Carbon nanothreads are a new one-dimensional sp3 carbon nanomaterial. They assemble into hexagonal crystals in a room temperature, nontopochemical solid-state reaction induced by slow compression of ...benzene to 23 GPa. Here we show that pyridine also reacts under compression to form a well-ordered sp3 product: C5NH5 carbon nitride nanothreads. Solid pyridine has a different crystal structure from solid benzene, so the nontopochemical formation of low-dimensional crystalline solids by slow compression of small aromatics may be a general phenomenon that enables chemical design of properties. The nitrogen in the carbon nitride nanothreads may improve processability, alters photoluminescence, and is predicted to reduce the bandgap.
Alzheimer's disease (AD) is defined by intracellular neurofibrillary tangles formed by the microtubule-associated protein tau and extracellular plaques formed by the β-amyloid peptide. AD tau tangles ...contain a mixture of tau isoforms with either four (4R) or three (3R) microtubule-binding repeats. Here we use solid-state NMR to determine how 4R and 3R tau isoforms mix at the molecular level in AD tau aggregates. By seeding differentially isotopically labeled 4R and 3R tau monomers with AD brain-derived tau, we measured intermolecular contacts of the two isoforms. The NMR data indicate that 4R and 3R tau are well mixed in the AD-tau seeded fibrils, with a 60:40 incorporation ratio of 4R to 3R tau and a small homotypic preference. The AD-tau templated 4R tau, 3R tau, and mixed 4R and 3R tau fibrils exhibit no structural differences in the rigid β-sheet core or the mobile domains. Therefore, 4R and 3R tau are fluently recruited into the pathological fold of AD tau aggregates, which may explain the predominance of AD among neurodegenerative disorders.
Molecular miscibility and homogeneity of amorphous solid dispersions (ASDs) are critical attributes that impact physicochemical stability, bioavailability, and processability. Observation of a single ...glass transition is utilized as a criterion for good mixing of drug substance and polymeric components but can be misleading and cannot quantitatively analyze the domain size at high resolution. While imaging techniques, on the other hand, can characterize phase separation on the particle surface at the nanometer scale, they often require customized sample preparation and handling. Moreover, a mixed system is not necessarily homogeneous. Compared to the numerous studies that have evaluated the mixing of drug substance and polymer in ASDs, inhomogeneity in the phase compositions has remained significantly underexplored. To overcome the analytical challenge, we have developed a 1H spin diffusion NMR technique to quantify molecular mixing of bulk ASDs at sub-100 nm resolution. It combines relaxation filtering (T 2H and T 1ρ) that leaves the active pharmaceutical ingredient (API) as the main source of 1H magnetization at the start of spin diffusion to the polymer matrix. A spray-dried nifedipine–poly(vinylpyrrolidone) (Nif–PVP) ASD at a 5 wt % drug loading was a homogeneous reference system that exhibited equilibration of magnetization transfer from API to polymer within a short spin diffusion time of ∼3 ms. While fast initial magnetization transfer proving mixing on the 1 nm scale was also observed in Nif–PVP ASDs prepared by hot-melt extrusion (HME) at 186 °C at a 40 wt % drug loading, incomplete equilibration of peak intensities documented inhomogeneity on the ≥30 nm scale. The nonuniformity was confirmed by the partial inversion of the Nif magnetization in the filter that resulted in an even more pronounced deviation from equilibration and by 1H–13C heteronuclear correlation (HETCOR) NMR. It is consistent with the observed differential 1H spin–lattice relaxation of Nif and PVP as well as a domain structure on the 20 nm scale observed in atomic force microscopy (AFM) images. The incomplete equilibration and differential relaxation were consistently reproduced in a model of two mixed phases of different compositions, e.g., 40 wt % of the ASD with a 15 wt % drug loading and the remaining 60 wt % with a 56 wt % drug loading. Hot-melt extrusion produced more inhomogeneous samples than spray drying for the samples examined in our study. To the best of our knowledge, this spin diffusion NMR method provides currently the highest-resolution quantification of inhomogeneous molecular mixing and phase composition in bulk samples of pharmaceutical dispersions produced with equipment, procedures, and drug loadings that are relevant to industrial drug development.
