Electroneutrality Breakdown and Specific Ion Effects in Nanoconfined Aqueous Electrolytes Observed by NMR

Ion distribution in aqueous electrolytes near the interface plays critical roles in electrochemical, biological and colloidal systems and is expected to be particularly significant inside nanoconfined regions. Electroneutrality of the total charge inside nanoconfined regions is commonly assumed a priori in solving ion distribution of aqueous electrolytes nanoconfined by uncharged hydrophobic surfaces with no direct experimental validation. Here, we use a quantitative nuclear magnetic resonance approach to investigate the properties of aqueous electrolytes nanoconfined in graphitic-like nanoporous carbon. Substantial electroneutrality breakdown in nanoconfined regions and very asymmetric responses of cations and anions to the charging of nanoconfining surfaces are observed. The electroneutrality breakdown is shown to depend strongly on the propensity of anions toward the water-carbon interface and such ion-specific response follows generally the anion ranking of the Hofmeister series. The experimental observations are further supported by numerical evaluation using the generalized Poisson-Boltzmann equationComment: 26 pages, 3 figure

Protein dynamics and thermodynamics crossover at 10°C: Different roles of hydration at hydrophilic and hydrophobic groups

Water at hydrophilic and hydrophobic groups interact differently with proteins. Particularly, hydration properties at hydrophobic groups undergo qualitative changes as temperature decreases below 10 °C. The influence of such interfacial changes on protein dynamics and thermodynamics remains largely unexplored. Here, nanosecond to microsecond protein dynamics and the free energy, enthalpy, and entropy of protein hydration are investigated by in-situ NMR as a function of hydration level and temperature. A crossover at 10 °C in protein dynamics and thermodynamics is revealed. The influence of water at hydrophilic groups shows little temperature dependence, whereas water at hydrophobic groups has stronger effect above 10 °C

Dominant Alcohol–Protein Interaction via Hydration-Enabled Enthalpy-Driven Binding Mechanism

Water plays an important role in weak associations of small drug molecules with proteins. Intense focus has been on binding-induced structural changes in the water network surrounding protein binding sites, especially their contributions to binding thermodynamics. However, water is also tightly coupled to protein conformations and dynamics, and so far little is known about the influence of water-protein interactions on ligand binding. Alcohols are a type of low-affinity drugs, and it remains unclear how water affects alcohol-protein interactions. Here, we present alcohol adsorption isotherms under controlled protein hydration using in-situ NMR detection. As functions of hydration level, Gibbs free energy, enthalpy, and entropy of binding were determined from the temperature dependence of isotherms. Two types of alcohol binding were found. The dominant type is low-affinity nonspecific binding, which is strongly dependent on temperature and the level of hydration. At low hydration levels, this nonspecific binding only occurs above a threshold of alcohol vapor pressure. An increased hydration level reduces this threshold, with it finally disappearing at a hydration level of h~0.2 (g water/g protein), gradually shifting alcohol binding from an entropy-driven to an enthalpy-driven process. Water at charged and polar groups on the protein surface was found to be particularly important in enabling this binding. Although further increase in hydration has smaller effects on the changes of binding enthalpy and entropy, it results in significant negative change in Gibbs free energy due to unmatched enthalpy-entropy compensation. These results show the crucial role of water-protein interplay in alcohol binding

Critical Role of Water in the Binding of Volatile Anesthetics to Proteins

Numerous small molecules exhibit drug-like properties by low-affinity binding to proteins. Such binding is known to be influenced by water, the detailed picture of which, however, remains unclear. One particular example is the controversial role of water in the binding of general anesthetics to proteins as an essential step in general anesthesia. Here we demonstrate that a critical amount of hydration water is a prerequisite for anesthetic-protein binding. Using nuclear magnetic resonance, the concurrent adsorption of hydration water and bound anesthetics on model proteins are simultaneously measured. Halothane binding on proteins can only take place after protein hydration reaches a threshold hydration level of ~0.31 gram of water per gram of proteins at the relative water vapor pressure of ~0.95. Similar dependence on hydration is also observed for several other proteins. The ratio of anesthetic partial pressures at which two different anesthetics reach the same fractional load is correlated with the anesthetic potency. The binding of nonimmobilizers, which are structurally similar to known anesthetics but unable to produce anesthesia, does not occur even after the proteins are fully hydrated. Our results provide the first unambiguous experimental evidence that water is absolutely required to enable anesthetic-protein interactions, shedding new light on the general mechanism of molecular recognition and binding

Magnetic susceptibility and microstructure of hydrogenated amorphous silicon measured by nuclear magnetic resonance on a single thin film

