High-field NMR Research Articles

Quinocidin, a Cytotoxic Antibiotic with an Unusual 3,4-Dihydroquinolizinium Ring and Michael Acceptor Reactivity toward Thiols.

Cytotoxicity-guided fractionation of the culture broth of Actinomadura sp. TP-A0019 led to the isolation of quinocidin (1), a cytotoxic antibiotic with an unusual 3,4-dihydroquinolizinium ring. The structural assignment was made on the basis of high-field NMR experiments and chemical synthesis. Comparison of the spectral properties of 1 with those of its synthetic counterparts revealed that 1 is a racemic mixture of two enantiomers, which showed similar cytotoxicity against HeLa-S3 cells. Nucleophile-trapping experiments demonstrated that 1 captured 2-mercaptoethanol and N-acetyl-l-cysteine by means of a Michael addition-type reaction, but was inert toward 2-aminoethanol and glycolic acid. Notably, the addition of 1 to thiols proceeded smoothly in neutral aqueous media at room temperature. In view of the thiol-trapping ability and the unusual structure, 1 provides a unique scaffold for designing drug leads and protein-labeling probes.

Determination of Protein ps-ns Motions by High-Resolution Relaxometry.

Many of the functions of biomacromolecules can be rationalized by the characterization of their conformational energy landscapes: the structures of the dominant states, transitions between states and motions within states. Nuclear magnetic resonance (NMR) spectroscopy is the technique of choice to study internal motions in proteins. The determination of motions on picosecond to nanosecond timescales requires the measurement of nuclear spin relaxation rates at multiple magnetic fields. High sensitivity and resolution are obtained only at high magnetic fields, so that, until recently, site-specific relaxation rates in biomolecules were only measured over a narrow range of high magnetic fields. This limitation was particularly striking for the quantification of motions on nanosecond timescales, close to the correlation time for overall rotational diffusion. High-resolution relaxometry is an emerging technique to investigate picosecond-nanosecond motions of proteins. This approach uses a high-field NMR spectrometer equipped with a sample shuttle device, which allows for the measurement of the relaxation rate constants at low magnetic fields, while preserving the sensitivity and resolution of a high-field NMR spectrometer. The combined analysis of high-resolution relaxometry and standard high-field relaxation data provides a more accurate description of the dynamics of proteins, in particular in the nanosecond range. The purpose of this chapter is to describe how to perform high-resolution relaxometry experiments and how to analyze the rates measured with this technique.

Photodegradation of novel oral anticoagulants under sunlight irradiation in aqueous matrices

Kinetics of photodegradation of novel oral anticoagulants dabigatran, rivaroxaban, and apixaban were studied under simulated solar light irradiation in purified, mineral, and river waters. Dabigatran and rivaroxaban underwent direct photolysis with polychromatic quantum yields of 2.2 × 10−4 and 4.4 × 10−2, respectively. The direct photodegradation of apixaban was not observed after 19 h of irradiation. Kinetics of degradation of rivaroxaban was not impacted by the nature of the aqueous matrix while photosensitization from nitrate ions was observed for dabigatran and apixaban dissolved in a mineral water. The photosensitized reactions were limited in the tested river water (Isle River, Périgueux, France) certainly due to the hydroxyl radical scavenging effect of the dissolved organic matter. The study of photoproduct structures allowed to identify two compounds for dabigatran. One of them is the 4-aminobenzamidine while the second one is a cyclization product. In the case of rivaroxaban, as studied by very high field NMR, only one photoproduct was observed i.e. a photoisomer. Finally, seven photoproducts were clearly identified from the degradation of apixaban under simulated solar light.

(Oxo)(Fluoro)-Aluminates in KF-Al2O3 System: Thermal Stability and Structural Correlation.

Precise investigation of part of the phase diagram of KF-Al2O3 system was performed in an experiment combining different techniques. Solidified mixtures of KF-Al2O3 were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO2, K2Al22O34, α-K3AlF6, KAlF4, and K2Al2O3F2. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF-Al2O3 with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K2Al2O3F2.

Restricted lithium ion dynamics in PEO-based block copolymer electrolytes measured by high-field nuclear magnetic resonance relaxation.

