Magnetic Resonance in Chemistry最新文献

Electron paramagnetic resonance (EPR) in connection with site-directed spin labeling is a structural biology tool that can be employed to obtain structural and conformational properties of various biological systems. Recent advances in methodological and technical improvements have made EPR spectroscopy a rapidly growing tool for gleaning important structural and conformational dynamics of membrane proteins. In this review, we discuss advancements in the popular site-directed spin labeling EPR approaches in brief and their applications to study the structure and conformations of biologically important membrane proteins. Recent examples of electron spin echo envelope modulation (ESEEM), double electron-electron resonance (DEER), and In-cell EPR studies for addressing structural and conformational-related questions of membrane proteins will be highlighted.

NMR and MRI provide a variety of customizable methods for process monitoring. A selection was applied to monitor structural and compositional changes in hazelnuts during thermal treatment, with particular focus on the roasting and aging behavior of hazelnut oil. Hazelnuts contain a high oil fraction stored in subcellular oleosomes, whose stability is crucial for product quality and shelf life. Thermal stress can alter these microscopic oil-containing structures, affecting oil mobility and oxidative stability. In situ MRI measurements were combined with pulsed field gradient stimulated echo (PFG-STE) NMR diffusion experiments to investigate structural changes across multiple length scales. MRI detected mesostructural alterations in the hazelnut matrix from ~50 μm to several millimeters, corresponding to features above the cellular level. At roasting temperatures below 150°C, only minor structural changes occurred, whereas at 200°C, pronounced void formation and cellular collapse were observed. A dedicated experimental setup enabled in situ measurements during roasting under controlled temperature, allowing spatially resolved monitoring of oil redistribution in coarse nut structure. Complementary PFG-STE NMR diffusion measurements provided insight into the microstructure (100 nm-10 μm), revealing subcellular structural changes and oil mobility. These results showed that oleosomes were largely destroyed already at 100°C. Furthermore, NMR spectroscopy demonstrated temperature-dependent oxidation kinetics of unsaturated fatty acids in hazelnut oil on a molecular level, with clear formation of oxidation products upon heating, whereas ambient storage caused only minor chemical changes. The combined use of MRI and NMR enables quasi-nondestructive, in situ monitoring of molecular, microstructural, and mesostructural transformations in hazelnuts and their oil under controlled thermal processing conditions.

Nuclear magnetic resonance (NMR) is a powerful analytical tool for wine analysis to identify and quantify a metabolite composition. However, a limiting factor of 1D ¹H NMR spectroscopy is the overlap of signals in complex mixtures. While conventional 2D NMR methods disperse the signals over two dimensions, they are associated with long experiment times. In the case of wine, interesting metabolites are also often masked by the large water and ethanol peaks. To improve wine analysis by NMR, a method that uses the advantages of 2D NMR while suppressing solvent signals and being within the timeframe of 1D NMR is highly desirable. Interleaved ultrafast COSY (iuf-COSY) offers a possibility for fast acquisition of a 2D spectrum and has been demonstrated as a powerful tool in metabolomics studies, as a complement to 1D NMR methods. Here, the iuf-COSY experiment has been adapted to suppress water and ethanol signals by using a shaped pulse and a NOESY block. This approach efficiently suppresses solvent signals and gives a 2D COSY spectrum of wine in approximately 20 min. Important metabolites that originally were covered by solvent signals could be annotated, while minimal interleaving artefacts were observed. This is an efficient method to acquire a COSY spectrum of a wine sample, which can aid with the identification and discrimination of metabolites in future wine studies through additional cross peaks, while working within a high-throughput time scale. This might be particularly interesting in the field of wine metabolomics, quality control, authenticity and fraud.

The NMR interaction tensors of ⁹Be and ¹¹B of hambergite, Be ${}_2$ BO ${}_3$ OH, were derived from single-crystal NMR experiments. In the orthorhombic crystal structure of hambergite (which we redetermined by single-crystal XRD, confirming the results of previous studies), both beryllium and boron atoms occupy Wyckoff position $8c$ , with atoms pairwise related by inversion symmetry. This leads to four magnetically independent ⁹Be and ¹¹B atoms per site, which are observable in the NMR spectra. Unequivocal assignment of these resonances to atomic positions in the unit cell is generally impossible, as an analysis of the symmetry relations shows. For the hambergite system, this assignment ambiguity could be resolved with the help of DFT calculations using the VASP code, with the resulting eigenvectors compared with the experimental ones. Examination of ⁹Be-¹H dipolar coupling effects, which could be detected in some of the ⁹Be spectra, in combination with XRD experiments to confirm the goniometer axis orientation, provided further spatial information and confirmed the assignment. The thus determined numerical values for the quadrupolar coupling constants $\chi$ and isotropic chemical shifts ${\delta }_{iso}$ are as follows: for ⁹Be[1] $222.6\pm 0.6$  kHz and 1.6 ppm, for ⁹Be[2] $-121.2\pm 0.4$  kHz and 1.4 ppm and for ¹¹B[1] $2.648\pm 0.004$  MHz and 18.1 ppm.

