Solid-state NMR Characterization Research Articles

Solid-State NMR Characterization of Mefloquine Resinate Complexes Designed for Taste-Masking Pediatric Formulations.

Mefloquine (MQ) is an antimalarial medication prescribed to treat or malaria prevention.. When taken by children, vomiting usually occurs, and new doses of medication frequently need to be taken. So, developing pediatric medicines using taste-masked antimalarial drug complexes is mandatory for the success of mefloquine administration. The hypothesis that binding mefloquine to an ion-exchange resin (R) could circumvent the drug's bitter taste problem was proposed, and solid-state 13C cross-polarization magic angle spinning (CPMAS) NMR was able to follow MQ-R mixtures through chemical shift and relaxation measurements. The nature of MQ-R complex formation could then be determined. Impedimetric electronic tongue equipment also verified the resinate taste-masking efficiency in vitro. Variations in chemical shifts and structure dynamics measured by proton relaxation properties (e.g., T1ρH) were used as probes to follow the extension of mixing and specific interactions that would be present in MQ-R. A significant decrease in T1ρH values was observed for MQ carbons in MQ-R complexes, compared to the ones in MQ (from 100-200 ms in MQ to 20-50 ms in an MQ-R complex). The results evidenced that the cationic resin interacts strongly with mefloquine molecules in the formulation of a 1:1 ratio complex. Thus, 13C CPMAS NMR allowed the confirmation of the presence of a binding between mefloquine and polacrilin in the MQ-R formulation studied.

Complex isomerism influencing the textural properties of organometallic [Cu(salen)] porous polymers: paramagnetic solid-state NMR characterization and heterogeneous catalysis.

Although organometallic porous polymer networks are recognized as promising heterogeneous catalysts, the relationship between ligand/monomer geometry and network parameters is usually not well understood due to the lack of atom-resolved characterization methods for the amorphous network matrix. In this work, a series of copper(II) salen-type metal complexes was synthesized, using trans- and cis-1,2-diaminocyclohexane segments, and thoroughly characterized by single-crystal X-ray diffraction and solution- and solid-state NMR spectroscopy. Terminal ethynyl groups of the complexes were then transformed into polyacetylene chains by coordination chain-growth homopolymerization, resulting in highly porous (458-655 m2 g-1) organometallic polymer networks with a copper(II) ion content of about 12 wt%. The presence of paramagnetic copper(II) moieties in these complexes and respective polymer networks required the application of tailored NMR techniques, which together with X-ray crystallography and DFT calculations of the paramagnetic NMR shifts made it possible to investigate the differences in the complex geometry in liquid, powder and crystalline form and compare it with the complex geometry in polymer networks. All prepared organometallic polymer networks were also tested as heterogeneous catalysts for styrene oxidation with uncommonly high substrate conversions and compared with their low-molecular-weight analogues. The high reusability of such heterogeneous polymer-based catalysts was also proven.

Catalyst-Free Transfer Hydrogenation from Amine-Borane Small Oligomers.

Amine-borane dimers and oligomers with varied steric and electronic profiles were prepared via capping agent-controlled AA/BB polycondensations. They were used for transfer hydrogenations to aldehydes, ketones, imines as well as electron-poor alkene/alkyne moieties. The amine-borane Lewis-paired oligomers and the congested bis(amine-borane)s provided the highest yields. This was likely helped by facilitated dissociation (oligomers) or H-bond assistance. In the case of the oligomers, the second equivalent of H2 present was also engaged in the reaction. Solid-state NMR characterization provides evidence that the boron-containing materials obtained after transfer dehydrogenation are highly similar to those obtained from thermal dehydrogenation. The oligomers bridge the gap between simple amine-borane molecular reductants and the poly-amine-boranes and provide a full picture of the reactivity changes at the different scales.

Passive Translocation of Phospholipids in Asymmetric Model Membranes: Solid-State 1H NMR Characterization of Flip-Flop Kinetics Using Deuterated Sphingomyelin and Phosphatidylcholine.

