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Dissolved organic matter (DOM), the most reactive fraction of forest soil organic matter, is increasingly impacted by wildfires worldwide. However, few studies have quantified the temporal changes in soil DOM quantity and quality after fire. Here, soil samples were collected after the Qipan Mountain Fire (3-36 months) from pairs of burned and unburned sites. DOM contents and characteristics were analyzed using carbon quantification and various spectroscopic and spectrometric techniques. Compared with the unburned sites, burned sites showed higher contents of bulk DOM and most DOM components 3 months after the fire but lower contents of them 6-36 months after the fire. During the sharp drop of DOM from 3 to 6 months after the fire, carboxyl-rich alicyclic molecule-like and highly unsaturated compounds had greater losses than condensed aromatics. Notably, the burned sites had consistently higher abundances of oxygen-poor dissolved black nitrogen and fluorescent DOM 3-36 months after the fire, particularly the abundance of pyrogenic C2 (excitation/emission maxima of <250/∼400 nm) that increased by 150% before gradually declining. This study advances the understanding of temporal variations in the effects of fire on different soil DOM components, which is crucial for future postfire environmental management.

Crystallization is a widely used purification technique in the manufacture of active pharmaceutical ingredients (APIs) and precursor molecules. However, when impurities and desired compounds have similar molecular structures, separation by crystallization may become challenging. In such cases, some impurities may form crystalline solid solutions with the desired product during recrystallization. Understanding the molecular structure of these recrystallized solid solutions is crucial to devise methods for effective purification. Unfortunately, there are limited analytical techniques that provide insights into the molecular structure or spatial distribution of impurities that are incorporated within recrystallized products. In this study, we investigated model solid solutions formed by recrystallizing salicylic acid (SA) in the presence of anthranilic acid (AA). These two molecules are known to form crystalline solid solutions due to their similar molecular structures. To overcome challenges associated with the long 1 H longitudinal relaxation times ( T 1 ( 1 H)) of SA and AA, we employed dynamic nuclear polarization (DNP) and 15 N isotope enrichment to enable solid-state NMR experiments. Results of solid-state NMR experiments and DFT calculations revealed that SA and AA are homogeneously alloyed as a solid solution. Heteronuclear correlation (HETCOR) experiments and plane-wave DFT structural models provide further evidence of the molecular-level interactions between SA and AA. This research provides valuable insights into the molecular structure of recrystallized solid solutions, contributing to the development of effective purification strategies and an understanding of the physicochemical properties of solid solutions.

Untargeted nuclear magnetic resonance (NMR) metabolomics was used to evaluate compositional changes during yogurt fermentation upon lupin enrichment compared to traditional conditions. Lupin significantly changed the sample metabolic profile and its time course dynamics, seemingly delaying microbial action. The levels of organic and amino acids were significantly altered, along with those of some sugars, nucleotides, and choline compounds. Lupin seemed to favor acetate and formate synthesis, compared to that of citrate and fumarate; a higher formate levels may suggest increased levels of Streptococcus thermophilus action, compared to Lactobacillus bulgaricus . Lupin-yogurt was poorer in hippurate, lactose (and hence lactate), galactose, glucose-1-phosphate, and galactose-1-phosphate, containing higher orotate levels (possibly related to increased uridine derivatives), among other differences. Trigonelline was confirmed as a lupin marker, possibly together with glutamate and histidine. Other metabolite trajectories remained unchanged upon lupin addition, unveiling unaffected underlying processes. These results demonstrate the usefulness of untargeted NMR metabolomics to understand/develop new foodstuffs and their production processes, highlighting the identity of a variety of bioactive metabolites with importance for human health.

Organic nitrates in the atmosphere are associated with photochemical pollution and are the main components of secondary organic aerosols, which are related to haze. An efficient method for determining organic nitrates in atmospheric fine particles (PM2.5) was established using synthesized standards. Four alkyl (C7–C10) nitrates and three aromatic nitrates (tolyl nitrate, phenethyl nitrate, and p-xylyl nitrate) were synthesized and characterized by 1H and 13C nuclear magnetic resonance spectroscopy. The optimal ions for quantifying and confirming the identities of the analytes were identified by analyzing the standards by gas chromatography tandem mass spectrometry. The tandem mass spectrometer was a triple quadrupole instrument. This method can obtain more accurate information of organic nitrates than on-line methods. Spiked recovery tests were performed using three spike concentrations, and the recoveries were 61.0–111.4 %, and the relative standard deviations were < 8.2% for all of the analytes. Limits of detection and quantification were determined, and the linearity of the method for each analyte was assessed. The applicability of the method was demonstrated by analyzing six PM2.5 samples. Overall, 87% of the analytes were detected in the samples. Phenethyl nitrate, heptyl nitrate, and octyl nitrate were detected in every sample. Phenethyl nitrate was found at a higher mean concentration (3.23 ng/m3) than the other analytes. [ABSTRACT FROM AUTHOR]

