Advanced Solid-state Nuclear Magnetic Resonance Research Articles

Chemical composition of soil humin in an organic soil profile

Humin is identified as the most recalcitrant fraction of soil organic matter, but its synthesis and depth variability in the soil profile are unknown. Here, we suggest that carboxyl-rich alicyclic structures (CRAM) and fused aromatic ring structures (FAR) can process significant components of the humin structure. We conducted molecular-level characterization of humin using advanced solid-state nuclear magnetic resonance spectroscopy (SSNMR) experiments and calculated the proportion of FAR and functionalities of CRAM (OCq, –C(CH)n, –CH3) structures. Dipolar dephased (dd) direct polarization magic angle spinning (DPMAS) NMR experiments provided insights into non-protonated carbon and mobile quaternary carbons. We calculated the proportions of non-protonated aromatic carbon and non-polar aliphatic C in the organic layers of the topsoil (Oa), mid-soil (Om), and mineral subsoil (Cg). It was evident that the total aliphatic carbon constitutes a significant proportion of humin, as is the non-protonated C attached to CH3. The 13C dd/DPMAS NMR spectrum of Cg humin clearly showed the formation and preservation of hydrophobic OCq, which is indicative of CRAM structures. CRAM structures could be attached to FAR, which makes the whole structure relatively more hydrophobic, a critical humin functionality in deeper soil. The findings from our research can be successfully utilized in terrestrial carbon sequestration studies to amplify the potential of humin as a recalcitrant carbon reservoir.

Peptides as Model Systems for Biofunctionalizations of Cellulose─Synthesis and Structural Characterization by Advanced Solid-State Nuclear Magnetic Resonance Techniques

Peptides as Model Systems for Biofunctionalizations of Cellulose─Synthesis and Structural Characterization by Advanced Solid-State Nuclear Magnetic Resonance Techniques

V2O5-P2O5-TeO2 glass anodes for Li-ion batteries

Understanding the role of network formers in affecting the electrochemical performances of the glass anodes for Li-ion batteries (LIBs) is of great importance for developing novel glass anode materials. In this work, we substitute the network former P2O5 for partial amount of TeO2 in V2O5-TeO2 glass composition and probe the effect of this substitution on both the microstructure and performances of the LIB anodes. The network structure of the glass before and after discharging/charging cycles is revealed at atomistic level by advanced Solid-State Nuclear Magnetic Resonance spectroscopy and X-ray photoelectron spectroscopy. The results show that, upon the insertion/extraction of Li+ ions, the structural network of the glass anode is irreversibly disassociated into many polyhedral species that congregate to form different kinds of nanoclusters. The substitution of P2O5 for TeO2 promotes the formation of Li2Te and the reduction of V5+ to V4+, and thereby enhances the capacity of the glass anodes.

Rotational decoupling between the hydrophilic and hydrophobic regions in lipid membranes

Cells use homeostatic mechanisms to ensure an optimal composition of distinct types of lipids in cellular membranes. The hydrophilic region of biological lipid membranes is mainly composed of several types of phospholipid headgroups that interact with incoming molecules, nanoparticles, and viruses, whereas the hydrophobic region consists of a distribution of acyl chains and sterols affecting membrane fluidity/rigidity related properties and forming an environment for membrane-bound molecules such as transmembrane proteins. A fundamental open question is to what extent the motions of these regions are coupled and, consequently, how strongly the interactions of phospholipid headgroups with other molecules depend on the properties and composition of the membrane hydrophobic core. We combine advanced solid-state nuclear magnetic resonance spectroscopy with high-fidelity molecular dynamics simulations to demonstrate how the rotational dynamics of choline headgroups remain nearly unchanged (slightly faster) with incorporation of cholesterol into a phospholipid membrane, contrasting the well-known extreme slowdown of the other phospholipid segments. Notably, our results suggest a new paradigm in which phospholipid dipole headgroups interact as quasi-freely rotating flexible dipoles at the interface, independent of the properties in the hydrophobic region.

Structural study of rare earth doped fluoride phosphate glasses by solid-state nuclear magnetic resonance

A series of glasses with the compositions of 10NaF-(90-x) NaPO3-xYF3 (x = 5, 10, 15 and 20 mol%) were prepared via the conventional melt-quenching method. The structures of these glasses were characterized by multiple advanced solid-state nuclear magnetic resonance (SSNMR) techniques. Multiple phosphorus species QmnY were resolved at atomic level. Here, the value m and n denote the number of P–O–P bonds and the number of Y3+ ion bounding to phosphorus oxygen tetrahedron, respectively, which were identified by two dimensional (2D) J-resolved spectroscopy. 19F MAS spectra, 19F↔23Na and 19F↔31P rotational echo double resonance (REDOR) prove that F−ions are involved into the glass networks through Y-F•••Na and P-F•••Na linkages. The 31P {19F} J-coupling based heteronuclear multiple quantum coherence (J-HMQC) spectrum proves that F− ions selectively connect with Q1 species. F− ions show significant preference to connecting with Y3+ over P5+.

