O NMR methods are emerging as a powerful tool for determination of structure and dynamics in materials and biological solids. We present experimental and theoretical frameworks for measurements of O NMR relaxation times in static solids focusing on the excitation of the central transition of the O spin 5/2 system. We employ O-enriched NaNO as a model compound, in which the nitrate oxygen atoms undergo 3-fold jumps. Rotating frame (T), transverse (T) and longitudinal (T) relaxation times as well as line shapes were measured for the central transition in the 280 to 195 K temperature range at 14.1 and 18.8 T field strengths. We conduct experimental and theoretical comparison between different relaxation methods and demonstrate the advantage of combining data from multiple relaxation time and line shape measurements to obtain a more accurate determination of the dynamics as compared to either of the techniques alone. The computational framework for relaxation of spin 5/2 nuclei is developed using the numerical integration of the Liouville - von Neumann equation.

Cellulose-lignin blends are proposed as alternative precursors for carbon fiber (CF) production, offering a potential sustainable and cost-effective alternative to the expensive fossil-based polymers currently used. The characteristics of the precursor fibers including their crystallinity, the incorporated chemical structures and the distribution of the biopolymers have a significant influence on their carbonization behavior and the properties of the CFs. They are partly determined by the composition of the bio-based resources and the conditions used during the fiber fixation, i.e. the coagulation, an important processing step. In this work, C solid and 2D solution NMR methodologies were applied to investigate the impact of coagulation and thermostabilization conditions on cellulose and cellulose-lignin blends using a thin film model. Solid state NMR spectroscopy showed that the choice of the anti-solvent influenced the proportion of cellulose II versus amorphous regions in the coagulated films. Independent of the presence of lignin, the choice of anti-solvent seems to impact the rate of thermal reactions. After thermostabilization at 245 °C, the samples were investigated using a solution NMR protocol devised for cellulosic materials. At 275 °C, most of the samples became insoluble for solution NMR. However, solid state NMR revealed further changes in the chemical composition, which were dependent on both the presence of lignin and the choice of anti-solvent. This multi-faceted approach combining solid state and 2D solution NMR techniques provides a comprehensive understanding of the cellulose structure and the products formed for cellulose-lignin-based CFs, which is crucial for optimizing their properties and potential applications.

The molecular structure of hydrochars produced from C-enriched glucose under various conditions has been elucidated based on advanced one- and two-dimensional (2D) H-C and C-C solid-state nuclear magnetic resonance (NMR) with spectral editing. Regardless of synthesis conditions, hydrochars consist mostly of oxygen-substituted arene rings (including diphenols) and furans connected by alkyl linkers rich in ketones. Cross-linking nonprotonated and methyne (C-H) alkyl carbons have been identified through spectrally edited 2D NMR. Alkenes and 'quaternary' C-O are observed only at low synthesis temperature, while some clusters of fused arene rings are generated at high temperature. Hydrochar composition is nearly independent of reaction time in the range from 1 to 5 h. Equilibration of C magnetization within 1 s shows that the materials are homogeneous on the 5-nm scale, refuting core-shell models of hydrochar microspheres. While furan C-O carbons bonded to alkyl groups or ketones show distinctive cross peaks in 2D NMR, phenolic C-OH is observed unambiguously by hydroxyl-proton selection. While methylene-linked furan rings are fairly common, the signal previously assigned to furan Cα-Cα linkages is shown to arise from abundant, stable catecholic ortho-diphenols, whose HO-C=C-OH structure is proved by 2DC-C NMR after hydroxyl-proton selection. Quantitative C NMR spectra of low- and high-temperature hydrochars have been matched by chemical-shift simulations for representative structural models. Mixed phenol and furan rings connected by ketones and alkyl linkers provide good fits of the experimental spectra, while literature models dominated by large clusters of fused rings and with few phenols or alkyl-linked ketones do not.