A nuclear magnetic resonance technique for precisely measuring the bulk magnetic susceptibility of micron-thick hydrogenated amorphous silicon (a-Si:H) film is introduced. The large disorder-induced diamagnetic enhancement exhibited by a-Si:H is shown to provide a sensitive bulk measurement for detecting variations in structural order in a-Si:H films. Furthermore, this approach is shown to be effective in revealing the details of microstructure in a-Si:H, including the presence of microstructural anisotropy

Structural Ordering and its Correlation to the Optoelectronic Properties of a-Si:H Films

Magnetic susceptibility was suggested theoretically to be sensitive to the overall structural order of a-Si:H and is measured precisely for various a-Si:H thin films using a new technique

Hydrophilic and Hydrophobic Characteristics of Reservoir Rocks Quantified by Nuclear Magnetic Resonance-Detected Water Isotherms

Accurate determination of rock wettability is crucial for evaluating petroleum reserves and optimizing oil production. Reservoir rocks, formations that contain accumulations of oil or gas, often comprise a large collection of mineral types and a wide range of pore sizes. This heterogeneity makes it difficult to measure the wettability of reservoir rocks by conventional techniques, such as contact angle measurement. Here, we resolve this problem by applying an in situ nuclear magnetic resonance (NMR) adsorption isotherm technique. Investigations of glass bead phantoms demonstrate that NMR-detected isotherms are capable of distinguishing between hydrophilic and hydrophobic surfaces within the pore network. This NMR-based isotherm methodology is further applied to reservoir rocks. Water isotherms of the pristine rocks provide information on two important rock properties - wettability and pore-size distribution. Such information can be used, among other things, to model the wettability of the formation and to maximize waterflood recovery efficiency

Temperature dependence of lysozyme hydration and the role of elastic energy

Water plays critical roles in protein dynamics and functions. However, the most basic property of hydration—the water sorption isotherm—remains inadequately understood. Surface adsorption is the commonly adopted picture of hydration. Since it does not account for changes in the conformational entropy of proteins, it is difficult to explain why protein dynamics and activity change upon hydration. The solution picture of hydration provides an alternative approach to describe the thermodynamics of hydration. Here, the flexibility of proteins could influence the hydration level through the change of elastic energy upon hydration. Using nuclear magnetic resonance to measure the isotherms of lysozyme in situ between 18 and 2°C, the present work provides evidence that the part of water uptake associated with the onset of protein function is significantly reduced below 8°C. Quantitative analysis shows that such reduction is directly related to the reduction of protein flexibility and enhanced cost in elastic energy upon hydration at lower temperature. The elastic property derived from the water isotherm agrees with direct mechanical measurements, providing independent support for the solution model. This result also implies that water adsorption at charged and polar groups occurring at low vapor pressure, which is known for softening the protein, is crucial for the later stage of water uptake, leading to the activation of protein dynamics. The present work sheds light on the mutual influence of protein flexibility and hydration, providing the basis for understanding the role of hydration on protein dynamics

Lithium Intercalation into Opened Single-Wall Carbon Nanotubes: Storage Capacity and Electronic Properties

The effects of structure and morphology on lithium storage in single-wall carbon nanotube (SWNT) bundles were studied by electrochemistry and nuclear magnetic resonance techniques. SWNTs were chemically etched to variable lengths and were intercalated with Li. The reversible Li storage capacity increased from LiC(6) in close-end SWNTs to LiC(3) after etching, which is twice the value observed in intercalated graphite. All the nanotubes became metallic upon intercalation of Li, with the density of states at the Fermi level increasing with increasing Li concentration. The enhanced capacity is attributed to Li diffusion into the interior of the SWNTs through the opened ends and sidewall defects

Gas adsorption in single-walled carbon nanotubes studied by NMR

Adsorption isotherms of methane and ethane in single-walled carbon nanotubes (SWNTs) were measured by 1H nuclear magnetic resonance (NMR) at room temperature. It is shown that the interior of SWNTs becomes available for methane and ethane adsorption after cutting of SWNTs. Such endohedral adsorption dominates methane and ethane adsorption in SWNTs, at least below 1 MPa. The average exchange time between molecules adsorbed inside SWNTs and free gas molecules outside was estimated to be on the order of 80 ms. It is shown that exposure to oxygen has no effect on methane and ethane endohedral adsorption in SWNTs, suggesting that the adsorption energy of oxygen molecules inside SWNTs is small compared to that of methane. 13C NMR indicates that under atmospheric pressure and room temperature helium atoms could access the interstitial sites of SWNT bundles whereas H2, CO2, and N2 molecules could not