31,873 research outputs found
Memory effects in biochemical networks as the natural counterpart of extrinsic noise
We show that in the generic situation where a biological network, e.g. a
protein interaction network, is in fact a subnetwork embedded in a larger
"bulk" network, the presence of the bulk causes not just extrinsic noise but
also memory effects. This means that the dynamics of the subnetwork will depend
not only on its present state, but also its past. We use projection techniques
to get explicit expressions for the memory functions that encode such memory
effects, for generic protein interaction networks involving binary and unary
reactions such as complex formation and phosphorylation, respectively.
Remarkably, in the limit of low intrinsic copy-number noise such expressions
can be obtained even for nonlinear dependences on the past. We illustrate the
method with examples from a protein interaction network around epidermal growth
factor receptor (EGFR), which is relevant to cancer signalling. These examples
demonstrate that inclusion of memory terms is not only important conceptually
but also leads to substantially higher quantitative accuracy in the predicted
subnetwork dynamics
Hierarchical bounding structures for efficient virial computations: Towards a realistic molecular description of cholesterics
We detail the application of bounding volume hierarchies to accelerate
second-virial evaluations for arbitrary complex particles interacting through
hard and soft finite-range potentials. This procedure, based on the
construction of neighbour lists through the combined use of recursive
atom-decomposition techniques and binary overlap search schemes, is shown to
scale sub-logarithmically with particle resolution in the case of molecular
systems with high aspect ratios. Its implementation within an efficient
numerical and theoretical framework based on classical density functional
theory enables us to investigate the cholesteric self-assembly of a wide range
of experimentally-relevant particle models. We illustrate the method through
the determination of the cholesteric behaviour of hard, structurally-resolved
twisted cuboids, and report quantitative evidence of the long-predicted phase
handedness inversion with increasing particle thread angles near the
phenomenological threshold value of . Our results further highlight
the complex relationship between microscopic structure and helical twisting
power in such model systems, which may be attributed to subtle geometric
variations of their chiral excluded-volume manifold
Physics-based visual characterization of molecular interaction forces
Molecular simulations are used in many areas of biotechnology, such as drug design and enzyme engineering. Despite the development of automatic computational protocols, analysis of molecular interactions is still a major aspect where human comprehension and intuition are key to accelerate, analyze, and propose modifications to the molecule of interest. Most visualization algorithms help the users by providing an accurate depiction of the spatial arrangement: the atoms involved in inter-molecular contacts. There are few tools that provide visual information on the forces governing molecular docking. However, these tools, commonly restricted to close interaction between atoms, do not consider whole simulation paths, long-range distances and, importantly, do not provide visual cues for a quick and intuitive comprehension of the energy functions (modeling intermolecular interactions) involved. In this paper, we propose visualizations designed to enable the characterization of interaction forces by taking into account several relevant variables such as molecule-ligand distance and the energy function, which is essential to understand binding affinities. We put emphasis on mapping molecular docking paths obtained from Molecular Dynamics or Monte Carlo simulations, and provide time-dependent visualizations for different energy components and particle resolutions: atoms, groups or residues. The presented visualizations have the potential to support domain experts in a more efficient drug or enzyme design process.Peer ReviewedPostprint (author's final draft
A multiscale approach to liquid crystal nematics via statistical field theory
We propose an approach to a multiscale problem in the theory of thermotropic
uniaxial nematics based on the method of statistical field theory. This
approach enables us to relate the coefficients , , , and
of the Landau-de Gennes free energy for the isotropic-nematic phase transition
to the parameters of a molecular model of uniaxial nematics, which we take to
be a lattice gas model of nematogenic molecules interacting via a short-ranged
potential. We obtain general constraints on the temperature and volume fraction
of nematogens for the Landau-de Gennes theory to be stable against molecular
orientation fluctuations at quartic order. In particular, for the case of a
fully occupied lattice, we compute the values of the isotropic-nematic
transition temperature and the order parameter discontinuity predicted by (i) a
continuum approximation of the nearest-neighbor Lebwohl-Lasher model and (ii) a
Lebwohl-Lasher-type model with a nematogenic interaction of finite range. We
find that the predictions of (i) are in reasonably good agreement with known
results of MC simulation.Comment: 12 pages, 2 figure
Ferroelectric Phase Transitions from First Principles
An effective Hamiltonian for the ferroelectric transition in is
constructed from first-principles density-functional-theory total-energy and
linear-response calculations through the use of a localized, symmetrized basis
set of ``lattice Wannier functions.'' Preliminary results of Monte Carlo
simulations for this system show a first-order cubic-tetragonal transition at
660 K. The involvement of the Pb atom in the lattice instability and the
coupling of local distortions to strain are found to be particularly important
in producing the behavior characteristic of the transition. A
tentative explanation for the presence of local distortions experimentally
observed above is suggested. Further applications of this method to a
variety of systems and structures are proposed for first-principles study of
finite-temperature structural properties in individual materials.Comment: 14 pages, harvmac, 4 uuencoded figure
Inclusions induced phase separation in mixed lipid film
The effect of rigid inclusions on the phase behavior of a film containing a
mixture of lipid molecules is investigated. In the proposed model, the
inclusion-induced deformation of the film, and the resulting energy cost are
strongly dependent upon the spontaneous curvature of the mixed film. The
spontaneous curvature is in turn strongly influenced by the composition of
film. This coupling between the film composition and the energy per inclusion
leads to a lateral modulation of the composition, which follows the local
curvature of the membrane. In particular, it is shown that the inclusion may
induce a global phase separation in a film which would otherwise be
homogeneously mixed. The mixed film is then composed of patches of different
average composition, separated by the inclusions. This process may be of
relevance to explain some aspects of lipid-protein association in biological
membranes.Comment: 19 pages, 5 figure
Fluctuation induced interactions between domains in membranes
We study a model lipid bilayer composed of a mixture of two incompatible
lipid types which have a natural tendency to segregate in the absence of
membrane fluctuations. The membrane is mechanically characterized by a local
bending rigidity which varies with the average local lipid
composition . We show, in the case where varies weakly with
, that the effective interaction between lipids of the same type can
either be everywhere attractive or can have a repulsive component at
intermediate distances greater than the typical lipid size. When this
interaction has a repulsive component, it can prevent macro-phase separation
and lead to separation in mesophases with a finite domain size. This effect
could be relevant to certain experimental and numerical observations of
mesoscopic domains in such systems.Comment: 9 pages RevTex, 1 eps figur
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