543 research outputs found
A novel iterative strategy for protein design
We propose and discuss a novel strategy for protein design. The method is
based on recent theoretical advancements which showed the importance to treat
carefully the conformational free energy of designed sequences. In this work we
show how computational cost can be kept to a minimum by encompassing negative
design features, i.e. isolating a small number of structures that compete
significantly with the target one for being occupied at low temperature. The
method is succesfully tested on minimalist protein models and using a variety
of amino acid interaction potentials.Comment: 9 pages, 8 figure
Phase diagram of force-induced DNA unzipping in exactly solvable models
The mechanical separation of the double helical DNA structure induced by
forces pulling apart the two DNA strands (``unzipping'') has been the subject
of recent experiments. Analytical results are obtained within various models of
interacting pairs of directed walks in the (1,1,...,1) direction on the
hypercubic lattice, and the phase diagram in the force-temperature plane is
studied for a variety of cases. The scaling behaviour is determined at both the
unzipping and the melting transition. We confirm the existence of a cold
denaturation transition recently observed in numerical simulations: for a
finite range of forces the system gets unzipped by {\it decreasing} the
temperature. The existence of this transition is rigorously established for
generic lattice and continuum space models.Comment: 19 pages, 5 eps figures; revised version with minor changes,
presentation simplified in the text with details in appendix. Accepted for
publication in Phys. Rev.
Swollen-Collapsed Transition in Random Hetero-Polymers
A lattice model of a hetero-polymer with random hydrophilic-hydrophobic
charges interacting with the solvent is introduced, whose continnuum
counterpart has been proposed by T. Garel, L. Leibler and H. Orland {J. Phys.
II France 4, 2139 (1994)]. The transfer matrix technique is used to study
various constrained annealed systems which approximate at various degrees of
accuracy the original quenched model. For highly hydrophobic chains an ordinary
-point transition is found from a high temperature swollen phase to a
low temperature compact phase. Depending on the type of constrained averages,
at very low temperatures a swollen phase or a coexistence between compact and
swollen phases are found. The results are carefully compared with the
corresponding ones obtained in the continuum limit, and various improvements in
the original calculations are discussed.Comment: 13 pages, 8 figures; revised version with minor changes, accepted for
publication in European Physical Journal
Global protein function prediction in protein-protein interaction networks
The determination of protein functions is one of the most challenging
problems of the post-genomic era. The sequencing of entire genomes and the
possibility to access gene's co-expression patterns has moved the attention
from the study of single proteins or small complexes to that of the entire
proteome. In this context, the search for reliable methods for proteins'
function assignment is of uttermost importance. Previous approaches to deduce
the unknown function of a class of proteins have exploited sequence
similarities or clustering of co-regulated genes, phylogenetic profiles,
protein-protein interactions, and protein complexes. We propose to assign
functional classes to proteins from their network of physical interactions, by
minimizing the number of interacting proteins with different categories. The
function assignment is made on a global scale and depends on the entire
connectivity pattern of the protein network. Multiple functional assignments
are made possible as a consequence of the existence of multiple equivalent
solutions. The method is applied to the yeast Saccharomices Cerevisiae
protein-protein interaction network. Robustness is tested in presence of a high
percentage of unclassified proteins and under deletion/insertion of
interactions.Comment: 5 pages, 2 figures, 2 supplementary table
Heteropolymers in a Solvent at an Interface
Exact bounds are obtained for the quenched free energy of a polymer with
random hydrophobicities in the presence of an interface separating a polar from
a non polar solvent. The polymer may be ideal or have steric self-interactions.