Using a quantitative 1 H NMR-based approach, molecular interactions between key taste active compounds and high-molecular-weight (HMW) polymers were directly investigated in red wine. Analysis of qualitative and quantitative 1 H NMR spectra over time allowed a distinction of three interaction scenarios: (i) no interactions for flavon-3-ol glycosides, ellagitannins, carbohydrates, and amino acids; (ii) changes in the chemical shift to lower frequencies for flavan-3-ols and phenolic acid ethyl esters; and (iii) changes in the chemical shift to higher frequencies for phenolic acids, organic acids, inorganic salts, and alditols. Additionally, using liquid chromatography-tandem mass spectrometry (LC-MS/MS), quantitative 1 H nuclear magnetic resonance (qHNMR), and high-performance ion chromatography (HPIC), a taste reconstitution model of Primitivo red wine was established for the first time. Human sensory experiments with the new taste recombinant and different HMW fractions demonstrated the influence of the tastant polymer interactions on the sour and salty taste perception of red wine and the intrinsic bitter and astringent taste of the polymers. Further, the influence of the molecular weight cutoff (MWCO) of the polymers and the pH value on the tastant polymer interactions was analyzed. Especially, the HMW fractions 30-50 kDa and >50 kDa caused strong shifts to lower and higher frequencies, respectively. NMR-based interaction studies at different pH values revealed a maximum of interactions at pH 4.0. Based on these results, flavor changes in red wine caused by tastant polymer interactions can be predicted on a molecular level in the future.

E-liquids usually contain significant nicotine, which will exist primarily in two forms, monoprotonated and free-base, the proportions of which are alterable through the effective pH of the medium. The fraction of nicotine in the free-base form is αfb, with 0 ≤ αfb ≤ 1. When dosed via aerosol, the two nicotine forms have different mechanisms and kinetics of delivery, as well as differing implications for harshness of the inhaled aerosol, so αfb is relevant regarding abuse liability. Previous attempts to determine αfb in electronic cigarette liquids and vapor have been flawed. We employed the exchange-averaged 1H NMR chemical shifts of nicotine to determine αfb in samples of e-liquids. This method is rapid and direct and can also be used with collected aerosol material. The e-liquids tested were found to have 0.03 ≤ αfb ≤ 0.84. The αfb values in collected aerosol liquid samples were highly correlated with those for the parent e-liquids. E-liquids designed to combine high total nicotine level (addictive delivery) with low αfb (for ease of inhalation) are likely to be particularly problematic for public health. [ABSTRACT FROM AUTHOR]

Here, the analysis of the peculiar homopolymeric O-chain, isolated from the lipopolysaccharide (LPS) of Rhodopseudomonas palustris strain CGA009, is reported. The O-chain is built up of a novel 4-C-branched sugar (12-deoxy-4-C-(d-altro-5,7,8,9-tetrahydroxyhexyl))-3-O-methyl-d-galactopyranose)) whose structure, absolute configuration, and conformational features were deduced by 2D NMR spectroscopy, optical rotation measurements, and molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

The amphipathic a-helical peptide KIA14 [(KIAGKIA)2-NH2] was studied in membranes using circular dichroism and solid-state NMR spectroscopy to obtain global as well as local structural information. By analyzing ²H NMR data from 10 analogues of KIA14 that were selectively labeled with Ala-d3, those positions that are properly folded into a helix could be determined within the membrane-bound peptide. The N-terminus was found to be unraveled, whereas positions 4-14 formed an ideal helix all the way to the C-terminus. The helicity did not change when Gly residues were replaced by Ala-d3 but was reduced when Ile was replaced, indicating that large hydrophobic residues are required for membrane binding and helix formation. The reduced helicity was strongly correlated with a decrease in peptide-induced leakage from lipid vesicles. The orientation of the short KIA14 peptide was assessed in several lipid systems and compared with that of the longer KIA21 sequence [(KIAGKIA)3-NH2]. In 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine, both peptides are aligned flat on the membrane surface, whereas in 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC)/1-myristoyl-2-hydroxy-sn-glycero-3-phosphatidylcholine (lyso-MPC) both are inserted into the membrane in an upright orientation. These two types of lipid systems had been selected for their strongly negative and positive spontaneous curvature, respectively. We propose that in these cases, the peptide orientation is largely determined by the lipid properties. On the other hand, in plain DMPC and 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine, which have only a slight positive curvature, a marked difference in orientation is evident: the short KIA14 lies almost flat on the membrane surface, whereas the longer KIA21 is more tilted. We thus propose that out of the lipid systems tested here, DMPC (with hardly any curvature) is the least biased lipid system in which peptide orientation and realignment can be studied, allowing to compare and discriminate the intrinsic effects of the properties of the peptides as such. [ABSTRACT FROM AUTHOR]

The 19F chemical shift is a sensitive NMR probe of structure and electronic environment in organic and biological molecules. In this report, we examine chemical shift parameters of 4F-, 5F-, 6F-, and 7F-substituted crystalline tryptophan by magic angle spinning (MAS) solid-state NMR spectroscopy and density functional theory. Significant narrowing of the 19F lines was observed under fast MAS conditions, at spinning frequencies above 50 kHz. The parameters characterizing the 19F chemical shift tensor are sensitive to the position of the fluorine in the aromatic ring and, to a lesser extent, the chirality of the molecule. Accurate calculations of 19F magnetic shielding tensors require the PBE0 functional with a 50% admixture of a Hartree-Fock exchange term, as well as taking account of the local crystal symmetry. The methodology developed will be beneficial for 19F-based MAS NMR structural analysis of proteins and protein assemblies. [ABSTRACT FROM AUTHOR]

