as K8/K18 and K5/K14, and to intermediate filament proteins in general
Charge-Density-Wave Ordering in the Metal-Insulator Transition Compound PrRu4P12
X-ray and electron diffraction measurements on the metal-insulator (M-I)
transition compound PrRuP have revealed the emergence of a periodic
ordering of charge density around the Pr atoms. It is found that the ordering
is associated with the onset of a low temperature insulator phase. These
conclusions are supported by the facts that the space group of the crystal
structure transforms from Im to Pm below the M-I transition
temperature and also that the temperature dependence of the superlattice peaks
in the insulator phase follows the squared BCS function. The M-I transition
could be originated from the perfect nesting of the Fermi surface and/or the
instability of the electrons.Comment: 4 pages, 5 figures, Phys. Rev. B (2004) (in press
Water permeation through stratum corneum lipid bilayers from atomistic simulations
Stratum corneum, the outermost layer of skin, consists of keratin filled
rigid non-viable corneocyte cells surrounded by multilayers of lipids. The
lipid layer is responsible for the barrier properties of the skin. We calculate
the excess chemical potential and diffusivity of water as a function of depth
in lipid bilayers with compositions representative of the stratum corneum using
atomistic molecular dynamics simulations. The maximum in the excess free energy
of water inside the lipid bilayers is found to be twice that of water in
phospholipid bilayers at the same temperature. Permeability, which decreases
exponentially with the free energy barrier, is reduced by several orders of
magnitude as compared to with phospholipid bilayers. The average time it takes
for a water molecule to cross the bilayer is calculated by solving the
Smoluchowski equation in presence of the free energy barrier. For a bilayer
composed of a 2:2:1 molar ratio of ceramide NS 24:0, cholesterol and free fatty
acid 24:0 at 300K, we estimate the permeability P=3.7e-9 cm/s and the average
crossing time \tau_{av}=0.69 ms. The permeability is about 30 times smaller
than existing experimental results on mammalian skin sections.Comment: latex, 8 pages, 6 figure
Variaciones hematológicas y de presión sanguínea en perros después de una picadura de abejas
RESUMEN
Objetivo. El objetivo de este estudio fue determinar los cambios hematológicos y de presión arterial en perros después de una picadura cutánea de abejas. Material y métodos. Se utilizaron 10 perros adultos, a los cuales se les inoculó veneno de abeja mediante una picadura cutánea controlada. De cada animal se extrajo una muestra de sangre previa y los 15 y 300 minutos post inoculación. Se registró la presión arterial sistólica (PAS), diastólica (PAD), media (PAM) y de pulso (PP) desde 15 minutos previos hasta 45 minutos posteriores a la inoculación. Resultados. El recuento de eritrocitos, el hematocrito y la concentración de hemoglobina se redujeron a los 15 minutos post-inoculación (p0.05). Conclusiones. La picadura cutánea de abejas produce un descenso transitorio del recuento de eritrocitos, hematocrito y concentración de hemoglobina, junto con una disminución en el recuento de reticulocitos y del recuento relativo de eosinófilos, sin alterar mayormente la presión sanguínea
Relativistic predictions of spin observables for exclusive proton knockout reactions
Within the framework of the relativistic distorted wave impulse approximation
(DWIA), we investigate the sensitivity of complete sets of polarization
transfer observables for exclusive proton knockout from the 3s,
2d and 2d states in Pb, at an incident laboratory
kinetic energy of 202 MeV, and for coincident coplanar scattering angles
(, ), to different distorting optical potentials,
finite-range (FR) versus zero-range (ZR) approximations to the DWIA, as well as
medium-modified meson-nucleon coupling constants and meson masses. Results are
also compared to the nonrelativistic DWIA predictions based on the
Schr\"{o}dinger equation.Comment: Submitted for publication to Physicical Review C, 23 pages, 7 figure
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