1,449 research outputs found
Mechanics of collective unfolding
Mechanically induced unfolding of passive crosslinkers is a fundamental
biological phenomenon encountered across the scales from individual
macro-molecules to cytoskeletal actin networks. In this paper we study a
conceptual model of athermal load-induced unfolding and use a minimalistic
setting allowing one to emphasize the role of long-range interactions while
maintaining full analytical transparency. Our model can be viewed as a
description of a parallel bundle of N bistable units confined between two
shared rigid backbones that are loaded through a series spring. We show that
the ground states in this model correspond to synchronized, single phase
configurations where all individual units are either folded or unfolded. We
then study the fine structure of the wiggly energy landscape along the reaction
coordinate linking the two coherent states and describing the optimal mechanism
of cooperative unfolding. Quite remarkably, our study shows the fundamental
difference in the size and structure of the folding-unfolding energy barriers
in the hard (fixed displacements) and soft (fixed forces) loading devices which
persists in the continuum limit. We argue that both, the synchronization and
the non-equivalence of the mechanical responses in hard and soft devices, have
their origin in the dominance of long-range interactions. We then apply our
minimal model to skeletal muscles where the power-stroke in acto-myosin
crossbridges can be interpreted as passive folding. A quantitative analysis of
the muscle model shows that the relative rigidity of myosin backbone provides
the long-range interaction mechanism allowing the system to effectively
synchronize the power-stroke in individual crossbridges even in the presence of
thermal fluctuations. In view of the prototypical nature of the proposed model,
our general conclusions pertain to a variety of other biological systems where
elastic interactions are mediated by effective backbones
Iterations of a Riddle: The Reconciliation of Individual and Collective Interests
Scholarships & Prizes Office. University of Sydne
Internal Dust Correction Factors for Star Formation Rates Derived for Dusty \HII Regions and Starburst Galaxies
Star formation rates in galaxies are frequently estimated using the Balmer
line fluxes. However, these can be systematically underestimated because dust
competes for the absorption of Lyman continuum photons in the ionized gas. Here
we present theoretical correction factors in a simple analytic form. T These
factors scale as the product of the ionization parameter, , and the
nebular O/H abundance ratio, both of which can now be derived from the
observation of bright nebular line ratios. The correction factors are only
somewhat dependent upon the photoelectron production by grains, but are very
sensitive to the presence of complex PAH-like carbonaceous molecules in the
ionized gas, providing that these can survive in such an environment.Comment: 13 pages, 1 figures, Accepted for publication in ApJ. (Feb 1, 2003
Stable Photoinduced Separated Charge State in Viologen Halometallates: Some Key Parameters
With the aim to define key parameters causing the photochromic properties of (MV)[Bi2Cl8] and (MV)(4)[Bi6Cl26] (MV2+, methylviologen; 1,1-dimethyl-4,4-bipyridinium), the effects of substituting Bi by Sb, Cl by Br, or MV2+ by MOV2+ (1,1-dimethoxy-4,4-bipyridinium) or MeMOV2+ (1-methyl-1-methoxy-4,4-bipyridinium) on the photoinduced charge transfer properties of such viologen halometallates are explored. It appears that only salts containing chlorobismuthate anions undergo a color change upon UV irradiation and that the nature of viologen entities has a key role in the process. We also suggest that a key parameter for observing the stable photoinduced separated charge state in chlorobismuthate viologen hybrids is a high chloride/viologen ratio, rather than the size of the anionic oligomer, as observed in the previously reported unique series (MV)((2n+2)/2)[Bi2nCl8n+2]
Pyrene-functionalized Foldamer: Structural Impact and Recognition Properties supported by Donor-Acceptor Interactions
An electroactive and luminescent foldamer based on an oligopyridine biscarboxamide skeleton was synthesized and characterised. Its conformation in the solid state proved to be strongly affected by the peripheral pyrene units. The latter also endow the target derivative with recognition abilities toward electron-withdrawing molecules, which allow tuning of the spectroscopic properties of the foldamer
Microscopic nanomechanical dissipation in gallium arsenide resonators
We report on a systematic study of nanomechanical dissipation in
high-frequency (approximatively 300 MHz) gallium arsenide optomechanical disk
resonators, in conditions where clamping and fluidic losses are negligible.
