Proton conductors, particularly hydrated solid membranes, have various applications in sensors, fuel cells, and cellular biological systems. Unraveling the intrinsic proton transfer mechanism is ...critical for establishing the foundation of proton conduction. Two scenarios on electrical conduction, the Grotthuss and the vehicle mechanisms, have been reported by experiments and simulations. But separating and quantifying the contributions of these two components from experiments is difficult. Here, we present the conductive behavior of a two-dimensional layered proton conductor, graphene oxide membrane (GOM), and find that proton hopping is dominant at low water content, while ion diffusion prevails with increasing water content. This change in the conduction mechanism is attributable to the layers of water molecules in GOM nanosheets. The overall conductivity is greatly improved by forming one layer of water molecules. It reaches the maximum with two layers of water molecules, resulting from creating a complete hydrogen-bond network within GOM. When more than two layers of water molecules enter the GOM nanosheets, inducing the breakage of the ordered lamellar structure, protons spread in both in-plane and out-of-plane directions inside the GOM. Our results validate the existence of two conduction mechanisms and show their distinct contributions to the overall conductivity. Furthermore, these findings provide an optimization strategy for the design of realizing the fast proton transfer in materials with water participation.
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The sugar molecule trehalose has been proven to be an excellent stabilizing cosolute for the preservation of biological materials. However, the stabilizing mechanism of trehalose has been much ...debated during the previous decades, and it is still not fully understood, partly because it has not been completely established how trehalose molecules structure around proteins. Here, we present a molecular model of a protein–water–trehalose system, based on neutron scattering results obtained from neutron diffraction, quasielastic neutron scattering, and different computer modeling techniques. The structural data clearly show how the proteins are preferentially hydrated, and analysis of the dynamical properties show that the protein residues are slowed down because of reduced dynamics of the protein hydration shell, rather than because of direct trehalose–protein interactions. These findings, thereby, strongly support previous models related to the preferential hydration model and contradict other models based on water replacement at the protein surface. Furthermore, the results are important for understanding the specific role of trehalose in biological stabilization and, more generally, for providing a likely mechanism of how cosolutes affect the dynamics of proteins.
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The maintenance of cell membrane fluidity is of critical importance for various cellular functions. At lower temperatures when membrane fluidity decreases, plants and cyanobacteria react by ...introducing unsaturation in the lipids, so that the membranes return to a more fluidic state. To probe how introduction of unsaturation leads to reduced membrane fluidity, a model cationic lipid dioctadecyldimethylammonium bromide (DODAB) has been chosen, and the effects of an unsaturated lipid monoolein (MO) on the structural dynamics and phase behavior of DODAB have been monitored by quasielastic neutron scattering and time-resolved fluorescence measurements. In the coagel phase, fluidity of the lipid bilayer increases significantly in the presence of MO relative to pure DODAB vesicles and becomes manifest in significantly enhanced dynamics of the constituent lipids along with faster hydration and orientational relaxation dynamics of a fluorophore. On the contrary, MO restricts both lateral and internal motions of the lipid molecules in the fluid phase (>330 K), which is consistent with relatively slow hydration and orientational relaxation dynamics of the fluorophore embedded in the mixed lipid bilayer. The present study illustrates how incorporation of an unsaturated lipid at lower temperatures (below the phase transition) assists the model lipid (DODAB) in regulating fluidity via enhancement of dynamics of the constituent lipids.
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Synthetic cationic lipids have garnered significant attention as promising candidates for gene/DNA transfection in therapeutic applications. The phase behavior of the vesicles formed by these lipids ...is intriguing, revealing intricate connections to the structure and dynamics of the membrane. These phenomena emerge from the complex interplay between hydrophobic and electrostatic interactions of the lipids. In this study, we explore the impact of an ionic liquid-based surfactant, 1-decyl-3-methylimidazolium bromide (DMIMBr), on the structural, dynamical, and phase behavior of cationic dihexadecyldimethylammonium bromide (DHDAB) vesicles. Our investigations indicate that the addition of DMIMBr increases the vesicle size while thinning the membrane. Further, DMIMBr also induces substantial changes in the membrane phase behavior. At 10 and 25 mol %, DMIMBr eliminates the pre-transition from coagel to intermediate crystalline (IC) phase and decreases the onset temperature of the main phase transition to the fluid phase. In the cooling cycle, the addition of DMIMBr further induces the formation of an intermediate gel phase. This behavior is reminiscent of the non-synchronous ordering observed in the DODAB membrane, a longer-chain counterpart of DHDAB. Interestingly, at 40 mol % of DMIMBr, the formation of the intermediate gel phase is largely suppressed. Neutron scattering data provide evidence that the addition of DMIMBr enhances lipid mobility in coagel and fluid phases, suggesting that DMIMBr acts as a plasticizer, enhancing membrane fluidity across all of the phases. Our findings infer that DMIMBr modulates the membrane’s phase behavior and fluidity, two essential ingredients for the efficient transport of cargo, by controlling the balance of electrostatic and hydrophobic interactions.
