5,143 research outputs found
Secondary metabolites of the argan tree (Morocco) may have disease prevention properties
The argan tree (Argania spinosa L. Skeels) is native to Morocco, where after the Holly oak it constitutes the second most common tree in the country. Recent studies suggest that dietary argan oil, an endemic seed oil from argan fruits, may have a relevant role in disease prevention, and its consumption could protect against atherosclerosis and cancer. Unfortunately, in less than a century, more than a third of the forest has disappeared. It is therefore imperative to improve the tree\'s production potential so that it can regain its key position in the agricultural systems of the region. On the basis of ethnobotanical knowledge, researchers are screening metabolites of this rare plant to identify bioactive compounds for the development of new therapeutic agents and food supplements. This includes studies on secondary metabolites with chemopreventive activities. In this review, a complete outline of components (triglycerides, unsaponifiable, phenolic antioxidants and aroma constituents) are described. Finally, a discussion of the biological functions of the polar and non-polar A. spinosa products which have been evaluated using a range of in vitro bioassays are described.African Journal of Biotechnology Vol. 4 (5), pp. 381-388, 200
Higher order glass-transition singularities in colloidal systems with attractive interactions
The transition from a liquid to a glass in colloidal suspensions of particles
interacting through a hard core plus an attractive square-well potential is
studied within the mode-coupling-theory framework. When the width of the
attractive potential is much shorter than the hard-core diameter, a reentrant
behavior of the liquid-glass line, and a glass-glass-transition line are found
in the temperature-density plane of the model. For small well-width values, the
glass-glass-transition line terminates in a third order bifurcation point, i.e.
in a A_3 (cusp) singularity. On increasing the square-well width, the
glass-glass line disappears, giving rise to a fourth order A_4 (swallow-tail)
singularity at a critical well width. Close to the A_3 and A_4 singularities
the decay of the density correlators shows stretching of huge dynamical
windows, in particular logarithmic time dependence.Comment: 19 pages, 12 figures, Phys. Rev. E, in prin
How does the relaxation of a supercooled liquid depend on its microscopic dynamics?
Using molecular dynamics computer simulations we investigate how the
relaxation dynamics of a simple supercooled liquid with Newtonian dynamics
differs from the one with a stochastic dynamics. We find that, apart from the
early beta-relaxation regime, the two dynamics give rise to the same relaxation
behavior. The increase of the relaxation times of the system upon cooling, the
details of the alpha-relaxation, as well as the wave vector dependence of the
Edwards-Anderson-parameters are independent of the microscopic dynamics.Comment: 6 pages of Latex, 4 figure
Glasslike Arrest in Spinodal Decomposition as a Route to Colloidal Gelation
Colloid-polymer mixtures can undergo spinodal decomposition into colloid-rich
and colloid-poor regions. Gelation results when interconnected colloid-rich
regions solidify. We show that this occurs when these regions undergo a glass
transition, leading to dynamic arrest of the spinodal decomposition. The
characteristic length scale of the gel decreases with increasing quench depth,
and the nonergodicity parameter exhibits a pronounced dependence on scattering
vector. Mode coupling theory gives a good description of the dynamics, provided
we use the full static structure as input.Comment: 14 pages, 4 figures; replaced with published versio
Confirmation of Anomalous Dynamical Arrest in attractive colloids: a molecular dynamics study
Previous theoretical, along with early simulation and experimental, studies
have indicated that particles with a short-ranged attraction exhibit a range of
new dynamical arrest phenomena. These include very pronounced reentrance in the
dynamical arrest curve, a logarithmic singularity in the density correlation
functions, and the existence of `attractive' and `repulsive' glasses. Here we
carry out extensive molecular dynamics calculations on dense systems
interacting via a square-well potential. This is one of the simplest systems
with the required properties, and may be regarded as canonical for interpreting
the phase diagram, and now also the dynamical arrest. We confirm the
theoretical predictions for re-entrance, logarithmic singularity, and give the
first direct evidence of the coexistence, independent of theory, of the two
coexisting glasses. We now regard the previous predictions of these phenomena
as having been established.Comment: 15 pages,15 figures; submitted to Phys. Rev.
