Potential of 129Xe NMR spectroscopy of adsorbed xenon for testing the chemical state of the surface of mesoporous carbon materials illustrated by the example of aggregates of diamond and onion-like carbon nanoparticles

The chemical shift in the 129Xe NMR spectrum of adsorbed xenon is very sensitive to the presence of oxygen-containing functional groups on the surface of mesoporous carbon materials. Well-characterized, structurally similar nanodiamond and onion-like carbon samples are considered here as model objects.

Secondary bonding in para-substituted diphenyltellurium dichlorides (p-XC6H4)2TeCl2 (X = H, Me, MeO) probed by 125Te MAS NMR spectroscopy. Crystal and molecular structure of (p-MeC6H4)2TeCl2

The solid-state structures of the previously known para-substituted diphenyltellurium dichlorides, (p-XC₆H₄)₂TeCl₂ (X=H (1), Me (2), MeO (3)) were investigated by ¹²⁵Te MAS NMR spectroscopy and in case of 2 by single crystal X-ray diffraction. The ¹²⁵Te-NMR shielding anisotropy (SA) was studied by tensor analyses based on relative intensities of the observed spinning sidebands. Solid-state NMR parameters, namely the isotropic chemical shift (δ_iso), anisotropy (ζ) and asymmetry (η), were discussed in relation to the molecular structures established by X-ray crystallography. The asymmetry (η) was found to be particularly sensitive to structural differences stemming mostly from the diverse secondary Te...Cl interactions, but no correlation with geometric parameters could be established.

4-Amino-1,8-naphthalimide-based anion receptors: employing the naphthalimide N-H moiety in the cooperative binding of dihydrogenphosphate

The 4-amino-1,8-naphthalimide-based anion receptor 3 binds dihydrogenphosphate with 1:1 stoichiometry through cooperative hydrogen bonding to a naphthalimide N–H and thiourea N–H groups. This was clearly established from ¹H NMR titration experiments in DMSO-d₆ where a substantial shift in the resonance for the naphthalimide N–H was observed concomitant with the expected thiourea N–H chemical shift migration upon successive additions of H₂PO₄⁻. However, whilst ¹H NMR titration experiments indicate that 3 was capable of binding other anions such as acetate, the naphthalimide N–H does not participate and the N–H resonance was essentially invariant during the titration. The lack of cooperative binding in this instance was justifiable on steric grounds.

Characterization of the drug binding specificity of rat liver fatty acid binding protein

Liver-fatty acid binding protein (L-FABP) is found in high levels in enterocytes and is involved in the cytosolic solubilization of fatty acids during fat absorption. In the current studies, the interaction of L-FABP with a range of lipophilic drugs has been evaluated to explore the potential for L-FABP to provide an analogous function during the absorption of lipophilic drugs. Binding affinity for L-FABP was assessed by displacement of a fluorescent marker, 1-anilinonaphthalene-8-sulfonic acid (ANS), and the binding site location was determined via nuclear magnetic resonance chemical shift perturbation studies. It was found that the majority of drugs bound to L-FABP at two sites, with the internal site generally having a higher affinity for the compounds tested. Furthermore, in contrast to the interaction of L-FABP with fatty acids, it was demonstrated that a terminal carboxylate is not required for specific binding of lipophilic drugs at the internal site of L-FABP.

The interaction of lipophilic drugs with intestinal fatty acid-binding protein

Intestinal fatty acid-binding protein (I-FABP) is a small protein that binds long-chain dietary fatty acids in the cytosol of the columnar absorptive epithelial cells (enterocytes) of the intestine. The binding cavity of I-FABP is much larger than is necessary to bind a fatty acid molecule, which suggests that the protein may be able to bind other hydrophobic and amphipathic ligands such as lipophilic drugs. Herein we describe the binding of three structurally diverse lipophilic drugs, bezafibrate, ibuprofen (both R- and S-isomers) and nitrazepam to I-FABP. The rank order of affinity for I-FABP determined for these compounds was found to be R-ibuprofen {approx} bezafibrate > S-ibuprofen >> nitrazepam. The binding affinities were not directly related to aqueous solubility or partition coefficient of the compounds; however, the freely water-soluble drug diltiazem showed no affinity for I-FABP. Drug-I-FABP interaction interfaces were defined by analysis of chemical shift perturbations in NMR spectra, which revealed that the drugs bound within the central fatty acid binding cavity. Each drug participated in a different set of interactions within the cavity; however, a number of common contacts were observed with residues also involved in fatty acid binding. These data suggest that the binding of non-fatty acid lipophilic drugs to I-FABP may increase the cytosolic solubility of these compounds and thereby facilitate drug transport from the intestinal lumen across the enterocyte to sites of distribution and metabolism.

