14 research outputs found
Boiling Temperature As a Scaling Parameter for the Microscopic Relaxation Dynamics in Molecular Liquids
At
sufficiently high temperatures, the center-of-mass microscopic
diffusion dynamics of liquids is characterized by a single component,
often with weak temperature dependence. In this regime, the effective
cage made by the neighbor particles cannot be sustained and readily
breaks down, enabling long-range diffusion. As the temperature is
decreased, the cage relaxation becomes impeded, leading to a higher
viscosity with more pronounced temperature dependence. On the microscopic
scale, the sustained caging effect leads to a separation between a
faster in-cage relaxation component and a slower cage-breaking relaxation
component. The evidence for the separate dynamic components, as opposed
to a single stretched component, is provided by quasielastic neutron
scattering experiments. We use a simple method to evaluate the extent
of the dynamic components separation as a function of temperature
in a group of related aromatic molecular liquids. We find that, regardless
of the glass-forming capabilities or lack thereof, progressively more
pronounced separation between the in-cage and cage-breaking dynamic
components develops on cooling down as the ratio of <i>T</i><sub>b</sub>/<i>T</i>, where <i>T</i><sub>b</sub> is the boiling temperature, increases. This reflects the microscopic
mechanism behind the empirical rule for the glass forming capability
based on the ratio of boiling and melting temperatures, <i>T</i><sub>b</sub>/<i>T</i><sub>m</sub>. When a liquidās <i>T</i><sub>b</sub>/<i>T</i><sub>m</sub> happens to
be high, the liquid can readily be supercooled below its <i>T</i><sub>m</sub> because the liquidās microscopic relaxation dynamics
is already impeded at <i>T</i><sub>m</sub>, as evidenced
by a sustained caging effect manifested through the separation of
the in-cage and cage-breaking dynamic components. Our findings suggest
certain universality in the temperature dependence of the microscopic
diffusion dynamics in molecular liquids, regardless of their glass-forming
capabilities. Unless the insufficiently low (with respect to <i>T</i><sub>b</sub>) melting temperature, <i>T</i><sub>m</sub>, intervenes and makes crystallization thermodynamically favorable
when cage-breaking is still unimpeded and the structural relaxation
is fast, the liquid is likely to become supercooled. The propensity
to supercooling and eventually forming a glass is thus determined
by a purely thermodynamic factor, <i>T</i><sub>b</sub>/<i>T</i><sub>m</sub>
Differential Microscopic Mobility of Components within a Deep Eutectic Solvent
From
macroscopic measurements of deep eutectic solvents such as
glyceline (1:2 molar ratio of choline chloride to glycerol), the long-range
translational diffusion of the larger cation (choline) is known to
be slower compared to that of the smaller hydrogen bond donor (glycerol).
However, when the diffusion dynamics are analyzed on the subnanometer
length scale, we find that the displacements associated with the localized
diffusive motions are actually larger for choline. This counterintuitive
diffusive behavior can be understood as follows. The localized diffusive
motions confined in the transient cage of neighbor particles, which
precede the cage-breaking long-range diffusion jumps, are more spatially
constrained for glycerol than for choline because of the stronger
hydrogen bonds the former makes with chloride anions. The implications
of such differential localized mobility of the constituents should
be especially important for applications where deep eutectic solvents
are confined on the nanometer length scale and their long-range translational
diffusion is strongly inhibited (e.g., within microporous media)
Temperature Dependence of Logarithmic-like Relaxational Dynamics of Hydrated tRNA
The dynamics of RNA within the Ī²-relaxation
region of 10
ps to 1 ns is crucial to its biological function. Because of its simpler
chemical building blocks and the lack of the side methyl groups, faster
relaxational dynamics of RNA compared to proteins can be expected.
