Probing the alkyl ligands on silylated mesoporous MCM-41 using hyperpolarized 129Xe NMR spectroscopy

Variable-temperature hyperpolarized (HP) 129Xe NMR spectroscopy has been employed to characterize surface properties of mesoporous MCM-41 modified by silylation treatment. The characteristic chemical shifts responsible for Xe-surface interactions exhibit strong correlations with both the surface coverage and chain length of the grafted alkylsilanes. Consequently, the deshielding medium contribution due to individual alkyl ligand can be deduced based on the group contribution analysis revealing the potential use of HP 129Xe NMR for probing the surface properties of organic-functionalized porous materials.     

Characterization of porosity in organic and metal-organic macrocycles by hyperpolarized 129Xe NMR spectroscopy

Hyperpolarized (129)Xe NMR spectroscopy is used to establish the solid-state porosity of shape-persistent macrocycles with either an organic or metal-organic framework. These studies show that even upon removal of cocrystallized solvent molecules, the macrocycles maintain a porous or channeled structure. The technique can provide valuable information about systems for which X-ray crystallographic analysis is not feasible. [structure: see text]     

Probing the porosity of cocrystallized MCM-49/ZSM-35 zeolites by hyperpolarized 129Xe NMR

One- and two-dimensional 129Xe NMR spectroscopy has been employed to study the porosity of cocrystallized MCM-49/ZSM-35 zeolites under the continuous flow of hyperpolarized xenon gas. It is found by variable-temperature experiments that Xe atoms can be adsorbed in different domains of MCM-49/ZSM-35 cocrystallized zeolites and the mechanically mixed counterparts. The exchange of Xe atoms in different types of pores is very fast at ambient temperatures. Even at very low temperature two-dimensional exchange spectra (EXSY) show that Xe atoms still undergo much faster exchange between MCM-49 and ZSM-35 analogues in the cocrystallized zeolites than in the mechanical mixture. This demonstrates that the MCM-49 and ZSM-35 analogues in cocrystallized zeolites may be stacked much closer than in the physical mixture, and some parts of intergrowth may be formed due to the partially similar basic structure of MCM-49 and ZSM-35.     

Probing aerogels by multiple quantum filtered (131)Xe NMR spectroscopy.

At the interface between solid surfaces and cavities filled with gaseous or liquid xenon, the nuclear magnetization of (131)Xe (S = (3)/(2)) is subject to quadrupolar interactions which may lead to higher rank single-quantum coherences that can be described by tensor elements T(2,)(+/-)(1) and T(3,)(+/-)(1). This can be demonstrated by multiple-quantum filtered (MQF) NMR experiments. In gaseous xenon on Pyrex surfaces, the primary source of such coherences was shown to be coherent evolution induced by a nonvanishing average quadrupolar coupling. In this contribution, MQF NMR is applied to aerogels filled with liquid xenon to demonstrate the potential of this technique for material sciences. Xenon in the liquid phase provides a sufficient spin density to obtain reasonable signal-to-noise ratios. Coherent evolution and relaxation both contribute to the creation of higher rank coherences depending on the presence or absence of water molecules on the surface. These two processes can be distinguished experimentally and provide complementary information about the surface of the host material.     

Distribution and modification of sorption sites in amphiphilic calixarene-based solid lipid nanoparticles from hyperpolarized 129Xe NMR spectroscopy

The site distribution and accessibility in amphiphilic calixarenes-based solid lipid nanoparticles were determined as a function of lipid chain length using in situ 129Xe NMR spectroscopy with flowing hyperpolarized Xe gas. The study illustrates that host cavities in as-prepared materials are increasingly occluded by the lipid chain for compounds with chain lengths from C6 to C12 and are almost completely occluded for C14 and C16 chain lengths. Host cavities present at the surface of the particles are still accessible to small atoms (xenon) and organic molecules (methylene chloride, etc). The Xe spectra show that the accessible void space can be increased remarkably by exposure of the particle surface to suitably sized guest molecules that appear to displace the occluding hydrocarbon chains from the host cavities by competitive adsorption. This postsynthesis treatment thus modifies the state of self-assembly and improves sorption capability. The HP Xe NMR approach presented is suitable for small samples (a few milligrams) of SLNs, likely also for other biomaterials such as vesicles, model membranes, etc.     

