Solid-state NMR Spectra Research Articles

Post-acquisition correction of NMR spectra distorted by dynamic and static field inhomogeneity of cryogen-free magnets

Post-acquisition correction of NMR spectra is an important part of NMR spectroscopy that enables refined NMR spectra to be obtained, clean from undesirable out-phasing, broadening and noising. We describe analytical and numerical mathematical methods for post-acquisition correction of NMR spectra distorted by static and dynamic magnetic field inhomogeneity caused by imperfections of main superconducting coils and the cold head operation, typical for cryogen-free magnets. For the dynamic inhomogeneity, we apply a variant of the general reference deconvolution method, complemented with our mathematical analysis of spectral parameters. For the static inhomogeneity, we apply the method of a delayed Fourier transform, also supported with our analytical calculations. We verify our approach by correction processing of high-field experimental liquid-state 1H NMR spectra of water and ethanol as well as solid-state 13C MAS NMR spectra of adamantane and obtain good results for both static and dynamic field distortions. This work complements our previous work on instrumental suppression of dynamic distortions caused by the cold head operation. The results presented contribute well to the general field of processing NMR spectra and serve towards a more extensive use of cryogen-free magnets in high-resolution NMR spectroscopy.

Modification of halloysite lumen with dopamine derivatives as filler for antibiofilm coating

HypothesisDevelopment of nanocomposite coating with antibiofilm properties is of fundamental importance to efficient fight biofilm formation preventing infections in biomedical area. In this context, halloysite nanotubes (HNTs), biocompatible and low-cost clay mineral, have been efficiently used as filler for different polymeric matrices affording several nanocomposites with appealing antimicrobial properties. The modification of HNTs surfaces represents a valuable strategy to improve the utilization of the clay for biological purposes. Experiments. Herein, the covalent modification of the HNTs lumen with properly designed dopamine derivatives with different perfluoroalkyl chain length is reported. The obtained nanomaterials are thoroughly characterized by several techniques. As proof of concept the antibiofilm properties on E. coli strain of the nanomaterials are assayed as well. Finally, the HNTs fillers were introduced into a polydopamine matrix allowing for the preparation of functional coatings, resistant to formation of microbial biofilms. FindingsAll characterization methods proved the selectivity of the modification and the increased hydrophobicity of the lumen. In particular 27Al solid state nuclear magnetic resonance (NMR) spectra showed a upfield shift of the Al signal. Studies on the antibiofilm properties highlighted different activities according to the length of perfluoroalkyl chains of organic molecules as proved by 19F solid state NMR spectra. The synthetized materials were promising for future application as coatings on medical implants.

Near-infrared-emitting silsesquioxane-based porous polymer containing thiophene for highly efficient adsorption and detection of iodine vapor and solution phase

The powerful gathering and potent detection of radioactive iodine, which may be created during the production of nuclear energy or the management of nuclear waste, are essential. Herein, near-infrared-emitting (NIR) porous polymeric materials with thiophene (PCS-DPBS) were employed for the first time to capture iodine vapor effectively and reversibly. The fluorophore monomer (CN-DPBS) with a donor–acceptor-donor (D-A-D) typed electronic configuration exhibiting near-infrared fluorescence emission was successfully produced by the Knoevenagel reaction. Then, CN-DPBS reacted with octavinylsilsesquioxane (OVS) via the Friedel-Crafts reaction, resulting in a series of porous polymers based on silsesquioxane (PCS-DPBS). The polymer was effectively synthesized, as evidenced by elemental analysis, FT-IR, solid-state 29Si NMR, and solid-state 13C NMR spectra. The morphology of PCS-DPBS was also demonstrated by XRD and SEM results. Interestingly, the resulting polymer has distinct properties and characteristics such as NIR emission (663 nm), adjustable porosity (specific surface area of 245 m2 g−1 and total pore volumes of 0.150 cm3 g−1), and thermal stability with Td5% and Td10% of 202 and 414 °C, respectively, in N2 atmosphere. With a large micropore volume and a high thiophene content in its polymer structure, this results in high iodine vapor adsorption capacities, with uptake capacities up to 2.81 g g−1 at 70 °C. The recycling ability is at least five times and the iodine uptake capacity is still >50%. The CN-DPBSunit provides PCS-DPBS with unique luminescence, allowing it to function as a probe to detect iodine solution with great sensitivity with KSV values of 1622.4 M−1. This study on a thiophene-containing adsorbent brings up new possibilities for the effective capture and detection of iodine in environment.