The intrinsic ionic conductivity of polyethylene oxide (PEO)-based block copolymer electrolytes is often assumed to be identical to the conductivity of the PEO homopolymer. Here, we use high-field 7Li nuclear magnetic resonance (NMR) relaxation and pulsed-field-gradient (PFG) NMR diffusion measurements to probe lithium ion dynamics over nanosecond and millisecond time scales in PEO and polystyrene (PS)-b-PEO-b-PS electrolytes containing the lithium salt LiTFSI. Variable-temperature longitudinal (T1) and transverse (T2) 7Li NMR relaxation rates were acquired at three magnetic field strengths and quantitatively analyzed for the first time at such fields, enabling us to distinguish two characteristic time scales that describe fluctuations of the 7Li nuclear electric quadrupolar interaction. Fast lithium motions [up to O(ns)] are essentially identical between the two polymer electrolytes, including sub-nanosecond vibrations and local fluctuations of the coordination polyhedra between lithium and nearby oxygen atoms. However, lithium dynamics over longer time scales [O(10 ns) and greater] are slower in the block copolymer compared to the homopolymer, as manifested experimentally by their different transverse 7Li NMR relaxation rates. Restricted dynamics and altered thermodynamic behavior of PEO chains anchored near PS domains likely explain these results.

Use of Hybrid Capillary Tube Apparatus on 400 MHz NMR for Quantitation of Crucial Low-Quantity Metabolites Using aSICCO Signal.

Metabolites of new chemical entities can influence safety and efficacy of a molecule and often times need to be quantified in preclinical studies. However, synthetic standards of metabolites are very rarely available in early discovery. Alternate approaches such as biosynthesis need to be explored to generate these metabolites. Assessing the quantity and purity of these small amounts of metabolites with a nondestructive analytical procedure becomes crucial. Quantitative NMR becomes the method of choice for these samples. Recent advances in high-field NMR (>500 MHz) with the use of cryoprobe technology have helped to improve sensitivity for analysis of small microgram quantity of such samples. However, this type of NMR instrumentation is not routinely available in all laboratories. To analyze microgram quantities of metabolites on a routine basis with lower-resolution 400 MHz NMR instrument fitted with a broad band fluorine observe room temperature probe, a novel hybrid capillary tube setup was developed. To quantitate the metabolite in the sample, an artificial signal insertion for calculation of concentration observed (aSICCO) method that introduces an internally calibrated mathematical signal was used after acquiring the NMR spectrum. The linearity of aSICCO signal was established using ibuprofen as a model analyte. The limit of quantification of this procedure was 0.8 mM with 10 K scans that could be improved further with the increase in the number of scans. This procedure was used to quantify three metabolites-phenytoin from fosphenytoin, dextrophan from dextromethorphan, and 4-OH-diclofenac from diclofenac-and is suitable for minibiosynthesis of metabolites from in vitro systems.

Complementary analyses of hollow cylindrical unioriented permanent magnet (HCM) with high permeability external layer

In this article, several studies based on analytical expressions and computational simulations on Hollow Cylindrical Magnets with an external soft ferromagnetic material (HCM magnets) are presented. Electromagnetic configurations, as well as permanent-magnet-based structures, are studied in terms of magnetic field strength and homogeneity. Permanent-magnet-based structures are further analyzed in terms of the anisotropy of the magnetic permeability. It was found that the HCM magnets produce a highly homogeneous magnetic field as long as the magnetic material is isotropic. The dependency of the magnetic field strength and homogeneity in terms of the anisotropy of the magnetic permeability is also explored here. These magnets can potentially be used in medium-resolution NMR spectrometers and high-field NMR spectrometers.

Screening of “spice” herbal mixtures: From high-field to low-field proton NMR

Forty one samples of herbal spices intended to be introduced into the European market and seized by the French customs were analysed with high-field 1H NMR. Nine synthetic cannabinoids (MAM-2201, JWH-073, JWH-210, JWH-122, JWH-081, JWH-250, UR-144, XLR-11 and AKB-48-5F) were detected and quantified. The ability of a compact benchtop low-field NMR spectrometer for a rapid screening of the content of herbal blends was then successfully explored for the first time. Even if low-field 1H NMR spectra are much less resolved than high-field spectra, we demonstrate that they provide valuable clues on the chemical structures of synthetic cannabinoids with the detection of some typical signals.

High-Field NMR Spectroscopy Reveals Aromaticity-Modulated Hydrogen Bonding in Heterocycles.