Phonon-phonon interactions are shown to contribute with a loss background
fading away at cryogenic temperatures (3 K). Atomic layer deposition of alumina
at the surface modifies the quality factor of resonators, pointing towards the
importance of surface dissipation. The temperature evolution is accurately
fitted by two-level systems models, showing that nanomechanical dissipation in
gallium arsenide resonators directly connects to their microscopic properties.
Two-level systems, notably at surfaces, appear to rule the damping and
fluctuations of such high-quality crystalline nanomechanical devices, at all
temperatures from 3 to 300K
An Analytical Approach to Neuronal Connectivity
This paper describes how realistic neuromorphic networks can have their
connectivity properties fully characterized in analytical fashion. By assuming
that all neurons have the same shape and are regularly distributed along the
two-dimensional orthogonal lattice with parameter , it is possible to
obtain the accurate number of connections and cycles of any length from the
autoconvolution function as well as from the respective spectral density
derived from the adjacency matrix. It is shown that neuronal shape plays an
important role in defining the spatial spread of network connections. In
addition, most such networks are characterized by the interesting phenomenon
where the connections are progressively shifted along the spatial domain where
the network is embedded. It is also shown that the number of cycles follows a
power law with their respective length. Morphological measurements for
characterization of the spatial distribution of connections, including the
adjacency matrix spectral density and the lacunarity of the connections, are
suggested. The potential of the proposed approach is illustrated with respect
to digital images of real neuronal cells.Comment: 4 pages, 6 figure
Mid-Infrared Emission Features in the ISM: Feature-to-Feature Flux Ratios
Using a limited, but representative sample of sources in the ISM of our
Galaxy with published spectra from the Infrared Space Observatory, we analyze
flux ratios between the major mid-IR emission features (EFs) centered around
6.2, 7.7, 8.6 and 11.3 microns, respectively. In a flux ratio-to-flux ratio
plot of EF(6.2)/EF(7.7) as a function of EF(11.3)/EF(7.7), the sample sources
form roughly a -shaped locus which appear to trace, on an overall
basis, the hardness of a local heating radiation field. But some driving
parameters other than the radiation field may also be required for a full
interpretation of this trend. On the other hand, the flux ratio of
EF(8.6)/EF(7.7) shows little variation over the sample sources, except for two
HII regions which have much higher values for this ratio due to an ``EF(8.6\um)
anomaly,'' a phenomenon clearly associated with environments of an intense
far-UV radiation field. If further confirmed on a larger database, these trends
should provide crucial information on how the EF carriers collectively respond
to a changing environment.Comment: 16 pages, 1 figure, 1 table; accepted for publication in ApJ Letter
The Value of a virtual Kitchen to assess the activities of Daily life in Alzheimer Disease
Patients with Alzheimer\u27s disease (AD patients) experience difficulties in everyday life activities that effect independence. These difficulties can be tested using virtual reality (VR) environments that simulate real life situations. The purpose of this study was to examine the value of a virtual kitchen as an assessment tool of everyday life activities in AD patients. We focus on the assessment results obtained in a group of AD patients on a virtual kitchen designed to assess their ability to prepare a virtual cup of coffee using a virtual coffee machine. An identical real daily living task was tested. Twenty-four AD patients were matched to 32 healthy elderly controls. Significant differences in performance between research and control groups were detected on the virtual and the real tasks, with the research group performing worse. In addition, regression analyses revealed that the number of errors in the virtual test was the best predictor for the real assessment and the Instrumental Activities of Daily Living score. This provides initial support for the sensitivity and the ecological validity of the virtual kitchen as an assessment tool of everyday life activities in AD patient
- …