Stereochemical elucidation of synthetic and natural compounds is a rate-determining step in the structure determination process. In the present work, we introduce a method called Chirality In Silico Structure Elucidation (CiSE), which employs ¹J_CC coupling constants to determine the correct stereochemistry of natural products. To determine the correct stereochemistry, the proposed method assigns a probability to all possible structures of a compound in question. For calculating these probabilities, the errors between the experimental and density functional theory (DFT) computed ¹J_CC couplings of a large number of compounds are fitted into Student's t-distribution, from which statistical parameters such as mean, standard deviation, and degrees of freedom are determined. Afterward, the probabilities for all possible candidate structures are calculated using Bayes's theorem, with the correctly assigned stereoisomer typically exhibiting a probability exceeding 95%.

1D and 2D NMR spectra of amide 3 and benzimidazole 4 were unequivocally assigned. Moreover, the aminolysis of D-glycero-D-gulo-heptono-1,4-lactone 1 with o-phenylenediamine 2 was explored using different methodologies. Results indicated that focused ultrasound selectively yielded the amide 3. Studies demonstrated that intensive cavitation can exceed the activation energy without disrupting the thermodynamic equilibrium. Thus, the kinetic product (amide 3) was favored over the thermodynamic product (benzimidazole 4). In contrast, the ultrasonic cleaner led to an incomplete and unselective reaction. On the other hand, thermal induction favored the synthesis of benzimidazole 4, while the mechanochemical synthesis was inefficient in promoting the aminolysis of the γ-lactone 1 with compound 2.

The drying of egg yolk tempera paint in the dark has been investigated from 24 h up to 2 years by nuclear magnetic resonance (NMR) relaxometry and spectroscopy. Variations around a reference recipe have been carried out to investigate several aspects: the amount of added water to the binder, the nature of the minerals or the pigments and the drying time. The ¹H signal measured here corresponds to the ¹H contained in the fatty acids of the egg yolk (EY). The $R_1$ NMR dispersion (NMRD) profile increases when the added water in the preparation of the tempera is above 50 wt% linked to a destructuration of egg yolk high- and low-density lipoproteins (HDL and LDL). In addition to the chemical nature of the minerals, the shape and the surface area also play an important role in the destructuration of egg yolk HDL and LDL. The most striking result is the discontinuous evolution of the NMRD with time with the appearance of a step around 1 or 2 months. Depending on the water content in the initial tempera, a steep change in $R_1$ is observed while before and after only a weak evolution occurs.

Biochar is a multifunctional soil amendment that improves soil structure, enhances water-holding capacity, and contributes to carbon sequestration. However, the dose–response relationship between biochar addition and soil behavior remains underexplored, particularly at high application rates. In this study, fifteen soil–biochar mixtures were prepared with biochar mass fractions from 0 to 1 (f_BC = 0–1) to evaluate in detail the changes induced in a Sicilian clay soil. The mixtures were investigated for pH, electrical conductivity, bulk density, water-holding capacity, and water activity (Aw). Biochar addition caused pronounced increases in alkalinity, porosity, and water retention, following nonlinear dose–response trends with clear thresholds beyond f_BC ≈ 0.3–0.5. FT-IR spectroscopy revealed the progressive appearance of oxygenated and aromatic functional groups, accompanied by a reduction in signals from adsorbed water and native soil polar groups. Fast Field-Cycling NMR relaxometry provided molecular-scale insight into soil–water interactions. At high biochar contents, water proton T₁ relaxation times were markedly lengthened, indicating a reduced overall efficiency of surface-driven relaxation. Correlation-time (τ_c) analysis further revealed the emergence of water populations with longer correlation times and a redistribution of relaxation pathways toward outer-sphere dominated mechanisms. Overall, the results indicate that biochar improves soil water retention not by strong surface adsorption but through effective pore-space storage, keeping water available for biological use. The combined spectroscopic and relaxometric approach establishes a direct link between molecular-level water dynamics and macroscopic soil properties, highlighting the value of FFC-NMR as a powerful tool for studying natural porous systems.

Pyridines are a crucial class of heterocycles with widespread applications in natural products, pharmaceuticals, and fluorescent organic materials. In this manuscript, we report the results from a kinetic and mechanistic investigation of an inverse-electron-demand Diels–Alder (IEDDA) cycloaddition involving an oxazole-type diene synthesized via an Ugi–Zhu multicomponent reaction (UZ-3CR). This heterodiene reacts efficiently with various dienophiles such as E-4-oxopentenoic acid, fumaric acid, and monoethyl maleate, yielding highly substituted pyridines in good to excellent yields. Reaction conditions were optimized, and the influence of solvent polarity on regioselectivity was evaluated. The necessity of protonation for successful cycloadditions was probed using structurally diverse dienophiles, revealing the essential role of the carboxylic acid group in triggering the reactions. Mechanistic insights were supported by a comprehensive NMR study (¹H, ¹³C, and ¹⁵N), which provided indirect evidence of in situ protonation of the oxazole ring. Notably, ¹⁵N NMR revealed significant downfield shifts of the oxazole nitrogen, consistent with its protonation, and the emergence of new nitrogen signals corresponding to pyridine products. This study demonstrates the synthetic utility of Ugi–Zhu-derived 5-aminooxazoles in IEDDA cycloadditions and highlights the critical role of acid-promoted activation in enabling efficient pyridine synthesis. We report the results from a kinetic and mechanistic investigation of an IEDDA cycloaddition involving an oxazole-type diene synthesized via an UZ-3CR.