NMR Signals Research Articles

Assessing the Surface Chemistry of 2D Transition Metal Carbides (MXenes): A Combined Experimental/Theoretical 13C Solid State NMR Approach.

The surface functionalization of 2D transition metal carbides or nitrides, so-called MXenes, is one of the fundamental levers allowing to deeply modify their physicochemical properties. Beyond new approaches to control this pivotal parameter, the ability to unambiguously assess their surface chemistry is thus key to expand the application fields of this large class of 2D materials. Using a combination of experiments and state of the art density functional theory calculations, we show that the NMR signal of the carbon─the element common to all MXene carbides and corresponding MAX phase precursors─is extremely sensitive to the MXene functionalization, although carbon atoms are not directly bonded to the surface groups. The simulations include the orbital part to the NMR shielding and the contribution from the Knight shift, which is crucial to achieve good correlation with the experimental data, as demonstrated on a set of reference MXene precursors. Starting with the Ti3C2Tx MXene benchmark system, we confirm the high sensitivity of the 13C NMR shift to the exfoliation process. Developing a theoretical protocol to straightforwardly simulate different surface chemistries, we show that the 13C NMR shift variations can be quantitatively related to different surface compositions and number of surface chemistry variants induced by the different etching agents. In addition, we propose that the etching agent affects not only the nature of the surface groups but also their spatial distribution. The direct correlation between surface chemistry and 13C NMR shift is further confirmed on the V2CTx, Mo2CTx, and Nb2CTx MXenes.

Creating Solid Solutions of Metallocenes: Migration of Nickelocene into the Ferrocene Crystal Lattice in the Absence of a Solvent

Ferrocene and nickelocene do not react with each other in solution; however, the large impact of the paramagnetic component on the ferrocene 1H NMR signal linewidth and relaxation times has been quantified. Co-crystallization of ferrocene and nickelocene at any ratio from a solvent can be explained with both pure substances crystallizing in the same space group P21/n. As a new phenomenon, when a ferrocene single crystal is exposed to polycrystalline nickelocene in the absence of a solvent, the nickelocene migrates into the ferrocene crystal lattice and a mixed crystal is formed that retains its macroscopic shape. This process has been proven visually by cutting the single crystal. Mixing polycrystalline ferrocene with polycrystalline nickelocene at different molar ratios with a mortar and pestle leads to crystalline solid solutions with the corresponding molar ratios of both components. This migration of one organometallic component into an existing crystal lattice of another at ambient temperature in the absence of a solvent has not been described previously. Paramagnetic 1H solid-state NMR spectroscopy of static and rotating samples of dry ferrocene/nickelocene mixtures at varying ratios is used to prove and quantify the mixing of both metallocenes at the molecular level. A single-crystal X-ray structure of a 50/50 mixed crystal corroborates the NMR results that nickelocene and ferrocene are randomly distributed in the lattice and that the space group P21/n is retained. All ferrocene molecules in the mixed crystal lattice show a broadening of their 1H wideline signals and residual magic-angle spinning (MAS) lines at ambient temperature. The broadening of the ferrocene signals correlates with the nickelocene content. 1H T1 relaxation time measurements for the signals of ferrocene in samples with different amounts of nickelocene corroborate the assumption that the signal broadening is due to paramagnetic dipole–dipole relaxation of ferrocene molecules in the vicinity of nickelocene. Spatially separated ferrocene and nickelocene powders in one rotor show the solid-state NMR characteristics of the individual polycrystalline metallocenes. The described formation of solid solutions of metallocenes in the absence of a solvent will open new pathways to homogeneously mixed nanoparticles with desired metal ratios and dual-atom catalysts.

The membrane activity of the antimicrobial peptide caerin 1.1 is pH dependent.