The bounds allow to prove that a ``neutral'' random polymer is localized near
the interface at any temperature, whereas a ``non-neutral'' chain is shown to
undergo a delocalization transition at a finite temperature. These results are
valid for a quite general a priori probability distribution for both
independent and correlated hydrophobic charges. As a particular case we
consider random AB-copolymers and confirm recent numerical studies.Comment: 4 pages, no figure
Species lifetime distribution for simple models of ecologies
Interpretation of empirical results based on a taxa's lifetime distribution
shows apparently conflicting results. Species' lifetime is reported to be
exponentially distributed, whereas higher order taxa, such as families or
genera, follow a broader distribution, compatible with power law decay. We show
that both these evidences are consistent with a simple evolutionary model that
does not require specific assumptions on species interaction. The model
provides a zero-order description of the dynamics of ecological communities and
its species lifetime distribution can be computed exactly. Different behaviors
are found: an initial power law, emerging from a random walk type of
dynamics, which crosses over to a steeper branching process-like
regime and finally is cutoff by an exponential decay which becomes weaker and
weaker as the total population increases. Sampling effects can also be taken
into account and shown to be relevant: if species in the fossil record were
sampled according to the Fisher log-series distribution, lifetime should be
distributed according to a power law. Such variability of behaviors in
a simple model, combined with the scarcity of data available, cast serious
doubts on the possibility to validate theories of evolution on the basis of
species lifetime data.Comment: 19 pages, 2 figure
Hidden scaling patterns and universality in written communication
The temporal statistics exhibited by written correspondence appear to be
media dependent, with features which have so far proven difficult to
characterize. We explain the origin of these difficulties by disentangling the
role of spontaneous activity from decision-based prioritizing processes in
human dynamics, clocking all waiting times through each agent's `proper time'
measured by activity. This unveils the same fundamental patterns in written
communication across all media (letters, email, sms), with response times
displaying truncated power-law behavior and average exponents near -3/2. When
standard time is used, the response time probabilities are theoretically
predicted to exhibit a bi-modal character, which is empirically borne out by
our new years-long data on email. These novel perspectives on the temporal
dynamics of human correspondence should aid in the analysis of interaction
phenomena in general, including resource management, optimal pricing and
routing, information sharing, emergency handling.Comment: 27 pages, 10 figure
Entropy production in systems with unidirectional transitions
The entropy production is one of the most essential features for systems
operating out of equilibrium. The formulation for discrete-state systems goes
back to the celebrated Schnakenberg's work and hitherto can be carried out when
for each transition between two states also the reverse one is allowed.
Nevertheless, several physical systems may exhibit a mixture of both
unidirectional and bidirectional transitions, and how to properly define the
entropy production in this case is still an open question. Here, we present a
solution to such a challenging problem. The average entropy production can be
consistently defined, employing a mapping that preserves the average fluxes,
and its physical interpretation is provided. We describe a class of stochastic
systems composed of unidirectional links forming cycles and detailed-balanced
bidirectional links, showing that they behave in a pseudo-deterministic
fashion. This approach is applied to a system with time-dependent stochastic
resetting. Our framework is consistent with thermodynamics and leads to some
intriguing observations on the relation between the arrow of time and the
average entropy production for resetting events.Comment: (Accepted for publication in Physical Review Research
Geometry of compact tubes and protein structures
Proteins form a very important class of polymers. In spite of major advances
in the understanding of polymer science, the protein problem has remained
largely unsolved. Here, we show that a polymer chain viewed as a tube not only
captures the well-known characteristics of polymers and their phases but also
provides a natural explanation for many of the key features of protein
behavior. There are two natural length scales associated with a tube subject to
compaction -- the thickness of the tube and the range of the attractive
interactions. For short tubes, when these length scales become comparable, one
obtains marginally compact structures, which are relatively few in number
compared to those in the generic compact phase of polymers. The motifs
associated with the structures in this new phase include helices, hairpins and
sheets. We suggest that Nature has selected this phase for the structures of
proteins because of its many advantages including the few candidate strucures,
the ability to squeeze the water out from the hydrophobic core and the
flexibility and versatility associated with being marginally compact. Our
results provide a framework for understanding the common features of all
proteins.Comment: 15 pages, 3 eps figure
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