11,622 research outputs found
Microscopic Theory of Protein Folding Rates.I: Fine Structure of the Free Energy Profile and Folding Routes from a Variational Approach
A microscopic theory of the free energy barriers and folding routes for
minimally frustrated proteins is presented, greatly expanding on the
presentation of the variational approach outlined previously [J. J. Portman, S.
Takada, P. G. Wolynes, Phys. Rev. Lett. {\bf 81}, 5237 (1998)]. We choose the
-repressor protein as an illustrative example and focus on how the
polymer chain statistics influence free energy profiles and partially ordered
ensembles of structures. In particular, we investigate the role of chain
stiffness on the free energy profile and folding routes. We evaluate the
applicability of simpler approximations in which the conformations of the
protein molecule along the folding route are restricted to have residues that
are either entirely folded or unfolded in contiguous stretches. We find that
the folding routes obtained from only one contiguous folded region corresponds
to a chain with a much greater persistence length than appropriate for natural
protein chains, while the folding route obtained from two contiguous folded
regions is able to capture the relatively folded regions calculated within the
variational approach. The free energy profiles obtained from the contiguous
sequence approximations have larger barriers than the more microscopic
variational theory which is understood as a consequence of partial ordering.Comment: 16 pages, 11 figure
Spatial networks with wireless applications
Many networks have nodes located in physical space, with links more common
between closely spaced pairs of nodes. For example, the nodes could be wireless
devices and links communication channels in a wireless mesh network. We
describe recent work involving such networks, considering effects due to the
geometry (convex,non-convex, and fractal), node distribution,
distance-dependent link probability, mobility, directivity and interference.Comment: Review article- an amended version with a new title from the origina
Stable Frank-Kasper phases of self-assembled, soft matter spheres
Single molecular species can self-assemble into Frank Kasper (FK) phases,
finite approximants of dodecagonal quasicrystals, defying intuitive notions
that thermodynamic ground states are maximally symmetric. FK phases are
speculated to emerge as the minimal-distortional packings of space-filling
spherical domains, but a precise quantitation of this distortion and how it
affects assembly thermodynamics remains ambiguous. We use two complementary
approaches to demonstrate that the principles driving FK lattice formation in
diblock copolymers emerge directly from the strong-stretching theory of
spherical domains, in which minimal inter-block area competes with minimal
stretching of space-filling chains. The relative stability of FK lattices is
studied first using a diblock foam model with unconstrained particle volumes
and shapes, which correctly predicts not only the equilibrium {\sigma} lattice,
but also the unequal volumes of the equilibrium domains. We then provide a
molecular interpretation for these results via self-consistent field theory,
illuminating how molecular stiffness regulates the coupling between
intra-domain chain configurations and the asymmetry of local packing. These
findings shed new light on the role of volume exchange on the formation of
distinct FK phases in copolymers, and suggest a paradigm for formation of FK
phases in soft matter systems in which unequal domain volumes are selected by
the thermodynamic competition between distinct measures of shape asymmetry.Comment: 40 pages, 22 figure
Cross-Recurrence Quantification Analysis of Categorical and Continuous Time Series: an R package
This paper describes the R package crqa to perform cross-recurrence
quantification analysis of two time series of either a categorical or
continuous nature. Streams of behavioral information, from eye movements to
linguistic elements, unfold over time. When two people interact, such as in
conversation, they often adapt to each other, leading these behavioral levels
to exhibit recurrent states. In dialogue, for example, interlocutors adapt to
each other by exchanging interactive cues: smiles, nods, gestures, choice of
words, and so on. In order for us to capture closely the goings-on of dynamic
interaction, and uncover the extent of coupling between two individuals, we
need to quantify how much recurrence is taking place at these levels. Methods
available in crqa would allow researchers in cognitive science to pose such
questions as how much are two people recurrent at some level of analysis, what
is the characteristic lag time for one person to maximally match another, or
whether one person is leading another. First, we set the theoretical ground to
understand the difference between 'correlation' and 'co-visitation' when
comparing two time series, using an aggregative or cross-recurrence approach.