Non-covalent chemical functionalization of micron-sized styrene-butadiene rubber with silica particles via solid-state cryogenic mixing process

The effective, high-value reutilization of reclaimed rubber, obtained from end-of-life tires, in the production of new high-performance tires remains an environmental and technological challenge. Cryogenically ground micron-sized rubber particles demonstrate a significant promise to realize satisfactory physical performance measures in reclaimed rubber-based tires. However, the maximum useable content of the cryogenically ground micron-sized rubber particles to be incorporated into tires is strictly limited by their ineffective interfacial chemical interactions with the host pristine rubber matrix during the post-polymerization process. Here, this work presents the non-covalent chemical functionalization of the cryogenically ground micron-sized styrene-butadiene rubber particles with reactive silica particles via a solid-state cryogenic mixing process. The highly-scalable solid-state mixing process enables the sufficiently uniform and near-homogenous distribution of the silica particles on the micron-sized rubber particles. Scanning electron microscope images highlight the micron-sized rubber particles decorated with individual silica particles. Fourier transform infrared and solid-state nuclear magnetic resonance spectra of the functionalized micron-sized rubber particles demonstrate a non-covalent conjugation mechanism between the silica and rubber particles in which the chemical fingerprint of the prime rubber backbone chains remains chemically intact. The chemically functionalized cryogenically ground micron-sized rubber particles possess reactive silica particle sites that are ultimately designed to facilitate the participation of the recycled rubber particles in post-polymerization processes with host matrix which shall allow higher loading levels than the state-of-the-art configurations.

Green chemistry fabrication of durable antimicrobial peptide-immobilized silk fibroin films for accelerated full-thickness wound healing

Silk fibroin (SF) has been intensively explored for wound dressings due to its excellent biological and mechanical properties. However, forming SF films with a combination of durable and excellent antibacterial properties as well as biocompatibility is still a challenge. Herein, a marine-derived antimicrobial peptide, actinomycin X2 (Ac.X2), was immobilized onto SF fibers and Ac. X2-immobilized SF (ASF) films was further prepared using all green chemical approaches. The immobilized amount of Ac. X2 can be precisely quantified by its characteristic UV–visible absorption peak at 443 nm. Both Fourier transform infrared spectroscopy (FTIR) and solid-state 13C NMR spectra revealed that the lactone bond on the α/β rings of Ac. X2 was covalently grafted with the amino groups of SF, which greatly improved the degree of immobilization. The ASF film exhibited excellent antimicrobial activity against Staphylococcus aureus and Methicillin-resistant S.aureus (MRSA), with no biofilm formation. Furthermore, the ASF film showed a suitable degradation rate, good hemocompatibility, and reduced cytotoxicity in vitro biocompatibility assay. In vivo rat full-thickness wound healing experiment, the ASF film exhibited effectively promote skin regeneration, wound vascularization, and restore skin function compared with the control and SF wound dressings. These results suggest that the versatile ASF film would be a potential wound dressing for clinical application.

Zwitterionic or Not? Fast and Reliable Structure Determination by Combining Crystal Structure Prediction and Solid-State NMR.

