Pulsed Field Gradient NMR Measurements Research Articles

Understanding Ion Dynamics in Closoborate-Type Lithium-Ion Conductors on Different Time-Scales.

The lithium-ion transport mechanism in 0.7Li(CB9H10)-0.3Li(CB11H12) complex hydride solid electrolyte was studied over a wide time-scale (ns-ms) by choosing appropriate techniques for assessing ionic motion on the desired time-scale using nuclear magnetic resonance (NMR) relaxation, AC impedance, and pulsed field gradient-NMR (PFG-NMR) measurements. The 7Li NMR line width decreased with increasing temperature, and the spin-lattice relaxation time T1 for the cation and anions showed a minimum near 303 K, indicating that the lithium ions and the anions were highly mobile. The activation energy estimated from the analysis of the NMR relaxation time matched well with the values estimated from the AC impedance and PFG-NMR. This confirms that the lithium-ion motion in 0.7Li(CB9H10)-0.3Li(CB11H12) is the same over a wide time-scale, suggesting steady Li-ion motion over a wide transport range. This understanding offers insights into strategies for designing complex hydride lithium superionic conductors.

Effect of Poloxamer Binding on the Elasticity and Toughness of Model Lipid Bilayers.

Poloxamers, also known by their trade name, Pluronics, are known to mitigate damage to cellular membranes. However, the mechanism underlying this protection is still unclear. We investigated the effect of poloxamer molar mass, hydrophobicity, and concentration on the mechanical properties of giant unilamellar vesicles, composed of 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine, using micropipette aspiration (MPA). Properties including the membrane bending modulus (κ), stretching modulus (K), and toughness are reported. We found that poloxamers tend to decrease K, with an impact largely dictated by their membrane affinity, i.e., both a high molar mass and less hydrophilic poloxamers depress K at lower concentrations. However, a statistically significant effect on κ was not observed. Several poloxamers studied here showed evidence of membrane toughening. Additional pulsed-field gradient NMR measurements provided insight into how polymer binding affinity connects to the trends observed by MPA. This model study provides important insights into how poloxamers interact with lipid membranes to further understanding of how they protect cells from various types of stress. Furthermore, this information may prove useful for the modification of lipid vesicles for other applications, including use in drug delivery or as nanoreactors.

Eutectic Electrolytes Composed of LiN(SO2F)2 and Sulfones for Li-Ion Batteries

Sulfones are polar molecules that can be used as thermally stable electrolyte solvents for Li-ion batteries (LIBs). Li salts form stoichiometric solvates with sulfones in the electrolytes. The melting points of the solvates tend to be higher than room temperature, thereby limiting the operating temperature range of the batteries. In this study, the applicability of ternary eutectic mixtures of LiN(SO2F)2 (LiFSA), sulfolane (SL), and dimethyl sulfone (DMS) as LIB electrolytes was assessed. Relative to the binary LiFSA–sulfone electrolytes, the ternary eutectic electrolytes remained liquid over a wide temperature range due to the increased entropy of mixing. Sulfone-bridged Li+–sulfone–Li+ and anion-bridged Li+–FSA––Li+ network structures were formed in the eutectic electrolyte with a composition of [LiFSA]/[SL]/[DMS] = 1/1.5/1.5. Pulsed-field gradient NMR measurements revealed that the Li+ ion dynamically exchanges sulfones and anions and diffuses more rapidly than these ligands, resulting in the relatively high Li+ transference number of the electrolyte. Highly reversible charge–discharge processes of the LiCoO2 and graphite electrodes were attained using the ternary eutectic electrolyte. The rate capability of the Li/LiCoO2 cell in the eutectic electrolyte was comparable to that of the cell in the conventional 1 M LiPF6 in an ethylene carbonate/dimethyl carbonate solution despite its lower ionic conductivity.

Self-diffusion of mixed xylene isomers in ZIF-71 crystals dispersed in a polymer to form a hybrid membrane