Solubility is frequently introduced at the high school and introductory college levels through the symbolic domain using net ionic equations and solubility product constants. Students may become proficient with spectator ion cancellation and skilled with algorithmic mathematical applications of solubility without obtaining a deeper understanding of the underlying concept of solubility. We present a convenient chemical system that allows for rapid data collection through benchtop nuclear magnetic resonance (NMR) spectroscopy and simple evaporation experiments. NMR spectroscopy can measure the particulate level properties of hydrogen nuclei, objects on the order of a femtometer in size. Solvent evaporation to macroscopic solute residue ties solubility back to a visual frame of reference for students. Data collected from these dissimilar methods of measurement lend themselves to relating the particulate, macroscopic, and symbolic domains of understanding in a hands-on laboratory setting. [ABSTRACT FROM AUTHOR]

31P{¹H} nuclear magnetic resonance spectroscopy is a particularly useful tool for studying the reactions of P-donor ligands such as phosphines and phosphites with transition metals and other Lewis bases because the reactions take place on the nonbonding pair of electrons on the phosphorus. In addition, 31P has a 100% natural abundance and a high gyromagnetic ratio resulting in a high sensitivity that allows the spectra to be recorded using small amounts of sample. An activity that combines air-sensitive synthesis of transition metal complexes of P-donor ligands and the characterization of these complexes with 31P{¹H} NMR spectroscopy has been developed for an upper-division, laboratory-based inorganic course. This laboratory familiarizes students with this useful nucleus for NMR spectroscopy and allows them to study the factors that affect the 31P{¹H} NMR chemical shifts of phosphorus-donor ligands and their transition metal complexes. Details of the activity, including procedure and pretest, are provided. Alternative methods and presentations are also proposed. [ABSTRACT FROM AUTHOR]

An iridium(III)-mediated C-H functionalization sequence involving a concerted cyclometalation-deprotonation/migratory insertion pathway is reported for the undergraduate chemistry laboratory. The air- and water-stable iridacycle intermediates are readily isolated and characterized by NMR spectroscopy. Both steps of the experiment are performed at room temperature under benchtop conditions, rendering the protocol suitable for a wide range of student skill levels. [ABSTRACT FROM AUTHOR]

This two-period undergraduate laboratory experiment involves the synthesis of a mixture of isomeric unknowns, isolation of the mixture by means of distillation, and characterization of the two products primarily by NMR spectroscopy (1D and 2D) supported with IR spectroscopy and GC-MS techniques. Subsequent calculation and examination of the relative energies of the products and of the lowest unoccupied molecular orbital of the products' common cationic intermediate provide a means of understanding the unique regioselectivity of the reaction. The combination of spectroscopic, spectrometric, and molecular modeling data obtained by students allows them not only to identify the products and propose a reasonable stepwise mechanism for the reaction, but also to rationalize why the non-Zaitsev product predominates. [ABSTRACT FROM AUTHOR]

Proton multiple-quantum time-domain NMR combined with time-temperature superposition is a powerful method to study entangled chain dynamics. Overcoming the previous limitation to regimes II-IV of the tube model, this study extends the method to regime I (localized Rouse motions) by use of a pulse sequence adapted to shorter times, thus covering all relevant regimes for the model case of poly(butadiene) with molecular weights (M) between 10 and 200 kDa. We determine a value for the entanglement time that is consistent with current rheological results and confirm a value below 1 for the time scaling exponent of the segmental orientation autocorrelation function (OACF) in regime I previously observed by other NMR techniques. The origins of deviations from tube-model predictions are assessed by focusing on the dynamics of the chain centers in end-chain deuterated triblock samples and by dilution of probe chains to 15% in a deuterated matrix with M of 2 MDa. The study is complemented by self-diffusion coefficients measured by pulsed-gradient NMR. Our OACF-based results for the terminal time reinforce the current consensus that below a chain length of 30-50 entanglements matrix effects cannot explain the nonideal M-scaling exponent of 3.4 but are responsible for an M-independent slowdown. The protracted approach of the pure local-reptation scaling in regime II is found to be only somewhat reduced for both chain centers and chains in a long matrix, confirming its generic intrachain origin. These microscopic insights could be compared with results from large-scale computer simulations and provide a gauge for theoretical approaches such as those dealing with constraint release and contour-length fluctuations. [ABSTRACT FROM AUTHOR]

Thermoresponsive polymers exhibit coil-globule transition in aqueous solution where the polymer undergoes transition from the coil-like morphology to a globular form with the change of temperature. Such transitions also reflect changes in the solvent dynamics captured by various spectroscopic methods. In this work, we construct a phenomenological model to capture the dynamics of the NMR relaxation of water molecules of an aqueous solution of thermoresponsive polymers that are known to form hydrogen bonds with the solvent water molecules. The model relies on the behavior of the polymer-solvent hydrogen bonds and the sharing of rotational kinetic energy of water molecules in the vicinity of the polymer chain and the bulk. This is shown to provide a direct estimate of the fractional change of the polymer-water hydrogen bonds across lower critical solution temperature from NMR relaxation data of solvent water along with a reliable estimate of the transition temperature. In addition, it also provides a measure of the dispersion of the strengths of these hydrogen bonds. We exemplify the validity of this model by successfully fitting the experimental data to show that the extracted parameters provide significant insights into the role played by the hydrogen bonds in the process. The possible extension of this model to solvents that form no hydrogen bonds with the polymers is also discussed. [ABSTRACT FROM AUTHOR]

