10,367 research outputs found
ORK1, a potassium-selective leak channel with two pore domains cloned from Drosophila melanogaster by expression in Saccharomyces cerevisiae.
A K+ channel gene has been cloned from Drosophila melanogaster by complementation in Saccharomyces cerevisiae cells defective for K+ uptake. Naturally expressed in the neuromuscular tissues of adult flies, this gene confers K+ transport capacity on yeast cells when heterologously expressed. In Xenopus laevis oocytes, expression yields an ungated K(+)-selective current whose attributes resemble the "leak" conductance thought to mediate the resting potential of vertebrate myelinated neurons but whose molecular nature has long remained elusive. The predicted protein has two pore (P) domains and four membrane-spanning helices and is a member of a newly recognized K+ channel family. Expression of the channel in flies and yeast cells makes feasible studies of structure and in vivo function using genetic approaches that are not possible in higher animals
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Erratum: ORK1, a potassium-selective leak channel with two pore domains cloned from Drosophila melanogaster by expression in Saccharomyces cerevisiae (Proceedings of the National Academy of Sciences of the USA (1996) 93:23 (13256-13261))
Configuration space connectivity across the fragile to strong transition in silica
We present a numerical analysis for SiO_2 of the fraction of diffusive
direction f_diff for temperatures T on both sides of the fragile-to-strong
crossover. The T-dependence of f_diff clearly reveals this change in dynamical
behavior. We find that for T above the crossover (fragile region) the system is
always close to ridges of the potential energy surface (PES), while below the
crossover (strong region), the system mostly explores the PES local minima.
Despite this difference, the power law dependence of f_diff on the diffusion
constant, as well as the power law dependence of f_diff on the configurational
entropy, shows no change at the fragile to strong crossover
Relationship between Fragility, Diffusive Directions and Energy Barriers in a Supercooled Liquid
An analysis of diffusion in a supercooled liquid based solely in the density
of diffusive directions and the value of energy barriers shows how the
potential energy landscape (PEL) approach is capable of explaining the
and relaxations and the fragility of a glassy system. We find that the
relaxation is directly related to the search for diffusive directions.
Our analysis shows how in strong liquids diffusion is mainly energy activated,
and how in fragile liquids the diffusion is governed by the density of
diffusive directions. We describe the fragile-to-strong crossover as a change
in the topography of the PEL sampled by the system at a certain crossover
temperature .Comment: 5 pages, 3 figure
Derivation of the Planck Spectrum for Relativistic Classical Scalar Radiation from Thermal Equilibrium in an Accelerating Frame
The Planck spectrum of thermal scalar radiation is derived suggestively
within classical physics by the use of an accelerating coordinate frame. The
derivation has an analogue in Boltzmann's derivation of the Maxwell velocity
distribution for thermal particle velocities by considering the thermal
equilibrium of noninteracting particles in a uniform gravitational field. For
the case of radiation, the gravitational field is provided by the acceleration
of a Rindler frame through Minkowski spacetime. Classical zero-point radiation
and relativistic physics enter in an essential way in the derivation which is
based upon the behavior of free radiation fields and the assumption that the
field correlation functions contain but a single correlation time in thermal
equilibrium. The work has connections with the thermal effects of acceleration
found in relativistic quantum field theory.Comment: 23 page
Quantum free energy differences from non-equilibrium path integrals: I. Methods and numerical application
The imaginary-time path integral representation of the canonical partition
function of a quantum system and non-equilibrium work fluctuation relations are
combined to yield methods for computing free energy differences in quantum
systems using non-equilibrium processes. The path integral representation is
isomorphic to the configurational partition function of a classical field
theory, to which a natural but fictitious Hamiltonian dynamics is associated.
It is shown that if this system is prepared in an equilibrium state, after
which a control parameter in the fictitious Hamiltonian is changed in a finite
time, then formally the Jarzynski non-equilibrium work relation and the Crooks
fluctuation relation are shown to hold, where work is defined as the change in
the energy as given by the fictitious Hamiltonian. Since the energy diverges
for the classical field theory in canonical equilibrium, two regularization
methods are introduced which limit the number of degrees of freedom to be
finite. The numerical applicability of the methods is demonstrated for a
quartic double-well potential with varying asymmetry. A general parameter-free
smoothing procedure for the work distribution functions is useful in this
context.Comment: 20 pages, 4 figures. Added clarifying remarks and fixed typo
Quantization from an exponential distribution of infinitesimal action
A statistical model of quantization based on an exponential distribution of
infinitesimal action is proposed. Trajectory which does not extremize the
action along an infinitesimal short segment of path is allowed to occur with a
very small probability following an exponential law. Planck constant is argued
to give the average deviation from the infinitesimal stationary action.Comment: 15 pages, accepted for publication in Physica
Participatory Security: Citizen Security, Participation, and the Inequities of Citizenship in Urban Peru
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90078/1/j.1470-9856.2011.00656.x.pd
Pine Bark and Green Tea Concentrated Extracts: Antioxidant Activity and Comprehensive Characterization of Bioactive Compounds by HPLC–ESI-QTOF-MS
The consumption of polyphenols has frequently been associated with low incidence of degenerative diseases. Most of these natural antioxidants come from fruits, vegetables, spices, grains and herbs. For this reason, there has been increasing interest in identifying plant extract compounds. Polymeric tannins and monomeric flavonoids, such as catechin and epicatechin, in pine bark and green tea extracts could be responsible for the higher antioxidant activities of these extracts. The aim of the present study was to characterize the phenolic compounds in pine bark and green tea concentrated extracts using high-performance liquid chromatography coupled to electrospray ionization mass spectrometry (HPLC–ESI-QTOF-MS). A total of 37 and 35 compounds from pine bark and green tea extracts, respectively, were identified as belonging to various structural classes, mainly flavan-3-ol and its derivatives (including procyanidins). The antioxidant capacity of both extracts was evaluated by three complementary antioxidant activity methods: Trolox equivalent antioxidant capacity (TEAC), ferric reducing antioxidant power (FRAP) and oxygen radical absorbance capacity (ORAC). Higher antioxidant activity values by each method were obtained. In addition, total polyphenol and flavan-3-ol contents, which were determined by Folin–Ciocalteu and vanillin assays, respectively, exhibited higher amounts of gallic acid and (+)-catechin equivalents.This work was supported by the project AGL2011-29857-C03-02 and AP2010-1551 (Spanish Ministry of Science and Innovation), as well as P09-CTS-4564, P10-FQM-6563 and P11-CTS-7625 (Andalusian Regional Government Council of Innovation and Science). The authors are also grateful to Instituto de Salud Carlos III for the Sara Borrell Grant (CD12/00672
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