13 C pulsed field gradient (PFG) NMR was used to quantify self-diffusion of a p -xylene/ o -xylene mixture inside zeolitic imidazolate framework-71 (ZIF-71) crystals dispersed in a Torlon polymer to form a ZIF-71/Torlon mixed-matrix membrane (MMM). The corresponding reference PFG NMR measurements with a bed of a loosely packed bed of ZIF-71 crystals were also performed. This study is motivated by a demonstrated potential of ZIF-based MMMs for liquid separations. Selective 13 C isotopic labelling of each xylene isomer was used to ensure that the self-diffusivity of each isomer in the mixture is measured independently and correctly. The diffusion measurements were performed at different diffusion times at 296 K. The observed dependencies of intra-ZIF self-diffusivities in ZIF-71/Torlon MMMs and the corresponding ZIF-71 crystal beds on diffusion time were explained by an influence of the external crystal surface on the studied diffusion process. Analysis of the time dependencies of the self-diffusivities measured for each xylene isomer allowed obtaining intra-ZIF self-diffusivities not perturbed by crystal surface effects as well as the corresponding diffusion selectivity for the studied sorbate mixture. The reported selectivity of around 4 demonstrates promising separation behavior of the considered membrane type. • First measurements of liquid mixtures in ZIF crystals located in a mixed-matrix membrane (MMM). • Intra-ZIF diffusivities of mixtures are quantified as a function of diffusion time by PFG NMR. • Diffusion selectivity is reported separately for a ZIF component of the MMM. • The reported diffusion selectivity shows promising potential of the studied MMM.

In Situ Determination of Carbon Number Distributions of Mixtures of Linear Hydrocarbons Confined within Porous Media Using Pulsed Field Gradient NMR.

Pulsed field gradient (PFG) NMR measurements, combined with a novel optimization method, are used to determine the composition of hydrocarbon mixtures of linear alkanes (C7-C16) in both the bulk liquid state and when imbibed within a porous medium of mean pore diameter 28.6 nm. The method predicts the average carbon number of a given mixture to an accuracy of ±1 carbon number and the mole fraction of a mixture component to within an average root-mean-square error of ±0.036 with just three calibration mixtures. Given that the method can be applied at any conditions of temperature and pressure at which the PFG NMR measurements are made, the method has the potential for application in characterizing hydrocarbon liquid mixtures inside porous media and at the operating conditions relevant to, for example, hydrocarbon recovery and heterogeneous catalysis.

Kinetics of oxygen transfer reactions on the graphene surface. Part II. H2O vs. CO2

This computational chemistry analysis compares the interactions of H2O and CO2 with zigzag graphene edges, and in particular their adsorption and desorption steps. We provide detailed information regarding the geometric and electronic factors that influence both their adsorption and desorption (of H2 and CO) processes. Density functional theory results are compared with experimental information to offer heretofore unavailable insights into key aspects of the rate-limiting steps, inhibition phenomena and nanoscale differences in these two reactions. Thus, for example, the orientation and rotation of the adsorbing H2O molecules are elucidated using intrinsic reaction coordinate calculations and molecular orbital analysis, and these results complement recent pulse field gradient NMR measurements and molecular dynamics calculations. Such mechanistic findings reveal the H2O adsorption process to be a slow rotational phenomenon whereas the low-temperature H2 desorption is geometrically constrained by the presence of contiguous (di)hydrogenated carbon atoms with or without hydroxyl groups. This surface-assisted desorption mechanism is proposed to be responsible for the formation of hexagonal pits during the graphite-H2O reaction. Similar insights were obtained for the graphene-CO2 reaction. Mechanistic schemes are proposed for the desorption products observed in experiments, distinguishing zigzag from armchair sites.

Obtaining Hydrodynamic Radii of Intrinsically Disordered Protein Ensembles by Pulsed Field Gradient NMR Measurements.

In the disordered state, a protein exhibits a high degree of structural freedom, in both space and time. For an ensemble of disordered or unfolded proteins, this means that the ensemble comprises a high diversity of structures, ranging from compact collapsed states to fully extended polypeptide chains. In addition, each chain is highly dynamic and undergoes conformational changes and local dynamics on both fast and slow timescales. The size properties of disordered proteins are thus best described as ensemble averages. A straightforward measure of the size is the hydrodynamic radius, RH, of the ensemble. Since the disordered state is conformationally fluid, the observed RH does not refer to a particular shape or fold. Instead, it should be interpreted as a measure for the average compaction of the structural ensemble. In addition to characterizing the disordered ensemble itself, RH can be used to, with good precision, monitor changes in the ensemble size properties upon functional interactions of the disordered protein, e.g., dimerization, ligand binding, and folding pathways. Here, we present a step-by-step protocol for diffusion measurements using pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. We describe how to calibrate the magnetic field gradient and offer different schemes for sample preparation. Finally, we describe how to obtain RH directly from the diffusion coefficient as well as from using an internal standard as a reference.