Histidine-rich antimicrobial peptides (AMPs) are an important class of membrane-active peptides that can provide a scaffold to induce pH-sensitive interactions with cell membranes. The Australian tree frog AMP, caerin 1.1 (Cae-1), has three histidine residues, which modify the interactions with neutral and negatively charged lipid systems. NMR and MD simulations showed that in an acidic environment in comparison to physiological conditions, Cae-1 induced greater perturbation of the lipid dynamics and water penetration into the membrane interior. Lipid packing, chemical environment, and dynamics of the phospholipid headgroup were monitored using 31P solid-state NMR. In particular, the CODEX technique revealed that Cae-1 has a large impact on the slow re-orientation of the lipids. 2H solid-state NMR showed that Cae-1 ordered the acyl chains across the hydrophobic core. Interestingly, titration of the paramagnetic Mn2+ into the lipid-AMP systems resulted in extensive quenching of the 13C solid-state NMR signals of the lipid headgroup and acyl chain carbons. These results supported the MD results that showed Cae-1 was mainly inserted within the lipid bilayer structure of both neutral and negatively charged membranes, with the charged residues dragging the water and phosphate groups inwards. This could be an early step in the mechanism of membrane disruption by histidine-rich AMPs and indicated that Cae-1 is potentially forming toroidal pores in bilayers of neutral and negatively charged lipid membranes, especially under acidic conditions.

A new perspective of molecular diffusion by nuclear magnetic resonance

The diffusion-weighted NMR signal acquired using Pulse Field Gradient (PFG) techniques, allows for extrapolating microstructural information from porous materials and biological tissues. In recent years there has been a multiplication of diffusion models expressed by parametric functions to fit the experimental data. However, clear-cut criteria for the model selection are lacking. In this paper, we develop a theoretical framework for the interpretation of NMR attenuation signals in the case of Gaussian systems with stationary increments. The full expression of the Stejskal–Tanner formula for normal diffusing systems is devised, together with its extension to the domain of anomalous diffusion. The range of applicability of the relevant parametric functions to fit the PFG data can be fully determined by means of appropriate checks to ascertain the correctness of the fit. Furthermore, the exact expression for diffusion weighted NMR signals pertaining to Brownian yet non-Gaussian processes is also derived, accompanied by the proper check to establish its contextual relevance. The analysis provided is particularly useful in the context of medical MRI and clinical practise where the hardware limitations do not allow the use of narrow pulse gradients.

Solvent Effects Used for Optimal Simultaneous Analysis of Amino Acids via 19F NMR Spectroscopy.

19F NMR has been extensively used in simultaneous analysis of multicomponent due to its 100% natural isotope abundance, high NMR-sensitivity, and wide-range chemical shifts. The solvent effects are usually observed in NMR spectroscopy and cause large changes in 19F chemical shifts. Herein, we propose that the simultaneous analysis of a complex mixture can be achieved using solvent effects via 19F NMR spectroscopy, such as a mixture solution of amino acids (AAs). AAs are not only cell-signaling molecules, but are also considered as biomarkers of some diseases. Hence, the analysis of AAs is important for human health and the diagnosis of diseases. In this work, the key to the success of sensing 19 biogenic AAs is the use of 2-fluorobenzaldehyde (2FBA) as a highly sensitive derivatizing agent and solvent effects to produce distinguishable 19F NMR signals. As a result, the resolution of 19F NMR spectroscopy of multiple 2FBA-labeled AAs is obviously higher than other methods based on 19F NMR. Moreover, 14 and 18 AAs can be satisfactorily differentiated and unambiguously identified in different complicated media supporting the growth of mammalian cells. Furthermore, quantification of the concentration of AAs can be made, and the limit of detection reaches 10 μM. Our work provides new insights into the simultaneous analysis of a multicomponent mixture based on solvent effects by 19F NMR spectroscopy.

SABRE Hyperpolarization with up to 200 bar Parahydrogen in Standard and Quickly Removable Solvents.

Parahydrogen (p-H2)-based techniques are known to drastically enhance NMR signals but are usually limited by p-H2 supply. This work reports p-H2-based SABRE hyperpolarization at p-H2 pressures of hundreds of bar, far beyond the typical ten bar currently reported in the literature. A recently designed high-pressure setup was utilized to compress p-H2 gas up to 200 bar. The measurements were conducted using a sapphire high-pressure NMR tube and a 43 MHz benchtop NMR spectrometer. In standard methanol solutions, it could be shown that the signal intensities increased with pressure until they eventually reached a plateau. A polarization of about 2%, equal to a molar polarization of 1.2 mmol L-1, could be achieved for the sample with the highest substrate concentration. While the signal plateaued, the H2 solubility increased linearly with pressure from 1 to 200 bar, indicating that p-H2 availability is not the limiting factor in signal enhancement beyond a certain pressure, depending on sample composition. Furthermore, the possibility of using liquefied ethane and compressed CO2 as removable solvents for hyperpolarization was demonstrated. The use of high pressures together with quickly removable organic/non-organic solvents represents an important breakthrough in the field of hyperpolarization, advancing SABRE as a promising tool for materials science, biophysics, and molecular imaging.

