2,803 research outputs found
Altered excitation-contraction coupling in human chronic atrial fibrillation
This review focuses on the (mal)adaptive processes in atrial excitation-contraction coupling occurring in patients with chronic atrial fibrillation. Cellular remodeling includes shortening of the atrial action potential duration and effective refractory period, depressed intracellular Ca<sup>2+</sup> transient, and reduced myocyte contractility. Here we summarize the current knowledge of the ionic bases underlying these changes. Understanding the molecular mechanisms of excitation-contraction-coupling remodeling in the fibrillating human atria is important to identify new potential targets for AF therapy
Disentangling protein and lipid interactions that control a molecular switch in photosynthetic light harvesting
In the photosynthetic apparatus of plants and algae, the major Light-Harvesting Complexes (LHCII) collect excitations and funnel these to the photosynthetic reaction center where charge separation takes place. In excess light conditions, remodeling of the photosynthetic membrane and protein conformational changes produces a photoprotective state in which excitations are rapidly quenched to avoid photodamage. The quenched states are associated with protein aggregation, however the LHCII complexes are also proposed to have an intrinsic capacity to shift between light harvesting and fluorescence-quenched conformational states. To disentangle the effects of protein-protein and protein-lipid interactions on the LHCII photoprotective switch, we compared the structural and fluorescent properties of LHCII lipid nanodiscs and proteoliposomes with very low protein -to-lipid ratios. We demonstrate that LHCII proteins adapta fully fluorescent state in nanodiscs and in proteoliposomes with highly diluted protein densities. Increasing the protein density induces a transition into a mildly-quenched state that reaches a plateau at a molar protein-to-lipid ratio of 0.001 and has a fluorescence yield reminiscent of the light-harvesting state in vivo. The low onset for quenching strongly suggests that LHCII-LHCII attractive interactions occur inside membranes. The transition at low protein densities does not involve strong changes in the excitonic circular-dichroism spectrum and is distinct from a transition occurring at very high protein densities that comprises strong fluorescence quenching and circular-dichroism spectral changes involving chlorophyll 611 and 612, correlating with proposed quencher sites of the photoprotective mechanism
Dynamics of Passive-Scalar Turbulence
We present the first study of the dynamic scaling or multiscaling of
passive-scalar and passive-vector turbulence. For the Kraichnan version of
passive-scalar and passive-vector turbulence we show analytically, in both
Eulerian and quasi-Lagrangian frameworks, that simple dynamic scaling is
obtained but with different dynamic exponents. By developing the multifractal
model we show that dynamic multiscaling occurs in passive-scalar turbulence
only if the advecting velocity field is itself multifractal. We substantiate
our results by detailed numerical simulations in shell models of passive-scalar
advection.Comment: published versio
Random spread on the family of small-world networks
We present the analytical and numerical results of a random walk on the
family of small-world graphs. The average access time shows a crossover from
the regular to random behavior with increasing distance from the starting point
of the random walk. We introduce an {\em independent step approximation}, which
enables us to obtain analytic results for the average access time. We observe a
scaling relation for the average access time in the degree of the nodes. The
behavior of average access time as a function of , shows striking similarity
with that of the {\em characteristic length} of the graph. This observation may
have important applications in routing and switching in networks with large
number of nodes.Comment: RevTeX4 file with 6 figure
A simple model for dynamic heterogeneity in glass-forming liquids
Liquids near the glass transition exhibit dynamical heterogeneity, i.e. local
relaxation rates fluctuate strongly over space and time. Here we introduce a
simple continuum model that allows for quantitative predictions for the
correlators describing these fluctuations. We find remarkable agreement of the
model predictions for the dynamic susceptibility with numerical
results for a binary hard-sphere (HARD) liquid and for a Kob-Andersen
Lennard-Jones (KALJ) mixture. We explain why the existence and position of the
peak of provides no information about the lifetime
of the heterogeneities. We show that depends weakly on , but find a way to use this weak dependence to estimate
from .Comment: Main text: 5 pages, 3 figures. Supplemental material: 2 pages, 1
figur
The bli regulon - a network of blue light inducible genes of N. crassa
Several physiological responses of N. crassa are observed when this fungus is exposed to blue light. Here, we do not intend to make a comprehensive list of all the light effects observed so far in N. crassa (for a review, see Degli Innocenti and Russo 1984. In Blue Light Effects in Biological Systems ed. H. Senger, Springer-Verlag. pp 213-219.), but point out only the underlying themes. First, the time interval between the light stimulus and the observed response can be very different, and ranges from a few minutes to several hours - or even days - depending on the nature of the physiological response in question
Characterization and control of small-world networks
Recently Watts and Strogatz have given an interesting model of small-world
networks. Here we concretise the concept of a ``far away'' connection in a
network by defining a {\it far edge}. Our definition is algorithmic and
independent of underlying topology of the network. We show that it is possible
to control spread of an epidemic by using the knowledge of far edges. We also
suggest a model for better advertisement using the far edges. Our findings
indicate that the number of far edges can be a good intrinsic parameter to
characterize small-world phenomena.Comment: 9 pages and 6 figure
Evidence for Excimer Photoexcitations in an Ordered {\pi}-Conjugated Polymer Film
We report pressure-dependent transient picosecond and continuous-wave
photomodulation studies of disordered and ordered films of
2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced
absorption (PA) bands in the disordered film exhibit very weak pressure
dependence and are assigned to intrachain excitons and polarons. In contrast,
the ordered film exhibits two additional transient PA bands in the midinfrared
that blueshift dramatically with pressure. Based on high-order configuration
interaction calculations we ascribe the PA bands in the ordered film to
excimers. Our work brings insight to the exciton binding energy in ordered
films versus disordered films and solutions. The reduced exciton binding energy
in ordered films is due to new energy states appearing below the continuum band
threshold of the single strand.Comment: 5.5 pages, 5 figure
Edge wetting of an Ising three-dimensional system
The effect of edge on wetting and layering transitions of a three-dimensional
spin-1/2 Ising model is investigated, in the presence of longitudinal and
surface magnetic fields, using mean field (MF) theory and Monte Carlo (MC)
simulations. For T=0, the ground state phase diagram shows that there exist
only three allowed transitions, namely: surface and bulk transition, surface
transition and bulk transition. However, there exist a surface intra-layering
temperature , above which the surface and the intra-layering surface
transitions occur. While the bulk layering and intra-layering transitions
appear above an other finite temperature . These
surface and bulk intra-layering transitions are not seen in the perfect
surfaces case.
Numerical values of and , computed by Monte Carlo
method are found to be smaller than those obtained using mean field theory.
However, the results predicted by the two methods become similar, and are
exactly those given by the ground state phase diagram, for very low
temperatures. On the other hand, the behavior of the local magnetizations as a
function of the external magnetic field, shows that the transitions are of the
first order type. and decrease when increasing the
system size and/or the surface magnetic field. In particular,
reaches the wetting temperature for sufficiently large system sizes.Comment: 11 Pages latex, 12 Figures P
- …