NMR determination of ionic structure in plasticized polyether-urethane polymer electrolytes

Solid polymer electrolytes based on amorphous polyether-urethane networks combined with lithium or sodium salts and a low molecular weight cosolvent (plasticizer) have been investigated in our laboratories for several years. Conductivity enhancements of up to two orders of magnitude can be obtained whilst still retaining solid elastomeric properties. In order to understand the effects of the plasticizers and their mechanism of conductivity enhancement, multinuclear NMR has been employed to investigate ionic structure in polymer electrolyte systems containing NaCF₃SO₃, LiCF₃SO₃ and LiClO₃ salts.

With increasing dimethyl formamide (DMF) and propylene carbonate (PC) concentration the increasing cation chemical shift with fixed salt concentration indicates a decreasing anion-cation association consistent with an increased number of charge carriers. ¹³C chemical shift data for the same systems suggests that whilst DMF also decreases cation-polymer interactions, PC does the opposite, presumably by shielding cation-anion interactions. Temperature dependent ⁷Li spin-lattice relaxation times indicate the expected increase in ionic mobility upon plasticization with a shift of the T₁ minimum to lower temperatures. The magnitude of T₁ at the minimum increases upon addition of DMF whereas there is a slight decrease when PC is added. This also supports the suggestion that the DMF preferentially solvates the cation whereas the action of PC is limited to coulomb screening, hence freeing the anion.

An nmr investigation of ionic structure and mobility in plasticized solid polymer electrolytes

²³Na and ¹⁹F nuclear magnetic resonance spectroscopy is used to investigate the effect of plasticizer addition on ionic structure and mobility in a urethane crosslinked polyether solid polymer electrolyte. The incorporation of dimethyl formamide and propylene carbonate plasticizers in a sodium triflate/polyether system results in an upfield chemical shift for the ²³Na resonance consistent with decreased anion-cation association and increased cation-plasticizer interactions. The ¹⁹F resonances appears less susceptible to changes in chemical environment with only minor chemical shift changes recorded. Spin lattice relaxation measurements for the ¹⁹F nucleus are also reported. Two minima are observed in the relaxation measurements consistent with both an inter and intramolecular relaxation mechanism.

Solid state NMR characterization of polyrrole : the nature of the dopant in PF6− doped films

³¹P and ¹⁹F solid state NMR have been used to study the nature of the PF₆− anion in polypyrrole films at various levels of oxidation. It appears that the symmetric PF₆− unit remains undistorted and unchanged throughout, suggesting that it is predominantly acting only as a counterion and not as a true ‘dopant’, since any distortion in the phosphorous environments would result at the very least in chemical shift anisotropy of the ³¹P nucleus. A second set of phosphorous and fluorine resonances, which are consistent with a difluoride phosphorous compound, appeared in the films. Upon electrochemical reduction of the polymer, the undistorted PF₆− anion leaves the film whereas the second phosphorous species remains. Re-oxidation of the polymer reverses the processes observed during reduction.

Recent insights on the role of cryoprotective agents in vitrification

In recent efforts to produce cryoprotective solutions which cause either complete, or almost complete, vitrification of the cell or tissue material, increasingly complex cocktails of solutes have invariably been used. Why some of these solutes are so much more effective in suppressing ice formation than other, related solutes has never been clear. To begin to compare and contrast the role of the solute in aiding vitrification we have examined the nature of the hydrogen bonding interactions between the solute and water and between the solute molecules themselves, via proton nuclear magnetic resonance experiments. These experiments, carried out on neat samples of the solutions, show marked differences between solutes such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, the family of butanediol isomers, dimethylsulfoxide, and so on, at fixed concentration. The solutions also show marked trends in the NMR chemical shift as a function of concentration in any given solution. Thus it appears that, from the point of view of the physical suppression of ice in aqueous solutions, cryoprotective agents which can act as moderately strong bases are optimum. The mechanism by which the solute promotes glass formation was also investigated in a separate series of NMR experiments using more dilute solutions of the solute in water. These experiments indicate that the role of the solute is twofold in that it must (i) effectively suppress the anomalous structuring which occurs in supercooled water and is responsible for the rapid nucleation of ice and (ii) provide a decrease in molecular mobility at low temperatures such that the nucleation probability is decreased and glass formation occurs at a relatively high temperature. It is argued that both such effects can be brought about by the strong hydrogen bonding interactions between water and solutes such as 2,3-butanediol.

Solid state NMR analysis of polypyrrole grown in a phosphonium ionic liquid

³¹P, ¹⁹F and ¹³C solid state NMR analysis has been used to investigate the intercalation/de-intercalation of both anions and cations in electrochemically synthesized polypyrrole films. Use of a phosphonium-based ionic liquid, tri(hexyl)(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide, allows the separate detection of the cation and anion by analysis of the phosphorous and fluorine resonances, respectively. Initial results indicate the incorporation of both cations and anions during film growth in the ionic liquid. There is a notable change in the ³¹P chemical shift of the cation on incorporation into the film, consistent with a significant change in environment compared to the pure ionic liquid. Despite its large size, the phosphonium cation can be completely expelled from the film by oxidation.