Hyperpolarizing DNA Nucleobases via NMR Signal Amplification by Reversible Exchange.

The present work investigates the potential for enhancing the NMR signals of DNA nucleobases by parahydrogen-based hyperpolarization. Signal amplification by reversible exchange (SABRE) and SABRE in Shield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) of selected DNA nucleobases is demonstrated with the enhancement (ε) of 1H, 15N, and/or 13C spins in 3-methyladenine, cytosine, and 6-O-guanine. Solutions of the standard SABRE homogenous catalyst Ir(1,5-cyclooctadeine)(1,3-bis(2,4,6-trimethylphenyl)imidazolium)Cl ("IrIMes") and a given nucleobase in deuterated ethanol/water solutions yielded low 1H ε values (≤10), likely reflecting weak catalyst binding. However, we achieved natural-abundance enhancement of 15N signals for 3-methyladenine of ~3300 and ~1900 for the imidazole ring nitrogen atoms. 1H and 15N 3-methyladenine studies revealed that methylation of adenine affords preferential binding of the imidazole ring over the pyrimidine ring. Interestingly, signal enhancements (ε~240) of both 15N atoms for doubly labelled cytosine reveal the preferential binding of specific tautomer(s), thus giving insight into the matching of polarization-transfer and tautomerization time scales. 13C enhancements of up to nearly 50-fold were also obtained for this cytosine isotopomer. These efforts may enable the future investigation of processes underlying cellular function and/or dysfunction, including how DNA nucleobase tautomerization influences mismatching in base-pairing.

Gem‐diol Generation in Copper and Zinc N‐methyl‐2‐imidazolecarboxaldehyde Complexes: Solid‐state NMR, EPR and Single‐Crystal X‐ray Studies

AbstractNovel coordination compounds, mono‐ (1) and binuclear copper complexes (2) and a zinc complex (3) were synthesized and studied through single‐crystal X‐ray crystallography, solid‐state NMR and EPR techniques to determine the chemical functionalization of the carbonyl group in N‐methyl‐2‐imidazolecarboxaldehyde ligand. Particularly, molecules containing carbonyl groups are versatile ligands that give rise to a wide range of new materials due to the high reactivity of the carbonyl group. However, the chemical identification of the functional group present in these coordination compounds is a challenge because the copper ion affects the NMR signals. In this sense, X‐ray crystallography becomes an indispensable tool for the analysis. The imidazole ligands in copper complexes 1 and 2 were found to be the aldehyde and the gem‐diol forms, respectively. Furthermore, the gem‐diol and carboxylate moieties were detected in the crystal lattice for the zinc complex 3 and studied by solution‐ and solid‐state NMR.

Influencing Factors of Hydration Number of Synthesized Methane Hydrates in Sediment

The low-field NMR method is introduced to explore the effect of various experimental conditions on hydration number during hydrate dissociation in methane hydrate-bearing sediments. It is shown that the NMR signal peak value of water is independent of the dry density of sediment, gas pressure, and salt concentration and significantly influenced by temperature. A relationship among NMR signal peak value, substance content, pressure, and temperature is developed to estimate the hydration number of pore hydrates during hydrate dissociation. It is revealed that the hydration number (Nh) of synthesized methane hydrates remains practically unchanged during hydrate dissociation in fine-grained sediments and is trivially influenced by experimental conditions, including initial pressure, density, initial water saturation, and salt content. Our experimental results indicated that the hydration number of methane hydrates is about 6.5, which is slightly higher than that of natural hydrates.