Although lateral and inter-leaflet lipid-lipid interactions in cell membranes play roles in maintaining asymmetric lipid bilayers, the molecular basis of these interactions is largely unknown. Here, we established a method to determine the distribution ratio of phospholipids between the outer and inner leaflets of asymmetric large unilamellar vesicles (aLUVs). The trimethylammonium group, (CH3)3N+, in the choline headgroup of N-palmitoyl-sphingomyelin (PSM) and 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) gave rise to a relatively sharp signal in magic-angle spinning solid-state 1H NMR (MAS-ss-1H NMR). PSM and DOPC have the same headgroup structure, but one phospholipid was selectively observed by deuterating the trimethylammonium group of the other phospholipid. The addition of Pr3+ to the medium surrounding aLUVs selectively shifted the chemical shift of the (CH3)3N+ group in the outer leaflet from that in the inner leaflet, which allowed estimation of the inter-leaflet distribution ratio of the unlabeled lipid in aLUVs. Using this method, we evaluated the translocation of PSM and DOPC between the outer and inner leaflets of the cholesterol-containing aLUVs, with PSM and DOPC mostly distributed in the outer and inner leaflets, respectively, immediately after aLUV preparation; their flip and flop rates were approximately 2.7 and 6.4 × 10-6 s-1, respectively. During the passive symmetrization of aLUVs, the lipid translocation rate was decreased due to changes in the membrane order, probably through the formation of the registered liquid-ordered domains. Comparison of the result with that of symmetric LUVs revealed that lipid asymmetry may not significantly affect the lipid translocation rates, while the lateral lipid-lipid interaction may be a dominant factor in lipid translocation under these conditions. These findings highlight the importance of considering the effects of lateral lipid interactions within the same leaflet on lipid flip-flop rates when evaluating the asymmetry of phospholipids in the cell membrane.

Bicalutamide polymorphism: solid-state NMR characterization of two crystalline forms

Bicalutamide polymorphism: solid-state NMR characterization of two crystalline forms

Raman, WAXS, and Solid-State NMR Characterizations of Regenerated Silk Fibroin Using Lanthanide Ions as Chaotropic Agents.

Bombyx mori silk fibroin (SF) has been reported as a convenient natural material for regenerative medicine, optoelectronics, and many other technological applications. SF owes its unique features to the hierarchical organization of the fibers. Many efforts have been made to set up protocols for dissolution since many applications of SF are based on regenerated solutions and fibers, but chaotropic conditions required to disassemble the packing of the polymer afford solutions with poor crystalline behavior. Our previous research has disclosed a dissolution and regeneration process of highly crystalline fibers involving lanthanide ions as chaotropic agents, demonstrating that each lanthanide has its own unique interaction with SF. Herein, we report elucidation of the structure of Ln-SF fibers by the combined use of Raman spectroscopy, wide-angle X-ray scattering (WAXS), and solid-state NMR techniques. Raman spectra confirmed the coordination of metal ions to SF, WAXS results highlighted the crystalline content of fibers, and solid-state NMR enabled the assessment of different ratios of secondary structures in the protein.

Phosphorus/Sulfur-Enriched Reduced Graphene Oxide Papers Obtained from Recycled Graphite: Solid-State NMR Characterization and Electrochemical Performance for Energy Storage

The reduction of graphene oxide (GO) by means of thermal and/or chemical treatments leads to the production of reduced graphene oxide (rGO)—a material with improved electrical conductivity and considered a viable and low-cost alternative to pure graphene in several applications, including the production of supercapacitor electrodes. In the present work, GO was prepared by the oxidation of graphite recycled from spent Li-ion batteries using mixtures of sulfuric and phosphoric acids (with different H2SO4/H3PO4 ratios), leading to the production of materials with significant S and P contents. These materials were then thermally reduced, resulting in rGO papers that were investigated by solid-state 13C and 31P nuclear magnetic resonance, along with other methods. The electrochemical properties of the produced rGO papers were evaluated, including the recording of cyclic voltammetry and galvanostatic charge–discharge curves, besides electrochemical impedance spectroscopy analyses. The samples obtained by thermal reduction at 150 °C exhibited good rate capability at high current density and high capacitance retention after a large number of charge–discharge cycles. The results evidenced a strong relationship between the electrochemical properties of the produced materials and their chemical and structural features, especially for the samples containing both S and P elements. The methods described in this work represent, then, a facile and low-cost alternative for the production of rGO papers using graphite recycled from spent batteries, with promising applications as supercapacitor electrodes.

