64 research outputs found
Molecular Dynamics Simulations Using a Capacitance–Polarizability Force Field
We
present molecular dynamics (MD) simulations using a capacitance–polarizability
force field. This force field allows an atomistic description of charge
migration within a particle and hence the image charge effects at
the interface of such a particle. By employing atomic capacitance
and polarizability as the key parameters that describe fluctuating
charges and dipoles, we can thus explore the effect of charge migration
on the structural dynamics. We illustrate the method by exploring
gold nanoparticles in aqueous solutions and compare with previous
simulation work. We reach the conclusion that the capacitance–polarizability
force field MD method serves as a promising tool for simulating gold–water
systems, indicating probable extensions to other metal solutions and
for studies of more complicated systems provided that a proper parametrization
of the capacitance force field can be made. For the particular system
studied, it is found that the water molecules interact with the surface
through oxygen atoms, leading to more hydrogen-bond donors than acceptors
at the gold–water interface. A prominent shoulder peak is found
in the radial distribution of oxygen atoms with respect to the gold
surface, due to the fact that the oxygen atoms adsorbed at the on-top
sites of the gold nanoparticle. The surface of the aqueous gold nanoparticle
carries negative charge, which is balanced by the positive charge
in the second outermost layer
Simulation of Gold Functionalization with Cysteine by Reactive Molecular Dynamics
The
anchoring mechanism of cysteine to gold in water solution is
characterized in detail by means of a combination of quantum chemistry
(QC) and reactive classical molecular dynamics (RC-MD) calculations.
A possible adsorption–reaction route is proposed, through RC-MD
simulations based on a modified version of the protein reactive force
field (ReaxFF), in which gold–protein interactions have been
included after accurate parametrization at the QC level. The computational
results confirm recent experimental findings regarding the mechanism
as a two-step binding, namely, a slow physisorption followed by a
fast chemisorption. The reaction barriers are estimated through the
nudged elastic band approach and checked by QC calculations. Surface
reconstructions, induced by the strong adsorption of the molecule,
are identified, and their
role, as further adsorbate stabilizers, is properly disclosed. The
satisfactory agreement with QC data and experiments confirm the reliability
of the simulations and the unique opportunity they provide to follow
locally molecule adsorption on selected materials
Functional Water Molecules in Rhodopsin Activation
G-protein-coupled
receptors (GPCRs) are integral membrane proteins
that mediate cellular response to an extensive variety of extracellular
stimuli. Studies of rhodopsin, a prototype GPCR, have suggested that
water plays an important role in mediating the activation of family
A GPCRs. However, our understanding of the function of water molecules
in the GPCR activation is still rather limited because resolving the
functional water molecules solely based on the results from existing
experiments is challenging. Using all-atom molecular dynamics simulations
in combination with inhomogeneous fluid theory, we identify in this
work the positioning of functional water molecules in the inactive
state, the Meta II state, and the constitutive active state of rhodopsin,
basing on the thermodynamic signatures of the water molecules. We
find that one hydration site likely functions as a switch to regulate
the distance between Glu181 and the Schiff base in the rhodopsin activation.
We observe that water molecules adjacent to the “NpxxY”
motif are not as stable in the Meta II state as in the inactive state
as indicated by the thermodynamics signatures, and we rationalize
that the behaviors of these water molecules are closely correlated
with the rearrangement of the water-mediated hydrogen-bond network
in the “NPxxY” motif, which is essential for mediating
the activation of the receptor. We thereby propose a hypothesis of
the water-mediated rhodopsin activation pathway
Two-Photon Absorption of Metal-Assisted Chromophores
Aiming to understand the effect of
a metal surface on nonlinear
optical properties and the combined effects of surface and solvent
environments on such properties, we present a multiscale response
theory study, integrated with dynamics of the two-photon absorption
of 4-nitro-4′-amino-<i>trans</i>-stilbene physisorbed
on noble metal surfaces, considering two such surfaces, Ag(111) and
Au(111), and two solvents, cyclohexane and water, as cases for demonstration.
A few conclusions of general character could be drawn: While the geometrical
change of the chromophore induced by the environment was found to
notably alter (diminish) the two-photon absorption cross section in
the polar medium, the effects of the metal surface and solvent on
the electronic structure of the chromophore surpasses the geometrical
effects and leads to a considerably enhanced two-photon absorption
cross section in the polar solvent. This enhancement of two-photon
absorption arises essentially from the metal charge image induced
enlargement of the difference between the dipole moment of the excited
state and the ground state. The orientation-dependence of the two-photon
absorption is found to connect with the lateral rotation of the chromophore,
where the two-photon absorption reaches its maximum when the polarization
of the incident light coincides with the long-axis of the chromophore.
Our results demonstrate a distinct enhancement of the two-photon absorption
by a metal surface and a polar medium and envisage the employment
of metal–chromophore composite materials for future development
of nonlinear optical materials with desirable properties
First Hyperpolarizability of Collagen Using the Point Dipole Approximation
The application of localized hyperpolarizabilities
to predict a
total protein hyperpolarizability is presented for the first time,
using rat-tail collagen as a demonstration example. We employ a model
comprising the quadratic Applequist point-dipole approach, the so-called
LoProp transformation, and a procedure with molecular fractionation
using conjugate caps to determine the atomic and bond contributions
to the net β tensor of the collagen [(PPG)<sub>10</sub>]<sub>3</sub> triple-helix. By using Tholes exponential damping modification
to the dyadic tensor in the Applequist equations, a correct qualitative
agreement with experiment is found. The intensity of the β<sub>HRS</sub> signal and the depolarization ratios are best reproduced
by decomposing the LoProp properties into the atomic positions and
using Tholes exponential damping with the original damping parameter.