Among the many natural biomaterials for which information on atomic-level structure and reorientational motion can offer essential clues to function, insoluble multi-component composites with limited degrees of order are among the most challenging to study. Despite its limited sensitivity, solid-state NMR (ssNMR) is often the technique of choice to ferret out these details in carbon- and nitrogen-rich materials: this spectroscopic approach can probe many biomaterials in their native or near-native states, either with or without the introduction of stable NMR-active isotopes, or with the assistance of dynamic nuclear polarization technology. During a span of close to four decades, such research targets and ssNMR approaches have been exemplified by insects, a diverse and evolutionarily agile group of organisms with global impacts that include ecology, agriculture, and human disease. In this short review, we present case studies on insect cuticles that range from protective exoskeletons and egg capsules to the wing structures that enable flight and showcase nature's awe-inspiring beauty, highlighting the use of ssNMR spectroscopy to profile chemical composition, elucidate macromolecular architecture, and monitor metabolic development in these fascinating biological assemblies.

People with the genetic disease cystic fibrosis (CF) often have chronic airway infections and produce airway secretions called sputum. A better understanding of sputum composition is desired in order to track changes in response to CF therapeutics and to improve laboratory models for the study of CF airway infections. The glycosylated protein mucin is a primary component. Along with extracellular DNA, mucin gives rise to the high viscoelasticity of sputum, which inhibits airway clearance and is thought to promote chronic airway infections in people with CF. Past studies of sputum composition identified additional biomolecular components of sputum including other proteins, both glycosylated and not glycosylated, free amino acids, and lipids. Typically, studies of sputum, as well as other complex biological materials, have focused on soluble or isolated components. Solid-state NMR is not limited to the study of soluble components. Instead, it can provide molecular-level information about insoluble biological samples. Additionally, solid-state NMR can provide information about sample composition without requiring any processing of the sample, eliminating the possibility of misestimating certain components due to insolubility or potential sample loss in isolation steps. In this study, we used both C and P CPMAS to investigate the total composition of sputum samples obtained from six people with CF. We compared these spectra to those of commercially available mucin, DNA, and phospholipid samples. Lastly, we performed complementary biochemical analyses to identify specific proteins present in the sputum samples. Overall, our findings provide insight into the composition of unprocessed sputum samples from people with CF, which can be used as a benchmark for future investigations of CF and infections in the airways of people with CF. Further, this study provides opportunities to expand the solid-state NMR approach to include dynamic nuclear polarization (DNP) to obtain high-resolution information of sputum and similar biological samples that are not feasible to isotopically enrich.

Planewave-corrected methods have proven effective for accurately modeling nuclear magnetic resonance (NMR) parameters in crystalline systems. Recent work extended the application of planewave-corrected calculations beyond the second row, predicting EFG tensor parameters for Cl using a simple molecular correction to projector augmented-wave (PAW) density functional theory (DFT). Here we extend this work using fragment and cluster-based calculations coupled with polarizable continuum (PCM) methods to improve further the accuracy of planewave-corrected Cl EFG tensor calculations. Benchmark data from a test set comprised of 105 individual Cl EFG tensor principal components for chlorine-containing molecular crystals and crystalline chloride salts shows fragment-corrected planewave calculations using the PBE0 hybrid density functional improve the accuracy of predicted EFG tensor components by 30 % relative to traditional planewave calculations. We compare the influence of different geometry optimization methods and density functionals on the accuracy of predicted Cl EFG tensor parameters. Four cases of spectral assignment are presented to demonstrate the utility of improving the accuracy of predicted Cl EFG tensor parameters.