Recent Advances in Solid-State Nuclear Magnetic Resonance Techniques for Materials Research

Establishing structure–property correlations is of paramount importance to materials research. The ability to selectively detect observable magnetization from transitions between quantized spin states of nuclei makes nuclear magnetic resonance (NMR) spectroscopy a powerful probe to characterize solids at the atomic level. In this article, we review recent advances in NMR techniques in six areas: spectral resolution, sensitivity, atomic correlations, ion dynamics, materials imaging, and hardware innovation. In particular, we focus on the applications of these techniques to materials research. Specific examples are given following the general introduction of each topic and technique to illustrate how they are applied. In conclusion, we suggest future directions for advanced solid-state NMR spectroscopy and imaging in interdisciplinary research.

Exploring water-soluble organic aerosols structures in urban atmosphere using advanced solid-state 13C NMR spectroscopy

Abstract Water-soluble organic matter (WSOM) in air particles has profound effects on climate and human health. At the heart of this environmental significance of WSOM lies a complex set of compounds, of which a major fraction still often remains undeciphered. Yet, not all environmental problems require delving into the molecular-level identification of WSOM constituents. Understanding the contribution of different functional groups to whole aerosol WSOM composition offers a highly important structural dataset that enables a better representation of WSOM in climate studies. For the first time, advanced solid-state 13C nuclear magnetic resonance (NMR) techniques, including nearly quantitative 13C multiple cross polarization/magic angle spinning (multiCP/MAS), multiCP/MAS with dipolar dephasing, multiCP/MAS with 13C chemical shift anisotropy filter, and two-dimensional 1H–13C heteronuclear correlation (2D HETCOR), are applied to acquire an accurate quantitative structural description of whole aerosol WSOM collected in an urban atmosphere. Two urban aerosol WSOM samples collected in two short periods of time, under different wintry weather conditions, were investigated. NMR data successfully pinpointed the variability of whole aerosol WSOM composition, allowing to suggest source-specific structural characteristics for each sample in two short periods of time. A new structural model of urban aerosol WSOM was build based on this compositional data, showing the presence of three independent classes of compounds that vary both in content and molecular diversity within short periods of time: heteroatom-rich aliphatic (either chain or branched), carbohydrate-like moieties, and highly substituted aromatic units. These findings establish advanced solid-state NMR as a promising tool for probing the chemical structures of inhomogeneous aerosol WSOM in rapidly changing atmospheric conditions, allowing to resolve discrepancies between modeled and measured aerosol WSOM.

The Monetite Structure Probed by Advanced Solid-State NMR Experimentation at Fast Magic-Angle Spinning.

We present a solid-state nuclear magnetic resonance (NMR) spectroscopy study of the local P and H environments in monetite [CaHPO; dicalcium phosphate anhydrous (DCPA)], as well as their relative spatial proximities. Each of the three H NMR peaks was unambiguously assigned to its respective crystallographically unique H site of monetite, while their pairwise spatial proximities were probed by homonuclear H–H double quantum–single quantum NMR experimentation under fast magic-angle spinning (MAS) of 66 kHz. We also examined the relative H–P proximities among the inequivalent {P1, P2} and {H1, H2, H3} sites in monetite; the corresponding shortest internuclear H–P distances accorded well with those of a previous neutron diffraction study. The NMR results from the monetite phase were also contrasted with those observed from the monetite component present in a pyrophosphate-bearing calcium phosphate cement, demonstrating that while the latter represents a disordered form of monetite, it shares all essential local features of the monetite structure.

The Chemical Structure of Carbon Nanothreads Analyzed by Advanced Solid-State NMR.

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 rateconstants 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.

Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR Experiments and MD Simulations: Consequences of B/Si Substitutions.