Some ramifications of the model for general protein property optimization
are briefly discussed
Microsecond Molecular Dynamics Simulations Provide Insight into the Allosteric Mechanism of the Gs Protein Uncoupling from the β<sub>2</sub> Adrenergic Receptor
Experiments
have revealed that in the β<sub>2</sub> adrenergic
receptor (β<sub>2</sub>AR)–Gs protein complex the α
subunit (Gαs) of the Gs protein can adopt either an “open”
conformation or a “closed” conformation. In the “open”
conformation the Gs protein prefers to bind to the β<sub>2</sub>AR, while in the “closed” conformation an uncoupling
of the Gs protein from the β<sub>2</sub>AR occurs. However,
the mechanism that leads to such different behaviors of the Gs protein
remains unclear. Here, we report results from microsecond molecular
dynamics simulations and community network analysis of the β<sub>2</sub>AR–Gs complex with Gαs in the “open”
and “closed” conformations. We observed that the complex
is stabilized differently in the “open” and “closed”
conformations. The community network analysis reveals that in the
“closed” conformation there exists strong allosteric
communication between the β<sub>2</sub>AR and Gβγ,
mediated by Gαs. We suggest that such high information flows
are necessary for the Gs protein uncoupling from the β<sub>2</sub>AR
Photochromic Diarylethenes with Heterocyclic Aromatic Rings: Correlation between Thermal Bistability and Geometrical Characters of Transition States
We
present a density functional theory study on the thermal bistability
of a number of photochromic diarylethenes, with emphasis on the free
energy barrier of the ground-state ring-opening process. We found
that the free energy barrier is correlated with the geometrical and
vibrational character of the transition state, in particular the distance
between the two reactive carbon atoms, the out-of-plane angles of
the methyl groups at the reactive carbon atoms, and the imaginary
vibrational frequency. Based on these relationships we propose a linear
fitting expression for the free energy barrier in terms of the three
aforementioned transition-state quantities and obtained a correlation
coefficient of <i>R</i><sup>2</sup> = 0.971. In this way
quantum chemical calculations may provide insight and structure–property
relationships, which can be applied in the development of novel photochromic
materials
Electronic Circular Dichroism of Surface-Adsorbed Molecules by Means of Quantum Mechanics Capacitance Molecular Mechanics
To
promote a more comprehensive understanding of the influence of metal–adsorbate
interaction for molecules at metallo surfaces or metallo nanoparticles
in solvent environments on their electronic circular dichroism (ECD)
spectra, we evaluate the application of a recently derived quantum
mechanics capacitance molecular mechanics (QMCMM) model for ECD. Using
helicene absorbed on gold surfaces in protic and aprotic solvents
as illustration, we elucidate the detailed effects on excitation energies,
transition moments, rotatory strengths, orientation dependence of
ECD spectra, and the different roles of aprotic and protic solvents
and the induced charge distribution patterns on the surface. These
changes are decomposed in terms of surface alone, solvent alone, and
combined surface–solvent influence, and furthermore into the
indirect contributions by the surface-induced restructuring of the
helicene. Much of the salient changes of the ECD can be rationalized
to the substantial redistribution of charge at the gold surface induced
by the presence of the helicene. The study indicates that through
the QMCMM model the effects of a metallic surface on the circular
dichroism spectra of adsorbed organic molecules can be tackled by
extended QM calculations coupled to polarizability–capacitance
force fields for large metallic clusters representing surfaces or
nanoparticles
Simulation of Gold Functionalization with Cysteine by Reactive Molecular Dynamics
The
anchoring mechanism of cysteine to gold in water solution is
characterized in detail by means of a combination of quantum chemistry
(QC) and reactive classical molecular dynamics (RC-MD) calculations.
A possible adsorption–reaction route is proposed, through RC-MD
simulations based on a modified version of the protein reactive force
field (ReaxFF), in which gold–protein interactions have been
included after accurate parametrization at the QC level. The computational
results confirm recent experimental findings regarding the mechanism
as a two-step binding, namely, a slow physisorption followed by a
fast chemisorption. The reaction barriers are estimated through the
nudged elastic band approach and checked by QC calculations. Surface
reconstructions, induced by the strong adsorption of the molecule,
are identified, and their
role, as further adsorbate stabilizers, is properly disclosed. The
satisfactory agreement with QC data and experiments confirm the reliability
of the simulations and the unique opportunity they provide to follow
locally molecule adsorption on selected materials
Functional Water Molecules in Rhodopsin Activation
G-protein-coupled
receptors (GPCRs) are integral membrane proteins
that mediate cellular response to an extensive variety of extracellular
stimuli. Studies of rhodopsin, a prototype GPCR, have suggested that
water plays an important role in mediating the activation of family
A GPCRs. However, our understanding of the function of water molecules
in the GPCR activation is still rather limited because resolving the
functional water molecules solely based on the results from existing
experiments is challenging. Using all-atom molecular dynamics simulations
in combination with inhomogeneous fluid theory, we identify in this
work the positioning of functional water molecules in the inactive
state, the Meta II state, and the constitutive active state of rhodopsin,
basing on the thermodynamic signatures of the water molecules. We
find that one hydration site likely functions as a switch to regulate
the distance between Glu181 and the Schiff base in the rhodopsin activation.
We observe that water molecules adjacent to the “NpxxY”
motif are not as stable in the Meta II state as in the inactive state
as indicated by the thermodynamics signatures, and we rationalize
that the behaviors of these water molecules are closely correlated
with the rearrangement of the water-mediated hydrogen-bond network
in the “NPxxY” motif, which is essential for mediating
the activation of the receptor. We thereby propose a hypothesis of
the water-mediated rhodopsin activation pathway
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