Carbon nanothreads are a new type of one-dimensional sp3-carbon nanomaterial formed by slow compression and decompression of benzene. We report characterization of the chemical structure of ...13C-enriched nanothreads by advanced quantitative, selective, and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments complemented by infrared (IR) spectroscopy. The width of the NMR spectral peaks suggests that the nanothread reaction products are much more organized than amorphous carbon. In addition, there is no evidence from NMR of a second phase such as amorphous mixed sp2/sp3-carbon. Spectral editing reveals that almost all carbon atoms are bonded to one hydrogen atom, unlike in amorphous carbon but as is expected for enumerated nanothread structures. Characterization of the local bonding structure confirms the presence of pure fully saturated “degree-6” carbon nanothreads previously deduced on the basis of crystal packing considerations from diffraction and transmission electron microscopy. These fully saturated threads comprise between 20% and 45% of the sample. Furthermore, 13C–13C spin exchange experiments indicate that the length of the fully saturated regions of the threads exceeds 2.5 nm. Two-dimensional 13C–13C NMR spectra showing bonding between chemically nonequivalent sites rule out enumerated single-site thread structures such as polytwistane or tube (3,0) but are consistent with multisite degree-6 nanothreads. Approximately a third of the carbon is in “degree-4” nanothreads with isolated double bonds. The presence of doubly unsaturated degree-2 benzene polymers can be ruled out on the basis of 13C–13C NMR with spin exchange rate constants tuned by rotational resonance and 1H decoupling. A small fraction of the sample consists of aromatic rings within the threads that link sections with mostly saturated bonding. NMR provides the detailed bonding information necessary to refine solid-state organic synthesis techniques to produce pure degree-6 or degree-4 carbon nanothreads.
Improved multiple cross polarization (multiCP) pulse sequences for quickly acquiring quantitative
C NMR spectra of organic solids are presented. Loss of
C magnetization due to imperfect read-out and ...storage pulses in multiCP has been identified as a significant mechanism limiting polarization enhancement for
C sites with weak couplings to
H. This problem can be greatly reduced by composite 90° pulses with non-orthogonal phases that flip the magnetization onto the spin-lock field and back to the longitudinal direction for the
H repolarization period; the observed loss is <3% for over ±10 kHz resonance offset and up to 20% flip-angle error. This composite-pulse multiCP (ComPmultiCP) sequence consistently provides performance superior to that of conventional multiCP, without any trade-off. The longer total CP time enabled by the composite pulses allows for a wider amplitude ramp during CP, which decreases the sensitivity to Hartmann-Hahn mismatch by a factor of two, with a <7% root-mean-square deviation within a 1-dB range for Boc-alanine. In samples with very short T
, under-polarization of non-protonated carbons can be compensated by slight dipolar dephasing of CH
signals resulting from relatively weak decoupling during the Hahn spin echo period before detection. Quantitative spectra have been obtained by ComPmultiCP for low-crystallinity branched polyethylene at 4.5 kHz MAS, and in combination with partial dipolar dephasing for soil organic matter at 14 kHz MAS.
The molecular connectivity of polymer-metal-organic framework (polyMOF) hybrid materials was investigated using density functional theory calculations and solid-state NMR spectroscopy. The ...architectural constraints that dictate the formation of polyMOFs were assessed by examining poly(1,4-benzenedicarboxylic acid) (pbdc) polymers in two archetypical MOF lattices (UiO-66 and IRMOF-1). Modeling of the polyMOFs showed that in the IRMOF-1-type lattice, six, seven, and eight methylene (-CH
-) groups between 1,4-benzenedicarboxylate (terephthalate, bdc
) units can be accommodated without significant distortions, while in the UiO-66-type lattice, an optimal spacing of seven methylene groups between bdc
units is needed to minimize strain. Solid-state NMR supports these predictions and reveals pronounced spectral differences for the same polymer in the two polyMOF lattices. With seven methylene groups, polyUiO-66-7a shows 7 ± 3% of uncoordinated terephthalate linkers, while these are undetectable (<4%) in the corresponding polyIRMOF-1-7a. In addition, NMR-detected backbone mobility is significantly higher in the polyIRMOF-1-7a than in the corresponding polyUiO-66-7a, again indicative of taut chains in the latter.