The short and intermediate range structures of a large series of bioactive borophosphosilicate (BPS) glasses were probed by solid-state nuclear magnetic resonance (NMR) spectroscopy and atomistic molecular dynamics (MD) simulations. Two BPS glass series were designed by gradually substituting SiO2 by B2O3 in the respective phosphosilicate base compositions 24.1Na2O-23.3CaO-48.6SiO2-4.0P2O5 ("S49") and 24.6Na2O-26.7CaO-46.1SiO2-2.6P2O5 ("S46"), the latter constituting the "45S5 Bioglass" utilized for bone grafting applications. The BPS glass networks are built by interconnected SiO4, BO4, and BO3 moieties, whereas P exists mainly as orthophosphate anions, except for a minor network-associated portion involving P-O-Si and P-O-B[4] motifs, whose populations were estimated by heteronuclear 31P{11B} NMR experimentation. The high Na+/Ca2+ contents give fragmented glass networks with large amounts of nonbridging oxygen (NBO) anions. The MD-generated glass models reveal an increasing propensity for NBO accommodation among the network units according to BO4 < SiO4 < BO3 ≪ PO4. The BO4/BO3 intermixing was examined by double-quantum-single-quantum correlation 11B NMR experiments, which evidenced the presence of all three BO3-BO3, BO3-BO4, and BO4-BO4 connectivities, with B[3]-O-B[4] bridges dominating. Notwithstanding that B[4]-O-B[4] linkages are disfavored, both NMR spectroscopy and MD simulations established their presence in these modifier-rich BPS glasses, along with non-negligible B[4]-NBO contacts, at odds with the conventional structural view of borosilicate glasses. We discuss the relative propensities for intermixing of the Si/B/P network formers. Despite the absence of pronounced preferences for Si-O-Si bond formation, the glass models manifest subtle subnanometer-sized structural inhomogeneities, where SiO4 tetrahedra tend to self-associate into small chain/ring motifs embedded in BO3/BO4-dominated domains.

Probing the aluminum complexation by Siberian riverine organic matter using solid-state DNP-NMR

In a Siberian river, the concentrations of chemical species vary with the hydrological regime. Dissolved Organic Matter (DOM) and aluminum ions show a parallel trend in the course of the hydrological year. However, the speciation of aluminum in this natural environment remains an open question. We propose here a combination of spectroscopic techniques to investigate the proximity between the aluminum atoms and DOM. First, one-dimensional (1D) solid-state Nuclear Magnetic Resonance (NMR), Electron Paramagnetic Resonance (EPR) and 1D and 2D solution NMR spectra were acquired, providing a clear overview of the DOM composition. Second, the sensitivity enhancement yielded by Dynamic Nuclear Polarization enabled the NMR detection of proximities between the 27Al and 13C nuclei. Hence, we show that 8.3±1.3% of the carboxylate groups observed by NMR are connected to the Al3+ ions in the DOM sample. We here demonstrate for the first time how advanced solid-state NMR methods can provide key information about the localization of aluminum in such complex natural materials.

Chemical nature of humic substances in two typical Chinese soils (upland vs paddy soil): A comparative advanced solid state NMR study

Knowledge of the structural features of humic substances (HSs) is essential for elucidating the mechanisms of humification in different soil environments and realizing their profound roles in environmental issues. The aim of this work was to investigate the chemical structures of fulvic acid (FA), humic acid (HA) and humin (HM) fractions isolated from an upland soil (Fluvisol) and a paddy soil (Anthrosol) typical in China using advanced solid-state 13C nuclear magnetic resonance (NMR) techniques. The results revealed that there were great structural differences of HSs between the two soils. The two FAs showed distinct quantitative differences in aliphatics with more polysaccharides in the FA from the upland soil than from the paddy soil. The HM from the upland soil differed from the paddy soil HM in having more proteins/peptides (23% vs 20%), total aromatics (21% vs 12%) as well as fewer lipids (24% vs 35%) and polysaccharides (27% vs 31%). The HM fractions represented the most different components of organic matter between the two soils. The degree of difference between the two HAs fell in between that of FAs and HM fractions. In particular, the HA from the upland soil had relatively greater degree of aromaticity. Our study indicated that the upland soil exhibited a higher degree of humification compared with the paddy soil. Among the three humic fractions, the FAs featured COO/N-CO groups, and the HAs were more enriched in protonated aromatic C for both soils. In contrast, the two HM fractions contained more O-alkyl C and fewer aromatics than did the other humic fractions, being closer to the original organic materials in soils. We speculate that the evolutionary route of HSs is likely to be the transformation of original organic materials into HM, followed by increased degradation, further oxidization and conversion into HA, and then into FA.

