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International audience; Magic angle spinning nuclear magnetic resonance spectroscopy experiments are widely employed in the characterization of solid media. The approach is incredibly versatile but deleteriously suffers from low sensitivity, which may be alleviated by adopting dynamic nuclear polarization methods, resulting in large signal enhancements. Paramagnetic metal ions such as Gd 3+ have recently shown promising results as polarizing agents for 1 H, 13 C, and 15 N nuclear spins. We demonstrate that the widely available and inexpensive chemical agent Gd(NO 3) 3 achieves significant signal enhancements for the 13 C and 15 N nuclear sites of [2-13 C, 15 N]glycine at 9.4 T and ∼105 K. Analysis of the signal enhancement profiles at two magnetic fields, in conjunction with electron paramagnetic resonance data, reveals the solid effect to be the dominant signal enhancement mechanism. The signal amplification obtained paves the way for efficient dynamic nuclear polarization without the need for challenging synthesis of Gd 3+ polarizing agents.

Solid-state 1H, 13C, and 15N nuclear magnetic resonance (NMR) spectroscopy has been an essential analytical method in studying complex molecules and biomolecules for decades. While oxygen-17 (17O) NMR is an ideal and robust candidate to study hydrogen bonding within secondary and tertiary protein structures for example, it continues to elude many. We discuss an improved multiple-turnover labeling procedure to develop a fast and cost-effective method to 17O label fluoroenylmethyloxycarbonyl (Fmoc)-protected amino acid building blocks. This approach allows for inexpensive ($0.25 USD/mg) insertion of 17O labels, an important barrier to overcome for future biomolecular studies. The 17O NMR results of these building blocks and a site-specific strategy for labeled N-acetyl-MLF-OH and N-formyl-MLF-OH tripeptides are presented. We showcase growth in NMR development for maximizing sensitivity gains using emerging sensitivity enhancement techniques including population transfer, high-field dynamic nuclear polarization, and cross-polarization magic-angle spinning cryoprobes.

Magic angle spinning nuclear magnetic resonance spectroscopy experiments are widely employed in the characterization of solid media. The approach is incredibly versatile but deleteriously suffers from low sensitivity, which may be alleviated by adopting dynamic nuclear polarization methods, resulting in large signal enhancements. Paramagnetic metal ions such as Gd

Solid-state

Made available in DSpace on 2021-06-25T11:07:45Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-11-19 The use of a hybrid sol-gel route to prepare silicate hybrid materials and glasses through stereolithography and other photopolymerizable based additive manufacturing techniques has become a major topic. In this work, we present a new synthesis of 2-hydroxyethyl methacrylate/aluminum-phosphate hybrid materials with potential use on additive manufacturing through stereolithography (SLA). The unique sol-gel chemistry of phosphates and their strong reactivity toward metal alkoxide precursors pose special challenges toward stable sols with low solvent concentration. Here, we were able to control condensation reactions, avoiding precipitation, yielding clear and long shelf-life stable 2-hydroxyethyl methacrylate/aluminum-phosphate hybrid sols with different Al/(Al + P) ratios (from 0% up to 50%), which can be further photopolymerized, yielding transparent crack-free monolithic materials. A detailed structural study through multinuclear solid-state NMR and infrared spectroscopies showed a high polymerization degree with inorganic Aly(PO4)x-like fragments acting as cross-linkers. These cross-linked structures are sensitive to the Al/(Al + P) ratio, and an increasing aluminum concentration results in a higher degree of inorganic condensation. The organic-inorganic structures are highly interconnected resulting in homogeneous materials. Chemistry Institute São Paulo State University UNESP, Rua Francisco Degni 55 São Carlos Institute of Physics University of São Paulo - USP, Avenida Trabalhardor Saocarlense 400 Bruker BioSpin Corporation, 15 Fortune Drive Centre de Optique Photonique et Laser-COPL Universite Laval, 2375 rue de la Terrase Chemistry Institute São Paulo State University UNESP, Rua Francisco Degni 55

A fast NMR data acquisition strategy is explored to detect and characterize carbocations on solid zeolites., Acidic zeolites are porous aluminosilicates used in a wide range of industrial processes such as adsorption and catalysis. The formation of carbocation intermediates plays a key role in reactivity, selectivity and deactivation in heterogeneous catalytic processes. However, the observation and determination of carbocations remain a significant challenge in heterogeneous catalysis due to the lack of selective techniques of sufficient sensitivity to detect their low concentrations. Here, we combine 13C isotopic enrichment and efficient dynamic nuclear polarization magic angle spinning nuclear magnetic resonance spectroscopy to detect carbocations in zeolites. We use two dimensional 13C–13C through-bond correlations to establish their structures and 29Si–13C through-space experiments to quantitatively probe the interaction between multiple surface sites of the zeolites and the confined hydrocarbon pool species. We show that a range of various membered ring carbocations are intermediates in the methanol to hydrocarbons reaction catalysed by different microstructural β-zeolites and highlight that different reaction routes for the formation of both targeted hydrocarbon products and coke exist. These species have strong van der Waals interaction with the zeolite framework demonstrating that their accumulation in the channels of the zeolites leads to deactivation. These results enable understanding of deactivation pathways and open up opportunities for the design of catalysts with improved performances.

