Journal of Magnetic Resonance Open最新文献

Although very effective in decreasing NMR relaxation of large proteins, homogeneous deuteration can be costly, and anyway unsuitable for recombinant production in metazoan systems. We sought to explore other deuteration schemes, which would be adapted to protein expression in mammalian cells. Here, we evaluate the benefits of the deuteration on alpha- and beta-positions of amino acids for a typical middle size protein domain, namely the model 40 kDa-large kinase p38α. We report the position-specific deuteration of free amino acids by using enzyme-assisted H/D exchange, executed by the cystathionine gamma-synthase and a newly designed high-performance mutant E325A. Then, we used cell-free expression in bacterial extracts to avoid any scrambling and back-protonation of the tested isotopically labelled amino acids (Ala, Leu, Lys, Ser, Asp, Glu, Gly). Our results show signal enhancements up to three in ¹H-¹⁵N spectra when these α/β-deuterated amino acids are integrated. Because our approach relies on single ²H_α/β-¹⁵N-amino acid labeling, an additional three-fold increase in sensitivity is obtained by the possible use of moderate resolution SOFAST-HMQC instead of the classical HSQC or TROSY experiments. This allows recording residue-resolved solution ¹H-¹⁵N NMR spectra of 100 μg of p38α in one hour with S/N∼10.

Dynamic Nuclear Polarization (DNP) is transforming the landscape of solid-state characterization for both biological solids and functional materials. By transferring electron spin polarization to coupled nuclear spins under microwave irradiation, DNP increases NMR sensitivity by several orders of magnitude. However, the mechanism of DNP transfer and its efficiency under magic-angle spinning (MAS) significantly differs from that under static conditions. This primer article provides a comprehensive and pedagogical explanation of the theoretical aspects of MAS-DNP, with a specific focus on the cross-effect mechanism. A clear understanding of the nuances of MAS-DNP is crucial for improving its efficiency and extending its application to high magnetic fields and fast MAS conditions. To this end, the article proposes a guideline for synthetic chemists to develop DNP polarizing agents under these experimental conditions.

In this article we use numerical simulations to study the effect of T₁ dispersion on fluid polarization buildup in oil flow to characterize the sensitivity of both a conventional NMR concept (ROI located inside the polarization magnet) and a Earth's field NMR concept (ROI outside and downstream of the polarization magnet) to T₁ dispersion of flowing samples. As a polarization field in both concepts we use a 90 cm long Halbach magnet. The T₁ dispersion behavior of the oils is based on a set of crude oils that span a viscosity range of 0.7 cP up to 2·10⁴ cP and T₁ relaxation measurements for Larmor frequencies between 10 kHz and 20 MHz. Numerical simulations based on solving the Bloch-Torrey equation for the longitudinal magnetization component show that fluid polarization levels in a ROI of a Earth's field NMR system concept are much more strongly affected by T₁ dispersion than in the conventional NMR system concept. As a result, we may conclude that the Earth's field NMR system design is less robust for measuring flowing samples that show strong T₁ dispersion behavior. In comparison, the conventional NMR system design is relatively insensitive to the effect of T₁ dispersion, as T₁ dispersion effects were found to form a relatively small correction to the magnetization buildup. The conventional NMR system design consequently is the preferred implementation of a NMR system that operates on fluids with strong T₁ dispersion behavior. We show that in the presence of T₁ dispersion s = vT₁(0)/L_m* may be used as a governing parameter for fluid polarization buildup, where T₁(0) is the T₁ relaxation time in the center of the polarization magnet, and we show how an modified analytical uniform field model can be used to describe fluid polarization for a uniform flow velocity distribution in the presence of T₁ dispersion with an accuracy within 1% for the samples and field distribution considered in this study at industrially relevant flow velocities.

Rapid-scan electron paramagnetic resonance spectroscopy is an emerging technique which substantially improves the signal-to-noise ratio and time resolution compared to conventional continuous-wave experiments. This allows the investigation of spin-labeled biomolecules and their structural dynamics on much shorter time scales than usually accessible. The EPR spectrum however is superimposed by a strong background that is caused by microphonic effects of the alternating magnetic field. This article discusses the use of non-quadratic cost functions for background removal of rapid-scan spectra. The method is validated for the most prominent type of spin-probes in the field of biochemistry: the nitroxide spin-label.

The receptivity of NMR spectroscopy is low when compared to other techniques. Historically, increasing the strength of the static magnetic field has been the major approach to increase NMR sensitivity. In recent years several polarization transfer protocols have been used to enhance the signal-to-noise ratio (SNR), although they require special accessories and/or sample preparation. In this paper, we consider both the challenges and opportunities of steady-state free precession (SSFP) pulse sequences as a simple and efficient alternative to enhance SNR, in standard high-resolution and benchtop low-resolution NMR spectrometers. The maximum gain in these sequences is obtained with the shortest time between the pulses (Tp). However, when Tp<T₂, the SSFP signal contains FID and echo components which lead to phase, intensity, and truncation artifacts on spectra obtained by Fast Fourier transform (FT). Several phase alternation SSFP sequences were used to cancel the echo component and minimize these problems in the FT spectra. Krylov base diagonalization method (KBDM) was used to eliminate the phase and truncation problems in spectra acquired by SSFP pulse sequences and can be a viable alternative to FT. The experiments were performed in high and low resolution (bench top) NMR spectrometers and significant enhancements in SNR of low receptivity nuclei such as ¹³C and ¹⁵N could be achieved. The SSFP sequences were also shown to enhance SNR in nuclei with high receptivity such as ¹⁹F and ³¹P, in very dilute samples, as is common in environmental and biological samples.