However, the situation is actually opposite. In this work, the relaxational
dynamics of tRNA is measured by quasielastic neutron scattering and
analyzed using the mode coupling theory, originally developed for
glass-forming liquids. Our results reveal that the dynamics of tRNA
follows a log-decay within the Ī²-relaxation region, which is
an important trait demonstrated by the dynamics of proteins. The dynamics
of hydrated tRNA and lysozyme compared in the time domain further
demonstrate that the slower dynamics of tRNA relative to proteins
originates from the difference in the folded states of tRNA and proteins,
as well as the influence of their hydration water
Mixed Ionic Liquid Improves Electrolyte Dynamics in Supercapacitors
Well-tailored mixtures
of distinct ionic liquids can act as optimal
electrolytes that extend the operating electrochemical window and
improve charge storage density in supercapacitors. Here, we explore
two room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bisĀ(trifluoromethylsulfonyl)Āimide
(EmimTFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF<sub>4</sub>). We study their electric double-layer behavior in the neat
state and as binary mixtures on the external surfaces of onion-like
carbon electrodes using quasielastic neutron scattering (QENS) and
classical density functional theory techniques. Computational results
reveal that a mixture with 4:1 EmimTFSI/EmimBF<sub>4</sub> volume
ratio displaces the larger [TFSI<sup>ā</sup>] anions with smaller
[BF<sub>4</sub><sup>ā</sup>] ions, leading to an excess adsorption
of [Emim<sup>+</sup>] cations near the electrode surface. These findings
are corroborated by the manifestation of nonuniform ion diffusivity
change, complementing the description of structural modifications
with changing composition, from QENS measurements. Molecular-level
understanding of ion packing near electrodes provides insight for
design of ionic liquid formulations that enhance the performance of
electrochemical energy storage devices
Quasi-elastic Neutron Scattering Reveals Ligand-Induced Protein Dynamics of a GāProtein-Coupled Receptor
Light
activation of the visual G-protein-coupled receptor (GPCR)
rhodopsin leads to significant structural fluctuations of the protein
embedded within the membrane yielding the activation of cognate G-protein
(transducin), which initiates biological signaling. Here, we report
a quasi-elastic neutron scattering study of the activation of rhodopsin
as a GPCR prototype. Our results reveal a broadly distributed relaxation
of hydrogen atom dynamics of rhodopsin on a picosecondānanosecond
time scale, crucial for protein function, as only observed for globular
proteins previously. Interestingly, the results suggest significant
differences in the intrinsic protein dynamics of the dark-state rhodopsin
versus the ligand-free apoprotein, opsin. These differences can be
attributed to the influence of the covalently bound retinal ligand.
Furthermore, an idea of the generic free-energy landscape is used
to explain the GPCR dynamics of ligand-binding and ligand-free protein
conformations, which can be further applied to other GPCR systems
Influence of Surface Oxidation on Ion Dynamics and Capacitance in Porous and Nonporous Carbon Electrodes
We investigate the influence of surface
chemistry and ion confinement
on capacitance and electrosorption dynamics of room-temperature ionic
liquids (RTILs) in supercapacitors. Using air oxidation and vacuum
annealing, we produced defunctionalized and oxygen-rich surfaces of
carbide-derived carbons (CDCs) and graphene nanoplatelets (GNPs).