Uncertainties in the structure determination of peptide analogs of boronic acid, exacerbated by the many coordination modes of boron, represent an obstacle in understanding their role in living organisms and thus also in developing the next generation of anticancer drugs. For that reason, we present here a general experimental-computational strategy allowing structure determination of complex boronic acid derivatives with extensive conformational variability. We demonstrate successful solution of the crystal structures of two nonsolvated polymorphs of ixazomib citrate directly from synchrotron powder diffraction data, which is challenging because the two molecules in the asymmetric unit cell that exhibit 32 degrees of conformational freedom push the limits of current solution procedures. We used a novel two-step Rietveld refinement based on DFT-D restraints to improve information quality derived from powder diffraction data to be comparable with that of single-crystal solutions. NMR crystallography was applied to verify the crystal structures, and the high potential value of using 11B NMR parameters toward the solution of unknown structures was demonstrated. Evolution of 11B–11B double-quantum coherences allows probing of interatomic distances up to 7 Å. Overall, we present an integrated approach that applies several techniques in conjunction to provide otherwise unavailable structural information. [ABSTRACT FROM AUTHOR]

NMR crystallographic techniques are used to validate a structure of β-piroxicam determined from powder X-ray diffraction (PXRD) with a relatively poor R-factor. Geometry optimization of PXRD- and single-crystal XRD- derived structures results in convergence to the same energy of the structures, with minimal atomic displacements, and good agreement of gauge-included projector augmented wave (GIPAW) calculated and experimentally determined NMR 1H, 13C, and 15N chemical shifts, and 14N quadrupolar parameters. Calculations on isolated molecules combined with 2D magic-angle spinning (MAS) 1H double-quantum (DQ) and 14N–1H NMR experiments confirm the 3D packing arrangement of β-piroxicam. NMR crystallography is shown to be an effective means of validating crystal structures that might otherwise be considered sceptically on the basis of diffraction data alone. [ABSTRACT FROM AUTHOR]

Conjugated polymers are anisotropic in shape and with regard to electronic properties. Little is known as to how electronic anisotropy impacts the underlying characteristics of the electron spin, such as the coupling to orbital magnetic moments. Using multi-frequency electrically detected magnetic resonance spectroscopy extending over 12 octaves in frequency, we explore the effect of spin-orbit coupling by examining the pronounced broadening of resonance spectra with increasing magnetic field. Whereas in three commonly used materials, the high-field spectra show asymmetric broadening, as would be expected from anisotropic g-strain effects associated with the molecular structure, in the conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) the spectra broaden isotropically, providing a direct measure of the microscopic distribution in g-factors. This observation implies that effective charge-carrier g-tensors are isotropic, which likely originates from motional narrowing in this high-mobility material. [ABSTRACT FROM AUTHOR]

The structural study of glycans and glycoconjugates is essential to assign their roles in homeostasis, health, and disease. Once dominated by nuclear magnetic resonance spectroscopy, mass spectrometric methods have become the preferred toolbox for the determination of glycan structures at high sensitivity. The patterns of such structures in different cellular states now allow us to interpret the sugar codes in health and disease, based on structure-function relationships. Dr. Catherine E. Costello was the 2017 recipient of the American Society for Mass Spectrometry’s Distinguished Contribution Award. In this Perspective article, we describe her seminal work in a historical and geographical context and review the impact of her research accomplishments in the field.8ᅟGraphical abstract [ABSTRACT FROM AUTHOR]

Mixtures that contain a known target component but are otherwise poorly specified are important in many fields. Previously, the activity of the target component, which is needed, e.g., to design separation processes, could not be predicted in such mixtures. A method was developed to solve this problem. It combines a thermodynamic group contribution method for the activity coefficient with NMR spectroscopy, which is used for estimating the nature and amount of the different chemical groups in the mixture. The knowledge of the component speciation of the mixture is not required. Test cases that are inspired by bioprocess engineering applications show that the new method gives surprisingly good results. [ABSTRACT FROM AUTHOR]