Then, we describe more formally the principles of cross-recurrence, and show
with the current package how to carry out analyses applying them. We end the
paper by comparing computational efficiency, and results' consistency, of crqa
R package, with the benchmark MATLAB toolbox crptoolbox. We show perfect
comparability between the two libraries on both levels
Topological Signals of Singularities in Ricci Flow
We implement methods from computational homology to obtain a topological
signal of singularity formation in a selection of geometries evolved
numerically by Ricci flow. Our approach, based on persistent homology, produces
precise, quantitative measures describing the behavior of an entire collection
of data across a discrete sample of times. We analyze the topological signals
of geometric criticality obtained numerically from the application of
persistent homology to models manifesting singularities under Ricci flow. The
results we obtain for these numerical models suggest that the topological
signals distinguish global singularity formation (collapse to a round point)
from local singularity formation (neckpinch). Finally, we discuss the
interpretation and implication of these results and future applications.Comment: 24 pages, 14 figure
Flow field computations for blunt bodies in planetary environments /equilibrium/ Final report
Radiative and convective heat transfer about two blunt bodies traveling at hypersonic speeds at zero angle of attack in assumed Mars atmosphere of nitrogen and carbon dioxid
Statics and Dynamics of Strongly Charged Soft Matter
Soft matter materials, such as polymers, membranes, proteins, are often
electrically charged. This makes them water soluble, which is of great
importance in technological application and a prerequisite for biological
function. We discuss a few static and dynamic systems that are dominated by
charge effects. One class comprises complexation between oppositely charged
objects, for example the adsorption of charged ions or charged polymers (such
as DNA) on oppositely charged substrates of different geometry. The second
class comprises effective interactions between similarly charged objects. Here
the main theme is to understand the experimental finding that similarly and
highly charged bodies attract each other in the presence of multi-valent
counterions. This is demonstrated using field-theoretic arguments as well as
Monte-Carlo simulations for the case of two homogeneously charged bodies.
Realistic surfaces, on the other hand, are corrugated and also exhibit
modulated charge distributions, which is important for static properties such
as the counterion-density distribution, but has even more pronounced
consequences for dynamic properties such as the counterion mobility. More
pronounced dynamic effects are obtained with highly condensed charged systems
in strong electric fields. Likewise, an electrostatically collapsed highly
charged polymer is unfolded and oriented in strong electric fields. At the end
of this review, we give a very brief account of the behavior of water at planar
surfaces and demonstrate using ab-initio methods that specific interactions
between oppositely charged groups cause ion-specific effects that have recently
moved into the focus of interest.Comment: 61 pages, 31 figures, Physics Reports (2005)-in press (high quality
figures available from authors
A New Mechanism of Model Membrane Fusion Determined from Monte Carlo Simulation
We have carried out extensive Monte Carlo simulations of the fusion of tense
apposed bilayers formed by amphiphilic molecules within the framework of a
coarse grained lattice model. The fusion pathway differs from the usual stalk
mechanism. Stalks do form between the apposed bilayers, but rather than expand
radially to form an axial-symmetric hemifusion diaphragm of the trans leaves of
both bilayers, they promote in their vicinity the nucleation of small holes in
the bilayers. Two subsequent paths are observed: (i) The stalk encircles a hole
in one bilayer creating a diaphragm comprised of both leaves of the other
intact bilayer, and which ruptures to complete the fusion pore. (ii) Before the
stalk can encircle a hole in one bilayer, a second hole forms in the other
bilayer, and the stalk aligns and encircles them both to complete the fusion
pore. Both pathways give rise to mixing between the cis and trans leaves of the
bilayer and allow for transient leakage.Comment: revised version, accepted to Biophys. J. (11 figures
- …