Stability Condition of Methane Hydrate in Fine-Grained Sediment

Stability condition is of critical importance for methane hydrate exploitation, transportation, and reserves. This study measured the stability conditions of methane hydrate in fine-grained sediment with different dry densities (ρd = 1.40, 1.50 and 1.60 g/cm3) and various initial water saturations by the multi-step heating method. The experimental result showed that the methane hydrate formation in fine-grained sediment required lower temperature and/or higher pressure compared to that in bulk state. At the same time, it is found that the deviation degree of P–T conditions of methane hydrate in fine-grained sediment with different dry density and initial water saturation are completely different from that in pure water. In addition, according to the nuclear magnetic resonance technique (NMR), the changes in NMR signal intensity during the formation and decomposition of methane hydrate in silt were analyzed. Regardless of formation and dissociation stages, liquid water always distributes in the small sediment pores. An empirical formula is developed to address the capillary suction of water and hydrate with respect to the unhydrated water within sediment. Furthermore, a phase equilibrium model is proposed to predict the stability conditions of hydrate-bearing fine-grained sediment.

Olefin Metathesis under Spatial Confinement and Continuous Flow: Investigation of Isomeric Side Reactions with a Grubbs–Hoveyda Type Catalyst

AbstractA 2nd‐generation Grubbs–Hoveyda type catalyst was immobilized inside mesoporous silica and used in the ring‐closing metathesis (RCM) of an α,ω‐diene to a large macro(mono)cycle. The goal was to investigate the relationship between substrate concentration, reaction time, and overall experiment time on the rate of isomerization under spatial (mesopore space) confinement with continuous‐flow microreactors. RCM reactions are commonly monitored by 1H NMR analysis, however, elucidation of reaction mixtures yielding large rings with a difference of only a single carbon atom remains difficult, because NMR signals are sometimes indistinguishable. In this work, an analytical platform with on‐line separation and detection of UV‐active substrate as well as (side) products by high‐performance liquid chromatography and a UV/Vis‐diode array detector (DAD) plus mass spectrometry served as enabling technology to quantify yield and selectivity under the respective reaction conditions. Using this setup, competitive reaction equilibria and isomerization reactions, in particular, could be resolved. Identification and quantification of relevant compounds of the reaction scheme under spatial confinement became possible despite chemical similarity. Kinetic data revealed that isomerization increases with higher substrate concentrations (up to 250 mM) and longer reaction times (from 1.2 to 18.6 min), but shows a distinct decline for prolonged overall experiment times (up to ∼250 min).

Enhancing the NMR signals of plant oil components using hyperpolarisation relayed via proton exchange.

In this work, the limited sensitivity of magnetic resonance is addressed by using the hyperpolarisation method relayed signal amplification by reversible exchange (SABRE-Relay) to transfer latent magnetism from para-hydrogen, a readily isolated spin isomer of hydrogen gas, to components of key plant oils such as citronellol, geraniol, and nerol. This is achieved via relayed polarisation transfer in which an [Ir(H)2(IMes)(NH2R)3]Cl type complex produces hyperpolarised NH2R free in solution, before labile proton exchange between the hyperpolarisation carrier (NH2R) and the OH-containing plant oil component generates enhanced NMR signals for the latter. Consequently, up to ca. 200-fold 1H (0.65% 1H polarisation) and 800-fold 13C NMR signal enhancements (0.65% 13C polarisation) are recorded for these essential oils in seconds. Remarkably, the resulting NMR signals are not only diagnostic, but prove to propagate over large spin systems via a suitable coupling network. A route to optimise the enhancement process by varying the identity of the carrier NH2R, and its concentration is demonstrated. In order to prove utility, these pilot measurements are extended to study a much wider range of plant-derived molecules including rhodinol, verbenol, (1R)-endo-(+)-fenchyl alcohol, (-)-carveol, and linalool. Further measurements are then described which demonstrate citronellol and geraniol can be detected in an off-the-shelf healthcare product rose geranium oil at concentrations of just a few tens of μM in single scan 1H NMR measurements, which are not visible in comparable thermally polarised NMR experiments. This work therefore presents a significant expansion of the types of molecules amenable to hyperpolarisation using para-hydrogen and illustrates a real-world application in the diagnostic detection of low concentration analytes in mixtures.

