5,433 research outputs found
Temperature Dependence of the Hydrophobic Hydration and Interaction of Simple Solutes: An Examination of Five Popular Water Models
We examine five different popular rigid water models (SPC, SPCE, TIP3P, TIP4P
and TIP5P) using MD simulations in order to investigate the hydrophobic
hydration and interaction of apolar Lennard-Jones solutes as a function of
temperature in the range between and . For all investigated
models and state points we calculate the excess chemical potential for the
noble gases and Methane.All water models exhibit too small hydration entropies,
but show a clear hierarchy. TIP3P shows poorest agreement with experiment
whereas TIP5P is closest to the experimental data at lower temperatures and
SPCE is closest at higher temperatures. A rescaling procedure inspired by
information theory model of Hummer et al. ({\em Chem.Phys.}258, 349-370 (2000))
suggests that the differences between the different models and real water can
be explained on the basis of the density curves at constant pressure. In
addition, the models that give a good representation of the water structure at
ambient conditions (TIP5P, SPCE and TIP4P) show considerably better agreement
with the experimental data than SPC and TIP3P. We calculate the hydrophobic
interaction between Xenon particles directly from a series of 60 ns simulation
runs.We find that the temperature dependence of the association is related to
the strength of the solvation entropy. Nevertheless, differences between the
models seem to require a more detailed molecular picture.The TIP5P model shows
by far the strongest temperature dependence.The suggested density-rescaling is
also applied to the Xenon-Xenon contact-pair configuration, indicating the
presence of a temperature where the hydrophobic interaction turns into purely
repulsive.The predicted association for Xenon in real water suggest the
presence a strong variation with temperature.Comment: 19 pages, 16 figures, revtex4 twocolums, removed typos, accepted for
publication in J.Chem. Phy
Heat Capacity Effects Associated with the Hydrophobic Hydration and Interaction of Simple Solutes: A Detailed Structural and Energetical Analysis Based on MD Simulations
We examine the SPCE and TIP5P water models to study heat capacity effects
associated with the hydrophobic hydration and interaction of Xenon particles.
We calculate the excess chemical potential for Xenon employing the Widom
particle insertion technique. The solvation enthalpy and excess heat capacity
is obtained from the temperature dependence of the chemical potentials and,
alternatively, directly by Ewald summation, as well as a reaction field based
method. All three different approaches provide consistent results. The reaction
field method allows a separation of the individual components to the heat
capacity of solvation into solute/solvent and solvent/solvent parts, revealing
the solvent/solvent part as the dominating contribution. A detailed spacial
analysis of the heat capacity of the water molecules around a pair of Xenon
particles at different separations reveals that the enhanced heat capacity of
the water molecules in the bisector plane between two Xenon atoms is
responsible for the maximum of the heat capacity observed at the desolvation
barrier, recently reported by Shimizu and Chan ({\em J. Am. Chem. Soc.},{\bf
123}, 2083--2084 (2001)). The about 60% enlarged heat capacity of water in the
concave part of the joint Xenon-Xenon hydration shell is the result of a
counterplay of strengthened hydrogen bonds and an enhanced breaking of hydrogen
bonds with increasing temperature. Differences between the two models
concerning the heat capacity in the Xenon-Xenon contact state are attributed to
the different water model bulk heat capacities, and to the different spacial
extension of the structure effect introduced by the hydrophobic particles.
Similarities between the different states of water in the joint Xenon-Xenon
hydration shell and the properties of stretched water are discussed.Comment: 14 pages, 16 figures, twocolumn revte
Evolution of the structure of amorphous ice - from low-density amorphous (LDA) through high-density amorphous (HDA) to very high-density amorphous (VHDA) ice
We report results of molecular dynamics simulations of amorphous ice for
pressures up to 22.5 kbar. The high-density amorphous ice (HDA) as prepared by
pressure-induced amorphization of Ih ice at T=80 K is annealed to T=170 K at
various pressures to allow for relaxation. Upon increase of pressure, relaxed
amorphous ice undergoes a pronounced change of structure, ranging from the
low-density amorphous ice (LDA) at p=0, through a continuum of HDA states to
the limiting very high-density amorphous ice (VHDA) regime above 10 kbar. The
main part of the overall structural change takes place within the HDA
megabasin, which includes a variety of structures with quite different local
and medium-range order as well as network topology and spans a broad range of
densities. The VHDA represents the limit to densification by adapting the
hydrogen-bonded network topology, without creating interpenetrating networks.
