2,375 research outputs found
Configurational space continuity and free energy calculations
Free energy is arguably the most importance function(al) for understanding of
molecular systems. A number of rigorous and approximate free energy
calculation/estimation methods have been developed over many decades. One
important issue, the continuity of an interested macrostate (or path) in
configurational space, has not been well articulated, however. As a matter of
fact, some important special cases have been intensively discussed. In this
perspective, I discuss the relevance of configurational space continuity in
development of more efficient and reliable next generation free energy
methodologies.Comment: 17 pages, 2 figure
Nonlinear backbone torsional pair correlations in proteins
Protein allostery requires dynamical structural correlations. Physical origin
of which, however, remain elusive despite intensive studies during last two
decades. Based on analysis of molecular dynamics (MD) simulation trajectories
for ten proteins with different sizes and folds, we found that nonlinear
backbone torsional pair (BTP) correlations, which are spatially more
long-ranged and are mainly executed by loop residues, exist extensively in most
analyzed proteins. Examination of torsional motion for correlated BTPs
suggested that aharmonic torsional state transitions are essential for such
non-linear correlations, which correspondingly occur on widely different and
relatively longer time scales. In contrast, BTP correlations between backbone
torsions in stable helices and strands are mainly linear and
spatially more short-ranged, and are more likely to associate with intra-well
torsional dynamics. Further analysis revealed that the direct cause of
non-linear contributions are heterogeneous, and in extreme cases canceling,
linear correlations associated with different torsional states of participating
torsions. Therefore, torsional state transitions of participating torsions for
a correlated BTP are only necessary but not sufficient condition for
significant non-linear contributions. These findings implicate a general search
strategy for novel allosteric modulation of protein activities. Meanwhile, it
was suggested that ensemble averaged correlation calculation and static contact
network analysis, while insightful, are not sufficient to elucidate mechanisms
underlying allosteric signal transmission in general, dynamical and time scale
resolved analysis are essential.Comment: 25 pages, 8 figure
Shot Range and High Order Correlations in Proteins
The main chain dihedral angles play an important role to decide the protein
conformation. The native states of a protein can be regard as an ensemble of a
lot of similar conformations, in different conformations the main chain
dihedral angles vary in a certain range. Each dihedral angle value can be
described as a distribution, but only using the distribution can't describe the
real conformation space. The reason is that the dihedral angle has correlation
with others, especially the neighbor dihedral angles in primary sequence. In
our study we analysis extensive molecular dynamics (MD) simulation trajectories
of eleven proteins with different sizes and folds, we found that in stable
second structure the correlations only exist between the dihedrals near to each
other in primary sequence, long range correlations are rare. But in unstable
structures (loop) long range correlations exist. Further we observed some
characteristics of the short range correlations in different second structures
({\alpha}-helix, {\beta}-sheet) and we found that we can approximate good high
order dihedral angle distribution good only use three order distribution in
stable second structure which illustrates that high order correlations (over
three order) is small in stable second structure
Entropically Dominant State of Proteins
Configurational entropy is an important factor in the free energy change of
many macromolecular recognition and binding processes, and has been intensively
studied. Despite great progresses that have been made, the global sampling
remains to be a grand challenge in computational analysis of relevant
processes. Here we propose and demonstrate an entropy estimation method that is
based on physical partition of configurational space and can be readily
combined with currently available methodologies. Tests with two globular
proteins suggest that for flexible macromolecules with large and complex
configurational space, accurate configurational entropy estimation may be
achieved simply by considering the entropically most important subspace. This
conclusion effectively converts an exhaustive sampling problem into a local
sampling one, and defines entropically dominant state for proteins and other
complex macromolecules. The conceptional breakthrough is likely to positively
impact future theoretical analysis, computational algorithm development and
experimental design of diverse chemical and biological molecular systems.Comment: 10 pages, 4 figure and 27 reference
Utility of potential energy span as an approximate free energy proxy
Free energy calculation is critical in predictive tasks such as protein
folding, docking and design. However, rigorous calculation of free energy
change is prohibitively expensive in these practical applications. The minimum
potential energy is therefore widely utilized to approximate free energy. In
this study, based on analysis of extensive molecular dynamics (MD) simulation
trajectories of a few native globular proteins, we found that change of minimum
and corresponding maximum potential energy terms exhibit similar level of
correlation with change of free energy. More importantly, we demonstrated that
change of span (maximum - minimum) of potential energy terms, which engender
negligible additional computational cost, exhibit considerably stronger
correlations with change of free energy than the corresponding change of
minimum and maximum potential energy terms. Therefore, potential energy span
may serve as an alternative efficient approximate free energy proxy.Comment: 18 pages, 8 figure
Configurational space discretization and free energy calculation in complex molecular systems
Trajectories provide dynamical information that is discarded in free energy
calculations, for which we sought to design a scheme with the hope of saving
cost for generating dynamical information. We first demonstrated that snapshots
in a converged trajectory set are associated with implicit conformers that have
invariant statistical weight distribution (ISWD). Based on the thought that
infinite number of sets of implicit conformers with ISWD may be created through
independent converged trajectory sets, we hypothesized that explicit conformers
with ISWD may be constructed for complex molecular systems through systematic
increase of conformer fineness, and tested the hypothesis in lipid molecule
palmitoyloleoylphosphatidylcholine (POPC). Furthermore, when explicit
conformers with ISWD were utilized as basic states to define conformational