From DNA base pairs to drug-receptor binding, hydrogen (H-)bonding and aromaticity are common features of heterocycles. Herein, the interplay of these bonding aspects is explored. H-bond strength modulation due to enhancement or disruption of aromaticity of heterocycles is experimentally revealed by comparing homodimer H-bond energies of aromatic heterocycles with analogs that have the same H-bonding moieties but lack cyclic π-conjugation. NMR studies of dimerization in C6 D6 find aromaticity-modulated H-bonding (AMHB) energy effects of approximately ±30 %, depending on whether they enhance or weaken aromatic delocalization. The attendant ring current perturbations expected from such modulation are confirmed by chemical shift changes in both observed ring C-H and calculated nucleus-independent sites. In silico modeling confirms that AMHB effects outweigh those of hybridization or dipole-dipole interaction.

NMR and DFT investigations of structure of colchicine in various solvents including density functional theory calculations

A detailed NMR investigation of the chemical shifts of hydrogen and carbon atoms associated with the structure of the naturally occurring alkaloid colchicine was conducted using high field NMR. Initially, the experimental chemical shifts for colchicine in chloroform and DMSO were compared to the values calculated using density functional theory (DFT). There were significant deviations observed for the chloroform solvent, but these were only slight in the DMSO solution. Dilution of the chloroform solution changed the experimental chemical shifts and improved agreement with the DFT calculations, suggesting self-aggregation at higher concentrations. A dimeric model was proposed for which agreement with the DFT calculated chemical shifts was better than for corresponding monomeric structures. Three further solvents were studied to evaluate changes in chemical shift values at different dilutions. Chloroform, benzene and water showed significant chemical shift changes implying self-aggregation, whereas DMSO and acetone did not show significant change upon dilution.

Full Correlations across Broad NMR Spectra by Two-Field Total Correlation Spectroscopy.

Total correlation spectroscopy (TOCSY) is a key experiment to assign nuclear magnetic resonance (NMR) spectra of complex molecules. Carbon-13 TOCSY experiments are essential to assign signals of protein side chains. However, the performance of carbon-13 TOCSY deteriorates at high magnetic fields since the necessarily limited radiofrequency irradiation fails to cover the broad range of carbon-13 frequencies. Here, we introduce a new concept to overcome the limitations of TOCSY by using two-field NMR spectroscopy. In two-field TOCSY experiments, chemical shifts are labelled at high field but isotropic mixing is performed at a much lower magnetic field, where the frequency range of the spectrum is drastically reduced. We obtain complete correlations between all carbon-13 nuclei belonging to amino acids across the entire spectrum: aromatic, aliphatic and carboxylic. Two-field TOCSY should be a robust and general approach for the assignment of uniformly carbon-13 labelled molecules in high-field and ultra-high field NMR spectrometers beyond 1000 MHz.

Are there limits to rational chemical synthesis?

Chemical synthesis is the purposeful execution of one or more reactions to obtain a product, or several products, which could modify the existing molecular frameworks, or make a complex (often natural) molecule from simple reagents. There are several possible reasons to make complex molecules by total chemical synthesis. A century ago the aim was often to identify a molecular structure. With the advent of routine X-ray crystallography and high-field FT NMR in the late 1960s, total chemical synthesis served as a necessary structural confirmation became much less compelling. Another reason that chemists synthesized natural products was because of their useful properties. In some cases, molecules could be cheaper to make from scratch than to obtain from nature source. The history of chemistry is essentially the history of finding out the structure of molecules and of developing new and efficient methods of making them. Putting these molecules to new uses is what underpins our modern world, but it was really a secondary goal for most of chemistry’s history. Great synthetic chemists of the mid-to-late twentieth century are revered not so much for what they made but for how they made it. Synthesis cultivates an understanding of the basic principles of chemistry: how and why reactions occur, the relationships between molecular shape and function, and so on. Today, synthetic chemists still need to pursue “ideality” in the way molecules are synthesized, and total synthesis should be able to provide large quantities of complex natural products with a minimum amount of labor and material expense. In future years, the emphasis of synthetic chemistry, also natural product synthesis, will shift from delivering structures to delivering functions. Natural products will continue to serve a central role in the discovery and development of pharmaceutical agents as well as the elucidation of new biological targets of therapeutic relevance. Synthetic chemists are in a unique position to define those very important molecules that we want, which allow us to access to non-natural derivatives that may offer superior biological and physical properties in comparison to the natural material. The research in natural products synthesis has indeed been the impetus for many fundamental discoveries and could also motivate future research in chemistry and biology.

Exploring the salt-cocrystal continuum with solid-state NMR using natural-abundance samples: implications for crystal engineering.