In situ switching of sorbent functionality as monitored with hyperpolarized (129)Xe NMR spectroscopy

In this contribution, we demonstrate that a material (organic zeolite mimetic coordination polymer [CuL(2)], where L = L(-) = CF(3)COCHCOC(OCH(3))(CH(3))(2)) can be endowed with its functionality in situ under molecular-level control. This process involves the isomerization of the ligands followed by phase interconversion from a dense to an open, porous form. The porous (beta) form of the complex reveals zeolite-like behavior but, unlike zeolites and many other hard porous frameworks, porosity may be created or destroyed at will by the application of suitable external stimuli. Contact with methylene chloride vapor was used to switch on the sorbent functionality, whereas switching off was accomplished with a temperature pulse. The transformations between functionally inactive alpha and active beta forms, as well as the amount of vacant pore space, were monitored in situ by observing the NMR spectrum of hyperpolarized (HP) Xe atom probes. For methylene chloride, the chemical shift of the coabsorbed HP Xe correlated directly with the amount of adsorbate in the pore system of the open framework, illustrating the use of HP Xe for following sorption kinetics. The adsorption of propane, as an inert adsorbate, was also monitored directly with (1)H NMR, with HP Xe and by BET measurements, revealing more complex behavior.     

NMR and MRI of blood-dissolved hyperpolarized Xe-129 in different hollow-fiber membranes

Magnetic resonance of hyperpolarized (129)Xe has found a wide field of applications in the analysis of biologically relevant fluids. Recently, it has been shown that the dissolution of hyperpolarized gas into the fluid via hollow-fiber membranes leads to bubble-free (129)Xe augmentation, and thus to an enhanced signal. In addition, hollow-fiber membranes permit a continuous operation mode. Herein, a quantitative magnetic resonance imaging and spectroscopy analysis of a customized hollow-fiber membrane module is presented. Different commercial hollow-fiber membrane types are compared with regard to their (129)Xe dissolution efficiency into porcine blood, its constituents, and other fluids. The presented study gives new insight into the suitability of these hollow-fiber membrane types for hyperpolarized gas dissolution setups.     

Exploring surfaces and cavities in lipoxygenase and other proteins by hyperpolarized xenon-129 NMR

This paper presents an exploratory study of the binding interactions of xenon with the surface of several different proteins in the solution and solid states using both conventional and hyperpolarized (129)Xe NMR. The generation of hyperpolarized (129)Xe by spin exchange optical pumping affords an enhancement by 3-4 orders of magnitude of its NMR signal. As a result, it is possible to observe Xe directly bound to the surface of micromolar quantities of lyophilized protein. The highly sensitive nature of the (129)Xe line shape and chemical shift are used as indicators for the conditions most likely to yield maximal dipolar contact between (129)Xe nuclei and nuclear spins situated on the protein. This is an intermediate step toward achieving the ultimate goal of NMR enhancement of the binding-site nuclei by polarization transfer from hyperpolarized (129)Xe. The hyperpolarized (129)Xe spectra resulting from exposure of four different proteins in the lyophilized, powdered form have been examined for evidence of binding. Each of the proteins, namely, metmyoglobin, methemoglobin, hen egg white lysozyme, and soybean lipoxygenase, yielded a distinctly different NMR line shape. With the exception of lysozyme, the proteins all possess a paramagnetic iron center which can be expected to rapidly relax the (129)Xe and produce a net shift in its resonance position if the noble gas atom occupies specific binding sites near the iron. At temperatures from 223 to 183 K, NMR signals were observed in the 0-40 ppm chemical shift range, relative to Xe in the gas phase. The signals broadened and shifted downfield as the temperature was reduced, indicating that Xe is exchanging between the gas phase and internal or external binding sites of the proteins. Additionally, conventional (129)Xe NMR studies of metmyoglobin and lipoxygenase in the solution state are presented. The temperature dependence of the chemical shift and line shape indicate exchange of Xe between adsorption sites on lipoxygenase and Xe in the solvent on the slow to intermediate exchange time scale. The NMR results are compared with N(2), Xe, and CH(4) gas adsorption isotherms. It is found that lipoxygenase is unique among the proteins studied in possessing a relatively high affinity for gas molecules, and in addition, demonstrating the most clearly resolved adsorbed (129)Xe NMR peak in the lyophilized state.     