A focus on detection of polymorphs by dynamic nuclear polarization solid-state nuclear magnetic resonance spectroscopy.

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy has found increasing application as a method for quantification and structure determination of solid forms (polymorphs) of organic solids and active pharmaceutical ingredients (APIs). However, ssNMR spectroscopy suffers from low sensitivity and resolution, making it challenging to detect dilute solid forms that may be present after recrystallization or reaction with co-formers. Cousin et al. (S. F. Cousin et al., Chem. Sci., 2023, https://doi.org/10.1039/D3SC02063K) have demonstrated that dynamic nuclear polarization (DNP) enhanced 13C cross-polarization (CP) saturation recovery experiments can be used to detect dilute polymorphic forms that are present within a mixture of solid forms. Enhancement of the NMR signal by DNP and differences in signal build-up rates for different polymorphs provide the sensitivity and contrast needed to resolve NMR signals from minor polymorphic forms. This method demonstrated by Cousin et al. should aid the discovery of solid drug forms.

Genetically Encoded Noncanonical Amino Acids in Proteins to Investigate Lysine Benzoylation.

Posttranslational modifications (PTMs) of lysine residues are major regulators of gene expression, protein-protein interactions, and protein localization and degradation. Histone lysine benzoylation is a recently identified epigenetic marker associated with active transcription, which has physiological relevance distinct from histone acetylation and can be regulated by debenzoylation of sirtuin 2 (SIRT2). Herein, we provide a protocol for the incorporation of benzoyllysine and fluorinated benzoyllysine into full-length histone proteins, which further serve as benzoylated histone probes with NMR or fluorescence signal for investigating the dynamics of SIRT2-mediated debenzoylation.

Advanced Solid-State NMR Analysis of Two Crystal Forms of Ranitidine Hydrochloride: Detection of 1H-14N Intra-/Intermolecular Correlations.

Understanding the characteristics of crystal polymorphism of active pharmaceutical ingredients and analyzing them with high sensitivity is important for quality of drug products, appropriate characterization strategies, and appropriate screening and selection processes. However, there are few methods to measure intra- and intermolecular correlations in crystals other than X-ray crystallography, for which it is sometimes difficult to obtain suitable single crystals. Recently, solid-state NMR has been recognized as a straightforward method for measuring molecular correlations. In this study, we selected ranitidine hydrochloride, which is known to exist in two forms, 1 and 2, as the model drug and investigated each form using solid-state NMR. In conducting the analysis, rotating the sample tube, which had a 1-mm inner diameter, increased the solid-state NMR resolution at 70 kHz. The 1H-14N dipolar-based heteronuclear multiple quantum coherence (D-HMQC) analysis revealed the intermolecular correlation of Form 1 between the N atom of the nitro group and a proton of the furan moiety, which were closer than those of the intramolecular correlation reported using single X-ray crystal analysis. Thus, 1H-14N D-HMQC analysis could be useful for characterizing intermolecular interaction in ranitidine hydrochloride crystals. In addition, we reassigned the 13C solid-state NMR signals of ranitidine hydrochloride according to the liquid-state and multiple solid-state NMR experiments.

Solid-state NMR for the characterization of retinal chromophore and Schiff base in TAT rhodopsin embedded in membranes under weakly acidic conditions.

TAT rhodopsin extracted from the marine bacterium SAR11 HIMB114 has a characteristic Thr-Ala-Thr motif and contains both protonated and deprotonated states of Schiff base at physiological pH conditions due to the low pKa. Here, using solid-state NMR spectroscopy, we investigated the 13C and 15N NMR signals of retinal in only the protonated state of TAT in the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho (1'-rac-glycerol) (POPE/POPG) membrane at weakly acidic conditions. In the 13C NMR spectrum of 13C retinal-labeled TAT rhodopsin, the isolated 14-13C signals of 13-trans/15-anti and 13-cis/15-syn isomers were observed at a ratio of 7:3. 15N retinal protonated Schiff base (RPSB) had a significantly higher magnetic field resonance at 160 ppm. In 15N RPSB/λmax analysis, the plot of TAT largely deviated from the trend based on the retinylidene-halide model compounds and microbial rhodopsins. Our findings indicate that the RPSB of TAT forms a very weak interaction with the counterion.