There has been significant recent interest in differentiating multicomponent solid forms, such as salts and cocrystals, and, where appropriate, in determining the position of the proton in the X-H⋯A-YX-⋯H-A+-Y continuum in these systems, owing to the direct relationship of this property to the clinical, regulatory and legal requirements for an active pharmaceutical ingredient (API). In the present study, solid forms of simple cocrystals/salts were investigated by high-field (700 MHz) solid-state NMR (ssNMR) using samples with naturally abundant 15N nuclei. Four model compounds in a series of prototypical salt/cocrystal/continuum systems exhibiting {PyN⋯H-O-}/{PyN+-H⋯O-} hydrogen bonds (Py is pyridine) were selected and prepared. The crystal structures were determined at both low and room temperature using X-ray diffraction. The H-atom positions were determined by measuring the 15N-1H distances through 15N-1H dipolar interactions using two-dimensional inversely proton-detected cross polarization with variable contact-time (invCP-VC) 1H→15N→1H experiments at ultrafast (νR ≥ 60-70 kHz) magic angle spinning (MAS) frequency. It is observed that this method is sensitive enough to determine the proton position even in a continuum where an ambiguity of terminology for the solid form often arises. This work, while carried out on simple systems, has implications in the pharmaceutical industry where the salt/cocrystal/continuum condition of APIs is considered seriously.

High‐field 95Mo and 183W static and MAS NMR study of polyoxometalates

The potential of high-field NMR to measure solid-state 95 Mo and 183 W NMR in polyoxometalates (POMs) is explored using some archetypical structures like Lindqvist, Keggin and Dawson as model compounds that are well characterized in solution. NMR spectra in static and under magic angle spinning (MAS) were obtained, and their analysis allowed extraction of the NMR parameters, including chemical shift anisotropy and quadrupolar coupling parameters. Despite the inherent difficulties of measurement in solid state of these low-gamma NMR nuclei, due mainly to the low spectral resolution and poor signal-to-noise ratio, the observed global trends compare well with the solution-state NMR data. This would open an avenue for application of solid-state NMR to POMs, especially when liquid-state NMR is not possible, e.g., for poorly soluble or unstable compounds in solution, and for giant molecules with slow tumbling motion. This is the case of Keplerate where we provide here the first NMR characterization of this class of POMs in the solid state. Copyright © 2017 John Wiley & Sons, Ltd.

Electrophilic Halogenation of closo-1,2-C2B8H10.

Initial studies on electrophilic halogenation of the dicarbaborane closo-1,2-C2B8H10 (1) have been carried out to reveal that the substitution takes place at B7 and B10 vertexes, which are the most removed from the CH positions. The course of the halogenation is strongly dependent on the nature of the halogenation agent and reaction conditions. Individual reactions led to the isolation of the monosubstituted compounds 1,2-C2B8H9-10-X (2) (where X = F, I) and 1,2-C2B8H9-7-X (3) (where X = Cl, I). Disubstituted carboranes 1,2-C2B8H8-7,10-X2 (4) (where X = Cl, Br, I) were obtained under more forcing conditions. Individual halo derivatives were characterized by mass spectrometry and high-field NMR (11B, 1H,13C) spectroscopy combined with two-dimensional [11B-11B]-COSY, 1H{11B(selective)}, and [11B-1H]-correlation NMR techniques. All of the derivatives bearing a halogen substituent in the B10 position exhibit a remarkable antipodal 13C and 1H NMR shielding at the CH1 vertex, increasing in the order H < I < Br < Cl < F. The structures of 1,2-C2B8H8-7,10-X2 derivatives (where X = Cl, I, 4b,d) were established by X-ray diffraction analyses.

The examination of a variety of different ions added to the crown ether derivatives with high field NMR spectrometer

The current study involves the measurements of T 1 relaxation time of complexes three different crown ethers (4-aminobenzo-15-Crown-5, 1-Aza-18-Crown-6, 1-Aza-15-Crown-5) and two different ions sodium perchlorate (NaClO4), lithium perchlorate (LiClO4) by a 400 MHz NMR spectrometer. The association constants (K a) of the complexes were determined on the basis of the evaluation of these measurements using the Benesi-Hildebrand method that we have modified. These association constants were consistent with those obtained from 1H NMR titration technique. As a result, it can be speculated that binding constants could be measured using T 1 relaxation time when chemical shifts are very small or cannot be observed.