When it comes to crystal structure determination, computational approaches such as Crystal Structure Prediction (CSP) have gained more and more attention since they offer some insight on how atoms and molecules are packed in the solid state, starting from only very basic information without diffraction data. Furthermore, it is well known that the coupling of CSP with solid-state NMR (SSNMR) greatly enhances the performance and the accuracy of the predictive method, leading to the so-called CSP-NMR crystallography (CSP-NMRX). In this paper, we present the successful application of CSP-NMRX to determine the crystal structure of three structural isomers of pyridine dicarboxylic acid, namely quinolinic, dipicolinic and dinicotinic acids, which can be in a zwitterionic form, or not, in the solid state. In a first step, mono- and bidimensional SSNMR spectra, i.e., 1H Magic-Angle Spinning (MAS), 13C and 15N Cross Polarisation Magic-Angle Spinning (CPMAS), 1H Double Quantum (DQ) MAS, 1H-13C HETeronuclear CORrelation (HETCOR), were used to determine the correct molecular structure (i.e., zwitterionic or not) and the local molecular arrangement; at the end, the RMSEs between experimental and computed 1H and 13C chemical shifts allowed the selection of the correct predicted structure for each system. Interestingly, while quinolinic and dipicolinic acids are zwitterionic and non-zwitterionic, respectively, in the solid state, dinicotinic acid exhibits in its crystal structure a "zwitterionic-non-zwitterionic continuum state" in which the proton is shared between the carboxylic moiety and the pyridinic nitrogen. Very refined SSNMR experiments were carried out, i.e., 14N-1H Phase-Modulated (PM) pulse and Rotational-Echo Saturation-Pulse Double-Resonance (RESPDOR), to provide an accurate N-H distance value confirming the hybrid nature of the molecule. The CSP-NMRX method showed a remarkable match between the selected structures and the experimental ones. The correct molecular input provided by SSNMR reduced the number of CSP calculations to be performed, leading to different predicted structures, while RMSEs provided an independent parameter with respect to the computed energy for the selection of the best candidate.

Quasi-Solid Polymer Electrolyte with Multiple Lithium-Ion Transport Pathways by In Situ Thermal-Initiating Polymerization.

Solid polymer electrolytes (SPEs) are considered to be attractive candidates for rechargeable batteries on account of their high safety and flexible processability. However, the restricted polymer segmental dynamics limit the Li+ conduction of SPEs. Herein, a composite electrolyte membrane was prepared via in situ thermal-initiating polymerization of diethylene glycol diacrylate (DEGDA) in a poly(vinylidene fluoride) frameworks (PVDF FMs) electrospun in advance. As a quasi-solid polymer electrolyte (QSPE), it provides multiple transport highways for Li+ built by the C═O or C-O or C═O/C-O groups in poly(diethylene glycol) diacrylate (PDEGDA), respectively, proved by density functional theory calculations together with the high-resolution 7Li solid-state nuclear magnetic resonance spectra. Since the interaction between Li+ and C═O is weaker than that between Li+ and C-O, Li+ tends to move along C═O dominating paths in PDEGDA/PVDF FMs QSPEs, even skipping back to C═O nodes from the original C-O dominating way. Multiple transport patterns facilitate Li+ migration within PDEGDA/PVDF FMs QSPEs, contributing to the ionic conductivity of 1.41 × 10-4 S cm-1 at 25 °C and the Li+ transference number of 0.454. Ascribing to the wetting capability of the monomer to the electrodes in use, compatible electrolyte/electrode interfaces with low interface resistance and compact cells were acquired by the in situ polymerization. Protective lithiated oligomers (RCOOLi) and LiF are enriched at the Li anode surface, promoting a lasting stable Li plating/stripping over 2000 h. By applying the QSPEs in LiFePO4 cell, a capacity of 157.7 mAh g-1 with almost 100% coulombic efficiency during 200 cycles is achieved at 25 °C.

Determining Local Magnetic Susceptibility Tensors in Paramagnetic Lanthanide Crystalline Powders from Solid-State NMR Chemical Shift Anisotropies

Exploring magnetic properties at the molecular level is a challenge that has been met by developing many experimental and theoretical solutions, such as polarized neutron diffraction (PND), muon-spin rotation (μ-SR), electron paramagnetic resonance (EPR), SQUID-based magnetometry measurements, and advanced modeling on open-shell systems and relativistic calculations. These methods are powerful tools that shed light on the local magnetic response in specifically designed magnetic materials such as contrast agents, for MRI, molecular magnets, magnetic tags for biological NMR, etc. All of these methods have their advantages and disadvantages. In order to complement the possibilities offered by these methods, we propose a new tool that implements a new approach combining simulation and fitting for high-resolution solid-state NMR spectra of lanthanide-based paramagnetic species. This method relies on a rigorous acquisition thanks to short high-power adiabatic pulses (SHAP) of high-resolution solid-state NMR isotropic and anisotropic data on a powdered magnetic material. It is also based on an efficient modeling of this data thanks to a semiempirical model based on a parametrization of the local magnetism and the crystal structure provided by diffraction methods. The efficiency of the calculation relies on a thorough simplification of the electron-nucleus interactions (point-dipole interaction, no Fermi contact) which is validated by experimental analysis. By taking advantage of the efficient calculation possibilities offered by our method, we can compare a great number of simulated spectra to experimental data and find the best-matching local magnetic susceptibility tensor. This method was applied to a series of isostructural lanthanide oxalates which are used as a benchmark system for many analytical methods. We present the results of thorough solid-state NMR and extensive modeling of the hyperfine interaction (including up to 400 paramagnetic centers) that yield local magnetic susceptibility tensor measurements that are self-consistent as well as consistent with bulk susceptibility measurements.