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) offers a recent approach to dramatically enhance NMR signals and has enabled detailed structural information to be obtained in a series of amorphous photocatalytic copolymers of alternating pyrene and benzene monomer units, the structures of which cannot be reliably established by other spectroscopic or analytical techniques. Large 13C cross-polarization (CP) magic angle spinning (MAS) signal enhancements were obtained at high magnetic fields (9.4–14.1 T) and low temperature (110–120 K), permitting the acquisition of a 13C INADEQUATE spectrum at natural abundance and facilitating complete spectral assignments, including when small amounts of specific monomers are present. The high 13C signal-to-noise ratios obtained are harnessed to record quantitative multiple contact CP NMR data, used to determine the polymers’ composition. This correlates well with the putative pyrene:benzene stoichiometry from the monomer feed ratio, enabling their structures to be understood.

We demonstrate that dynamic nuclear polarization (DNP) of membrane proteins in lipid bilayers may be achieved using a novel polarizing agent: pairs of spin labels covalently bound to a protein of interest interacting at an intermolecular interaction surface. For gramicidin A, nitroxide tags attached to the N-terminal intermolecular interface region become proximal only when bimolecular channels forms in the membrane. We obtained signal enhancements of 6-fold for the dimeric protein. The enhancement affect was comparable to that of a doubly tagged sample of gramicidin C, with intramolecular spin pairs. This approach could be a powerful and selective means for signal enhancement in membrane proteins, and for recognizing intermolecular interfaces.

Dynamic nuclear polarization (DNP) solid-state NMR was used to obtain natural abundance (13)C and (15)N CP MAS NMR spectra of microporous organic polymers with excellent signal-to-noise ratio, allowing for unprecedented details in the molecular structure to be determined for these complex polymer networks. Sensitivity enhancements larger than 10 were obtained with bis-nitroxide radical at 14.1 T and low temperature (∼105 K). This DNP MAS NMR approach allows efficient, high-throughput characterization of libraries of porous polymers prepared by combinatorial chemistry methods.

We show that natural abundance oxygen-17 NMR of solids could be obtained in minutes at a moderate magnetic field strength by using dynamic nuclear polarization (DNP). Electron spin polarization could be transferred either directly to (17)O spins or indirectly via (1)H spins in inorganic oxides and hydroxides using an oxygen-free solution containing a biradical polarization agent (bTbK). The results open up a powerful method for rapidly acquiring high signal-to-noise ratio solid-state NMR spectra of (17)O nuclear spins and to probe sites on or near the surface, without the need for isotope labeling.

International audience; Dynamic nuclear polarization (DNP)/solid-state nuclear magnetic resonance(NMR) spectroscopy bears great potential for the investigation of membraneassociatedpolypeptides which can often be produced only in small amounts and whichneed to be ‘diluted’ in lipid bilayer environments to adopt or maintain their functionalstructure. Here we present investigations using biradicals, such as TOTAPOL andbTbK, for solid-state NMR signal enhancement using DNP in the context of lipidmembranes. By transferring polarization from electron to nuclear spins using microwave irradiation signal enhancement factors of up to 13 are obtained withTOTAPOL and up to 17 with bTbK. The possible reasons why these factors are belowthose obtained in glassy samples of bulk solvents (40–60 under similar conditions) areevaluated and discussed. In order to further ameliorate the enhancement factors thephysico-chemical characteristics of TEMPOL, TOTAPOL, bTbK, and bCTbK, suchas their partitioning between hydrophilic and hydrophobic solvents or their stabilityunder different environmental conditions are presented. Finally, having providedproof-of-concept that DNP/solid-state NMR measurements can be performed withoriented membrane samples work in progress is presented on the development of a flatcoilprobe for DNP/solid-state NMR experiments on oriented membranes.