Commercial mesoporous oxides, such as silica, are not stable in liquid-phase reactions, particularly aqueous-phase reactions at elevated temperatures, which are corrosive to oxide supports. We have ...shown previously that the hydrothermal stability of silica is significantly improved by coating the surface with thin carbon layers. Herein, we show that controlled pyrolysis of sugars also provides a facile approach for coating supported metal catalysts, leading to improved dispersion of the active metal phase in the hydrothermally aged catalyst. The carbon overcoats are synthesized at mild temperatures, resulting in an open structure, as revealed by 13C NMR, which helps explain why the overcoats do not significantly block the active sites. We compare two approaches–depositing Pd on carbon-coated silica and depositing carbon overcoats on Pd/silica. The carbon overcoating approach leads to better performance after hydrothermal aging, as determined by using a probe reaction (CO oxidation) to quantify the number of active sites. The efficacy of the carbon overcoating was demonstrated by the improved stability of Pd/silica catalysts for aqueous phase acetone hydrogenation. Likewise, carbon-overcoated Cu/alumina catalyst was found to be more stable for aqueous-phase furfural hydrogenation compared with the uncoated catalyst.
With the constant promotion of the "comprehensive and accurate" highway maintenance policy, the demand for application of tunnel maintenance informatization is expanding, and the standardization of ...hierarchical coding for the tunnel engineering is critically urgent. In this paper, the hierarchical informatization and coding system of related industries are analysed firstly, and then the hierarchical coding structure and classification code with high accuracy on the level of structural units and affiliated facilities are designed according to the combination of highway tunnel properties and maintenance characteristics. At last, the modular congruence algorithm is adopted for automatic verification, and a set of hierarchical coding system for highway maintenance information is preliminarily established. Furthermore, the research of the hierarchical coding system application has laid a solid foundation for the whole life cycle maintenance management of highway tunnel.
Delay is a major challenge in teleoperation. To teleoperate a robot in real-time with specific human motion such as walking, prediction and compensation algorithms are needed to minimize the tracking ...delay. Human walking possesses the inherent features of synergy and quasi-periodicity with nonlinearity. By analyzing these features, three prediction methods were designed. A delay-coordinate phase space reconstruction method is based on the delay embedding theorem and uses the historical data. The echo prediction method uses data from the coupling joints of the other body side and takes the Euclidean distances in the delay-coordinate phase space reconstruction method for data search and match. The fused prediction method combines the delay-coordinate phase space reconstruction and echo prediction methods, uses the best matching historical data of the same joint and the corresponding joint of the other limb. These methods were implemented for teleoperation of an exoskeletal robot with inertial measurement unit measurements of human walking. The predicted trajectories were used to compensate the teleoperation delay and improve the robot tracking performance. Experimental results demonstrated that the fused method brought apparent improvement over other methods and can serve as a promising solution for time delay estimation, prediction, and compensation of human walking tracking and teleoperation.
The coexistence of rigid and mobile molecules or molecular segments abounds in biomolecular assemblies. Examples include the carbohydrate-rich cell walls of plants and intrinsically disordered ...proteins that contain rigid β-sheet cores. In solid-state nuclear magnetic resonance (NMR) spectroscopy, dipolar polarization transfer experiments are well suited for detecting rigid components, whereas scalar-coupling experiments are well suited for detecting highly mobile components. However, few NMR methods are available to detect the segments that undergo intermediate-amplitude fast motion. Here, we introduce two NMR experiments, a two-dimensional T 2H-filtered CP-hCH correlation and a three-dimensional J-INADEQUATE CCH correlation, to observe this intermediate-amplitude motion. Both experiments involve 1H detection under fast magic-angle spinning (MAS). By combining 1H transverse relaxation (T 2H) filters with dipolar polarization transfer, we suppress the signals of both highly rigid and highly mobile species, thus revealing the signals of intermediate mobile species. 1H detection under fast MAS is crucial for distinguishing the different motional amplitudes. We demonstrate these techniques on several plant cell wall samples and show that they allow the selective detection and resolution of certain hemicellulose and pectin signals, which are usually masked by the signals of the rigid cellulose and the highly dynamic pectins in purely dipolar and scalar NMR spectra.
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