While syringyl units are the most abundant monolignols in hardwood lignin, their phenolic (i.e. hydroxyl) end group concentration has not been measured. In two uniformly C-enriched young hardwoods, from beech and oak, the syringyl units were quantitatively investigated by advanced solid-state C NMR. Small signals of OH-terminated syringyl units were resolved in spectrally edited two-dimensional C-C NMR spectra of the two hardwoods. Their distinct peak positions predicted based on literature data were validated via the abundant OH-terminated syringyl units in hydrolyzed C-beechwood. In a two-dimensional C-C exchange spectrum with diagonal-ridge suppression, a well-resolved peak of phenolic syringyl units was observed at the characteristic C-H peak position of syringyl rings, without significant overlap from guaiacyl peaks. Accurate C chemical shifts of regular and end-group syringyl units were obtained. Through spectrally edited 2D NMR after H inversion recovery, phenols of condensed tannin complexed with arginine were carefully analyzed and shown to overlap minimally with signals from phenolic syringyl units. The local structure and resulting spin dynamics of ether (chain) and hydroxyl (end-group) syringyl units are nearly the same, enabling quantification by peak integration or deconvolution, which shows that phenolic syringyl end groups account for 2 ± 1 % of syringyl units in beechwood and 5 ± 2 % in oakwood. The observed low end-group concentration needs to be taken into account in realistic molecular models of hardwood lignin structure.

This study builds upon our prior researches and seeks to investigate and clarify the influences of various characteristics of hydrogen bonds (H-bonds) and charge transfer (CT) interactions, which were detected within the inhibitor binding pockets (labeled as the QM models I-IV) of MraY-capuramycin, MraY-carbacaprazamycin, MraY-3'-hydroxymureidomycin A, and MraY-muraymycin D2 complexes by QTAIM and NBO analyses from DFT QM/MM MD calculations, on the O chemical shielding (CS) and electric field gradient (EFG) tensors of carboxylate (Oδ), carbonyl (C═O), and hydroxyl (O-H) oxygens in these models. The O CS and EFG tensors of these three types of oxygens in QM models I-IV were calculated at the M06-2X/6-31G** level by including the solvent effects using the polarizable continuum model. From the computed O CS and EFG tensors in these models, it was found that the nuclear shielding, σ, for carboxylate or carbonyl oxygen increases (shielding effect) as the H-bond length decreases and the percentage p-character of n/n lone pair partner in the CT interaction enhances. In contrast, the σ (O-H) decreases (deshielding effect) with a reduction in the H-bond length as well as with an enhancement in percentage s-character of the n lone pair/σ* antibond. By reducing the H-bond length or by increasing p-character of the n/n lone pair, the Oδ/O═C quadrupole coupling constant smoothly decreases, while the Oδ/O═C asymmetry parameter smoothly increases. Moreover, these calculated parameters are in a good agreement with the experimental values. The information garnered here is valuable particularly for further understanding of empirical correlations between O NMR spectroscopic and H-bonding characteristics in the protein-ligand complexes.

In this work, we elucidated the structural organization of stimuli-responsive peptide-polydiacetylene (PDA) conjugates that can self-assemble as 1D nanostructures under neutral aqueous conditions. The amino acid sequences bear positively or negatively charged domains at the periphery of the peptide segments to promote solubility in water while also driving assembly of the individual and combined components into β-sheets. The photopolymerization of PDA, as well as the sensitivity of the resulting optical properties of the polymeric material to external stimuli, highly depends on the structural organization of the assembly of amphiphilic peptide-diacetylene units into 1D-nanostructures. Solid-state NMR measurements on C-labeled and N-labeled samples show that positively charged and negatively charged peptide amphiphiles are each capable of self-assembly, but self-assembly favors antiparallel β-sheet structure. When positively and negatively charged peptide amphiphiles interact in stoichiometric solutions, cooperative coassembly dominates over self-assembly, resulting in the desired parallel β-sheet structure with a concomitant increase in structural order. These results reveal that rational placement of oppositely charged residues can control β-strand organization in a peptide amphiphile coassembly, which would have implications on the adaptive properties of stimuli-responsive biomaterials such as the peptide-PDAs studied here.