High Surface Area Mesoporous GaPO4–SiO2 Sol–Gel Glasses: Structural Investigation by Advanced Solid-State NMR

Mesoporous silica–gallium phosphate glasses along the composition line, xGaPO4–(1 – x)SiO2 (x = 0.5, 0.33, 0.20, 0.14, and 0.11, respectively) were prepared via the sol–gel route. This glass-forming range is significantly wider than that accessible by previously reported routes. The glasses exhibit a mesoporous structure with surface areas around 400 m2/g, after calcination at 650 °C. The structural evolution from liquid to gel to glass was analyzed by liquid and advanced solid-state nuclear magnetic resonance techniques. The NMR results indicate that the glasses consist of GaPO4 and SiO2 nanodomains. With increasing GaPO4 content, the sizes of the GaPO4 domains become larger. Evidence for the connection of both domains at their interfaces by P–O–Si and Ga–O–Si linkages is presented by advanced 71Ga, 31P, and 29Si high-resolution dipolar solid-state NMR methods.

Fermentation and Cost-Effective 13 C/ 15 N Labeling of the Nonribosomal Peptide Gramicidin S for Nuclear Magnetic Resonance Structure Analysis

Gramicidin S (GS) is a nonribosomally synthesized decapeptide from Aneurinibacillus migulanus. Its pronounced antibiotic activity is attributed to amphiphilic structure and enables GS interaction with bacterial membranes. Despite its medical use for over 70 years, the peptide-lipid interactions of GS and its molecular mechanism of action are still not fully understood. Therefore, a comprehensive structural analysis of isotope-labeled GS needs to be performed in its biologically relevant membrane-bound state, using advanced solid-state nuclear magnetic resonance (NMR) spectroscopy. Here, we describe an efficient method for producing the uniformly (13)C/(15)N-labeled peptide in a minimal medium supplemented by selected amino acids. As GS is an intracellular product of A. migulanus, we characterized the producer strain DSM 5759 (rough-convex phenotype) and examined its biosynthetic activity in terms of absolute and biomass-dependent peptide accumulation. We found that the addition of either arginine or ornithine increases the yield only at very high supplementing concentrations (1% and 0.4%, respectively) of these expensive (13)C/(15)N-labeled amino acids. The most cost-effective production of (13)C/(15)N-GS, giving up to 90 mg per gram of dry cell weight, was achieved in a minimal medium containing 1% (13)C-glycerol and 0.5% (15)N-ammonium sulfate, supplemented with only 0.025% of (13)C/(15)N-phenylalanine. The 100% efficiency of labeling is corroborated by mass spectrometry and preliminary solid-state NMR structure analysis of the labeled peptide in the membrane-bound state.

Insight into local structure and molecular dynamics in organic solid-state ionic conductors.

Elucidating the rate and geometry of molecular dynamics is particularly important for unravelling ion-conduction mechanisms in electrochemical materials. The local molecular motions in the plastic crystal 1-ethyl-1-methylpyrrolidinium tetrafluoroborate ([C2 mpyr][BF4 ]) are studied by a combination of quantum chemical calculations and advanced solid-state nuclear magnetic resonance spectroscopy. For the first time, a restricted puckering motion with a small fluctuation angle of 25° in the pyrrolidinium ring has been observed, even in the low-temperature phase (-45 °C). This local molecular motion is deemed to be particularly important for the material to maintain its plasticity, and hence, its ion mobility at low temperatures.

ChemInform Abstract: Solid‐State NMR as a Spectroscopic Tool for Characterizing Phosphane‐Borane Frustrated Lewis Pairs

AbstractReview: 65 refs.

Composition of clay-fraction organic matter in Holocene paleosols revealed by advanced solid-state NMR spectroscopy

Identifying which components of organic matter are preserved over millennia in paleosols is important for understanding the stabilization mechanisms of soil organic matter (SOM) and in evaluating the portion of the terrestrial carbon stock that might be attributed to paleosols. Using advanced solid-state nuclear magnetic resonance (NMR) techniques, we examined the chemical composition of clay-associated SOM in a chronosequence of four paleosols and one modern soil collected from a site in the eastern Great Plains in Kansas, USA. The NMR spectra of clay SOM of the paleosols indicated the presence of OCH and CH2 moieties, which represent polysaccharides and lipids, respectively. The paleosol SOM also showed an appreciable peptide component, as indicated by signals of NCH and NCO. As in the modern soil, lignin residues in the paleosols contributed little to the SOM, and condensed aromatics contributed modestly to the clay SOM. The Roberts Creek paleosol samples were exceptions. Their NMR spectra included a considerable nonprotonated aromatic carbon component, reflecting fused aromatic rings and suggesting the presence of clay-fraction charcoal residues, probably originating from paleofires. In the clay SOM of both the paleosols and the modern soil, the polysaccharides and peptides indicated by the NMR spectra are consistent with the presence of microbially derived peptidoglycan and perhaps chitin residues. We conclude that lipids, charcoal, and polysaccharides, probably in the form of microbial residues, are likely to be the dominant SOM components in the paleosol clay fractions.