Kinetics from indirectly detected hyperpolarized NMR spectroscopy by using spatially selective coherence transfers

An important recent development in NMR spectroscopy is the advent of ex situ dynamic nuclear polarization (DNP) approaches, which are capable of yielding liquid‐state sensitivities that exceed considerably those afforded by the highest‐field spectrometers. This increase in sensitivity has triggered new research avenues, particularly concerning the in vivo monitoring of metabolism and disease by NMR spectroscopy. So far such gains have mainly materialized for experiments that focus on nonprotonated, low‐γ nuclei; targets favored by relatively long relaxation times T₁, which enable them to withstand the transfer from the cryogenic hyperpolarizer to the reacting centers of interest. Recent studies have also shown that transferring this hyperpolarization to protons by indirectly detected methods could successfully give rise to ¹H NMR spectra of hyperpolarized compounds with a high sensitivity. The present study demonstrates that, when merged with spatially encoded methods, indirectly detected ¹H NMR spectroscopy can also be exploited as time‐resolved hyperpolarized spectroscopy. A methodology is thus introduced that can successfully deliver a series of hyperpolarized ¹H NMR spectra over a minutes‐long timescale. The principles and opportunities presented by this approach are exemplified by following the in vitro phosphorylation of choline by choline kinase, a potential metabolic marker of cancer; and by tracking acetylcholine’s hydrolysis by acetylcholine esterase, an important enzyme partaking in synaptic transmission and neuronal degradation.     

Probing the surface structure of hydroxyapatite using NMR spectroscopy and first principles calculations

The surface characteristics of hydroxyapatite (HA) are probed using a combination of NMR spectroscopy and first principles calculations. The NMR spectrum is taken from a bone sample and the first principles calculations are performed using a plane-wave density functional approach within the pseudopotential approximation. The computational work focuses on the (100) and (200) surfaces, which exhibit a representative range of phosphate, hydroxyl and cation bonding geometries. The shielding tensors for the 31P, 1H and 17O nuclei are calculated from the relaxed surface structures using an extension of the projector augmented-wave method. The calculated 31P chemical shifts for the surface slab are found to be significantly different from the bulk crystal and are consistent with the NMR data from bone and also synthetically prepared nanocrystalline samples of HA. Rotational relaxations of the surface phosphate ions and the sub-surface displacement of other nearby ions are identified as causing the main differences. The investigation points to further calculations of other crystallographic surfaces and highlights the potential of using NMR with ab initio modelling to fully describe the surface structure and chemistry of HA, which is essential for understanding its reactivity with the surrounding organic matrix.     

Direct observation of atoms entering and exiting L-alanyl-L-valine nanotubes by hyperpolarized xenon-129 NMR

The kinetics of gas --> channel and channel --> gas exchange of Xe in self-assembled L-alanyl-L-valine (AV) nanotubes was facilitated by continuous flow hyperpolarized xenon-129 two-dimensional exchange NMR spectroscopy. Cross-peaks due to gas atoms entering or exiting were observed at Xe pressures of 92, 1320, and 3300 mbar, corresponding to Xe fractional occupancies ranging from 0.047 to 0.64. At each pressure, the rate of desorption from the channels was determined by fitting the mixing time dependence of the cross-peak and diagonal-peak signals to a magnetization exchange model, assuming steady-state Langmuir adsorption under hyperpolarized gas flow conditions. The observed rate constant for desorption of Xe from AV nanotubes decreased from 4.5 s-1 to 2.0 s-1 over the occupancy range studied, a finding that is consistent with a decrease in the diffusivity in the channels.     

Probing the initial events in the spontaneous emulsification of trans-anethole using dynamic NMR spectroscopy

The spontaneous emulsification of alcoholic solutions of trans-anethole (t-A) in water is investigated using EXSY and DOSY NMR techniques. The system investigated (5-10 mM t-A in 5% EtOH/H2O solution) is exceptional in providing sharp, clearly resolved signals for both t-A that is dissolved in the aqueous phase (free t-A) and t-A that is incorporated in aggregates (3-6 nm diameter) thus allowing both fractions to be probed simultaneously. This feature is utilized to explore the initial events that occur during the spontaneous emulsification process. Upon mixing, the majority of the t-A (ca. 75%) undergoes nucleation to form small aggregates (ca. 10 nm diameter), while 15% (corresponding to [t-A] = 7.5 x 10(-4) M) is dissolved in the aqueous phase. The kinetic rates governing the exchange process between aggregated and free t-A are found to be time-dependent and slow on the NMR time scale (k = 0.8-2 s(-1)). DOSY experiments indicate that the initially formed small aggregates undergo rapid coalescence to form larger droplets. Ostwald ripening of these droplets at the expense of the remaining small aggregates is responsible for the subsequent, slower time-evolution of the system.     