Structural conservation of HBV-like capsid proteins over hundreds of millions of years despite the shift from non-enveloped to enveloped life-style

The discovery of nackednaviruses provided new insight into the evolutionary history of the hepatitis B virus (HBV): The common ancestor of HBV and nackednaviruses was non-enveloped and while HBV acquired an envelope during evolution, nackednaviruses remained non-enveloped. We report the capsid structure of the African cichlid nackednavirus (ACNDV), determined by cryo-EM at 3.7 Å resolution. This enables direct comparison with the known capsid structures of HBV and duck HBV, prototypic representatives of the mammalian and avian lineages of the enveloped Hepadnaviridae, respectively. The sequence identity with HBV is 24% and both the ACNDV capsid protein fold and the capsid architecture are very similar to those of the Hepadnaviridae and HBV in particular. Acquisition of the hepadnaviral envelope was thus not accompanied by a major change in capsid structure. Dynamic residues at the spike tip are tentatively assigned by solid-state NMR, while the C-terminal domain is invisible due to dynamics. Solid-state NMR characterization of the capsid structure reveals few conformational differences between the quasi-equivalent subunits of the ACNDV capsid and an overall higher capsid structural disorder compared to HBV. Despite these differences, the capsids of ACNDV and HBV are structurally highly similar despite the 400 million years since their separation.

Fluorine-Labeled N-Boc-l-proline as a Marker for Solid-State NMR Characterization of Biofunctionalizations on Paper Substrates

An efficient approach employing 4-dimethylaminopyridine and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride as the coupling reagent is presented, which enables the functionalization of cotton linter paper substrates with the 19F spin label N-boc-cis-4-fluoro-l-proline. This spin label can be easily quantified by 19F magic angle spinning (MAS) NMR experiments to determine its loading on paper substrates. During the functionalization, the spin label stays intact, as confirmed by the 1H–19F heterocorrelation (1H → 19F CP MAS FSLG HETCOR) experiments. In combination with dynamic nuclear polarization (19F MAS DNP), the N-boc-cis-4-fluoro-l-proline spin label allows us to inspect 1 μmol/g and even lower molecule loadings on paper substrates, providing a highly sensitive local probe to analyze the structure of biofunctionalizations at the nanoscale on paper substrates in the future.

Interface modulation of Pt/Al2O3 catalyst and their roles in thermal stability

Interfacial local environment of noble metal nanoparticles (NPs) and support is crucial for the development and design of thermally and chemically stable noble metal heterogeneous catalysts. The unique chemical environment can strengthen the NPs interaction with the support surface, which can effectively avoid the agglomeration of NPs. The strategy is developed to modulate the interfacial chemical environment of supported Pt NPs with the help of the unsaturated Al3+ in Al2O3N (nanobelt) support, resulting in a sintering-resistant Pt/Al2O3N catalyst. Al solid-state MAS NMR characterization shows that Al2O3N support contains a large amount of unsaturated Alpenta3+ (about 22.6%). XAS confirms that Pt NPs deposited on the surface of Al2O3 with rich unsaturated pentahedral coordinated Al3+ sites have a unique coordination structure at the interface (more Pt-O coordination). When catalyzing the CO oxidation as a probe reaction, the catalytic activity and the particle sizes of Pt NPs do not change significantly even after the reaction at 400 °C for 15 days, and the complete CO conversion temperature increases only by 40 °C when the catalyst is screened at 600 °C for 5 days. This work demonstrates a new strategy to develop stable large-scale anti-sintering noble metal-based heterogeneous catalysts for many important high-temperature industrial applications.

