Transition to superdiffusive behavior in intracellular actin-based
transport mediated by molecular motors
Abstract: Intracellular transport of large cargoes, such as organelles,
vesicles or large proteins, is a complex dynamical process that involves
the interplay of ATP-consuming molecular motors, cytoskeleton filaments
and the viscoelastic cytoplasm. In this work we investigate the motion
of pigment organelles (melanosomes) driven by myosin-V motors in
Xenopus laevis melanocytes using a high spatiotemporal resolution
tracking technique. By analyzing the obtained trajectories, we show
that the melanosomes mean-square displacement undergoes a transition
from a subdiffusive to a superdiffusive behavior. A stochastic
theoretical model, which explicitly considers the collective action of
the molecular motors, is introduced to generalize the interpretation of
our data. Starting from a generalized Langevin equation, we derive an
analytical expression for the mean square displacement, which also takes
into account the experimental noise. By fitting theoretical expressions
to experimental data we were able to discriminate the exponents that
characterize the passive and active contributions to the dynamics and to
estimate the "global" motor forces correctly. Then, our model gives
a quantitative description of active transport in living cells with a
reduced number of parameters.
***
LE12418
Dynamical engineering of a quantum hybrid
Water can exist in three different states: solid, liquid, or gaseous.
However, it can never be in
two phases at the same time.
In contrast, such hybrid phases can occur in the quantum world. For
instance, a "supersolid"
phase has been postulated as the crossover between a frictionless
quantum liquid (a
"superfluid") and a regular solid.
In our theoretical work we propose a way of creating such a supersolid
state, using a mixture
of heavy and light atoms trapped by laser light. These atoms are
initially prepared in a
crystalline arrangement, which they "remember" even after they have
entered a superfluid
state. We end up with crystalline matter flowing without friction, a
state which has no
analogue in our everyday experience.
***
LA11941
SEISMIC HAZARD EVALUATION AFTER THE 2009 L'AQUILA EARTHQUAKE
On April 6, 2009, a 5.8 magnitude earthquake struck central Italy, causing
295 casualties, the collapse of more than 4000 buildings and thousands of people to loose their homes. The earthquake has been followed by an intense seismic activity including two big shocks of magnitude 5.3 and 5.1,occurred respectively two days and three days after and producing damages comparable with those caused by the main event. The two aftershocks were unexpected on the basis of standard models for hazard evaluation that usually assumes that the largest expected aftershock magnitude is about 1.2 smaller than the main shock magnitude. Furthermore, the hazard evaluated by the standard approach is very small at distances as large as 15 kms from the mainshock where the M=5.3 aftershock occurred.
Our analysis of seismic sequences in California, has showed that the aftershock spatial organization evolves in time consistently with a static stress diffusion mechanism. These findings define a new model for seismic hazard evaluation. In the attached Figure we apply our model to the L'aquila sequence, focusing in particular to one hour before the two big aftershocks. Hazard is compared with the one obtained by the standard ETAS model currently in use at the INGV department. The comparison shows that the probability of the two shocks is remarkably high, about 100 times larger than the one obtained by the standard method. Furthermore epicenters, indicated as green stars, are very close to the maximum hazard evaluated according to our model. The above results confirm that static stress diffusion is the main mechanism responsible for aftershock triggering and represents a crucial ingredient for the construction of more accurate post-seismic hazard maps.
***
LC12416
Reaching for magnetic monopoles – an analogy for a point source of
magnetic field is found
Two researchers at Helsinki University of Technology (Finland) and the
University of New South Wales (Australia) have found a way to create
so-called Dirac monopoles in Bose-Einstein condensates using methods
routinely employed in experiments. The condensate is only tens of
micrometers in diameter and composes of dilute alkali atom gas cooled
below one millionth of a degree above the absolute zero. Monopoles are
created into an optically-trapped condensate simply by changing currents
in nearby conductors. These Dirac monopoles provide an ideal analogy for
magnetic monopoles, point charges of magnetic field, which have been
theoretically predicted to have formed in the cool-down of the early
universe. They have been intensively sought for decades without success.
The method reported here provides a very promising technique for the
first experimental observation of an analogous monopole thus opening
pathways for studying monopole interactions, decay, and dynamics. Future
experimental studies will, perhaps, bring us a deeper understanding of
our universe.
***
BBR1150
Spin currents without magnetism
The spin of the electron - the electrons magnetic momentum - is the
smallest building block for magnetic phenomena. In particular, the
discovery of the giant magneto resistance has led to the introduction of
magnetoresistive random access memory into logic electronic circuits and
has truly revolutionized information processing technology. However, up
to date both magnetic fields and magnetic materials are needed, and the
switching is both slow and power consuming. In this work, we present
studies on systems, in which the magnetic moments of the electrons can
be controlled in a collective fashion without the need of any external
magnetic field. This is achieved by the so-called Rashba effect, which
allows a controlled rotation of the electron spin along its path through
a material. We show that this effect can be tuned by changing the mixing
ratio of a bismuth/lead alloy formed on a silver surface. Further we
find that such Rashba systems can be used to inject magnetic currents
into non-magnetic materials without the need of magnetic materials or
external magnetic fields. Such concepts are essential in the field of
spintronics, which could lead to more effective information processing
or even quantum computing.