Lithium-Ion and Chloride-Ion Transfer in Chiral and Racemic 3-Hydroxyquinuclidinium Perchlorate: Influence of Defects and a Mixed Plastic Crystal Matrix

Organic ionic plastic crystals (OIPCs) are promising solid electrolytes. To observe the effect of defects and a mixed OIPC matrix on ionic conductivity, lithium-ion transfer and chloride-ion transfer in chiral and racemic (+/±)-3-hydroxyquinuclidinium perchlorate plastic crystals were studied. The pure chiral material and racemic mixture and their corresponding doped mixtures showed different thermal behaviors. Powder X-ray diffraction revealed that the cubic crystal structure appeared in both the high-temperature phase of the pure chiral material and the whole temperature range for the racemic system. Solid-state nuclear magnetic resonance spectra indicated the possible ion mobility of the two undoped matrices. The lithium-ion conductivity in the chiral material (above −10 °C) and racemic mixture (in the whole temperature region) increased 2.5–3 orders of magnitude, while the chloride-ion conductivity in the chiral material only increased obviously above 50 °C. Positron annihilation lifetime spectroscopy demonstrated the dependence of ionic conductivity on defect volumes of the pure chiral material and its doped mixtures, and the defects of the doped mixtures expanded largely. The ionic conductivity and defect volume of the doped mixtures containing lithium ions and chloride ions qualitatively obeyed the Cohen–Turnbull free volume theory.

Early stages and final results of amyloid-beta self-assembly.

Amyloid-β (Aβ) peptides self-assemble into a wide variety of states, ranging from partially structured oligomers comprising 2-12 molecules to polymorphic, highly structured, micron-long fibrils comprising roughly 5000 molecules. I will present results from several recent studies in which we have characterized structural features of oligomers in the earliest stages of Aβ self-assembly, using novel time-resolved solid state NMR and time-resolved light scattering measurements, and the full structures of 40- and 42-residue Aβ fibrils that develop in human brain tissue, using solid state NMR and cryoEM methods. In the time-resolved studies, we find that Aβ peptides develop β-strand secondary structure within 1 ms of the initiation of self-assembly by a rapid change in pH, when the average assembly is only a dimer. Remarkably, the solid state NMR spectra of freeze-trapped samples show very little change as the average oligomer size increases from 2 to >20 molecules over a 30 s period. In the structural studies of brain-derived fibrils, we find that the predominant brain-derived 40-residue Aβ40 fibril polymorph is nearly identical to one of the polymorphs that can be grown in vitro under quiescent conditions. In contrast, structures of two predominant 42-residue Aβ42 fibril polymorphs derived from brain tissue differ significantly from structures of in vitro polymorphs that have been solved previously by other labs.

Structure, topology, oligomerization and dynamics of the MHC II transmembrane domains and the role of highly specific shingomyelin interactions.

