Journal of Magnetic Resonance Open最新文献

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.

In this work we present a 2D NMR experiment that provides insight into the full chemical exchange saturation transfer (CEST) network present in a sample. It yields all CEST profiles between any signals in a spectrum at once. The method relies on a combination of slice selective saturation during the preparation period, combined with an inverse read-out gradient applied during the evolution time. The resulting 2D spectrum yields gradient profiles in F1 with dips at the frequencies of signals that show a CEST to the corresponding signal in F2.

Aimed at fundamental understanding and design of advanced magnetic resonance experiments on basis of Hamiltonians, we describe highly convergent and exact effective Hamiltonian methods which alleviate important deficits of current less accurate methods. This involves single-spin vector effective Hamiltonian theory (SSV-EHT) to first order in the interaction frame of rf and chemical shift offsets as well as exact effective Hamiltonian theory (EEHT) being an exact approach to average Hamiltonian theory not relying on interaction frame transformations. Bringing these methods together, we present tools to analyze challenging experiments in need of considering large static components in Hamiltonian (e.g., offsets) while economizing with radiofrequency irradiation power. It is demonstrated how the two complementary tools may provide important new insight into the detailed effective Hamiltonians of advanced NMR experiments, noting that the methods are by no means restricted to NMR. This is demonstrated for isotropic mixing in liquid-state NMR and dipolar recoupling in solid-state NMR where insight into the delicate interplay between bilinear two-spin and linear single-spin terms in the effective Hamiltonian may increase understanding of determinants for broadband excitation and the formation of recoupling resonances. Furthermore, we demonstrate how simple products single-spin effective Hamiltonians may be used as generators of multiple-spin effective Hamiltonians and though this a new approach to density operator calculations for large multiple-spin systems.

In this work a rapid RNA assignment approach by combining chemical and enzymatic ¹³C and ¹⁵N stable isotope labeling is introduced. We exemplify the assignment strategy for imino N1H1 purine and N3H3 pyrimidine and aromatic C6H6 pyrimidine, C8H8 purine and C2H2 adenine resonances for a non-coding RNA comprising 66 nucleotides. The assignment strategy is based on position specific labeling by chemical solid phase synthesis and dilute stable isotope ¹³C/¹⁵N-labeling by mixing labeled and commercially available unlabeled RNA phosphoramidites. The assignment process is further facilitated by nucleotide specific labeling using T7 RNA polymerase in vitro transcription with in house produced atom specific ¹³C labeled ribonucleotide triphosphates. The approach is fast with a total NMR measurement time of only 22 h and also competitive in terms of costs as compared to the standard methodology relying on in vitro transcription using ²H, ¹⁵N and ¹³C/¹⁵N uniformly labeled ribonucleotide triphosphates. Furthermore, the assignment procedure revealed a slow exchange process on the NMR chemical shift time scale in the 66 nt non-coding RNA with possible biological implications in the regulation of bacterial competence.

Metabolomics is one of the leading approaches for understanding the toxic-mode-of-action of environmental contaminants. Nuclear Magnetic Resonance (NMR) spectroscopy has been commonly used in metabolomic studies; however, its main drawback is its relatively low sensitivity, making it challenging to study mass limited but environmentally crucial samples. In this work a 1 mm microlitre probe modified with a separate lock chamber to address this challenge, provided substantial improvements in mass sensitivity relative to conventional 5 mm NMR probes. The 1 mm probe is used to analyze various components of the model organism Daphnia magna, including hemolymph, parthenogenetic eggs, dormant eggs, and neonates. A μL volume flow system is designed for the 1 mm probe to perform an in-vivo exposure of neonates to high salt concentrations. The metabolic investigation of these samples was only achieved due to the minimum sample requirements and high salt tolerance of the probe, demonstrating that the 1 mm microlitre probe modified with a separate lock chamber holds significant potential for future metabolomic studies of mass limited samples.