64 research outputs found
Minimal model for the secondary structures and conformational conversions in proteins
Better understanding of protein folding process can provide physical insights on the function of proteins and makes it possible to benefit from genetic information accumulated so far. Protein folding process normally takes place in less than seconds but even seconds are beyond reach of current computational power for simulations on a system of all-atom detail. Hence, to model and explore protein folding process it is crucial to construct a proper model that can adequately describe the physical process and mechanism for the relevant time scale. We discuss the reduced off-lattice model that can express α-helix and β-hairpin conformations defined solely by a given sequence in order to investigate a protein folding mechanism of conformations such as a β-hairpin and also to investigate conformational conversions in proteins. The first two chapters introduce and review essential concepts in protein folding modelling physical interaction in proteins, various simple models, and also review computational methods, in particular, the Metropolis Monte Carlo method, its dynamic interpretation and thermodynamic Monte Carlo algorithms. Chapter 3 describes the minimalist model that represents both α-helix and β-sheet conformations using simple potentials. The native conformation can be specified by the sequence without particular conformational biases to a reference state. In Chapter 4, the model is used to investigate the folding mechanism of β-hairpins exhaustively using the dynamic Monte Carlo and a thermodynamic Monte Carlo method an effcient combination of the multicanonical Monte Carlo and the weighted histogram analysis method. We show that the major folding pathways and folding rate depend on the location of a hydrophobic. The conformational conversions between α-helix and β-sheet conformations are examined in Chapter 5 and 6. First, the conformational conversion due to mutation in a non-hydrophobic system and then the conformational conversion due to mutation with a hydrophobic pair at a different position at various temperatures are examined
Capturing the essence of folding and functions of biomolecules using Coarse-Grained Models
The distances over which biological molecules and their complexes can
function range from a few nanometres, in the case of folded structures, to
millimetres, for example during chromosome organization. Describing phenomena
that cover such diverse length, and also time scales, requires models that
capture the underlying physics for the particular length scale of interest.
Theoretical ideas, in particular, concepts from polymer physics, have guided
the development of coarse-grained models to study folding of DNA, RNA, and
proteins. More recently, such models and their variants have been applied to
the functions of biological nanomachines. Simulations using coarse-grained
models are now poised to address a wide range of problems in biology.Comment: 37 pages, 8 figure
Advances in Computational Solvation Thermodynamics
The aim of this thesis is to develop improved methods for calculating the free energy, entropy and enthalpy of solvation from molecular simulations. Solvation thermodynamics of model compounds provides quantitative measurements used to analyze the stability of protein conformations in aqueous milieus. Solvation free energies govern the favorability of the solvation process, while entropy and enthalpy decompositions give insight into the molecular mechanisms by which the process occurs. Computationally, a coupling parameter λ modulates solute-solvent interactions to simulate an insertion process, and multiple lengthy simulations at a fixed λ value are typically required for free energy calculations to converge; entropy and enthalpy decompositions generally take 10-100 times longer. This thesis presents three advances which accelerate the convergence of such calculations: 1) Development of entropy and enthalpy estimators which combine data from multiple simulations; 2) Optimization of λ schedules, or the set of parameter values associated with each simulation; 3) Validation of Hamiltonian replica exchange, a technique which swaps λ values between two otherwise independent simulations. Taken together, these techniques promise to increase the accuracy and precision of free energy, entropy and enthalpy calculations. Improved estimates, in turn, can be used to investigate the validity and limits of existing solvation models and refine force field parameters, with the goal of understanding better the collapse transition and aggregation behavior of polypeptides
Solvation thermodynamics of organic molecules by the molecular integral equation theory : approaching chemical accuracy
The integral equation theory (IET) of molecular liquids has been an active area of academic research in theoretical and computational physical chemistry for over 40 years because it provides a consistent theoretical framework to describe the structural and thermodynamic properties of liquid-phase solutions. The theory can describe pure and mixed solvent systems (including anisotropic and nonequilibrium systems) and has already been used for theoretical studies of a vast range of problems in chemical physics / physical chemistry, molecular biology, colloids, soft matter, and electrochemistry. A consider- able advantage of IET is that it can be used to study speci fi c solute − solvent interactions, unlike continuum solvent models, but yet it requires considerably less computational expense than explicit solvent simulations