MHC class II receptors carry important function in adaptive immunity and their malfunctioning is associated with multiple diseases such as diabetes type I, inflammatory and autoimmune diseases. Investigations of the transmembrane domains of the DQ alpha-1 and DQ beta-1 type I membrane protein subunits and their interactions resulting in the formation of homo- and heterodimers were performed using oriented solid-state NMR spectroscopy. Two populations can be distinguished by their differential dynamics probably corresponding the mono- and homodimers. Furthermore, the DQ alpha 1 chain carries a sequence motif that has been identified to specifically interact with sphingomyelin-C18 (SM-C18). Whereas this monomer-dimer equilibrium is hardly affected by the addition of 5 mole% SM-C18 a single population is visible in DMPC lipid bilayers suggesting that the lipid saturation is an important parameter. When the transmembrane domains were reconstituted in POPC/SM-C18 lipid bilayers where the fatty acyl chain of either the phosphatidylcholine or of the sphingolipid have been deuterated the resulting 2H solid-state NMR spectra show unexpected details on the protein-lipid interactions. The data are suggestive that SM-C18 together with the transmembrane domains form structures imposing positive curvature strain on the surrounding POPC lipids. This effect is attenuated when SM-C18 is recognized by the protein. The ensemble of data suggests a mechanisms of membrane protein regulation driven by the physico-chemical characteristics of the membrane rather than string direct interactions.

Effect of the Amine Hydrochloride Salt on the Antifogging Properties of Amino-Functionalized Polysilsesquioxane

A poly[3-(2-aminoethylamino)propyl]silsesquioxane (PAEAPS) film was immersed in 0.5 mol/L hydrogen chloride (HCl) methanolic solution and a poly[3-(2-aminoethylamino)propyl]silsesquioxane hydrochloride (PAEAPS-Cl) film containing the amine hydrochloride salt was prepared for the development of high-performance antifogging materials based on polysilsesquioxane. The PAEAPS-Cl film exhibited a higher water uptake (WU) of 29% and scratch resistance of 4.3 estimated by a nanoindenter than the PAEAPS film (WU = 22% and scratch resistance = 3.2). The introduction of the amine hydrochloride salt structure increased the antifogging properties and surface hardness. 29Si solid-state nuclear magnetic resonance (NMR) spectra indicated that the structure of the siloxane network in the PAEAPS film drastically changed upon hydrolysis by amino groups after exposure to water vapor at 40 °C and 95% relative humidity for 1 h. In the PAEAPS-Cl film, the cleavage of the siloxane network was prevented by the introduction of the amine hydrochloride salt as the neutral group, but the amine hydrochloride salt sometimes induced the morphological change of the PAEAPS-Cl film because of strong hydration and cracks and dimples were observed after exposure to water vapor at 40 °C and 95% relative humidity for 1 h. The poly[3-(2-aminoethylamino)propyl-co-methyl]silsesquioxane hydrochloride (P(AEAP-co-Me)S-Cl) film was prepared by the hydrolysis and polycondensation of 3-(2-aminoethylaminopropyl)triethoxysilane and methyltriethoxysilane, followed by immersion in 0.5 mol/L HCl methanolic solution. The obtained P(AEAP-co-Me)S-Cl film exhibited a WU of 17%, lower than those of the PAEAPS and PAEAPS-Cl films, but cracks and dimples were not observed in the P(AEAP-co-Me)S-Cl film even after exposure to water vapor at 40 °C and 95% relative humidity for 1 h. Furthermore, the introduction of the amine hydrochloride salt structure did not decrease transparency and the PAEAPS-Cl and P(AEAP-co-Me)S-Cl films showed high transparency, similar to the PAEAPS film.

Atomic Resolution Insights into pH Shift Induced Deprotonation Events in LS-Shaped Aβ(1-42) Amyloid Fibrils.

Alzheimer's disease is a neurodegenerative disorder associated with the deposition of misfolded aggregates of the amyloid-β protein (Aβ). Aβ(1-42) is one of the most aggregation-prone components in senile plaques of AD patients. We demonstrated that relatively homogeneous Aβ(1-42) fibrils with one predominant fold visible in solid-state NMR spectra can be obtained at acidic pH. The structure of these fibrils differs remarkably from some other polymorphs obtained at neutral pH. In particular, the entire N-terminal region is part of the rigid fibril core. Here, we investigate the effects of a pH shift on the stability and the fold of these fibrils at higher pH values. Fibril bundling at neutral pH values renders cryo-EM studies impractical, but solid-state NMR spectroscopy, molecular dynamics simulations, and biophysical methods provide residue-specific structural information under these conditions. The LS-fold of the Aβ(1-42) fibrils does not change over the complete pH range from pH 2 to pH 7; in particular, the N-terminus remains part of the fibril core. We observe changes in the protonation state of charged residues starting from pH 5 on a residue-specific level. The deprotonation of the C-terminal carboxyl group of A42 in the intermolecular salt bridge with D1 and K28 is slow on the NMR time scale, with a local pKa of 5.4, and local conformations of the involved residues are affected by deprotonation of A42. Thus, we demonstrate that this fibril form is stable at physiological pH values.