Glass to crystal transformation in the ternary BaO[sbnd]Nb2O5[sbnd]P2O5 system

Study of the glass to crystal transformation was realized for two ternary compounds belonging to the glass-forming region of the BaONb2O5P2O5 system: Ba3Nb2(PO4)4O2 with 16.66 mol% Nb2O5 (glass composition 3BaO.1Nb2O5.2P2O5) and BaNb2P2O11 with 33.33 mol% Nb2O5 (glass composition 1BaO.1Nb2O5.1P2O5). Physico-chemical properties of the corresponding glasses and crystals were compared, as well as their Raman, 31P and 93Nb NMR spectra. The density of polycrystalline samples is higher by 0.24–0.25 g cm−3 than the density of the corresponding glasses. 31P MAS NMR spectra show a close similarity of phosphorus environments in glasses and the corresponding crystalline samples. High-field (18.8T) NMR and 93Nb decoupling were necessary to obtain enough resolution on 31P NMR spectra to separate the 2 different short range environments of phosphorus atoms in Ba3Nb2(PO4)4O2. Static 93Nb NMR spectra showed the presence of distorted NbO6 octahedra both in the 3BaO.1Nb2O5.2P2O5 glass and the corresponding Ba3Nb2(PO4)4O2 crystals, while the presence of more symmetric NbO6 octahedra was detected in the BaNb2P2O11 compound, which is in accordance with the crystalline structures. Raman and NMR spectra showed similar short-range structural features in glasses and polycrystalline samples, whereas Raman spectra could detect some differences in the medium-range order of the BaO.Nb2O5.P2O5 glass and the corresponding BaNb2P2O11 crystal.

Photos of NMR pizza oven and grill, photo of laboratory lamp

Firing up the NMR Back in 1984, when Rich Haack first laid eyes on a high-field nuclear magnetic resonance spectrometer, he had a vision for the instrument that went beyond analyzing chemical compounds. “I thought if you could hollow it out, it would make a neat smoker, outdoor oven, or fire pit,” he tells Newscripts. Would that make it an NMR-B-Q ? Haack, a chemist who works in industry, never gave up on the idea. He just thought he would never get his hands on an NMR to hack. That all changed two years ago when he had the opportunity to acquire a defunct 400-MHz instrument from Open Technologies, a company that buys, sells, and services NMRs. Don Frank, the company’s chief executive officer, says the instrument was going to be scrapped. He knew of Haack’s plans, so he drove the old NMR from Open Technologies’ warehouse in Indiana to

Direct Measurement of the Angular Pair Correlation Coefficients in Molecular Liquids Using NMR. Benchmarking Force Fields for Atomistic Simulations

High-field deuterium NMR spectroscopy is used to characterize a number of molecular liquids and their mixtures in order to probe the directional part of the intermolecular interactions through the orientational ordering induced in the isotropic liquid phase by the spectrometer magnetic field. The systems studied include benzene, chloroform, hexafluorobenzene, and thiophene at various concentrations and in mixtures. Dilution with the magnetically isotropic tetramethylsilane provides quantification of ordering at "infinite magnetic dilution", that is, in the absence of magnetic intermolecular correlations, and thereby allows identification of the contribution of these correlations to the orientational ordering in neat phases and at various degrees of magnetic dilution. Such contributions are conveyed by angular pair correlation coefficients, which, in addition to being accessible to direct NMR measurement, are also possible to evaluate directly from molecular dynamics simulations. By using various force fields, simulations provide benchmark quantities for testing and possibly further improving the force field performance, particularly with respect to the directional components of the intermolecular interactions. The latter are critical for the simulation of self-assembly generally and particularly in biological systems.

Karmitoxin: An Amine-Containing Polyhydroxy-Polyene Toxin from the Marine Dinoflagellate Karlodinium armiger.

Marine algae from the genus Karlodinium are known to be involved in fish-killing events worldwide. Here we report for the first time the chemistry and bioactivity of a natural product from the newly described mixotrophic dinoflagellate Karlodinium armiger. Our work describes the isolation and structural characterization of a new polyhydroxy-polyene named karmitoxin. The structure elucidation work was facilitated by use of 13C enrichment and high-field 2D NMR spectroscopy, where 1H-13C long-range correlations turned out to be very informative. Karmitoxin is structurally related to amphidinols and karlotoxins; however it differs by containing the longest carbon-carbon backbone discovered for this class of compounds, as well as a primary amino group. Karmitoxin showed potent nanomolar cytotoxic activity in an RTgill-W1 cell assay as well as rapid immobilization and eventual mortality of the copepod Acartia tonsa, a natural grazer of K.armiger.