Molecular dynamics simulation of the fragile glass former ortho-terphenyl: a flexible molecule model
We present a realistic model of the fragile glass former orthoterphenyl and
the results of extensive molecular dynamics simulations in which we
investigated its basic static and dynamic properties. In this model the
internal molecular interactions between the three rigid phenyl rings are
described by a set of force constants, including harmonic and anharmonic terms;
the interactions among different molecules are described by Lennard-Jones
site-site potentials. Self-diffusion properties are discussed in detail
together with the temperature and momentum dependencies of the
self-intermediate scattering function. The simulation data are compared with
existing experimental results and with the main predictions of the Mode
Coupling Theory.Comment: 20 pages and 28 postscript figure
Dielectronic Recombination (via N=2 --> N'=2 Core Excitations) and Radiative Recombination of Fe XX: Laboratory Measurements and Theoretical Calculations
We have measured the resonance strengths and energies for dielectronic
recombination (DR) of Fe XX forming Fe XIX via N=2 --> N'=2 (Delta_N=0) core
excitations. We have also calculated the DR resonance strengths and energies
using AUTOSTRUCTURE, HULLAC, MCDF, and R-matrix methods, four different
state-of-the-art theoretical techniques. On average the theoretical resonance
strengths agree to within <~10% with experiment. However, the 1 sigma standard
deviation for the ratios of the theoretical-to-experimental resonance strengths
is >~30% which is significantly larger than the estimated relative experimental
uncertainty of <~10%. This suggests that similar errors exist in the calculated
level populations and line emission spectrum of the recombined ion. We confirm
that theoretical methods based on inverse-photoionization calculations (e.g.,
undamped R-matrix methods) will severely overestimate the strength of the DR
process unless they include the effects of radiation damping. We also find that
the coupling between the DR and radiative recombination (RR) channels is small.
We have used our experimental and theoretical results to produce
Maxwellian-averaged rate coefficients for Delta_N=0 DR of Fe XX. For kT>~1 eV,
which includes the predicted formation temperatures for Fe XX in an optically
thin, low-density photoionized plasma with cosmic abundances, our experimental
and theoretical results are in good agreement. We have also used our R-matrix
results, topped off using AUTOSTRUCTURE for RR into J>=25 levels, to calculate
the rate coefficient for RR of Fe XX. Our RR results are in good agreement with
previously published calculations.Comment: To be published in ApJS. 65 pages with 4 tables and lots of figure
Comparative simulation study of colloidal gels and glasses
Using computer simulations, we identify the mechanisms causing aggregation
and structural arrest of colloidal suspensions interacting with a short-ranged
attraction at moderate and high densities. Two different non-ergodicity
transitions are observed. As the density is increased, a glass transition takes
place, driven by excluded volume effects. In contrast, at moderate densities,
gelation is approached as the strength of the attraction increases. At high
density and interaction strength, both transitions merge, and a logarithmic
decay in the correlation function is observed. All of these features are
correctly predicted by mode coupling theory
Evidence for Unusual Dynamical Arrest Scenario in Short Ranged Colloidal Systems
Extensive molecular dynamics simulation studies of particles interacting via
a short ranged attractive square-well (SW) potential are reported. The
calculated loci of constant diffusion coefficient in the
temperature-packing fraction plane show a re-entrant behavior, i.e. an increase
of diffusivity on cooling, confirming an important part of the high
volume-fraction dynamical-arrest scenario earlier predicted by theory for
particles with short ranged potentials. The more efficient localization
mechanism induced by the short range bonding provides, on average, additional
free volume as compared to the hard-sphere case and results in faster dynamics.Comment: 4 pages, 3 figure
Asymptotic laws for tagged-particle motion in glassy systems
Within the mode-coupling theory for structural relaxation in simple systems
the asymptotic laws and their leading-asymptotic correction formulas are
derived for the motion of a tagged particle near a glass-transition
singularity. These analytic results are compared with numerical ones of the
equations of motion evaluated for a tagged hard sphere moving in a hard-sphere
system. It is found that the long-time part of the two-step relaxation process
for the mean-squared displacement can be characterized by the -relaxation-scaling law and von Schweidler's power-law decay while the
critical-decay regime is dominated by the corrections to the leading power-law
behavior. For parameters of interest for the interpretations of experimental
data, the corrections to the leading asymptotic laws for the non-Gaussian
parameter are found to be so large that the leading asymptotic results are
altered qualitatively by the corrections. Results for the non-Gaussian
parameter are shown to follow qualitatively the findings reported in the
molecular-dynamics-simulations work by Kob and Andersen [Phys. Rev. E 51, 4626
(1995)]
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