Relationship between ethane and ethylene diffusion inside ZIF-11 crystals confined in polymers to form mixed-matrix membranes

Self-diffusivities of ethane were measured by multinuclear pulsed field gradient (PFG) NMR inside zeolitic imidazolate framework-11 (ZIF-11) crystals dispersed in several selected polymers to form mixed-matrix membranes (MMMs). These diffusivities were compared with the corresponding intracrystalline self-diffusivities in ZIF-11 crystal beds. It was observed that the confinement of ZIF-11 crystals in ZIF-11/Torlon MMM can lead to a decrease in the ethane intracrystalline self-diffusivity. Such diffusivity decrease was observed at different temperatures used in this work. PFG NMR measurements of the temperature dependence of the intracrystalline self-diffusivity of ethylene in the same ZIF-11/Torlon MMM revealed similar diffusivity decrease as well as an increase in the diffusion activation energy in comparison to those in unconfined ZIF-11 crystals in a crystal bed. These observations for ethane and ethylene were attributed to the reduction of the flexibility of the ZIF-11 framework due to the confinement in Torlon leading to a smaller effective aperture size of ZIF-11 crystals. Surprisingly, the intra-ZIF diffusion selectivity for ethane and ethylene was not changed appreciably by the confinement of ZIF-11 crystals in Torlon in comparison to the selectivity in a bed of ZIF-11 crystals. No ZIF-11 confinement effects leading to a reduction in the intracrystalline self-diffusivity of ethane and ethylene were observed for the other two studied MMM systems: ZIF-11/Matrimid and ZIF-11/6FDA-DAM. The absence of the confinement effect in the latter MMMs can be related to the lower values of the polymer bulk modulus in these MMMs in comparison to that in ZIF-11/Torlon MMM. In addition, there may be a contribution from possible differences in the ZIF-11/polymer adhesion in different MMM types.

Enhanced lithium-ion transport in organosilyl electrolytes for lithium-ion battery applications

Abstract

Anomalous Solute Diffusivity in Ionic Liquids: Label-Free Visualization and Physical Origins

Dynamic diffusion of molecular solutes in concentrated electrolytes plays a critical role in many applications but is notoriously challenging to measure and model. This challenge is particularly true in the extreme case of ionic liquids (ILs), fluids composed entirely of cations and anions. Solute diffusivities in ILs show a strong concentration dependence, broadening the already vast IL design space and rendering conventional, sample-by-sample measurements impractical for screening. To gain better mechanistic insight into transport in this class of fluids, here we demonstrate a method to visualize the spatiotemporal evolution of concentration fields using microfluidic Fabry-Perot interferometry, enabling diffusivity measurements over an entire composition range within a single experiment. We focus on the absorption and diffusion of water, as both a model solute and a ubiquitous contaminant, within alkylmethylimidazolium-halide ILs. Notably, the Stokes-Einstein relation underpredicts water diffusivities ten- to 50-fold, indicating that water does not experience these ILs as continuum liquids. Based on these measurements, together with wide-angle x-ray scattering and pulsed-field gradient NMR measurements, we propose a new mechanistic framework in which water molecules hop between ion pairs within the IL, which acts as an immobile matrix over timescales relevant for water diffusion. In this case, diffusion is an activated process, with hops between hydrogen-bonding sites over an energetic barrier that decreases linearly with the water fraction. The functional form of the activation energy is consistent with NMR chemical shift measurements, which indicate that hydrogen bonding weakens in linear proportion to the water fraction. This simple model contains the key ingredients required to accurately predict the measured trends in diffusivity—an (Arrhenius) temperature dependence and an exponential composition dependence—for a range of cations, anions, water contents, and temperatures. Our results suggest a general mechanism for anomalously fast diffusion in ILs, where solutes “hop” between binding sites more quickly than the ions rearrange.6 MoreReceived 10 August 2018Revised 5 November 2018DOI:https://doi.org/10.1103/PhysRevX.9.011048Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAnomalous diffusionDiffusionPhysical SystemsIonic fluidsTechniquesFabry-Pérot interferometryInterferometryCondensed Matter, Materials & Applied PhysicsFluid DynamicsStatistical PhysicsPolymers & Soft MatterInterdisciplinary PhysicsAtomic, Molecular & OpticalGeneral Physics

Effect of β-Cyclodextrin on Physicochemical Properties of an Ionic Liquid Electrolyte Composed of N-Methyl-N-Propylpyrrolidinium bis(trifluoromethylsulfonyl)amide.