Pure Isotropic Proton NMR Spectra in Solids using Deep Learning.

The resolution of proton solid-state NMR spectra is usually limited by broadening arising from dipolar interactions between spins. Magic-angle spinning alleviates this broadening by inducing coherent averaging. However, even the highest spinning rates experimentally accessible today are not able to completely remove dipolar interactions. Here, we introduce a deep learning approach to determine pure isotropic proton spectra from a two-dimensional set of magic-angle spinning spectra acquired at different spinning rates. Applying the model to 8 organic solids yields high-resolution 1 H solid-state NMR spectra with isotropic linewidths in the 50-400 Hz range.

An investigation into the adsorption mechanism of n-butanol by ZIF-8: a combined experimental and ab initio molecular dynamics approach.

The zeolitic imidazolate framework, ZIF-8, has been shown by experimental methods to have a maximum saturation adsorption capacity of 0.36 g g-1 for n-butanol from aqueous solution, equivalent to a loading of 14 butanol molecules per unit cell or 7 molecules per sodalite β-cage. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) shows the presence of hydrogen bonding between adsorbed butanol molecules within the cage; the presence of three different O-H stretching modes indicates the formation of butanol clusters of varying size. Ab initio molecular dynamics simulations show the formation of intermolecular hydrogen bonding between the butanol molecules, with an average hydrogen-bond coordination number of 0.9 after 15 ps simulation time. The simulations also uniquely demonstrate the presence of weaker interactions between the alcohol O-H group and the π-orbital of the imidazole ring on the internal surface of the cage during early stages of adsorption. The calculated adsorption energy per butanol molecule is -33.7 kJ mol-1, confirming that the butanol is only weakly bound, driven primarily by the hydrogen bonding. Solid-state MAS NMR spectra suggest that the adsorbed butanol molecules possess a reasonable degree of mobility in their adsorbed state, rather than being rigidly held in specific sites. 2D 13C-1H heteronuclear correlation (HETCOR) experiments show interactions between the butanol aliphatic chain and the ZIF-8 framework experimentally, suggesting that O-H interactions with the π-orbital are only short lived. The insight gained from these results will allow the design of more efficient ways of recovering and isolating n-butanol, an important biofuel, from low-concentration solutions.

Solid-state NMR chemical shift analysis for determining the conformation of ATP bound to Na,K-ATPase in its native membrane.

Structures of membrane proteins determined by X-ray crystallography and, increasingly, by cryo-electron microscopy often fail to resolve the structural details of unstable or reactive small molecular ligands in their physiological sites. This work demonstrates that 13C chemical shifts measured by magic-angle spinning (MAS) solid-state NMR (SSNMR) provide unique information on the conformation of a labile ligand in the physiological site of a functional protein in its native membrane, by exploiting freeze-trapping to stabilise the complex. We examine the ribose conformation of ATP in a high affinity complex with Na,K-ATPase (NKA), an enzyme that rapidly hydrolyses ATP to ADP and inorganic phosphate under physiological conditions. The 13C SSNMR spectrum of the frozen complex exhibits peaks from all ATP ribose carbon sites and some adenine base carbons. Comparison of experimental chemical shifts with density functional theory (DFT) calculations of ATP in different conformations and protein environments reveals that the ATP ribose ring adopts an C3'-endo (N) conformation when bound with high affinity to NKA in the E1Na state, in contrast to the C2'-endo (S) ribose conformations of ATP bound to the E2P state and AMPPCP in the E1 complex. Additional dipolar coupling-mediated measurements of H-C-C-H torsional angles are used to eliminate possible relative orientations of the ribose and adenine rings. The utilization of chemical shifts to determine membrane protein ligand conformations has been underexploited to date and here we demonstrate this approach to be a powerful tool for resolving the fine details of ligand-protein interactions.

Fine control for the preparation of ceria nanorods (111).