γ-Cyclodextrin metal-organic framework (γ-CD-MOF) is a new type of highly porous carrier for potential loading of therapeutic or diagnostic gas like sulfur hexafluoride (SF6). Here, loading of SF6 into γ-CD-MOF was investigated for its mechanism by molecular simulation and quantitative determination of SF6 using quantitative nuclear magnetic resonance (qNMR). For the SF6 loading, γ-CD-MOF was first degassed to remove the air without thermal decomposition or loss of framework crystallinity, then placed in the copper tube, and sealed to adsorb SF6 under 1.2 MPa and 25 °C for 12 h. The qNMR was employed for the determination of SF6 loaded in γ-CD-MOF using Span 80 as suspending agent and trifluoroacetic acid as internal standard. Then, the thermodynamic parameters had been estimated. Finally, molecular modeling combining with 19F NMR spectra was conducted to reveal the status of SF6 molecules in γ-CD-MOF. The results demonstrated that the content of SF6 loaded in γ-CD-MOF was 2.67 ± 0.46 wt %. After exposing to the environment of free SF6 at 0.1 MPa for 10 days, the relative content was 74.7%. It was confirmed that SF6 preferred to stay in the cavity of γ-CD-MOF cubes rather than in the γ-CD molecular pairs, which was a nonchemical adsorptive process. In conclusion, this research has established qNMR method and molecular simulation to demonstrate SF6 molecules in γ-CD-MOF and its loading mechanism. [ABSTRACT FROM AUTHOR]

Site-selective organic transformations are commonly required in the synthesis of complex molecules. By employing a bespoke metal-organic framework (MOF, 1⋅[Mn(CO)3N3]), in which coordinated azide anions are precisely positioned within 1D channels, we present a strategy for the site-selective transformation of dialkynes into alkyne-functionalized triazoles. As an illustration of this approach, 1,7-octadiyne-3,6-dione stoichiometrically furnishes the mono-"click" product N-methyl-4-hex-5'-ynl-1',4'-dione-1,2,3-triazole with only trace bis-triazole side-product. Stepwise insights into conversions of the MOF reaction vessel were obtained by X-ray crystallography, demonstrating that the reactive sites are "isolated" from one another. Single-crystal to single-crystal transformations of the Mn(I)-metalated material 1⋅ [Mn(CO)3(H2O)]Br to the corresponding azide species 1⋅ [Mn(CO)3N3] with sodium azide, followed by a series of [3+2] azide-alkyne cycloaddition reactions, are reported. The final liberation of the "click" products from the porous material is achieved by N-alkylation with MeBr, which regenerates starting MOF 1⋅ [Mn(CO)3(H2O)]Br and releases the organic products, as characterized by NMR spectroscopy and mass spectrometry. Once the dialkyne length exceeds the azide separation, site selectivity is lost, confirming the critical importance of isolated azide moieties for this strategy. We postulate that carefully designed MOFs can act as physical protecting groups to facilitate other site-selective and chemoselective transformations. [ABSTRACT FROM AUTHOR]

MOF-808, a Zr(IV)-based metal-organic framework, has been proven to be a very effective heterogeneous catalyst for the hydrolysis of the peptide bond in a wide range of peptides and in hen egg white lysozyme protein. The kinetic experiments with a series of Gly-X dipeptides with varying nature of amino acid side chain have shown that MOF-808 exhibits selectivity depending on the size and chemical nature of the X side chain. Dipeptides with smaller or hydrophilic residues were hydrolyzed faster than those with bulky and hydrophobic residues that lack electron rich functionalities which could engage in favorable intermolecular interactions with the btc linkers. Detailed kinetic studies performed by ¹H NMR spectroscopy revealed that the rate of glycylglycine (Gly-Gly) hydrolysis at pD 7.4 and 60 ºC was 2.69 x 10-4 s-1 (t1/2 = 0.72 h), which is more than 4 orders of magnitude faster compared to the uncatalyzed reaction. Importantly, MOF-808 can be recycled several times without significantly compromising the catalytic activity. A detailed quantum-chemical study combined with experimental data allowed to unravel the role of the {Zr6O8} core of MOF-808 in accelerating Gly-Gly hydrolysis. A mechanism for the hydrolysis of Gly-Gly by MOF-808 is proposed in which Gly-Gly binds to two Zr(IV) centers of the {Zr6O8} core via the oxygen atom of the amide group and the N-terminus. The activity of MOF-808 was also demonstrated toward the hydrolysis of hen egg white lysozyme, a protein consisting of 129 amino acids. Selective fragmentation of the protein was observed with 55% yield after 25 h under physiological pH. [ABSTRACT FROM AUTHOR]

Quaternary clathrate-I phases with an overall composition of Ba8M16+yP30-y (M = Cu,Zn) exhibit complex structural chemistry. Characterization of the electronic structures and chemical bonding using quantum-chemical calculations and 31P solid state NMR spectroscopy demonstrated that the Cu-Zn-P framework is flexible and able to accommodate up to six Zn atoms per formula unit via bonding rearrangements, such as partial Zn/P substitution and the formation of Cu-Zn bonds. Such perturbations of the framework's bonding affect the thermal and charge transport properties. The overall thermoelectric figure-of-merit, ZT, of Ba8Cu14Zn2P30 is 0.62 at 800 K, which is 9 times higher than the thermoelectric performance of the ternary parent phase Ba8Cu16 P30 . Through a combination of inelastic neutron scattering and single crystal X-ray diffraction experiments at 10 K, low-energy rattling of the Ba guest atoms inside the large tetrakaidecahedral cages are shown to be the reason for the low thermal conductivities observed for the studied clathrates. [ABSTRACT FROM AUTHOR]