Similarities in chemical composition of soil organic matter across a millennia-old paddy soil chronosequence as revealed by advanced solid-state NMR spectroscopy

This study examined the chemical composition of soil organic matter (SOM) along a 2,000-year paddy soil chronosequence in eastern China by use of advanced solid-state nuclear magnetic resonance (NMR) spectroscopy as well as Fourier transform infrared spectroscopy (FTIR), aiming to identify changes in the chemical composition of SOM over a millennium timescale. The results showed that soil organic carbon concentration in the surface soil reached a steady state after 100 years of rice (Oryza sativa L.)–wheat (Triticum sp.) cropping on coastal tidal flats. The 13C NMR spectra and fractions of structural groups or components of the whole SOM samples differed little along the chronosequence, suggesting a similar chemical composition in SOM samples regardless of the duration of rice cultivation. The FTIR spectral pattern and relative intensities of some resolved functional groups or components of whole SOM were also similar along the soil chronosequence. The similarities in chemical composition of SOM can be attributed to the rice–wheat cropping system, in which SOM has undergone ongoing turnover under periodical fresh plant material input and wet–dry cropping alternation, leading to a similar chemical composition of bulk SOM.

Coexistence of Helical Morphologies in Columnar Stacks of Star-Shaped Discotic Hydrazones

Discotic hydrazone molecules are of particular interest as they form discotic phases where the discs are rigidified by intramolecular hydrogen bonds. Here, we investigate the thermotropic behavior and solid-state organizations of three discotic hydrazone derivatives with dendritic groups attached to their outer peripheries, containing six, eight, and ten carbons of linear alkoxy chains. On the basis of two-dimensional wide angle X-ray scattering (2DWAXS), the elevated temperature liquid crystalline (LC) phases were assigned to a hexagonal columnar (Colh) organization with nontilted hydrazone discs for all three compounds. With WAXS, advanced solid-state nuclear magnetic resonance (SSNMR) techniques, and ab initio computations, the compounds with six and ten carbons of achiral alkoxy side chains were further subjected to studies at 25 °C, revealing complex crystalline phases with rigid columns and flexible side chains. This combined approach led to models of coexisting helical columnar stacking morphologies for both systems with two different tilt/pitch angles between successive hydrazone molecules. The differences in tilt/pitch angles between the two compounds illustrate that the columns with short alkoxy chains (six carbons) are more influenced by the presence of other stacks in their vicinity, while those with long side chains are less tilted due to a larger alkoxy (ten carbons) buffer zone. The formation of different packing morphologies in the crystalline phase of a columnar LC has rarely been reported so far, which suggests the possibility of complex stacking structures of similar organic LC systems, utilizing small molecules as potential materials for applications in organic electronics.

Chemical structure changes in kerogen from bituminous coal in response to dike intrusions as investigated by advanced solid-state 13C NMR spectroscopy

Coal maturation due to igneous intrusions offers the ideal opportunity to study the chemical-structural evolution of rapidly heated coal. Variations in the chemical structures of kerogens from high volatile bituminous coal near two igneous dike intrusions in the Illinois Basin were characterized in detail by advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. With decreasing distance to dike contacts and increasing thermal maturity (vitrinite reflectance Ro from 0.62% to 5.03%), the new insights into the chemical structural changes in coal kerogens include (i) the C(CH2)C groups removed faster than CCH3 groups, suggesting that the major cleavage of aliphatics should not occur at aryl rings, (ii) a progressive decrease of the relative abundance of protonated aromatic carbons, possibly due to replacement of aromatic hydrogens by aryl groups via cross-linking, and (iii) an increase in the aromatic cluster size, in addition to the well established changes of the elimination of aliphatics and aromatic C-O groups and an increase in aromaticity. For the first time the growth of aromatic cluster sizes with increasing maturity was quantitatively estimated. Another novel finding is that coal kerogen (kerogen #22) directly at the contact with the large dike retained considerable aliphatic components with olefinic and COO groups. These aliphatic components could have been entrapped and failed to diffuse out rapidly enough because intense and very fast heating rendered this kerogen partially fluidized. Our advanced solid-state NMR techniques provided deep insights into the structural changes in kerogen from bituminous coal in response to dike intrusions which cannot be achievable using routine 13C cross polarization/magic angle spinning technique.