The morphologies and exposed surfaces of ceria nanocrystals are important factors in determining their performance. In order to establish a structure-property relationship for ceria nanomaterials, it is essential to have materials with well-defined morphologies and specific exposed facets. This is also crucial for acquiring high resolution 17O solid-state NMR spectra. In this study, we explore the synthesis conditions for preparing CeO2 nanorods with exposed (111) facets. The effects of alkali concentration, hydrothermal temperature and time, cerium source and oxidation agent are investigated and optimal synthesis conditions are found. The resulting CeO2 nanorods show very narrow 17O NMR peaks for the oxygen ions in the first, second and third layers, providing a foundation for future research on mechanisms involving ceria materials using 17O solid-state NMR spectroscopy.

Solid-state NMR - a complementary technique for protein framework characterization.

Protein frameworks are an emerging class of biomaterial with medical and technological applications. Frameworks are studied mainly by X-ray diffraction or scattering techniques. Complementary strategies are required. Here, we report solid-state NMR analyses of a microcrystalline protein-macrocycle framework and the rehydrated freeze-dried protein. This methodology may aid the characterization of low-crystallinity frameworks.

Solid-state NMR spectroscopy of roasted and ground coffee samples: Evidences for phase heterogeneity and prospects of applications in food screening

The advancement in the use of spectroscopic techniques to investigate coffee samples is of high interest especially considering the widespread problems with coffee adulteration and counterfeiting. In this work, the use of solid-state nuclear magnetic resonance (NMR) is investigated as a means to probe the various chemically-distinct phases existent in roasted coffee samples and to detect the occurrence of counterfeiting or adulterations in coffee blends. Routine solid-state 1H and 13C NMR spectra allowed the distinction between different coffee types (Arabica/Robusta) and the evaluation of the presence of these components in coffee blends. On the other hand, the use of more specialized solid-state NMR experiments revealed the existence of phases with different molecular mobilities (e.g., associated with lipids or carbohydrates). The results illustrate the usefulness of solid-state NMR spectroscopy to examine molecular mobilities and interactions and to aid in the quality control of coffee-related products.

Pulse sequence induced variability combined with multivariate analysis as a potential tool for 13C solid-state NMR signals separation, quantification, and classification

Multivariate Curve Resolution (MCR) is a multivariate analysis procedure commonly used to analyze spectroscopic data providing the number of components coexisting in a chemical system, the pure spectra of the components as well as their concentration profiles. Usually, this procedure relies on the existence of distinct systematic variability among spectra of the different samples, which is provided by different sources of variation associated to differences in samples origin, composition, physical chemical treatment, etc. In solid-state NMR, MCR has been also used as a post-processing method for spectral denoising or editing based on a given NMR property. In this type of use, the variability is induced by the incrementation of a given parameter in the pulse-sequence, which encodes the separation property in the acquired spectra. In this article we further explore the idea of using a specific pulse sequence to induce a controlled variability in the 13C solid-state NMR spectra and then apply MCR to separate the pure spectra of the components according to the properties associated to the induced variability. We build upon a previous study of sugarcane bagasse where a series of 13C solid-state NMR spectra acquired with the Torchia-T1 CPMAS pulse sequence, with varying relaxation periods, was combined with different sample treatments, to estimate individual 13C solid-state NMR spectra of different molecular components (cellulose, xylan and lignin). Using the same pulse sequence, we show other application examples to demonstrate the potentiality, parameter optimization and/or establish the limitations of the procedure. As a first proof of principle, we apply the approach to commercial semicrystalline medium density polyethylene (MDPE) and polyether ether ketone (PEEK) providing the estimation of the individual 13C ssNMR spectra of the polymer chains in the amorphous (short T1) and crystalline (long T1) domains. The analysis also provided the relative intensities of each estimated pure spectra, which are related to the characteristic T1 decays of the amorphous and crystalline domain fractions. We also apply the analysis to isotactic poly (1-butene) (iPB-I) as an example in which the induced T1 variability occurs due to the mobility difference between the polymer backbone and side-chains. A jack-knifing procedure and a student t text allow us to stablish the minimum number of spectra and the range of relaxation periods that need to be used to achieve a precise estimation of the individual pure spectra and their relative intensities. A detail discussion about possible drawbacks, applications to more complex systems, and potential extensions to other type of induced variability are also presented.