Journal of Magnetic Resonance Open最新文献

Magnetic resonance imaging (MRI) is a well-known and widespread imaging modality for neuroscience studies and the clinical diagnoses of neurological disorders, mainly due to its capability to visualize brain microstructures and quantify various metabolites. Additionally, its noninvasive nature makes possible the correlation of high-resolution MRI from ex vivo brain samples with histology, supporting the study of neurodegenerative disorders such as Alzheimer's or Parkinson's disease. However, the quality and resolution of ex vivo MRI highly depend on the availability of specialized radiofrequency coils with maximized filling factors for the different sizes and shapes of the samples to be studied. For instance, small, dedicated radiofrequency (RF) coils are not always commercially available in ultrahigh field whole-body MRI scanners. Even for ultrahigh field preclinical scanners, specific RF coils for ex vivo MRI are expensive and not always available. Here, we describe the design and construction of two RF coils based on the solenoid geometry for ex vivo MRI of human brain tissues in a 7T whole-body scanner and for ex vivo MRI of marmoset brain samples in a 9.4T preclinical scanner. We designed the 7T solenoid RF coil to maximize the filling factor of human brain samples conditioned on cassettes for histology, while the 9.4T solenoid was constructed to accommodate marmoset brain samples conditioned in 50 ml centrifuge tubes. Both solenoid designs operate in transceiver mode. The measured B₁⁺ maps show a high level of homogeneity in the imaging volume of interest, with a high signal-to-noise ratio over the imaging volume. High-resolution (80 µm in plane, 500 µm slice thickness) images of human brain samples were acquired with the 7T solenoid, while marmoset brain samples were acquired with an isotropic resolution of 60 µm using the 9.4T solenoid coil.

Oxide-based materials are of key technological importance in different areas including advanced functional materials, solid state chemistry and catalysis. Many of the key questions concerning these areas involve understanding the chemical bond between the metal and the oxygen ions in the first or subsequent coordinating shells. The spectroscopic study of oxygen is therefore of fundamental importance to elucidate the complex interfacial coordination chemistry that underlies the development of metal-oxide supported catalysts and other advanced materials. Oxygen atoms at solid surfaces or lining the pores of zeolite frameworks play a vital role in stabilizing and defining the electronic and geometric structure of single metal atoms or clusters that act as catalytically active sites. In the case of paramagnetic species, EPR and its related hyperfine techniques offer a unique opportunity to explore and understand the nature of the chemical bonding in metal-oxide systems through the detection of the ¹⁷O hyperfine interaction. In this perspective we offer an overview of experimental considerations and relevant examples specific to ¹⁷O hyperfine spectroscopy of transition metal ions in zeolites relevant to catalysis. ¹⁷O hyperfine coupling values are obtained, which allow discriminating σ- and π-bonding channels in metal-oxygen bonds involving first-row transition metal ions. An exhaustive collection of ¹⁷O hyperfine and nuclear quadrupole couplings in different systems including molecular and biomolecular chemistry is provided, emphasizing the connection between interfacial and molecular inorganic coordination chemistry.

Molecules are dynamic entities, and understanding intra- and inter-molecular reactions and changes in conformation is one of the most fascinating, important and complex subjects in NMR. Conformational changes and chemical reactions result in observed spins exchanging between different magnetic environments, and the sensitivity of NMR spectra to such dynamic processes has been recognised since the earliest days of the field. Careful analysis of such spectra, acquired using one- or two-dimensional experiments, can provide insight into structural, thermodynamic, kinetic and mechanistic aspects of the underlying exchange process. The theoretical principles of these lineshape analysis methods will be introduced in this article, alongside a practical discussion of calculation methods, data acquisition and analysis software.

A novel closed loop, continuous flow (CF) reactor system for parahydrogen enhanced nuclear magnetic resonance (NMR) of liquids via heterogeneous catalysis is introduced which enables recycling of unreacted liquid substrate reactant. This system consists of an HPLC pump, a liquid substrate reservoir incorporating a gas diffuser, an all-metal packed bed catalytic reactor, and an AF-2400 tube-in-tube gas permeable membrane for removal of normal H₂. Two types of supported metal nanoparticle catalysts were tested: mesoporous silica encapsulated Pt₃Sn intermetallic nanoparticles and a Rh on anatase TiO₂ support. In the CF hydrogenation of propargyl acetate to allyl acetate, the hyperpolarized signals exhibited stability over 20 min of recirculation, with signal enhancements of up to 626 using 99% p-H₂ and negligible leaching of the catalyst into the flowing solutions. These results demonstrate the practicality of performing systematic optimization of conditions for continuous flow catalysis and polarization transfer to heteronuclei with important implications for biomedical magnetic resonance imaging.