Polyether sulfone and Li6.4La3Zr1.4Ta0.6O12 based polymer-in-ceramic electrolyte with enhanced conductivity at low temperature for solid state lithium batteries

Developing solid state lithium batteries (SSLBs) is an essential step to overcome current safety and energy density issues of traditional Li-ion batteries. As the core component of SSLBs, a solid electrolyte with high ionic conductivity, great physical and chemical properties is still under exploration. Herein, a novel “polymer in ceramic” composite polymer-ceramic electrolytes (CPEs) is reported, which is composed of a polyether sulfone (PESF) polymer and cubic Li6.4La3Zr1.4Ta0.6O12 (LLZTO) ceramic with high contents (up to 80 wt. %). The as-prepared PESF-LLZTO CPEs possess good heat resistance and high mechanical strength to maintain structural integrity. The optimized 0.2PESF-0.8LLZTO CPEs exhibits an excellent ionic conductivity of 4.13×10−4 S cm−1 with a tLi+value as high as 0.64 at 20 °C after wetted by trace wetting agent (6.0 ul cm−2). More impressively, the ionic conductivity is still reached 1.49×10−4 S cm−1 at -10 °C and 9.42×10−4 S cm−1 at 80oC. Meanwhile, the 0.2PESF-0.8LLZTO CPEs shows a broad electrochemical window (≤4.8 V vs. Li+/Li) and an excellent compatibility with Li metal. Density functional theory calculations and 6Li solid-state NMR characterizations suggest that strong interactions among LLZTO, PESF and wetting agents contribute to the enhanced conductivity. As a proof for practical application, the Li||0.2PESF-0.8LLZTO CPEs||LiFePO4 full cells deliver a specific discharge capacity of 133.2 mAh g−1 at 2.0 C, and successfully run over 80 cycles at 0.5 C without capacity attenuation at 20 °C. The full cell also demonstrates a possibility for the application at low temperature down to -10oC and high temperature up to 60oC. This work brings a new perspective for high-performance SSLBs.

Solid-state 17O NMR study of α-d-glucose: exploring new frontiers in isotopic labeling, sensitivity enhancement, and NMR crystallography.

We report synthesis and solid-state 17O NMR characterization of α-d-glucose for which all six oxygen atoms are site-specifically 17O-labeled. Solid-state 17O NMR spectra were recorded for α-d-glucose/NaCl/H2O (2/1/1) cocrystals under static and magic-angle-spinning (MAS) conditions at five moderate, high, and ultrahigh magnetic fields: 14.1, 16.4, 18.8, 21.1, and 35.2 T. Complete 17O chemical shift (CS) and quadrupolar coupling (QC) tensors were determined for each of the six oxygen-containing functional groups in α-d-glucose. Paramagnetic Cu(ii) doping was found to significantly shorten the spin–lattice relaxation times for both 1H and 17O nuclei in these compounds. A combination of the paramagnetic Cu(ii) doping, new CPMAS CryoProbe technology, and apodization weighted sampling led to a sensitivity boost for solid-state 17O NMR by a factor of 6–8, which made it possible to acquire high-quality 2D 17O multiple-quantum (MQ) MAS spectra for carbohydrate compounds. The unprecedented spectral resolution offered by 2D 17O MQMAS spectra permitted detection of a key structural difference for a single hydrogen bond between two types of crystallographically distinct α-d-glucose molecules. This work represents the first case where all oxygen-containing functional groups in a carbohydrate molecule are site-specifically 17O-labeled and fully characterized by solid-state 17O NMR. Gauge Including Projector Augmented Waves (GIPAW) DFT calculations were performed to aid 17O and 13C NMR signal assignments for a complex crystal structure where there are six crystallographically distinct α-d-glucose molecules in the asymmetric unit.

Functionalization of synthetic saponite: Identification of grafting sites and application for anions sequestration