Dysiarenone (1), a dimeric C21 meroterpenoid featuring an unprecedented 2-oxaspiro[bicyclo[3.3.1]nonane-9,1'-cyclopentane] carbon skeleton, was isolated from the marine sponge Dysidea arenaria. The structure of 1 was determined by HRMS and NMR spectroscopic analyses coupled with ECD calculations. Dysiarenone showed inhibitory activities against COX-2 expression and the production of prostaglandin E2 with an IC50 value of 6.4 µM in LPS-stimulated RAW264.7 macrophages. [ABSTRACT FROM AUTHOR]

The use of chiral auxiliaries, which derivatize enantiomers to diastereomers, is an established technique for NMR spectroscopic analysis of chirality and enantiomeric excess (ee). Here we report that some small prochiral molecules exhibit ee-dependent splitting of 1H NMR signals at room temperature based on acid/base interactions with chiral analytes, especially when either a chiral or prochiral acid contains a phenoxy group at the α-position of the carboxylic acid. As a representative case, the benzylamine (BA)/2-phenoxylpropionic acid (PPA) complex was comprehensively investigated by using various methods. Notably, X-ray crystallographic analysis shows that there are multipoint interactions in the BA/PPA complex, implying that "fixing" of molecular conformation is critical for efficient intermolecular transfer of magnetic anisotropy. Our results suggest that a wide range of prochiral molecules are available for NMR determination of ee when intermolecular interactions between prochiral molecules and chiral analytes are adequately designed. [ABSTRACT FROM AUTHOR]

This study demonstrates the usefulness derived from relying on hyperpolarized water obtained by dissolution DNP, for site-resolved biophysical NMR studies of intrinsically disordered proteins. Thanks to the facile amide-solvent exchange experienced by protons in these proteins, 2D NMR experiments that like HMQC rely on the polarization of the amide protons, can be enhanced using hyperpolarized water by several orders of magnitude over their conventional counterparts. Optimizations of the DNP procedure and of the subsequent injection into the protein sample are necessary to achieve these gains while preserving state-of-the-art resolution; procedures enabling this transfer of the hyperpolarized water and the achievement of foamless hyperpolarized protein solutions are demonstrated. These protocols are employed to collect 2D 15N-¹H HMQC NMR spectra of a-synuclein, showing residue-specific enhancements =100× over their thermal counterparts. These enhancements, however, vary considerably throughout the residues. The biophysics underlying this residue-specific behavior upon injection of hyperpolarized water is theoretically examined, the information that it carries is compared with results arising from alternative methods, and its overall potential is discussed. [ABSTRACT FROM AUTHOR]

In contrast to the well-studied crystalline cellulose of microbial and animal origins, cellulose in plant cell walls is disordered due to its interactions with matrix polysaccharides. Plant cell wall (PCW) is an undisputed source of sustainable global energy; therefore, it is important to determine the molecular structure of PCW cellulose. The most reactive component of cellulose is the exocyclic hydroxymethyl group: when it adopts the tg conformation, it stabilizes intrachain and interchain hydrogen bonding, while gt and gg conformations destabilize the hydrogen-bonding network. So far, information about the hydroxymethyl conformation in cellulose has been exclusively obtained from 13C chemical shifts of monosaccharides and oligosaccharides, which do not reflect the environment of cellulose in plant cell walls. Here, we use solid-state Nuclear Magnetic Resonance (ssNMR) spectroscopy to measure the hydroxymethyl torsion angle of cellulose in two model plants, by detecting distance-dependent polarization transfer between H4 and H6 protons in 2D 13C-13C correlation spectra. We show that the interior crystalline portion of cellulose microfibrils in Brachypodium and Arabidopsis cell walls exhibits H4-H6 polarization transfer curves that are indicative of a tg conformation, whereas surface cellulose chains exhibit slower H4-H6 polarization transfer that is best fit to the gt conformation. Joint constraints by the H4-H6 polarization transfer curves and 13C chemical shifts indicate that it is unlikely for interior cellulose to have a significant population of the gt and gg conformation mixed with the tg conformation, while surface cellulose may adopt a small percentage of the gg conformation. These results provide new constraints to the structure and matrix interactions of cellulose in plant cell walls, and represent the first direct determination of a torsion angle in an important noncrystalline carbohydrate polymer. [ABSTRACT FROM AUTHOR]

The addition of Ph2P(2-C6H4OH) to [(η6-p-cymene)RuCl(µ-Cl)]2 led to the formation of three distinct products, depending on the experimental conditions. The previously reported [(η6-p-cymene)RuCl{ʎ2P,O-PPh2(2-C6H4O)}], 1, was obtained in 90% yield using MeOH as the solvent, in the presence of a base (Et3N). Conversely, [(η6-p-cymene)RuCl{ʎ2P,O-PPh2(2-C6H4OH)}]Cl, [2]Cl, was afforded (88% yield) in MeOH without using the base. However, [(η6-p-cymene)RuCl2{κP-PPh2(2-C6H4OH)}], 3, was the major species in chlorinated solvents. Compounds 1, [2]Cl, and 3 were characterized by analytical and spectroscopic (IR, NMR) methods. The X-ray structures of [2]+ and 3 were determined, with the latter forming hydrogen bond dimeric units in the solid state. The dissolution of [2]Cl into a series of deuterated solvents, except methanol, led to rapid equilibration between [2]Cl and 3, revealing the hemilabile behavior of the phosphino-phenol ligand. According to Pulsed-Field Gradient Spin Echo (PGSE)-NMR and DFT studies, the dimeric nature of 3 persists in solution; thus, the hydrogen-bonding properties of MeOH are crucial to stabilize [2]Cl with respect to 3, with the latter being 3.4 kcal mol-1 more stable than the former in the gas phase. Compound 1 underwent electrophilic additions by HCl, HBF4, and MeCOCl to give, respectively, [2]Cl (the reverse process being viable with Et3N), [2]BF4, and the ester [(η6-p-cymene)RuCl2{κP-PPh2(2-C6H4OCOMe)}], 5. The reaction of 1 with AgNO3 afforded the nitrate complex [(η6-p-cymene)Ru(NO3){ʎ2P,O-PPh2(2-C6H4O)}], 4. Compounds [2]BF4, 4, and 5 were isolated in the solid state in good to high yields and characterized by elemental analysis and IR and NMR spectroscopy. [ABSTRACT FROM AUTHOR]