¹H and ¹⁹F NMR methods based on chemical shift measurements of different hydrogen bond donors used for quantifying hydrogen bond strength were analyzed and compared. The extracted values from these different methods are shown to be highly correlated with each other and with several experimental and ab initio computed quantities characterizing hydrogen bond formation. The titration method based on ¹⁹F NMR spectroscopy was performed for detecting and quantifying the formation of very weak hydrogen bond complexes such as those involving fluorine atoms as hydrogen bond acceptors. This approach represents a powerful and reliable method for studying and characterizing these complexes that, although weak, are very relevant.

Peak overlap hampers quantification in one-dimensional (1D) ¹H NMR. 2D ¹H -¹³C HSQC spectrum provides resolution superior to 1D ¹H NMR. However, quantifying the components in a complex mixture with HSQC is not straightforward as in 1D ¹H NMR. Quantification using HSQC could open up new avenues for studying metabolism. The variations in ¹H–¹³C scalar couplings, T₁, T₂, and pulse imperfections contribute to this problem. Although T₁ and T₂ can be suitably chosen to minimize their deleterious effects, the differential polarization transfer for different resonances owing to large variations in ¹H -¹³C couplings does not allow the cross-peak intensities to be directly correlated to the quantity of metabolites. Existing approaches are time-consuming. We show that spatial encoding of the polarization transfer delays in HSQC using sweep frequency pulses in the presence of a magnetic field gradient allows one to have a transfer of polarization from ¹H to ¹³C insensitive to variations in ¹H -¹³C couplings improving the quantitative aspect of HSQC. Comparisons to other QHSQC and perfected HSQC variants are also provided.

Methyl substitution in ortho position causes a deshielding of 6–7 ppm on the ¹³C NMR chemical shift of the own methyl group and the carbon nucleus bonded to iodine atom (ipso) in iodobenzene-like molecules. In contrast, the carbon ipso is 3–4 ppm shielded when methyl is in para. To understand how the position of methyl substitution perturbs nuclear magnetic responses in iodobenzene and diacetoxyiodobenzene derivatives, shielding mechanisms are theoretically investigated via density functional theory calculations. We show the relative ortho position between iodine and methyl allows through-space and through-bond interactions to take place, generating additional paramagnetic currents and affecting the spin-orbit coupling propagation. Relevant paramagnetic couplings that explain the para methyl substitution behavior are also presented. Shielding mechanisms discussed here for monomethylated compounds can be summed to predict the ¹³C NMR chemical shift in multi methyl substituted iodine-containing compounds.

NMR studies exploit spin relaxation in a multitude of different ways, providing information on molecular structure and dynamics. Calculating the relaxation rates of NMR active nuclei in multi-spin systems is often a prerequisite for the proper analysis of experimental data. For many researchers the calculations appear complex, often involving different basis sets or expressions describing relaxation in different frames. In this tutorial paper we derive expressions for dipolar relaxation of an I-S two spin spin-system in the presence of a B₁ radio frequency field, where spins I and S can be either like or unlike. We consider two different approaches for the derivation of relaxation elements that have been used in the literature, including one where a series of transformations are carried out to the interaction representation of the effective field, comprising B₁ and Zeeman components. A second procedure is based on the well-known Solomon equations. We show that both approaches lead to identical results, in the process presenting a pedagogical description of relaxation theory.

This article is the content of the closing lecture delivered at the XIVth Manuel Rico Advanced NMR Course, held in Jaca (Spain) in June 2022. It is my personal impression on the technical frontiers where I expect new advances to expand even further the fields of NMR application, especially in the fields of biomedicine and biotechnology. It is dedicated to Robert Konrat on the occasion of his 60th birthday as an acknowledgement of his wide range of interest and vast scientific culture.