The present work reports the study of the mechanism of grafting of an alkoxysilane on the surface of a synthetic saponite. XRD and SEM/EDX characterizations confirmed the effective synthesis of the clay mineral with structural formula INTNa0.50TET[Si3.50Al0.50]OCT[Mg3]O10(OH)2. The functionalization of this material in its pristine or acid pre-treated form with a cationic silane (tetradecyldimethyl(3-trimethoxysilylpropyl)ammonium chloride) yielded a nanohybrid organoclay with interesting structural and chemical properties highlighted by 29Si solid state NMR, XRD, FTIR, TGA and electrochemical characterizations. The monodentate and bidentate grafting of the alkoxysilane was observed regardless the starting material was pristine or acid pre-treated saponite. The more significant layer-to-layer distance increase observed after the functionalization of acid pre-treated saponite indicated some interlayer intercalation/grafting of the alkoxysilane. Electrochemical characterization (cyclic voltammetry and electrochemical impedance spectroscopy) showed that the modified clay mineral exhibited anionic exchange property due to the presence of grafted cationic alkoxysilane. This anionic exchange capacity was confirmed when successfully applied for the adsorption of the anionic dye Congo Red. In all cases, the acid pre-treatment tended to improve the anion exchange property of the organoclay.

Thermally Conductive Excipient Expands KinetiSol® Processing Capabilities.

We report for the first time that incorporation of a thermally conductive excipient (TCE) modifies the thermal conductivity of the ternary drug-polymer-TCE compositions such that high-energy mixing can occur for prolonged periods at a selected steady-state temperature during the KinetiSol process. In this study, candurin, a TCE, is incorporated within a composition that is processed by high-energy mixing from the KinetiSol process to increase the thermal conductivity of the ternary composition. The improved thermal conductivity promotes heat transfer and enables the high-energy mixing applied during the KinetiSol process to be continued for prolonged time intervals at a selected steady-state temperature, instead of undergoing a continued increase in temperature when the TCE is not present in the composition. The addition of candurin does not impact the molecular structure and mixing of the drug and polymer in ASDs from solid-state NMR characterizations. Compositions with candurin achieved a steady-state processing temperature with + 5°C of the target temperature, and these compositions demonstrated the ability to mix for prolonged time periods while maintaining within this steady-state temperature range, thus enabling the formation of an ASD at a temperature that the drug does not chemically degrade. This study demonstrated that inclusion of the TCE modified the composition's thermal conductivity to efficiently dissipate heat to achieve a selected steady-state temperature during the KinetiSol process, thus providing prolonged mixing times at a lower temperature for dissolving the drug into the polymer to achieve an ASD without sacrificing product performance.

Molecular Mobility of Tert-butyl Alcohol Confined in a Breathing MIL-53 (Al) Metal-Organic Framework.

We present a detailed solid-state NMR characterization of the molecular dynamics of tert-butyl alcohol (TBA) confined inside breathing metal-organic framework (MOF) MIL-53(Al). 27 Al MAS NMR has demonstrated that TBA adsorption induces the iX phase of MIL-53 material with partially shrunk channels. 2 H solid-state NMR has shown that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two axes and librations of the molecule as a whole about the axis passing through the TBA C-O bond. These librations are realized by two distinct ways: fast molecule orientation change during the translational jump diffusion along the channel with characteristic time τD of about 10-9 s at 300 K; slow local librations at a single coordination site, representing framework hydroxyl groups, with τl ≈10-6 s at 300 K. Self-diffusion coefficient of the alcohol in the MOF has been estimated: D=3.4×10-10 m2 s-1 at 300 K. It has been inferred that both the framework flexibility and the interaction with framework hydroxyl groups define the dynamics of TBA confined in the channels of MIL-53 (Al).

A combined solid-state 17O NMR, crystallographic, and computational study of oxiranes

We report the synthesis and solid-state 17O NMR characterization of three 17O-labeled oxiranes: (2S*,3S*)-2,3-bis(4-nitrophenyl)-[17O]oxirane, (2S*,3R*)-2,3-bis(4-nitrophenyl)-[17O]oxirane, and 2,2,3-triphenyl-[17O]oxirane. In addition, we have determined the crystal structure of (2S*,3R*)-2,3-bis(4-nitrophenyl)oxirane by X-ray crystallography. When the experimentally determined 17O NMR tensors for oxiranes (where the C–O–C bond angle is about 60°) are compared with those for dimethyl ether (where the C–O–C bond angle is 113°) and other R–O–R′ functional groups, we found that the highly constrained geometry of oxiranes results in distinct tensor orientations in the molecular frame of reference. The experimental results are complemented by quantum chemical computations. This study represents the first time that 17O chemical shift and quadrupole coupling tensors are simultaneously determined for oxirane compounds.