The reaction of the 2,6-bis(imino)phenylchlorogermylene [LGeCl] (1, L = C6H3-2,6-(HC-NtBu)2) with silver(I) trifluoromethanesulfonate (AgOTf) in a molar ratio of 2:3 in toluene afforded a mixture of the bis(germylene)silver(I) complex [{L(TfO)Ge}2Ag(OTf)] (2) and AgCl. Compound 2 was then reacted with 2 equiv of 1,3-dimethyl-4,5-dimethylimidazol-2-ylidene (IMe,:C{N(Me)C(Me)}2) in toluene to afford 3, which comprises the bis(germyliumylidene)silver(I) complex dication [{L(IMe)Ge}2Ag(OTf)]2+. Compounds 2 and 3 were characterized by NMR spectroscopy and X-ray crystallography. [{L(IMe)Ge}2Ag(OTf)]2+ in compound 3 was further elucidated by DFT studies. [ABSTRACT FROM AUTHOR]

Rechargeable lithium-sulfur (Li-S) batteries have drawn significant attention as next-generation energy storage systems. Sulfur-copolymers are promising alternative cathode materials to elemental sulfur in Li-S batteries as they provide high reversible capacity. However, the redox mechanisms of these materials are not well understood owing to the difficulty in characterizing amorphous structures and identifying individual ionic species. Here, we use solid-state NMR techniques together with electrochemistry experiments and quantum calculations to investigate the structural evolution of the prototype S-copolymer cathodes, sulfur-diisopropenylbenzene copolymers (poly(S-co-DIB)), during cycling. We demonstrate that polysulfides with different chain lengths can be distinguished by 13C and 7Li NMR spectroscopy, revealing that the structure of the copolymers can be tuned in terms of polysulfide chain lengths and resulting reaction pathways during electrochemical cycling. Our results show that the improved cyclability of these cathodes originates from the role of organic moieties acting as anchors that fixate polysulfides to the polymeric network during cycling, thus preventing their diffusion into the electrolyte. We provide a new methodological concept for the mechanistic studies to track the intermediate species and phase transition in Li-S batteries. [ABSTRACT FROM AUTHOR]

The conformational preferences of proteinogenic glutamic acid esterified (GluOMe) and N-acetylated (AcGluOMe) derivatives have been determined in solution for the first time. Theoretical calculations at the ωB97X-D/aug-cc-pVTZ made possible the assignment of six and eight stable conformers for GluOMe and AcGluOMe, respectively. The conformational equilibrium of the studied compounds was evaluated in different organic solvents using a combination of the integral equation formalism polarizable continuum model (IEF-PCM) and ¹H NMR spectroscopy data. The results showed that the conformational equilibrium of both derivatives change in the presence of solvent. According to the quantum theory of atoms in molecules (QTAIM), non-covalent interactions (NCI), and natural bond orbitals (NBO) analyses, the conformational preferences observed for GluOMe and AcGluOMe are not dictated by the presence of a specific interaction but are due to a combination of hyperconjugative and steric effects. [ABSTRACT FROM AUTHOR]

Short fragments of the core-crystalline micelles formed by a sample of poly(ferrocenyldimethylsilane)-block-poly(isoprene) (PFS-b-PI) block copolymer (BCP) underwent self-seeding in decane when heated above its dissolution temperature. Variable temperature (VT) ¹H NMR and diffusion-ordered pulsed-gradient spin-echo (DOSY) NMR were used to monitor the behavior of micelles that dissolved as a function of increasing temperature. We examined a sample of micelle fragments of PFS65-b-PI637 characterized by Ln = 39 nm and Lw/Ln = 1.13. The PI corona had high mobility and gave a ¹H NMR signal in both micellar and unimer forms. In contrast, the PFS component could only be detected for the dissolved unimer. We found from ¹H NMR that essentially all the BCP molecules were incorporated into the micelles at temperatures up to and including 50 °C, at the limit of NMR detection. Both PFS and PI resonances could be detected between 70 and 100 °C, and the integration ratio of the PFS-to-PI peaks increased with temperature. DOSY NMR measured the self-diffusion coefficients (Ds) of the micelle fragments and unimer at these temperatures. The hydrodynamic radii (Rh) for these species were calculated from Ds using the Stokes-Einstein equation. The PFS signals gave Rh values in the range of 5-6 nm at temperatures between 80 and 100 °C, consistent with unimer diffusion. PI signals were fitted by an exponential decay at 25 °C with Rh = 38 nm characteristic of the micelle fragments and at 90, 95, and 100 °C with Rh ≈ 6 nm, corresponding to unimer. At intermediate temperatures (70-85 °C), PI signals were fitted to a sum of two exponential terms, consistent with a fast diffusing species and a slow diffusing species. Interestingly, we noticed that the size of the micelle fragments at elevated temperatures (80 and 85 °C) was sensitive to sample history; samples heated directly to the elevated temperatures were found to be shorter than those heated stepwise. [ABSTRACT FROM AUTHOR]