Through-space heteronuclear correlation experiments under magic-angle spinning (MAS) conditions can provide unique insights into inter-atomic proximities. In particular, it has been shown that experiments based on two consecutive coherence transfers, H → I → H, like D-HMQC (dipolar-mediated heteronuclear multiple-quantum correlation), are usually more sensitive for the indirect detection via protons of spin-3/2 quadrupolar nuclei with low gyromagnetic ratio. Nevertheless, the resolution is often decreased by the second-order quadrupolar broadening along the indirect dimension. To circumvent this issue, we incorporate an MQMAS (multiple-quantum MAS) quadrupolar filter into the t evolution period of the D-HMQC sequence, which results in a novel pulse sequence called D-HMQC-MQ. The triple-quantum coherences evolving during this filter are excited and reconverted using cosine-modulated long-pulses synchronized with the sample rotation to avoid spinning sidebands in the indirect dimension. The desired coherence transfer pathways during this sequence are selected using two nested cogwheel phase cycles with 56 steps. This high-resolution heteronuclear correlation technique is demonstrated experimentally for the indirect detection via H of spin-3/2 isotopes, such as B, Na and Cl, in zinc borate hydrate, NaHPO and l-histidine hydrochloride, respectively. We show that this experiment can be applied at high magnetic fields up to 28.2 T for protons subject to chemical shift anisotropies larger than 20 ppm, provided the MAS frequency is sufficiently stable since the D-HMQC-MQ experiment, like the parent D-HMQC, is sensitive to MAS fluctuations, which can produce t-noise.

We present a high-resolution magic-angle spinning (MAS) solid-state NMR (ssNMR) study to characterize nontuberculous mycobacteria (NTM). We studied two different NTM strains, Mycobacterium smegmatis, a model, non-pathogenic strain, and Mycobacterium abscessus, an emerging and important human pathogen. Hydrated NTM samples were studied at natural abundance without isotope-labelling, as whole-cells versus cell envelope isolates, and native versus fixed sample preparations. We utilized 1DC and 2D H-C ssNMR spectra and peak deconvolution to identify NTM cell-wall chemical sites. More than ∼100 distinct C signals were identified in the ssNMR spectra. We provide tentative assignments for ∼30 polysaccharides by using well resolved H/C chemical shifts from the 2D INEPT-based H-C ssNMR spectrum. The signals originating from both the flexible and rigid fractions of the whole-cell bacteria samples were selectively analyzed by utilizing either CP or INEPT based C ssNMR spectra. CP buildup curves provide insights into the dynamical similarity of the cell-wall components for NTM strains. Signals from peptidoglycan, arabinogalactan and mycolic acid were identified. The majority of the C signals were not affected by fixation of the whole cell samples. The isolated cell envelope NMR spectrum overlap with the whole-cell spectrum to a large extent, where the latter has more signals. As an orthogonal way of characterizing these bacteria, electron microscopy (EM) was used to provide spatial information. ssNMR and EM data suggest that the M. abscessus cell-wall is composed of a smaller peptidoglycan layer which is more flexible compared to M. smegmatis, which may be related to its higher pathogenicity. Here in this work, we used high-resolution 2D ssNMR first time to characterize NTM strains and identify chemical sites. These results will aid the development of structure-based approaches to combat NTM infections.

Solid-state NMR has great potential for investigating molecular structure, dynamics, and organization of the stratum corneum, the outer 10-20 μm of the skin, but is hampered by the unfeasibility of isotope labelling as generally required to reach sufficient signal-to-noise ratio for the more informative multidimensional NMR techniques. In this preliminary study of pig stratum corneum at 35 °C and water-free conditions, we demonstrate that cryogenic probe technology offers sufficient signal boost to observe previously undetectable minor resonances that can be uniquely assigned to fluid cholesterol, ceramides, and triacylglycerols, as well as enables H-H spin diffusion monitored by 2D H-C HETCOR to estimate 1-100 nm distances between specific atomic sites on proteins and lipids. The new capabilities open up for future multidimensional solid-state NMR studies to answer long-standing questions about partitioning of additives, such as pharmaceutically active substances, between solid and liquid domains within the protein and lipid phases in the stratum corneum and the lipids of the sebum.