Ionic liquids (ILs) are promising electrolyte materials for developing next-generation rechargeable batteries. In order to improve their properties, several kinds of additives have been investigated. In this study, β-cyclodextrin (β-CD) was chosen as a new additive in IL electrolytes because it can form an inclusion complex with bis(trifluoromethylsulfonyl)amide (TFSA) anions. We prepared the composites by mixing N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)amide/LiTFSA and a given amount of triacetyl-β-cyclodextrin (Acβ-CD). The thermal behaviors and electrochemical properties of the composites were analyzed by several techniques. In addition, pulse field gradient NMR measurements were conducted to determine the self-diffusion coefficients of the component ions. The addition of Acβ-CD to the IL electrolytes results in the decrease in the conductivity value and the increase in the viscosity value. In contrast, the addition of Acβ-CD to the IL electrolytes induced an improvement in the anodic stability because of the formation of an inclusion complex between the Acβ-CD and TFSA anions. CDs are potential candidates as additives in IL electrolytes for electrochemical applications.

Liquid Structures and Transport Properties of Lithium Bis(fluorosulfonyl)amide/Glyme Solvate Ionic Liquids for Lithium Batteries

The liquid structures and transport properties of electrolytes composed of lithium bis(fluorosulfonyl)amide (Li[FSA]) and glyme (triglyme (G3) or tetraglyme (G4)) were investigated. Raman spectroscopy indicated that the 1:1 mixtures of Li[FSA] and glyme (G3 or G4) are solvate ionic liquids (SILs) comprising a cationic [Li(glyme)]+ complex and the [FSA]− anion. In Li[FSA]-excess liquids with Li[FSA]/glyme molar ratios greater than 1, anionic Lix[FSA]y(y–x)– complexes were formed in addition to the cationic [Li(glyme)]+ complex. Pulsed field gradient NMR measurements revealed that the self-diffusion coefficients of Li+ (DLi) and glyme (Dglyme) are identical in the Li[FSA]/glyme=1 liquid, suggesting that Li+ and glyme diffuse together and that a long-lived cationic [Li(glyme)]+ complex is formed in the SIL. The ratio of the self-diffusion coefficients of [FSA]− and Li+, DFSA/DLi, was essentially constant at ~1.1–1.3 in the Li[FSA]/glyme<1 liquid. However, DFSA/DLi increased rapidly as the amount of Li[FSA] increased in the Li[FSA]/glyme>1 liquid, indicating that the ion transport mechanism in the electrolyte changed at the composition of Li[FSA]/glyme=1. The oxidative stability of the electrolytes was enhanced as the Li[FSA] concentration increased. Furthermore, Al corrosion was suppressed in the electrolytes for which Li[FSA]/glyme>1. A battery consisting of a Li metal anode, a LiNi1/3Mn1/3Co1/3O2 cathode, and Li[FSA]/G3=2 electrolyte exhibited a discharge capacity of 105mAhg−1 at a current density of 1.3mAcm−2, regardless of its low ionic conductivity of 0.2mScm−1.

Advanced Nuclear Magnetic Resonance Techniques for Characterizing Ionic Liquids for Lithium Ion Battery Applications; High Presssure NMR and Fast Field Cycling Relaxometry

Ionic liquids (ILs) have received widespread acclaim for their potential use as a safe and effective alternative to unstable and flammable organic solvents used in electrolytes for lithium-ion secondary batteries. The thermal and chemical stability of ILs, as well as the tunable properties of ILs by altering cation and anion species, have made them promising materials for use in multiple applications: such as textile dyeing, nuclear waste recycling, and solar cells. A limiting characteristic for IL use in secondary lithium-ion batteries is a notable lower ionic conductivity in most known species. Characterizing the effect of cation and anion structure on ionic diffusion in ILs, as well as studying molecular dynamics in IL species is of great importance for increasing their potential utility in battery chemistries. To this end, the techniques of Fast Field Cycling Nuclear Magnetic Resonance (FFCNMR) and pressure dependent NMR diffusometry are used to study ionic liquid species of interest. Fast field cycling measurements of spin-lattice relaxation times (T1) allow for access to time dependent molecular dynamics in ionic systems, as functions of both magnetic field strength and temperature. FFC measurements are complementary to high-pressure NMR experiments, which reveal pressure dependence of reorientation and tumbling processes, as well as the effects of alkyl chain length on ionic mobility in specific species. Presented here is the application of these two techniques to 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (CnMIM TFSA) ILs. Specifically, a selectively deuterated species of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIM TFSA) is observed and characterized. FFC results were used to extract cation self-diffusion coefficients, showing good agreement with Pulse Field Gradient (PFG) NMR measurements. High pressure T1 data on selectively deuterated cation isotopologues revealed site dependent interactions on the cation alkyl chain. The work at BNL (JFW and JH) was supported by the US-DOE Office of Science, Division of Chemical Sciences, Geosciences and Biosciences under contract DE-SC0012704.