An inclusion complex of β-Cyclodextrin-L-Phenylalanine : 1H NMR and molecular docking studies

An inclusion host-guest complex between β-cyclodextrin (β-CD) and L-phenylalanine (LPhe) was investigated using ¹H nuclear magnetic resonance spectroscopy and molecular docking techniques. ¹H chemical shift changes of β-CD were used to calculate the stability constant (K_stb) of the complex. On the basis of the Hildebrand-Benesi method, the K_stb of the 1:1 complex in D₂O solution at 300 K, pD 7.6 was of 25.5 M^-1, implying a fast intermolecular exchange rate process. Interestingly, docking simulation indicates the toroidal space can be occupied by L-Phe with two favorable arrangements. For the predicted model with the higher probability score, the L-Phe aromatic ring is facing to the secondary hydroxyl groups of β-CD. Results from NMR and docking simulation are in good agreement with the x-ray structures of β-CD/L-phenylalanine derivatives.

The "refoldability" of selected proteins in ionic liquids as a stabilization criterion, leading to a conjecture on biogenesis

The folding of proteins is usually studied in dilute aqueous solutions of controlled pH, but it has recently been demonstrated that reversible unfolding can occur in other media. Particular stability is conferred on the protein (folded or unfolded) when the process occurs in ‘protic ionic liquids’ (pILs) of controlled proton activity. This activity (‘effective pH’) is determined by the acid and base components of the pIL and is characterized in the present study by the proton chemical shift of the N–H proton. Here we propose a ‘refoldability’ or ‘refolding index’ (RFI) metric for assessing the stability of folded biomolecules in different solvent media, and demarcate high RFI zones in hydrated pIL media using ribonuclease A and hen egg white lysozyme as examples. Then we show that, unexpectedly, the same high RFIs can be obtained in pIL media that are 90% inorganic in character (simple ammonium salts). This leads us to a conjecture related to the objections that have been raised to ‘primordial soup’ theories for biogenesis, objections that are based on the observation that all the bonds involved in biomacromolecule formation are hydrolyzed in ordinary aqueous solutions unless specifically protected. The ingredients for primitive ionic liquids (NH3, CO, HCN, CO2, and water) were abundant in the early earth atmosphere, and many experiments have shown how amino acids could form from them also. Cyclical concentration in evaporating inland seas could easily produce the type of ambient-temperature, non-hydrolyzing, media that we have demonstrated here may be hospitable to biomolecules, and that may be actually encouraging of biopolymer assembly. Thus a plausible variant of the conventional ‘primordial soup’ model of biogenesis is suggested.

Ion conduction and phase morphology in sulfonate copolymer ionomers based on ionic liquid–sodium cation mixtures

A series of sulfonate based copolymer ionomers based on a combination of ionic liquid and sodium cations have been prepared in different ratios. This system was designed to improve the ionic conductivity of ionomers by partially replacing sodium cations with bulky cations that are less associated with anion centres on the polymer backbone. This provides more conduction sites for sodium to ‘hop’ to in the ionomers. Characterization showed the glass transition and 15N chemical shift of the ionomers did not vary significantly as the amount of Na+ varied, while the ionic conductivity increased with decreasing Na+ content, indicating conductivity is increasingly decoupled from Tg. Optical microscope images showed phase separation in all compositions, which indicated the samples were inhomogeneous. The introduction of low molecular weight plasticizer (PEG) reduced the Tg and increased the ionic conductivity significantly. The inclusion of PEG also led to a more homogeneous material.

Xe NMR spectroscopy of adsorbed xenon: Possibilites for exploration of microporous carbon materials

Despite the extensive use of 129Xe NMR for characterization of high surface-to-volume porous solids, particularly zeolites, this method has not been widely used to explore the properties of microporous carbon materials. In this study, commercial amorphous carbons of different origin (produced from different precursors) and a series of activated carbons obtained by successive cyclic air oxidation/pyrolysis treatments of a single precursor were examined. Models of 129Xe chemical shift as a function of local Xe density, mean pore size, and temperature are discussed. The virial coefficient arising from binary xenon collisions, σXe-Xe, varied linearly with the mean pore size given by N2 adsorption analysis; σ Xe-Xe appeared to be a better probe of the mean pore size than the chemical shift extrapolated to zero pressure, σS. © 2008 MAIK Nauka.

Insight into local structure and molecular dynamics in organic solid-state ionic conductors

Elucidating the rate and geometry of molecular dynamics is particularly important for unravelling ion-conduction mechanisms in electrochemical materials. The local molecular motions in the plastic crystal 1-ethyl-1-methylpyrrolidinium tetrafluoroborate ([C2 mpyr][BF4 ]) are studied by a combination of quantum chemical calculations and advanced solid-state nuclear magnetic resonance spectroscopy. For the first time, a restricted puckering motion with a small fluctuation angle of 25° in the pyrrolidinium ring has been observed, even in the low-temperature phase (-45 °C). This local molecular motion is deemed to be particularly important for the material to maintain its plasticity, and hence, its ion mobility at low temperatures.