Novel method for the measurement of xenon gas solubility using 129Xe NMR spectroscopy

A novel method is presented for determining xenon partitioning between a gas phase and a liquid phase. An experimental setup which permits the simultaneous measurement of the 129Xe chemical shift in both the gas and the liquid phases, that is, under the same experimental conditions, has been designed. Xenon solubility is obtained via 129Xe chemical shift measurements in the gas phase. The method was validated against xenon solubility data from the literature; in general, the agreement is found to be within 3%. The solubility of xenon in three solvents for which data have not been previously reported (acetone, acetonitrile, and 1,1,2,2-tetrachloroethane) was determined using this novel method. 129Xe chemical shifts for dissolved xenon are also reported; it is found that xenon-xenon interactions may play a significant role in the liquid phase even at low equilibrium xenon pressures.     

129Xe NMR spectroscopy study of porous cyanometallates

Zinc and cadmium hexacyanocobaltates(III) were prepared, and their porous networks were explored using 129Xe spectroscopy. The crystal structures of these two compounds are representative of porous hexacyanometallates, cubic (Fm-3m) for cadmium and rhombohedral (R-3c) for zinc. In the cubic structure, the porosity is related to systematic vacancies created from the elemental building block (i.e., the hexacyanometallate anion), whereas the rhombohedral (R-3c) structure is free of vacant sites but has tetrahedral coordination for the zinc atom, which leads to relatively large ellipsoidal pores communicated by elliptical windows. According to the Xe adsorption isotherms, these porous frameworks were found to be accessible to the Xe atom. The structure of the higher electric field gradient at the pore surface (Fm-3m) appears and is accompanied by a stronger guest-host interaction for the Xe atoms and a higher capacity for Xe sorption. For cadmium, the 129Xe NMR signal is typical of isotropic movement for the Xe atom, indicating that it remains trapped within a spherical cavity. From spectra recorded for different amounts of adsorbed Xe, the cavity diameter was estimated. For the zinc complex, 129Xe NMR spectra are asymmetric because of the Xe atom movement within an elongated cavity. The line-shape asymmetry changes when the Xe loading within the porous framework increases, which was ascribed to Xe-Xe interactions through the cavity windows. The Xe adsorption revealed additional structural information for the studied materials.     

Nanoporosity of an organo-clay shown by hyperpolarized xenon and 2D NMR spectroscopy

Interlayer nanoporosity of hectorite pillared by tetraethylammonium ions is explored by hyperpolarized xenon NMR and relevant gases such as carbon dioxide revealing the adsorption capacity of the open galleries.     

Probing the transferase activity of glycosidases by means of in situ NMR spectroscopy

This paper describes the conditions in which in situ NMR spectroscopy is a suitable technique to use when following the course of enzymatic transglycosylation reactions. Using this methodology, the reactions must be carried out in D₂O. Our experiments indicate that the rate of the transglycosylation reaction is reduced in this solvent while the rate of the hydrolysis of the disaccharides produced is enhanced depending on the nature of the anomeric substituent. However, this undesirable effect is generally weak because the rates of the transglycosylation reactions are always faster than the rates of the hydrolysis whatever the solvent. The great advantage of NMR spectroscopy lies in its potential to detect, in a single experiment, all the components of the reaction without any disturbance of the reaction medium.     

Availability and interconnectivity of pores in mesostructured ZSM-5 zeolites by the adsorption and diffusion of mesitylene

Probing the mesopore architecture in mesoporous zeolites is of importance for large scale applications of the materials. In this work, the adsorption and diffusion of mesitylene with larger molecule size in mesoporous ZSM-5 zeolites were carried out, in order to acquaint the availability and interconnectivity of mesopores in zeolite crystals. The comparisons of the shape of adsorption isotherms and the mesopore volume calculated from mesitylene and N₂ adsorption in mesoporous ZSM-5 zeolites with different mesoporosities can be used to discriminate two cases of mesopores: accessible mesopores connected to external surface of the zeolite crystals and non-accessible meso-voids that are occluded in the microporous matrix. Furthermore, the effective diffusivity and activation energy of mesitylene in mesoporous ZSM-5 extracted from ZLC desorption curves as a function of mesopore volume calculated from mesitylene adsorption reveal the enhancement of mesopore interconnectivity to molecule diffusion in zeolite crystals.