Electron-decoupled MAS DNP with N@C60.

Frequency-chirped microwaves decouple electron- and 13C-spins in magic-angle spinning N@C60:C60 powder, improving DNP-enhanced 13C NMR signal intensity by 12% for 7 s polarization, and 5% for 30 s polarization. This electron decoupling demonstration is a step toward utilizing N@C60 as a controllable electron-spin source for magic-angle spinning magnetic resonance experiments.

Ultra-selective 1D clean in-phase correlation spectroscopy.

Selective 1D COSY can unambiguously identify coupled spins but is often limited both by lack of selectivity, and by unfavourable multiplet lineshapes. Here, ultra-selective GEMSTONE excitation is employed with CLIP-COSY to provide through-bond correlations for nuclei whose NMR signals overlap. The new method is illustrated using the coccidiostat lasalocid and the immunosuppressant cyclosporin.

Az MRI története, dinamikus szájüregi és maxillofaciális alkalmazása

The incredible evolution of medicine in the 20th century was made possible by the appearance and rapid innovation of diagnostic procedures. The discovery of the medical application of X-rays gave the opportunity to image the human body without disrupting tissue structure. The use of X-rays spread around the world at an incredible speed and almost all medical fields, including dentistry, began to use them. In the second half of the 20th century, exponential improvement of computing technology replace the summation scans with a new three-dimensional technique, Computer Tomography. Cone Bean CT scanners (CBCT), which use a non-traditional cone-shaped beam as a radiation source, have made three-dimensional imaging widely available in dentistry. However, the harmful side effects of X-rays soon came to the attention of the public, and further research led to the development of a previously unknown technique, magnetic resonance imaging (MRI). This device was the result of the collaborative work of many scientists, mainly physicists. From the experiment by Isiodor I. Rabi, who proved that the atomic spin of nuclei can be changed in a magnetic field, to the scientists involved in the development of nuclear magnetic resonance spectroscopy, to the experiment of Paul C. Lautenburg, who was the first to create an image using this method. Raymond Damadien demonstrated that the NMR signal of tumour tissue differs from that of healthy tissue, thus providing the basis for the medical application of the method. Sir Peter Mansfield's new method greatly reduced the scanning time, allowing fast-moving organs such as the heart to be imaged. The measurement of contrast medium, and soon afterwards of blood oxygenation, made it possible to perform functional MRI examinations. The use of positron emission tomography combined with three-dimensional imaging techniques gives accurate diagnostic results, can be preferable in certain clinical situations and provides similar results at lower radiation levels. In terms of maxillofacial applications, CT and MRI scans are mainly used in cases of oral cavity tumour abnormalities. In dentistry, the imaging technique used in everyday practice is the CBCT, a non-traditional cone beam technique, which allows better quality images to be obtained in a shorter exposure time and using a lower dose of radiation.

Oligosaccharide analysis of the backbone structure of the characteristic polysaccharide of Dendrobium officinale

The backbone of Dendrobium officinale polysaccharide (DOP) was reported to be glucomannan. But the dispute continues due to the limitation of conventional analysis methods (e.g., the overlapped NMR signals). Herein, we describe an oligosaccharide sequencing analysis that successfully corrects the backbone to mannan. This analysis involves producing oligosaccharide fragments from mild acid hydrolysis of DOP followed by p -aminobenzoic ethyl ester (ABEE) derivatization. Then, a series of oligosaccharides with varied polymerization degrees (DP = 2–10) were isolated and identified by LC-MS profiling, monosaccharide composition analysis, linkage analysis, and 1D-/2D-NMR elucidation. Finally, we conclude that the backbone of DOP is a β -D-1,4-mannan which is confirmed by other analyses such as comparison with reference mannan. This oligosaccharide analysis is more straightforward and reliable by transforming the complicated polymers into oligomers which have more accurate and convincing MS and NMR data. • An oligosaccharide analysis method was proposed to characterize the DOP. • A series of oligomers and isomers from hydrolyzed DOP were isolated and identified. • An LC-MS method was established to test the oligomers' sugar sequence. • The backbone structure of DOP was corrected to be a β -D-1,4-Mannan.