The controlled shaping and surface functionalization of colloidal particles has provided opportunities for the development of new materials and responsive particles. The possibility of creating hollow particles with semipermeable walls allows modulating molecular transport properties on colloidal length scales. While shapes and sizes can typically be observed by optical means, the underlying chemical and physical properties are often invisible. Here, we present measurements of cross-membrane transport via pulsed field gradient NMR in packings of hollow colloidal particles. The work is conducted using a systematic selection of particle sizes, wall permeabilities, and osmotic pressures and allows tracking organic molecules as well as ions. It is also shown that, while direct transport of molecules can be measured, indirect markers can be obtained for invisible species via the osmotic pressure as well. The cross-membrane transport information is important for applications in nanoconfinement, nanofiltration, nanodelivery, or nanoreactor devices. [ABSTRACT FROM AUTHOR]

The strong binding of the adamantyl moiety to the cavity of β-cyclodextrin makes it a common binding motif in supramolecular chemistry and a common model system. Despite the attention, there are still unresolved questions regarding the orientation of the adamantane derivatives in the inclusion complexes--do they protrude from the wide or narrow opening of the cyclodextrin hosts? A combined analysis of ROESY NMR and molecular dynamics simulations allows the conclusion that positively charged adamantane derivatives are oriented with the hydrophilic group protruding from the wider opening of the cyclodextrin, while negatively charged adamantane derivatives form two coexisting types of complexes where the hydrophilic group respectively protrudes from the wide and narrow opening. Interestingly, structural modifications of the cyclodextrin host only have a slight impact on the guest orientation. [ABSTRACT FROM AUTHOR]

We have examined the 17O quadrupole-central-transition (QCT) NMR signal from [17O]nicotinamide (vitamin B3) dissolved in glycerol. Measurements were performed at five magnetic fields ranging from 9.4 to 35.2 T between 243 and 363 K. We found that, in the ultraslow motion regime, cross-correlation between the second-order quadrupole interaction and magnetic shielding anisotropy is an important contributor to the transverse relaxation process for the 17O QCT signal of [17O]nicotinamide. While such a cross-correlation effect has generally been predicted by relaxation theory, we report here the first experimental evidence for this phenomenon in solution-state NMR for quadrupolar nuclei. We have discussed the various factors that determine the ultimate resolution limit in QCT NMR spectroscopy. The present study also highlights the advantages of performing QCT NMR experiments at very high magnetic fields (e.g., 35.2 T). [ABSTRACT FROM AUTHOR]

The microscopic-order-macroscopic-disorder (MOMD) approach for ²H NMR line shape analysis is applied to dry and hydrated 3-fold- and 2-fold-symmetric amyloid-Aβ40 fibrils and protofibrils of the D23N mutant. The methyl moieties of L17, L34, V36 (C-CD3), and M35 (S-CD3) serve as probes. Experimental ²H spectra acquired previously in the 147-310 K range are used. MOMD describes local probe motion as axial diffusion (R tensor) in the presence of a potential, u, which represents the spatial restrictions exerted by the molecular surroundings. We find that R|| = (0.2-3.3) x 104 s-1, R⊥ = (2.2--2.5) x 10² s-1, and R is tilted from the ²H quadrupolar tensor at 60-75°. The strength of u is in the (2.0-2.4) kT range; its rhombicity is substantial. The only methyl moieties affected by fibril hydration are those of M35, located at fibril interfaces. The associated local potentials change form abruptly around 260 K, where massive water freezing occurs. An independent study revealed unfrozen "tightly-peptide-bound" water residing at the interfaces of the 3-fold-symmetric Aβ40 fibrils and at the interfaces of the E22G and E22Δ Aβ40-mutant fibrils. Considering this to be the case in general for Aβ40-related fibrils, the following emerges. The impact of water freezing is transmitted selectively to the fibril structure through interactions with tightly-peptide-bound water, in this case of M35 methyl moieties. The proof that such waters reside at the interfaces of the 2-fold-symmetric fibril, and the protofibril of the D23N mutant, is new. MOMD provides information on the surroundings of the NMR probe directly via the potential, u, which is inherent to the model; a prior interpretation of the same experimental data does so partially and indirectly (see below). Thus, MOMD analysis of NMR line shapes as applied to amyloid fibrils/protein aggregates emerges as a consistent new tool for elucidating the properties of, and processes associated with, molecular environments in the fibril. [ABSTRACT FROM AUTHOR]