Synthesis and Solution-Phase Characterization of Sulfonated Oligothioetheramides.

Nature has long demonstrated the importance of chemical sequence to induce structure and tune physical interactions. Investigating macromolecular structure and dynamics is paramount to understand macromolecular binding and target recognition. To that end, we have synthesized and characterized flexible sulfonated oligothioetheramides (oligo-TEAs) by variable temperature pulse field gradient (PFG) NMR, double electron-electron resonance (DEER), and molecular dynamics (MD) simulations to capture their room temperature structure and dynamics in water. We have examined the contributions of synthetic length (2-12mer), pendant group charge, and backbone hydrophobicity. We observe significant entropic collapse, driven in part by backbone hydrophobicity. Analysis of individual monomer contributions revealed larger changes due to the backbone compared to pendant groups. We also observe screening of intramolecular electrostatic repulsions. Finally, we comment on the combination of DEER and PFG NMR measurements via Stokes-Einstein-Sutherland diffusion theory. Overall, this sensitive characterization holds promise to enable de novo development of macromolecular structure and sequence-structure-function relationships with flexible, but biologically functional macromolecules.

Nafion® nanocomposite membranes with enhanced properties at high temperature and low humidity environments

Highly proton conductive nanocomposite membranes were synthesized by incorporating modified silica nanoparticles bearing different kinds of acid functionalities into Nafion. The short oligomeric corona, containing either phosphonate or sulfonate functional groups, leads to membranes with non-aggregated, discrete nanoparticles acting synergistically with the polymer. The new membranes exhibit significantly increased proton conductivity in all relative humidities and temperatures, however unprecedented behavior is observed at elevated temperatures (>80 °C) and/or low relative humidity. Pulse Field Gradient NMR measurements demonstrate that, in contrast to neat Nafion, which shows a precipitous decrease in the self-diffusion of water above 80 °C, the nanocomposite membranes were able to maintain high proton diffusivities pointing to adequate hydration levels for several hours at 130 °C without any external humidification. Furthermore, thermal and dynamic mechanical analysis reveal that the nanocomposite membranes retain their stiffness at much higher temperatures up to 200 °C compared to recast Nafion opening the way for high temperatures applications. The overall high ionic conductivity of the nanocomposite membranes especially above 80 °C coupled with their exceptional water retention capability and mechanical robustness makes them very attractive for potential use in fuel cell applications.

Structure and molecular dynamics of sodium dodecylsulfate micelles modified with hydrophilic short-chain imidazolium salts in aqueous solution

This study uses pyrene solubilization and NMR experiments to investigate the effects of two hydrophilic short-chain ionic liquids (ILs), 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) and 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) on the micellization of sodium dodecylsulfate (SDS) in the aqueous phase. The results of the pyrene solubilization experiments indicate that the added short-chain ILs not only promote the micellization of SDS but also modify micelle properties. For NMR studies, the present study focuses on (1) the compositions and spatial arrangements of component molecules and (2) the molecular dynamics of DS− in the Rmim-modified SDS micelle. Using pulsed field gradient (PFG) NMR measurements, the composition of the Rmim-modified SDS micelle was calculated by the diffusion data and was found to be correlated with the [SDS]. Two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) experiments confirm that the 1-alkyl-3-methylimidazolium cation (Rmim+) is located inside the Rmim-modified SDS micelle. Corresponding to the microstructure of the Rmim-modified SDS micelle, in terms of 1H T1 relaxation, the fast motion of SDS α-CH2 and β-CH2 segments are markedly restricted by the attached Rmim+ to the sulfate group of DS−, whereas the central carbons and the terminal CH3 group of DS− are only slightly affected. The chemical shift analysis indicates that the surface of the Rmim-modified SDS micelle is dehydrated by the absorbed Rmim+ and becomes more hydrophobic than that of the pure SDS micelle. Insertion of Rmim+ into the Rmim-modified SDS micelle appears to be driven through the hydrophobic interaction, and it is shown to combine with SDS molecules to constitute the hydrophobic part of the Rmim-modified SDS micelle.