A recently developed dynamic mean-field theory for disordered spins (spinDMFT) is shown to capture the spin dynamics of nuclear spins very well. The key quantities are the spin autocorrelations. In order to compute the free induction decay (FID), pair correlations are needed in addition. They can be computed on spin clusters of moderate size which are coupled to the dynamic mean fields determined in a first step by spinDMFT. We dub this versatile approach non-local spinDMFT (nl-spinDMFT). It is a particular asset of nl-spinDMFT that one knows from where the contributions to the FID stem. We illustrate the strengths of nl-spinDMFT in comparison to experimental data for CaF. Furthermore, spinDMFT provides the dynamic mean fields explaining the FID of the nuclear spins of C in adamantane up to some static noise. The spin Hahn echo in adamantane is free from effects of static noise and agrees excellently with the spinDMFT results without further fitting.

Energy transfer from Zeeman to dipolar order discovered by Jeener et al. is usually observed in solids with a strong dipole-dipole interaction of nuclear spins. It is not observed in liquids, where fast molecular motion completely averages this interaction. The intermediate case, when the dipole-dipole interaction of nuclear spins is only partially averaged, has been poorly studied. We report on the first measurement of an angular-dependent proton spin relaxation of a dipolar reservoir in mobile water molecules confined in the interlayer pores of a vermiculite single crystal. In this layered crystal, the intramolecular dipole-dipole interactions of nuclear spins are only partially averaged due to the restricted anisotropic molecular motion in nanopores. We show that this allows the formation of dipolar echo. We measured the spin-lattice relaxation times of the dipolar order T at different angles between the normal to the crystal surface and the applied magnetic field and obtained a distinct angular dependence of T. The minimum relaxation rate R was found around the magic angle of 54.74°.

A numerical simulation method, namely, SDNMR-WEBFIT, is reported for simulating proton spin diffusion NMR based on the Levenberg-Marquardt algorithm and a pseudo-2D diffusion model. This method is used for the precise quantification of dynamics heterogeneity of the interphase within multiphase polymer systems. The numerical simulation method provides measurements of spin-lattice relaxation time (T), proton density (ρ), lamellar thickness (d), and spin diffusion coefficient (D) for each component. The pseudo-2D diffusion model is employed to simulate the proton spin diffusion build-up/decay curves, simultaneously calculating the lateral fraction of island-like structures (x-ratio). Such approach was successfully applied to various polymer systems, such as semi-crystalline polymer (Poly(ε-caprolactone), PCL), block copolymers (Styrene-butadiene-styrene triblock copolymer, SBS), and plasticized semi-polymers (Polvinyl alcohol, PVA).

We report solid-state H, C, and O NMR determination of hyperfine coupling tensors (A-tensors) in several paramagnetic Cu(II) (d, S = 1/2) complexes: trans-Cu(DL-Ala)·HO, Cu([1-C]acetate)·HO, Cu([2-C]acetate)·HO, and Cu(acetate)·HO. Using these new experimental results and some A-tensor data available in the literature for trans-Cu(L-Ala) and KCuCl·2HO, we were able to examine the accuracy of A-tensor computation from a periodic DFT method implemented in the BAND program. We evaluated A-tensors on H (I = 1/2), C (I = 1/2), N (I = 1), O (I = 5/2), K (I = 3/2), Cl (I = 3/2), and Cu (I = 3/2) nuclei over a range spanning more than 3 orders of magnitude. We found that the BAND code can reproduce reasonably well the experimental results for both A-tensors and nuclear quadrupole coupling tensors.

