Diversity of a peptide phase diagram
Proteins are chain molecules that have an intrinsic ability to form
structural motifs such as alpha-helices (right-handed coiled or spiral
conformations) and beta-sheets (fully extended stretches of amino
acids). This leads to a complex phase behavior in which proteins can
assemble into various types of aggregates including highly ordered
crystalline phases, dense liquid-like phases, and highly ordered
amyloid fibrils associated with devastating diseases such as
Alzheimer's disease. Although a phase diagram is a prerequisite to
understand fundamental aspects of formation of ordered structures of
proteins, the experimental and theoretical determination of such a
diagram is extremely challenging. Here we use a coarse-grained protein
model that enables us to perform kinetic Monte Carlo simulations for
determining the phase diagram of alpha-helical and beta-sheet forming
peptides. The simulations reveal the existence of various metastable
peptide phases. The dense liquid-like phases are metastable with
respect to the fibrillar phases, and there is a hierarchy of metastability.
The phase diagram provides a fundamental insight into the peptide
self-assembly, insight that may be used to design novel biomaterials or
to prevent the peptides from forming disease-related amyloid fibrils.
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LJ11846E
Oscillating Drops
The oscillation of free drops has attracts much attention in various fields, such as fluid physics, aerography, astronomy, material science and chemical engineering, with focus on the underlying mechanism, practical applications as well as models to astral and nuclear physics. There have been extensive investigations about the axisymmetric oscillation of free drops; however, few attempts are made about the more difficult case of non-axisymmetric oscillation. Here we report the non-axisymmetric sectorial oscillations of acoustically levitated drops up to the 7th mode. These oscillations are found to be parametrically excited and could be controlled by modulating the sound field. The oscillation frequency increases with increasing mode number but decreases with equatorial radius for each mode. The data can be well described by a modified Rayleigh equation, without use of additional parameters. These results may bring in a new approach to the noncontact measurement of surface tension for liquids, as well as a method of imposing forced convection within acoustically levitated drops during containerless processing. Supplementary movies for the 2nd~7th mode sectorial oscillations are presented in EPAPS Document.
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LH12640
Structure determination of an epitaxial graphene layer on a metal surface
Graphene/metal systems are playing an increasingly important role in graphene research, mainly because the epitaxial growth of graphene on metal surfaces offers one promising route to a controlled synthesis of graphene monolayers. The contact with the metal affects the electronic structure of the graphene layer and its geometry, e.g., by causing a corrugation. The magnitude of the geometrical effects has not been experimentally determined before. The main reason is that the large unit cells of the typical moir‚ structures, resulting from the mismatch between the graphene and the metal lattice, make a structure determination very difficult. By an extensive Low Energy Electron Diffraction (LEED)-I(V) analysis the structure of the moir‚ superstructure of graphene on Ru(0001) has been determined. It is found that, in quantitative agreement with the DFT result, the graphene layer is strongly corrugated (1.5 angstroms), and that the distance to the metal is quite short at the corrugation minima (2.1 angstroms). The findings provide an explanation for the electronic structure changes in the graphene layer that have been observed before.
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AQ10464
Filaments of white light take curved paths
Propagation of powerful femtosecond light pulses in transparent solids
reveals surprising features on the nature of light and matter
interactions. One of its ultimate manifestations is self-focusing and
spontaneous break-up of intense elliptical laser beams into
self-organized periodic arrays of narrow white-light beams, termed light
filaments. In this paper, we demonstrate that although white-light
filaments emerge in apparently regular patterns, the individual
filaments propagate in curved trajectories (see Figure). The full
three-dimensional picture of a filament bundle, captured with high
spatial and temporal resolution, resembles optically turbulent
propagation in fused silica slab. Our observation unveils an exciting
physics of the nonlinear light and matter interactions, which facilitate
the beam break-up process and force the light to propagate along the
curved paths. These paths emerge as a result of the generation of new
optical fields, whose coherence is neither spatial nor temporal, but is
rather skewed in the unified space-time domain.
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LM11965
Weak and empty... and yet, dominant
Gravity: the weakest of all the fundamental interactions, has always
been considered to play only a sub-dominant role when it comes to
quantum phenomena. (Even black holes, which are the arena where gravity
reigns almighty, are only able to induce particle creation, the Hawking
radiation, at a rate which, in spite of its conceptual importance, is
virtually unobservable in realistic astrophysical situations.) Vacuum:
the state whose rich structure unveiled by quantum physics is usually as
evasive as the ``emptiness'' of classical physics. Separately, each
is believed to play only very subtle roles when it comes to observable
consequences involving quantum processes. Together, however, they may
engender an explosive combination! We report on the discovery of a novel
effect according to which well-behaved gravitational fields play a
crucial role by exponentially amplifying the energy density of the
vacuum. This leads to a scenario where the vacuum eventually dominates,
through its own gravitational field, the evolution of the system. The
formation of compact objects (e.g., neutron stars) and large-scale
structures are promising contexts where this gravity-induced vacuum
dominance may be triggered, which would lead to unexpected implications
for astrophysics and cosmology.
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LN11877
New data from recent thought experiment: Speed of light still constant
Albert Einstein is probably best known for his thought experiments leading
to the theory of Special Relativity, which then gave rise to
General Relativity, explaining gravity as the curvature of space-time.
Einstein's theory of gravity has so far withstood
all attempts to marry it with quantum theory. Recently it has
been claimed that photons from distant gamma ray bursts, measured
with the Fermi Space Telescope, could reveal modifications
of Special Relativity that are believed to arise from quantum effects of
gravity in a certain class of models. These modifications would show up
as an energy-dependence of the speed of light while still preserving the
relativity of restframes. It would mean that high energetic photons
were slower than low energetic ones.
A thought experiment however reveals that such modification of
Special Relativity would have more severe consequences than previously
realized. It would be incompatible with data confirming Special Relativity
on the one hand and extensively studied elementary particle interactions
on the other hand - data that has been available since more than three
decades. Using this data, it can be shown that to excellent accuracy the
speed of light must be constant.
This is the probably least expensive high precision test of fundamental
physics you'll ever get: a thought experiment combined with decades
old data. Sometimes a pen, a notebook, and a theoretical physicist is
enough.
***
LN11873
The retina is inverted – for a reason
Using optical analysis, it is possible for the first time to understand how the structure of the retina helps sharpen our view of the world. Our eyes are built like a digital camera, with a lens in front and a detector – the retina - at the back. At the far side of the retina lie the detecting photocells, strangely covered with transparent layers of neurons. The neurons serve as wiring that process and pass the detected image to the brain, but also distort the same image. It is not clear why this wiring is not behind the detector cells, and why this feature is common to so many animals. Then three years ago it was found that glial (Muller) cells, which intersect the retina across the neural layers, are able to transmit light. Now researchers at the Technion have constructed an optical model of the retina, and passed light through this volume. They found out that only light which came through the center of the pupil was captured in the glial cells and guided directly to the photocells. Light leaking from the neighboring cells or coming from the periphery, which would clutter our sight, was rejected and scattered away. This feat could not be achieved if the photocells came before the neural layers.