The connection between structure and metastability of various forms upon
decompression and heating is studied and discussed. We also discuss the analogy
with amorphous and crystalline silica. Finally, some conclusions concerning the
relation between amorphous ice and supercooled water are drawn.Comment: 11 pages, 12 postscript figures. To be published in The Journal of
Chemical Physic
Gas Enrichment at Liquid-Wall Interfaces
Molecular dynamics simulations of Lennard-Jones systems are performed to
study the effects of dissolved gas on liquid-wall and liquid-gas interfaces.
Gas enrichment at walls is observed which for hydrophobic walls can exceed more
than two orders of magnitude when compared to the gas density in the bulk
liquid. As a consequence, the liquid structure close to the wall is
considerably modified, leading to an enhanced wall slip. At liquid-gas
interfaces gas enrichment is found which reduces the surface tension.Comment: main changes compared to version 1: flow simulations are included as
well as different types of gase
Coarse-Grained Model for Phospholipid/Cholesterol Bilayer
We construct a coarse-grained (CG) model for dipalmitoylphosphatidylcholine
(DPPC)/cholesterol bilayers and apply it to large-scale simulation studies of
lipid membranes. Our CG model is a two-dimensional representation of the
membrane, where the individual lipid and sterol molecules are described by
point-like particles. The effective intermolecular interactions used in the
model are systematically derived from detailed atomic-scale molecular dynamics
simulations using the Inverse Monte Carlo technique, which guarantees that the
radial distribution properties of the CG model are consistent with those given
by the corresponding atomistic system. We find that the coarse-grained model
for the DPPC/cholesterol bilayer is substantially more efficient than atomistic
models, providing a speed-up of approximately eight orders of magnitude. The
results are in favor of formation of cholesterol-rich and cholesterol-poor
domains at intermediate cholesterol concentrations, in agreement with the
experimental phase diagram of the system. We also explore the limits of the
novel coarse-grained model, and discuss the general validity and applicability
of the present approach
Structure and dielectric properties of polar fluids with extended dipoles: results from numerical simulations
The strengths and short-comings of the point-dipole model for polar fluids of
spherical molecules are illustrated by considering the physically more relevant
case of extended dipoles formed by two opposite charges separated by a
distance (dipole moment ). Extensive Molecular Dynamics
simulations on a high density dipolar fluid are used to analyse the dependence
of the pair structure, dielectric constant \eps and dynamics as a function of
the ratio (\sig is the molecular diameter), for a fixed dipole
moment . The point dipole model is found to agree well with the extended
dipole model up to d/\sig \simeq 0.3. Beyond that ratio, \eps shows a
non-trivial variation with d/\sig. When d/\sig>0.6, a transition is
observed towards a hexagonal columnar phase; the corresponding value of the
dipole moment, \mu^2/\sig^3 k T=3, is found to be substantially lower than
the value of the point dipole required to drive a similar transition.Comment: 10 pages, 11 figures; Paper submitted to Molecular Physic
Metastability of life
The physical idea of the natural origin of diseases and deaths has been
presented. The fundamental microscopical reason is the destruction of any
metastable state by thermal activation of a nucleus of a nonreversable change.
On the basis of this idea the quantitative theory of age dependence of death
probability has been constructed. The obtained simple Death Laws are very
accurately fulfilled almost for all known diseases.Comment: 3 pages, 4 figure
Anharmonicity and self-similarity of the free energy landscape of protein G
The near-native free energy landscape of protein G is investigated through
0.4 microseconds-long atomistic molecular dynamics simulations in explicit
solvent. A theoretical and computational framework is used to assess the
time-dependence of salient thermodynamical features. While the quasi-harmonic
character of the free energy is found to degrade in a few ns, the slow modes
display a very mild dependence on the trajectory duration. This property
originates from a striking self-similarity of the free energy landscape
embodied by the consistency of the principal directions of the local minima,
where the system dwells for several ns, and of the virtual jumps connecting
them.Comment: revtex, 6 pages, 5 figure
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