Enhanced Ion Conduction in Li2.5Zn0.25PS4 via Anion Doping

Li1+2xZn1–xPS4 was computationally predicted to have superionic conductivity of over 50 mS/cm. However, experimental efforts so far have only yielded ionic conductivities on the order of 10–4 S/cm, due to difficulty in producing phase-pure crystalline products. Here, to improve phase purity and enhance ionic conductivity, Cl- doping is employed to synthesize Li1+2x–yZn1–xPS4–yCly, which is shown to stabilize Li1+2xZn1–xPS4 from decomposing to Li3PS4. Li2.5Zn0.25PS4 which shows the highest ionic conductivity among the Li1+2xZn1–xPS4 samples is chosen as an example to study. After incorporating only a small amount of Cl, the ionic conductivity increases from 0.6 mS/cm for Li2.5Zn0.25PS4 to 3.5 mS/cm for Li2.4Zn0.25PS3.9Cl0.1 at 25 °C. In addition, the activation energy is reduced from 0.33 to 0.27 eV. The phase purity and fine structure of Li1+2x–yZn1–xPS4–yCly are determined with XRD, and 7Li, 31P, 35Cl solid-state NMR characterizations. The experimental data confirm the success of Cl– doping. In addition, Li2.4Zn0.25PS3.9Cl0.1 exhibits a low electronic conductivity of 1.21 × 10–9 S/cm and a wide electrochemical stability window.

Defect engineering of polyethylene-like polyphosphoesters: solid-state NMR characterization and surface chemistry of anisotropic polymer nanoplatelets

Polyethylene-like polyphosphoesters crystallized from dilute solution into anisotropic nanoplatelets. As proven by solid-state NMR, the phosphate groups are expelled to the surface and on-surface chemistry was conducted leaving the crystals intact.

14N-1H HMQC solid-state NMR as a powerful tool to study amorphous formulations - an exemplary study of paclitaxel loaded polymer micelles.

Amorphous drug-polymer formulations are complex materials and often challenging to characterize, even more so if the small molecule component itself is increasingly complex. In this work, we present 14N-1H HMQC magic-angle spinning (MAS) NMR experiments in the solid state as a promising tool to study amorphous formulations. Poly(2-oxazoline) based polymer micelles loaded with different amounts of the cancer drug paclitaxel serve to highlight the possibilities offered by these experiments: while the dense core of these polymeric micelles prevents NMR spectroscopic analysis in solution and the very similar 15N chemical shifts hamper a solid-state NMR characterization based on this nucleus, 14N is a very versatile alternative. 14N-1H HMQC experiments yield well-separated signals, which are spread over a large ppm range, and provide information on the symmetry of the nitrogen environment and probe 14N-1H through-space proximities. In this way, the overall complexity can be narrowed down to specific N-containing environments. The results from the experiments presented here represent a valuable puzzle piece, which helps to improve the structural understanding of drug-polymer formulations. It can be straightforwardly combined with complementary NMR spectroscopic experiments and other analytical techniques.

N-Hydroxysuccinimide-activated esters as a functionalization agent for amino cellulose: synthesis and solid-state NMR characterization

We propose a mild and versatile synthesis protocol based on N-hydroxysuccinimide-activated esters for the introduction of new functionalities to cellulose, using as starting point established protocols for the tosylation of cellulose and its subsequent reaction with a diamine linker. As a proof of concept, we describe the functionalization of microcrystalline cellulose with a N-hydroxysuccinimide-activated ester of benzophenone, a photoreactive functional group. Irradiation of the final product with UV light yields a self-standing polymer film and is expected to result in cross-linking among cellulose chains. To monitor structural changes at the molecular level, each functionalization step is characterized by a multinuclear solid-state NMR approach. DNP-enhanced 15N CP MAS NMR experiments reveal the formation of the amide bond to the photoreactive linker and deliver further information about the binding situation of nitrogen-containing groups in these materials. The flexible synthesis protocol described here can be easily extended to a broad range of other functionalities of interest, both for the cellulose and macromolecular research.