Sequence-dependent DNA deformability studied using molecular dynamics simulations
Proteins recognize specific DNA sequences not only through direct contact between amino acids and bases, but also indirectly based on the sequence-dependent conformation and deformability of the DNA (indirect readout). We used molecular dynamics simulations to analyze the sequence-dependent DNA conformations of all 136 possible tetrameric sequences sandwiched between CGCG sequences. The deformability of dimeric steps obtained by the simulations is consistent with that by the crystal structures. The simulation results further showed that the conformation and deformability of the tetramers can highly depend on the flanking base pairs. The conformations of xATx tetramers show the most rigidity and are not affected by the flanking base pairs and the xYRx show by contrast the greatest flexibility and change their conformations depending on the base pairs at both ends, suggesting tetramers with the same central dimer can show different deformabilities. These results suggest that analysis of dimeric steps alone may overlook some conformational features of DNA and provide insight into the mechanism of indirect readout during protein–DNA recognition. Moreover, the sequence dependence of DNA conformation and deformability may be used to estimate the contribution of indirect readout to the specificity of protein–DNA recognition as well as nucleosome positioning and large-scale behavior of nucleic acids
Ab initio Prediction of the Conformation of Solvated and Adsorbed Proteins
Proteins are among the most important groups of biomolecules, with their
biological functions ranging from structural elements to signal transducers between
cells. Apart from their biological role, phenomena related to protein behaviour in
solutions and at solid interfaces can find a broad range of engineering applications
such as in biomedical implants, scaffolds for artificial tissues, bioseparations,
biomineralization and biosensors. For both biological and engineering applications,
the functionality of a protein is directly related to its three-dimensional structure (i.e.
conformation). Methods such as homology and threading that depend on a large
database of existing experimental knowledge are the most popular means of
predicting the conformation of proteins in their native environment. Lack of
sufficient experimentally-derived information for non-native environments such as
general solutions and solid interfaces prevents these knowledge-based methods being
used for such environments. Resort must, instead, be made to so-called ab initio
methods that rely upon knowledge of the primary sequence of the protein, its
environment, and the physics of the interatomic interactions. The development of
such methods for non-native environments is in its infancy – this thesis reports on the
development of such a method and its application to proteins in water and at
gas/solid and water/solid interfaces. After introducing the approach used – which is
based on evolutionary algorithms (EAs) – we first report a study of polyalanine
adsorbed at a gas/solid interface in which a switching behaviour is observed that, to
our knowledge, has never been reported before. The next section reports work that
shows the combination of the Langevin dipole (LD) solvent method with the Amber
potential energy (PE) model is able to yield solvation energies comparable to those
of more sophisticated methods at a fraction of the cost, and that the LD method is
able to capture effects that arise from inhomogenities in the water structure such as
H-bond bridges. The third section reports a study that shows that EA performance
and optimal control parameters vary substantially with the PE model. The first three
parts form the basis of the last part of the thesis, which reports pioneering work on
predicting ab initio the conformation of proteins in solutions and at water/solid
interfaces
Simulations in statistical physics and biology: some applications
One of the most active areas of physics in the last decades has been that of
critical phenomena, and Monte Carlo simulations have played an important role
as a guide for the validation and prediction of system properties close to the
critical points. The kind of phase transitions occurring for the Betts lattice
(lattice constructed removing 1/7 of the sites from the triangular lattice)
have been studied before with the Potts model for the values q=3, ferromagnetic
and antiferromagnetic regime. Here, we add up to this research line the
ferromagnetic case for q=4 and 5. In the first case, the critical exponents are
estimated for the second order transition, whereas for the latter case the
histogram method is applied for the occurring first order transition.
Additionally, Domany's Monte Carlo based clustering technique mainly used to
group genes similar in their expression levels is reviewed. Finally, a control
theory tool --an adaptive observer-- is applied to estimate the exponent
parameter involved in the well-known Gompertz curve. By treating all these
subjects our aim is to stress the importance of cooperation between distinct
disciplines in addressing the complex problems arising in biology.
Contents: Chapter 1 - Monte Carlo simulations in stat. physics; Chapter 2: MC
simulations in biology; Chapter 3: Gompertz equationComment: 82 pages, 33 figures, 4 tables, somewhat reduced version of the M.Sc.
thesis defended in Jan. 2006 at IPICyT, San Luis Potosi, Mx. (Supervisers:
Drs. R. Lopez-Sandoval and H.C. Rosu). Last sections 3.3 and 3.4 can be found
at http://lanl.arxiv.org/abs/physics/041108
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