Metal chalcogenide aerogels and xerogels are unique materials that combine semiconducting nanostructures with porosity and are thus of interest for photocatalysis and sensing. To assess the feasibility for applications dependent on molecular transport, the pore structure and interconnectivity in CdS aerogels and xerogels were probed by a combination of conventional techniques and hyperpolarized (HP) 129Xe NMR. HP 129Xe NMR of the aerogels was consistent with two distinct types of accessible pores (adsorption sites), and 2-D EXSY NMR data suggest that these are connected. In contrast, a single resonance of low intensity with a temperature dependence consistent with the dissolution of Xe within a polymer matrix, that is, transport facilitated at high temperatures due to chain motion, is observed in xerogels. 13C NMR and thermal gravimetric analysis data reveal the presence of residual organics in both xerogels and aerogels, but a relatively higher proportion in the former, due, in part, to the surfactant used in their preparation. These data, combined with the absence of any meaningful adsorption isotherm for N2 at 77 K (where limited thermal chain motion precludes gas adsorption) are consistent with a dynamic blocking of the xerogel pores. The results reveal the important role of residual organics and drying regimens on the accessibility of pores in metal chalcogenide gel architectures.

Metal-organic frameworks have demonstrated functionality stemming from both robustness and pliancy and as such, offer promise for a broad range of new materials. The flexible aspect of some of these solids is intriguing for so-called 'smart' materials in that they could structurally respond to an external stimulus. Herein, we present an open-channel metal-organic framework that, on dehydration, shifts structure to form closed pores in the solid. This occurs through multiple single-crystal-to-single-crystal transformations such that snapshots of the mechanism of solid-state conversion can be obtained. Notably, the gas composing the atmosphere during dehydration becomes trapped in the closed pores. On rehydration, the pores open to release the trapped gas. Thus, this new material represents a thermally robust and porous material that is also capable of dynamically capturing and releasing gas in a controlled manner.

The pore environments of a series of isoreticular metal-organic frameworks (IRMOF) have been studied using hyperpolarized (HP) 129Xe nuclear magnetic resonance (NMR) spectroscopy. Xenon gas behaved as an efficient probe molecule for interrogating the variability of adsorption sites in functionalized IRMOF materials through variations in the NMR chemical shift of the adsorbed xenon. The xenon adsorption enthalpies extracted from variable-temperature HP 129Xe NMR were found to be lower than published values for the physisorption of xenon. The low heats of adsorption were corroborated by xenon adsorption measurements that revealed two atoms per pore under one atmosphere of pressure at 19 °C. Average pore diameters estimated from the empirical chemical shift and pore size correlations based on a geometrical model were compared with X-ray crystallography data. The exchange processes of xenon in IRMOFs also were explored using 2D exchange spectroscopy (EXSY) 129Xe NMR. It was found the exchange of xenon from ad...

The bonding and dynamic properties of perfluoroalkanoic acid self-assembled monolayers (SAMs) on zirconia and titania powders were characterized by Fourier transform infrared and solid-state 19F magic-angle spinning NMR spectroscopy. The perfluoro fatty acids investigated included C(n)F(2n+1)CO2H, n = 7, 13, 15 and 17. The acids bind to both metal oxides via ionic carboxylate bonds, but complete monolayers are only formed on the zirconia. The shift of the CF3 group from -83 ppm in the bulk state to -85 ppm for the adsorbed monolayers is assigned to CF3 groups located at the air/monolayer interface. With the exception of the perfluorooctanoic acid, 19F spin lattice relaxation measurements indicate that the fluorocarbon chains of the adsorbed acids, even in the case of densely packed monolayers, are significantly more mobile than those in the bulk state. The motions associated with the enhanced mobility of the adsorbed acids are proposed to involve reorientations along the long chain axes. No evidence for chain melting in the fluorocarbon SAMs is found for temperatures well above the melting temperature of the bulk acids.

The structures formed by adsorption of carboxyalkylphosphonic acids on metal oxide powders were characterized by solid-state NMR and FTIR-PAS (Fourier transform infrared photoacoustic spectroscopy). A series of diacids, HO2C(CH2)nPO3H2 (n = 2, 3, 11, and 15), were deposited on nonporous TiO2 and ZrO2 powders and nanocrystalline ZrO2 with average particle diameters of 21, 30, and 5 nm, respectively. Solid-state 31P NMR, combined with FTIR-PAS, indicates that the phosphonic acid group binds selectively to the surface, producing a monolayer of carboxylic acid terminated chains. The average chain conformation depends on the substrate in addition to the chain length. Ordered samples display thermal order/disorder transitions similar to other self-assembled monolayer systems. The more restricted chain mobility relative to analogous methyl-terminated chains is attributed to hydrogen-bonding among the pendant carboxylic acid groups. These results demonstrate that phosphonic acids are useful for selectively introd...

