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.

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.

The determination of the relative and absolute configuration of natural compounds is a very challenging task. Among other anisotropic NMR parameters, residual chemical shift anisotropy (RCSA) induced by anisotropic media is an invaluable tool to determine relative configurations of natural and synthetic organic molecules in solution. This review introduces various RCSA-based methodologies for the structural elucidation of natural products. The current availability of alignment media in organic solvents for RCSA measurements is also discussed as are applications of RCSAs for structural analysis of various natural products.

The molecular dynamics of ionic liquids (ILs) can be probed using fast field cycling (FFC) NMR relaxometry. Conventionally, such studies focus on ILs where only one ionic species carries NMR-active nuclei or on systems combining $�{}^{�1�}$H nuclei on the cations with $�{}^{�19�}$F nuclei on the anions. This way, the dynamics of cations and anions can be resolved individually. However, the situation becomes considerably more complex in fully protonated systems where both ions contain protons, because the various relaxation pathways can no longer be disentangled. Here we report the first FFC NMR investigation of such a case, using the IL triethylammonium methanesulfonate ([TEA][OMs]). Our strategy exploits selective partial deuteration of the ionic species, which enables the separate evaluation of cation and anion dynamics. We demonstrate for the first time that, from the known partial relaxation rates together with the determined interionic distances and self-diffusion coefficients, the relaxation contribution arising from cation–anion interactions can be quantified. Remarkably, this approach even allows reconstruction of the total relaxation rate observed experimentally for the fully protonated IL. This methodology provides a fundamentally new route to overcoming the limited spectral resolution of FFC NMR relaxometry at low fields. More broadly, it establishes a framework for disentangling relaxation processes in complex multicomponent systems, thereby extending the applicability of FFC NMR to more challenging classes of ILs and related materials.

A rapid, reagent-free method for the quantification of chitosan in aqueous solutions was developed using ¹H nuclear magnetic resonance (NMR) relaxometry. Transverse relaxation times (T₂) of chitosan solutions (5–1000 mg/L) were measured with a Carr–Purcell–Meiboom–Gill (CPMG) sequence on a benchtop NMR relaxometer. The data were processed using three approaches: monoexponential fitting (Bruker software), inverse Laplace transformation (CONTIN), and one-component curve fitting (Excel Solver). Calibration curves derived from these models demonstrated high linearity and reproducibility, particularly when concentration intervals were segmented. Among the approaches, the most robust correlation was achieved by plotting the absolute area (AA) of the T₂ distribution obtained from CONTIN analysis versus chitosan concentration, yielding R² values of up to 0.9978 for 5–100 mg/L. Comparative analysis at 40°C and 21°C confirmed the method's temperature stability, with improved sensitivity at elevated temperature. Unlike conventional spectrophotometric or chromatographic methods, the proposed protocol requires no chemical derivatization or complex sample preparation. This technique provides a fast, accurate, and non-destructive alternative for chitosan quantification, especially suitable for material science applications where precise concentration monitoring is critical, such as surface modification of nanomaterials.