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Curcumin, the main ingredient in turmeric, has attracted attention due to its potential anti-inflammatory, anticancer, wound-healing, and antioxidant properties. Though curcumin efficacy is related ...to its interaction with biomembranes, there are few reports on the effects of curcumin on the lateral motion of lipids, a fundamental process in the cell membrane. Employing the quasielastic neutron scattering technique, we explore the effects of curcumin on the lateral diffusion of the dipalmotylphosphatidylcholine (DPPC) membrane. Our investigation is also supported by Fourier transform infrared spectroscopy, dynamic light scattering, and calorimetry to understand the interaction between curcumin and the DPPC membrane. It is found that curcumin significantly modulates the packing arrangement and conformations of DPPC lipid, leading to enhanced membrane dynamics. In particular, we find that the presence of curcumin substantially accelerates the DPPC lateral motion in both ordered and fluid phases. The effects are more pronounced in the ordered phase where the lateral diffusion coefficient increases by 23% in comparison to 9% in the fluid phase. Our measurements provide critical insights into molecular mechanisms underlying increased lateral diffusion. In contrast, the localized internal motions of DPPC are barely altered, except for a marginal enhancement observed in the ordered phase. In essence, these findings indicate that curcumin is favorably located at the membrane interface rather than in a transbilayer configuration. Further, the unambiguous evidence that curcumin modulates the membrane dynamics at a molecular level supports a possible action mechanism in which curcumin can act as an allosteric regulator of membrane functionality.
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•Increase in pore diffusion rate under gas flow.•Selective measurement from condensed phase, not gas.•Insight into diffusion under dynamic conditions.
Quasielastic neutron scattering ...(QENS) was used to investigate the diffusion of propane in zeolite ZSM-5 under static and macroscopic flow conditions. The rate of self-diffusion in the adsorbed phase was slightly faster under the flow conditions studied. Control measurements showed that the broadening seen in the quasielastic signal must result from propane condensed in the zeolitic pore and not from gas, either in the bulk or inter-particle voids.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
We measure the activation energy for the local segmental dynamics of polymer chains densely grafted to nanoparticles (NPs) using quasielastic neutron scattering. We aim to understand the underpinning ...physics of the experimentally measured enhanced gas transport in polymer grafted nanoparticle-based membranes relative to the neat polymer (without NPs), especially the permeability maximum, which occurs at intermediate chain lengths. We find that the activation energy goes through a minimum as a function of chain length, while the elementary jump size goes through a maximum around the same chain length. These results, likely, are the dynamic consequence of a structural transition of the grafted polymer brush from “extended” to “interpenetrated” with increasing chain length at fixed grafting density. Evidently, the regimes of different graft chain lengths near this structural transition are associated with lower activation energy, likely due to fluctuation effects, which also lead to enhanced gas transport.
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Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic ...neutron scattering measurements showing that dipolar CH3NH3(+) ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1-200-ps time window. Monte Carlo simulations of interacting CH3NH3(+) dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3(+) in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3(+) to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1-1 ms, faster than most observed hysteresis.
The addition of nanofillers to rubber matrices is a powerful route to improve the mechanical properties. Here, we focus on a molecular understanding of basic mechanisms that are important for the ...reinforcement in rubbers. The key role in this process is ascribed to bound rubber (BR) that engages with the matrix as well as with adjacent nanofillers. To date, this understanding has been impeded by the lack of experimental tools to directly probe the BR chains buried in a polymer matrix composed of the same polymer. To tackle this challenge, we combine neutron scattering/spectroscopy techniques with isotope-labeling and molecular dynamics simulations. The system is a simplified carbon-black-filled polybutadiene. The combined experimental and computational results provide new insights into the local structural and dynamical heterogeneities of BR chains and their interactions with the matrix polymer, highlighting (i) the structural partition of the bound chains into three components (i.e., trains, loops, and tails) and their fractions; (ii) their dynamical hierarchies, i.e., the trains that remain immobile on the filler surface, the loops that are fairly large and hence allow the interdigitation of matrix chains, and the tails with their unique characteristics to reach far out into the matrix and entangle with matrix chains. These multiple roles of the constituent components of the BR chains promote the formation of a well-developed adhesive polymer–filler interface, enhancing the elastic property of a filled rubber. The comprehensive understanding derived and validated by the model rubber will be translatable to many other polymer nanocomposites.
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Enzymatic activity is heavily influenced by pH, but the rationale for the dynamical mechanism of pH-dependent enzymatic activity has not been fully understood. In this work, combined neutron ...scattering techniques, including quasielastic neutron scattering (QENS) and small angle neutron scattering (SANS), are used to study the structural and dynamic changes of a model enzyme, xylanase, under different pH and temperature environments. The QENS results reveal that xylanase at optimal pH exhibits faster relaxational dynamics and a lower energy barrier between conformational substates. The SANS results demonstrate that pH affects both xylanase’s stability and monodispersity. Our findings indicate that enzymes have optimized stability and function under their optimal pH conditions, with both structure and dynamics being affected. The current study offers valuable insights into enzymatic functionality mechanisms, allowing for broad industrial applications.
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