Deuterium NMR spectroscopy has been used to probe the dynamics of deuterated octadecylphosphonate (−1,1−d2) (ODPA−d2) monolayers on nonporous ZrO2 powder (surface area ≈ 40 m2/g) over the temperature range of 200−340 K. At 200 K, a broadened Pake doublet with distinct nonrigid characteristics and a horn splitting of approximately 120 kHz was observed. With increasing temperature, the 2H spectrum gradually transforms into a relatively narrow and featureless peak. Spectral simulations are performed with the help of plausible motional models, and the results show that, over the whole temperature range studied, the C1−D bonds have substantial motional freedom with respect to the characteristic 2H NMR time scale. In addition, at each temperature, a weighted superposition of several simulated line shapes with different rates and site populations is required to account for the observed spectral features, indicating the presence of considerable motional heterogeneity within the ODPA monolayers. The results are fu...

Long-chain alkanoic acids (CH3(CH3)nCO2H, n > 15) form conformationally ordered monolayers on zirconium oxide powder via a chelating bidentate zirconium carboxylate surface bond. ω-Hydroxyalkanoic ...

Dynamic Nuclear Polarization (DNP) experiments transfer polarization from electron spins to nuclear spins with microwave irradiation of the electron spins for enhanced sensitivity in nuclear magnetic resonance (NMR) spectroscopy. Design and testing of a spectrometer for magic angle spinning (MAS) DNP experiments at 263 GHz microwave frequency, 400 MHz 1H frequency is described. Microwaves are generated by a novel continuous-wave gyrotron, transmitted to the NMR probe via a transmission line, and irradiated on a 3.2 mm rotor for MAS DNP experiments. DNP signal enhancements of up to 80 have been measured at 95 K on urea and proline in water–glycerol with the biradical polarizing agent TOTAPOL. We characterize the experimental parameters affecting the DNP efficiency: the magnetic field dependence, temperature dependence and polarization build-up times, microwave power dependence, sample heating effects, and spinning frequency dependence of the DNP signal enhancement. Stable system operation, including DNP performance, is also demonstrated over a 36 h period., National Institutes of Health (U.S.) (NIH grant EB-002804), National Institutes of Health (U.S.) (NIH grant EB-002026)

International audience; Oriented membrane samples encompassing the biradical bTbK and a transmembrane peptide carrying a single (15)N labeled residue have been prepared on polymer sheets with sample geometries that fit into a 3.2 mm MAS rotor. The proton-decoupled (15)N cross-polarization spectra of the peptide were characterized by a single line at fast magic angle spinning speeds of similar to 8 kHz. Irradiating these samples with p-waves resulted in Dynamic Nuclear Polarization and a concomitant 18-fold signal enhancement which considerably shortened the NMR acquisition times. Furthermore, the sideband patterns of magic angle oriented sample spinning (MAOSS) solid-state NMR spectra at slow spinning speeds (similar to 1 kHz) are indicative that the lipids and peptides form well-oriented bilayers at 100 K despite the narrow inner diameter of the rotor (2.2 mm) and the presence of considerable amounts of biradicals. The DNP signal enhancement opens up enhanced possibilities for multidimensional solid-state NMR investigation of oriented membrane polypeptides.

Hyperpolarized (HP) 129Xe NMR was used to probe the porosity and interconnectivity of pores in crystalline mesoporous TiO2. We have demonstrated that HP 129Xe NMR can be used to differentiate between similar sized pores within different crystalline phases. Pores of 4 nm size resident in mixed anatase and rutile mesoporous TiO2 phases were identified. Complementary to other pore characterization techniques, HP 129Xe NMR is able to probe the interconnectivity between pores present in these different phases. The cross peaks in 2D exchange (EXSY) NMR spectra between the signals of xenon in two types of pores are visible on millisecond timescale, indicating substantial pore interconnectivity. The obtained information on porosity and interconnectivity is important for the understanding of ion transport mechanisms in mesoporous TiO2 anode materials.

Our recent discovery of a new dipeptide gelator that can be activated by ultrasound in alkanes and polar solvents[8] allows one to envision several unusual and unexpected applications. A striking feature of this dipeptide is the readily ultrasound-induced gelation in hexane containing CdSe/ZnS core/shell QDs. In this case, ultrasound allows a blurring of the frontier between ‘‘soft’’ multi-responsive supramolecular gels and ‘‘hard’’ light-emitting semiconductor NPs, affording new materials displaying properties from each component. Here, we describe the preparation of QD-dipeptide nanocomposite gels which show bright luminescence under UV light and which can be switched from liquid to solid by ultrasound and back to liquid when heated. Applications using the gel and the dry material (xerogel) are also demonstrated regarding chemical sensing.