While oxidized surfaces of porous CDCs improve capacitance and rate
handling abilities of ions, defunctionalized nonporous GNPs improve
charge storage densities on planar electrodes. Quasi-elastic neutron
scattering (QENS) and inelastic neutron scattering (INS) probed the
structure, dynamics, and orientation of RTIL ions confined in divergently
functionalized pores. Oxidized, ionophilic surfaces draw ions closer
to pore surfaces and enhance potential-driven ion transport during
electrosorption. Molecular dynamics (MD) simulations corroborated
experimental data and demonstrated the significance of surface functional
groups on ion orientations, accumulation densities, and capacitance
<i>In Vivo</i> Protein Dynamics on the Nanometer Length Scale and Nanosecond Time Scale
Selectively labeled
GroEL protein was produced in living deuterated
bacterial cells to enhance its neutron scattering signal above that
of the intracellular milieu. Quasi-elastic neutron scattering shows
that the in-cell diffusion coefficient of GroEL was (4.7 Ā± 0.3)
Ć 10<sup>ā12</sup> m<sup>2</sup>/s, a factor of 4 slower
than its diffusion coefficient in buffer solution. Internal protein
dynamics showed a relaxation time of (65 Ā± 6) ps, a factor of
2 slower compared to the protein in solution. Comparison to the literature
suggests that the effective diffusivity of proteins depends on the
length and time scale being probed. Retardation of in-cell diffusion
compared to the buffer becomes more significant with the increasing
probe length scale, suggesting that intracellular diffusion of biomolecules
is nonuniform over the cellular volume. The approach outlined here
enables investigation of protein dynamics within living cells to open
up new lines of research using āin-cell neutron scatteringā
to study the dynamics of complex biomolecular systems
Collective Excitations in Protein as a Measure of Balance Between its Softness and Rigidity
In this article, we elucidate the
protein activity from the perspective
of protein softness and flexibility by studying the collective phonon-like
excitations in a globular protein, human serum albumin (HSA), and
taking advantage of the state-of-the-art inelastic X-ray scattering
(IXS) technique. Such excitations demonstrate that the protein becomes
softer upon thermal denaturation due to disruption of weak noncovalent
bonds. On the other hand, no significant change in the local excitations
is detected in ligand- (drugs) bound HSA compared to the ligand-free
HSA. Our results clearly suggest that the protein conformational flexibility
and rigidity are balanced by the native protein structure for biological
activity
Hydration Control of the Mechanical and Dynamical Properties of Cellulose
The mechanical and dynamical properties
of cellulose, the most
abundant biomolecule on earth, are essential for its function in plant
cell walls and advanced biomaterials. Cellulose is almost always found
in a hydrated state, and it is therefore important to understand how
hydration influences its dynamics and mechanics. Here, the nanosecond-time
scale dynamics of cellulose is characterized using dynamic neutron
scattering experiments and molecular dynamics (MD) simulation. The
experiments reveal that hydrated samples exhibit a higher average
mean-square displacement above ā¼240 K. The MD simulation reveals
that the fluctuations of the surface hydroxymethyl atoms determine
the experimental temperature and hydration dependence. The increase
in the conformational disorder of the surface hydroxymethyl groups
with temperature follows the cellulose persistence length, suggesting
a coupling between structural and mechanical properties of the biopolymer.
In the MD simulation, 20% hydrated cellulose is more rigid than the
dry form, due to more closely packed cellulose chains and water molecules
bridging cellulose monomers with hydrogen bonds. This finding may
have implications for understanding the origin of strength and rigidity
of secondary plant cell walls. The detailed characterization obtained
here describes how hydration-dependent increased fluctuations and
hydroxymethyl disorder at the cellulose surface lead to enhancement
of the rigidity of this important biomolecule
Enhanced Dynamics of Hydrated tRNA on Nanodiamond Surfaces: A Combined Neutron Scattering and MD Simulation Study
Nontoxic, biocompatible nanodiamonds
(ND) have recently been implemented
in rational, systematic design of optimal therapeutic use in nanomedicines.
However, hydrophilicity of the ND surface strongly influences structure
and dynamics of biomolecules that restrict <i>in situ</i> applications of ND. Therefore, fundamental understanding of the
impact of hydrophilic ND surface on biomolecules at the molecular
level is essential. For tRNA, we observe an enhancement of dynamical
behavior in the presence of ND contrary to generally observed slow
motion at strongly interacting interfaces. We took advantage of neutron
scattering experiments and computer simulations to demonstrate this
atypical faster dynamics of tRNA on ND surface. The strong attractive
interactions between ND, tRNA, and water give rise to unlike dynamical
behavior and structural changes of tRNA in front of ND compared to
without ND. Our new findings may provide new design principles for
safer, improved drug delivery platforms