Under normal experimental conditions in an achiral environment, NMR spectra of enantiomers have chemical shifts and J couplings which are not differentiable. In this work, the reproducibility of spectral intensities for pairs of amino acid enantiomers, as well as factors influencing these intensities, is assessed using C and N cross-polarization magic-angle spinning (CP/MAS) NMR spectroscopy. Prompted by a recent literature debate over a possible influence of the chirality-induced spin selectivity (CISS) effect on spectral intensities obtained in CP/MAS NMR experiments carried out on enantiomers, a number of control experiments were performed with recycle delays of at least 5T. These included the analysis of proton-decoupled Bloch decay solid-state NMR spectra as well as solution NMR spectra where the cross polarization process is absent. Bloch decay and CP/MAS NMR spectra yield the same relative intensities for pairs of enantiomers while solution NMR spectra provide relative intensities closest to unity. Differences of plus-or-minus a few percent in the D/L spectral intensity ratios observed in all solid-state NMR experiments are due to sample preparation (i.e., grinding, particle size, partial amorphization) and limitations on sample purity. As previously described in the literature, more drastic intensity differences on the order of 50% are easily created by ball milling the samples. Finally, apodization is shown to invert the apparent D/L ratio in low signal-to-noise N CP/MAS NMR spectra of aspartic acid enantiomers. In summary, no spectral intensity differences attributable to enantiomerism are identified.

The NMR lineshapes produced by half-integer quadrupolar nuclei are sensitive to 11 distinct fit parameters per inequivalent site. To date, automatic fitting routines have failed to replace manual parameter insertion and evaluation due to the importance of local minima and the need for fitting multiple-field magic-angle spinning (MAS) and static spectra simultaneously. Herein we introduce a new tool, AMES-Fit (Automatic Multiple Experiment Simulation and Fitting), to automatically find the global best-fit simulation parameters for a series of multiple-field NMR lineshapes. AMES-Fit uses an adaptive step size random search algorithm to dynamically probe parameter space and requires minimal human input. The best fits are obtained in a few minutes of computation time that would otherwise have required several person-hours of work. The program is freely available and open-source.

N NMR of magnetically oriented microcrystals is reported. With a home-built H-C-N probe capable of modulating the rotation of the sample around the axis normal to the magnetic field, magnetically oriented microcrystal suspension (MOMS) of l-alanine is made. N NMR spectra acquired with various timings during intermittent rotation lead to a rotation pattern of the MOMS similar to that of a single crystal. The effect of orientational distribution of the microcrystals to broadening of the resonance line is discussed.

The development of NMR crystallography methods requires a reliable database of chemical shifts measured for systems with known crystal structure. We measured and assigned carbon and hydrogen chemical shifts of twenty solid natural amino acids of known polymorphic structure, meticulously determined using powder X-ray diffraction. We then correlated the experimental data with DFT-calculated isotropic shieldings. The small size of the unit cell of most amino acids allowed for advanced computations using various families of DFT functionals, including generalized gradient approximation (GGA), meta-GGA and hybrid DFT functionals. We tested several combinations of functionals for geometry optimizations and NMR calculations. For carbon shieldings, the widely used GGA functional PBE performed very well, although an improvement could be achieved by adding shielding corrections calculated for isolated molecules using a hybrid functional. For hydrogen nuclei, we observed the best performance for NMR calculations carried out with structures optimized at the hybrid DFT level. The high fidelity of the calculations made it possible to assign additional signals that could not be assigned based on experiments alone, for example signals of two non-equivalent molecules in the unit cell of some of the amino acids.

Double-rotation (DOR) solid-state NMR spectroscopy is a high-resolution technique developed in the late 1980s. Although multiple-quantum magic-angle spinning (MQMAS) became the most widely used high-resolution method for half-integer spin quadrupoles after 1995, development and application of DOR NMR to a variety of chemical and materials science problems has endured. This Trend article recapitulates the development of DOR NMR, discusses various applications, and describes possible future directions. The main technical limitations specific to DOR NMR are simply related to the size of the double rotor system. The relatively large outer rotor (and thus coil) used for most applications over the past 35 years translates into relatively low rotor spinning frequencies, a low filling factor, and weak radiofrequency powers available for excitation and for proton decoupling. Ongoing developments in NMR instrumentation, including ever-shrinking MAS rotors and spherical NMR rotors, could solve many of these problems and may augur a renaissance for DOR NMR.