Li+ Transference Numbers in Liquid Electrolytes Obtained by Very-Low-Frequency Impedance Spectroscopy at Variable Electrode Distances

The Li+ transference numbers of three different liquid electrolytes for Li-ion batteries were measured in a symmetrical Li | electrolyte | Li cell by means of very-low-frequency impedance spectroscopy (VLF-IS). The electrolytes were: (i) The standard battery electrolyte LP30; (ii) an equimolar mixture of tetraglyme (G4) and lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI); (iii) Li-TFSI dissolved in the ionic liquid 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI). We found that the Li+ transference numbers of the two electrolytes LP30 and G4/Li-TFSI are much smaller than the Li+ transport numbers obtained from pulsed-field gradient NMR measurements. On the other hand, in the case of BMP-TFSI/Li-TFSI, the values for and are more similar. In order to rationalize the large differences between and found for LP30 and G4/Li-TFSI, we combined the Onsager reciprocal relations with linear response theory, and we derived expressions for , which take into account all correlations between ionic movements in the electrolyte. Thereby, we show that can be considerably smaller than , if strong directional correlations exist between the movements of cations and anions. Finally, we discuss differences in Li+ transference numbers obtained by VLF-IS and by potentiostatic polarization measurements.

Gas Transport Coefficients of Phthalide-Containing High-Tg Glassy Polymers Determined by Gas-Flux and NMR Measurements

The synthesis of three aromatic polymers containing phthalide cardo groups having high inherent viscosities is described. Polyimides PIC-6F and PIC-TB were prepared from a cardo diamine and two different dianhydrides, and poly(phthalidylidenearylene) (PPDAr) was prepared by a precipitative Friedel–Crafts homopolycondensation of 3-(4-biphenylyl)-3-chlorophthalide. All polymers presented high glass transition temperatures varying between 600 and 690 K. Dense membranes prepared by casting from N,N-dimethylacetamide solutions exhibited good mechanical properties and decomposition temperatures over 770 K under a N2 atmosphere. Results of gas transport measurements of O2, N2, and CO2 were comparatively analyzed based on the chemical structure of the polymers. Additionally, the solubility, diffusion, and permeability coefficients of [13C]O2 in these membranes were determined by 13C NMR spectroscopy and pulsed-field gradient NMR measurements, and the results were in good agreement with those determined with press...

Conformational regulation of the C‐terminal random coil in S100A4 by Ca2+ ions

S100A4 or metastasin plays a central role in the progression of metastatic diseases. Like many other examples, the protein S100A4 contains ordered and disordered regions. The aim of this study was to determine how the disordered C‐terminal region influences the binding to Ca2+ and in turn how Ca2+ binding alters the dynamics of the C‐terminal random coil. The combination of small angle X‐ray scattering, pulsed field gradient NMR measurements revealed a conformational change upon binding in the wild‐type protein, but not in the C‐terminal deleted mutant. Comparative isothermal titration calorimetry measurements also indicated that Ca2+ ions bind more tightly to the C‐terminal deleted mutant than the wild‐type protein. Molecular dynamics simulations provide a rational: in the wild type protein the positively charged C‐terminal random coil region interacts strongly with the EF hand residues when they do not coordinate Ca2+ , whereas the C‐terminal region is more free to move in the Ca2+‐bound form of the protein. The C‐terminal random coil never appear to be completely static, but occupies a different conformational space depending on the functional state of is Ca2+ binding protein. Thus this study provides a unique insight to the regulation of random coil regions.

The origin of a large apparent tortuosity factor for the Knudsen diffusion inside monoliths of a samaria-alumina aerogel catalyst: a diffusion NMR study.

Pulsed field gradient (PFG) NMR was applied to measure tortuosity factors for carbon dioxide diffusion in the Knudsen and gas regimes inside monoliths of a samaria-alumina aerogel catalyst, a high porosity material containing micropores in addition to meso- and macropores. The apparent tortuosity factor obtained from PFG NMR measurements for the Knudsen diffusion in the meso- and macropores of the catalyst has an unexpectedly large value of approximately 6 if carbon dioxide adsorption in the micropores and other types of surface adsorption sites of the catalyst is ignored. At the same time, the corresponding apparent tortuosity factor in the gas regime was found to be around 2. Application of a proposed model which describes fast molecular exchange between the surface adsorption sites and the main pore volume of the catalyst yields corrected tortuosity factors which depend only on the pore system geometry. Using this model, the corrected tortuosity factors were found to be around 2 for both diffusion regimes, in agreement with the expectations based on a high porosity of the studied catalyst.