entropy, change of which between two given macrostates was found to be
equivalent to change of free energy except a mere difference of a negative
temperature factor, and change of enthalpy essentially cancels corresponding
change of average intra-conformer entropy. These findings suggest that entropy
enthalpy compensation is inherently a local phenomenon in configurational
space. By implicitly taking advantage of entropy enthalpy compensation and
forgoing all dynamical information, constructing explicit conformers with ISWD
and counting thermally accessible number of which for interested end
macrostates is likely to be an efficient and reliable alternative end point
free energy calculation strategy.Comment: 27 pages, 8 figures, 1 tabl
Non-reciprocal Radio Frequency Transduction in a Parametric Mechanical Artificial Lattice
Generating non-reciprocal radio frequency transduction plays important roles
in a wide range of research and applications, and an aspiration is to integrate
this functionality into micro-circuit without introducing magnetic field,
which, however, remains challenging. By designing a 1D artificial lattice
structure with neighbor-interaction engineered parametrically, we predicted a
non-reciprocity transduction with giant unidirectionality. We then
experimentally demonstrated the phenomenon on a nano-electromechanical chip
fabricated by conventional complementary metal-silicon processing. A
unidirectionality with isolation as high as 24dB is achieved and several
different transduction schemes are realized by programming the control voltages
topology. Apart from being used as a radio frequency isolator, the system
provides a way to build practical on-chip programmable device for many
researches and applications in radio frequency domain.Comment: 12 pages, 4 figure
A simple neural network implementation of generalized solvation free energy for assessment of protein structural models
Rapid and accurate assessment of protein structural models is essential for
protein structure prediction and design. Great progress has been made in this
regard, especially by recent development of ``knowledge-based'' potentials.
Various machine learning based protein structural model quality assessment was
also quite successful. However, performance of traditional ``physics-based''
potentials have not been as effective. Based on analysis of computational
limitations of present solvation free energy formulation, which partially
underlies unsatisfactory performance of ``physics-based'' potentials, we
proposed a generalized sovation free energy (GSFE) framework. GSFE is
intrinsically flexible for multi-scale treatments and is amenable for machine
learning implementation. In this framework, each physical comprising unit of a
complex molecular system has its own specific solvent environment. One
distinctive feature of GSFE is that high order correlations within selected
solvent environment might be captured through machine learning, in contrast to
present empirical potentials (both ``knowledge-based'' and ``physics-based'')
that are mainly based on pairwise interactions. Finally, we implemented a
simple example of backbone and side-chain orientation based residue level
protein GSFE with neural network, which was found to have competitive
performance when compared with highly complex latest ``knowledge-based'' atomic
potentials in distinguishing native structures from decoys
Pathway-based feature selection algorithms identify genes discriminating patients with multiple sclerosis apart from controls
Introduction The focus of analyzing data from microarray experiments and
extracting biological insight from such data has experienced a shift from
identification of individual genes in association with a phenotype to that of
biological pathways or gene sets. Meanwhile, feature selection algorithm
becomes imperative to cope with the high dimensional nature of many modeling
tasks in bioinformatics. Many feature selection algorithms use information
contained within a gene set as a biological priori, and select relevant
features by incorporating such information. Thus, an integration of gene set
analysis with feature selection is highly desired. Significance analysis of
microarray to gene-set reduction analysis (SAM-GSR) algorithm is a novel
direction of gene set analysis, aiming at further reduction of gene set into a
core subset. Here, we explore the feature selection trait possessed by SAM-GSR
and then modify SAM-GSR specifically to better fulfill this role. Results and
Conclusions Training on a multiple sclerosis (MS) microarray data using both
SAM-GSR and our modification of SAM-GSR, excellent discriminative performance
on an independent test set was achieved. To conclude, absorbing biological
information from a gene set may be helpful for classification and feature
selection. Discussion Given the fact the complete pathway information is far
from completeness, a statistical method capable of constructing biologically
meaningful gene networks is in demand. The basic requirement is that interplay
among genes must be taken into account
Ideal gas behavior of rotamerically defined conformers in native globular proteins
Protein conformational transitions, which are essential for function, may be
driven either by entropy or enthalpy when molecular systems comprising solute
and solvent molecules are the focus. Revealing thermodynamic origin of a given
molecular process is an important but difficult task, and general principles
governing protein conformational distributions remain elusive. Here we
demonstrate that when protein molecules are taken as thermodynamic systems and
solvents being treated as the environment, conformational entropy is an
excellent proxy for free energy and is sufficient to explain protein
conformational distributions. Specifically, by defining each unique combination
of side chain torsional state as a conformer, the population distribution (or
free energy) on an arbitrarily given order parameter is approximately a linear
function of conformational entropy. Additionally, span of various microscopic
potential energy terms is observed to be highly correlated with both
conformational entropy and free energy. Presently widely utilized free energy
proxies, including minimum potential energy, average potential energy terms by
themselves or in combination with vibrational entropy\cite, are found to
correlate with free energy rather poorly. Therefore, our findings provide a
fundamentally new theoretical base for development of significantly more
reliable and efficient next generation computational tools, where the number of
available conformers,rather than poential energy of microscopic configurations,
is the central focus. We anticipate that many related research fields,
including structure based drug design and discovery, protein design, docking
and prediction of general intermolecular interactions involving proteins, are
expected to benefit greatly.Comment: 6 figures in the main tex
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