Solid composite electrolytes combining an ionic molecular phase to facilitate ion transport with a polymeric component to provide mechanical strength are promising material for solid-state batteries. However, the structure-property relationships of these complex composites are not fully understood. Herein we study composites combining the non-flammability and thermal stability of the organic ionic plastic crystal (OIPC) N-methyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl) amide [C₂mpyr][TFSI] with the mechanical strength of acrylic polymer nanoparticles functionalised with sulphonamide groups having lithium counter-cations. The effect of the formation of interfaces and interfacial regions between the OIPC and polymer nanoparticle on the thermal stability, ion transport, morphology and ion dynamics were studied. It was found that the composites where an interphase was formed by local mixing of the polymer with the OIPC upon heating showed higher local disorder in the OIPC phase and enhanced ion transport in comparison with the as-prepared composites. In addition, doping the composite with LiTFSI salt led to further structural disorder in the OIPC and a selective increase in lithium-ion mobility. Such an improved fundamental understanding of structure, dynamics and interfacial regions in solid electrolyte composites can inform the design of OIPC-polymer nanoparticle composites with enhanced properties for application as solid electrolyte in batteries.

Chemical exchange saturation transfer (CEST) NMR is widely used for enhancing the sensitivity of low-abundance exchanging sites in general, and for the water-based detection of labile metabolite protons under in vivo conditions in particular. CEST, however, faces a number of limitations when targeting multiple metabolites, including a radiofrequency (RF)-induced broadening of the detected peaks, and relatively long acquisition times deriving from its continuous-wave nature. Methods have been proposed to overcome these limitations, including a Fourier-encoded version of CEST –the Frequency-Labeled EXchange (FLEX) experiment– and the incorporation of background gradients during the RF saturation time. This work explores an alternative avenue, based on spatiotemporally encoded ultrafast (UF) 2D NMR. UF NMR can compress the time-domain indirect-dimension encoding of 2D NMR into a single shot; to exploit these potential time savings, an UF version of the FLEX experiment was taken as starting point, and the multiple t₁-incremented amplitude modulation cycles that the FLEX experiment normally requires were replaced by a single-shot spatiotemporal encoding. The ensuing UF 2D FLEX experiment was then used to monitor the spectral signatures of multiple moieties as they exchange with the solvent, by imprinting these onto the water resonance as in the original experiment –but now all within a single shot. Upon incorporating two-scan phase cycling and quadrature detection, the resulting method showed an experimental performance similar to t₁-encoded FLEX, while providing significant time savings plus imaging information that could be of further use in in vivo studies. The main advantages, features and drawbacks observed for UF 2D FLEX are briefly discussed.

Nuclear magnetic resonance is widely used to probe the dynamics of fluids confined in porous media, where structural and functional properties of complex systems can be determined. The application includes a large variety of research or industrial areas such as medicine, gas and oil extraction, soil studies, the development of materials for drug delivery, and catalysis, among many other applications. This review covers the use of different NMR experiments applied to study the liquid/surface interaction, mobility, tortuosity, pore connectivity, and exchange phenomena in different porous matrixes. This article, included in the special issue devoted to NMR in Latin America, provides a review of the most applied techniques and a summary of different applications carried out at the Universidad Nacional de Córdoba in Argentina.

In this manuscript we deal with recent advances in low-field Magnetic Resonance Imaging (MRI). The development of low-cost MRI solutions allowing portability and trustable diagnosis is a hot topic worldwide by these days. We analyze basic technical issues of recent examples of fixed-field instruments operating at low-field. Then we discuss pros and cons of the pre-polarized approach, from both physical and technical perspectives. Permanent magnet and electromagnet technology are confronted. Finally, magnetic field-cycling is introduced as an alternative MRI technique, where field-dependent experiments can be exploded for the development of new contrast mechanisms that are not feasible for fixed-field MRI instruments. As field cycled machines usually deals with switched currents in electromagnets, magnetic field instability and inhomogeneity are the main limiting factors affecting image quality. We finalize this manuscript discussing how it turns possible to overcome these limitations, thus opening new possibilities for the development of cost effective MRI technology.

Nanoparticles of LiNbO₃ and NaNbO₃ were obtained for the first time by microwave-assisted combustion. Preliminary experiments reveal that the synthetic conditions influence their microstructure and optoelectronic features. Therefore, there is a need for performing the structural characterization of these materials, obtained by this new route. In the case of NaNbO₃, there are two polimorphs which are stable at room temperature, space groups P2₁ma and Pbma. Powder x-Ray diffraction experiments were not capable to identify the crystalline phases present in the nanoparticles. Therefore, we have performed a detailed structural characterization of the nanoparticles by 1D and 2D solid state ²³Na and ⁹³Nb Nuclear Magnetic Resonance (NMR) techniques. ²³Na results reveal the presence of both phases, Pbma and P2₁ma, for samples prepared using NaNO₃ precursor in a 1:1 Na:Nb ratio or NaCl in excess. On the other hand, the P2₁ma polymorph could be isolated in the synthesis using NaCl salt in 1:1 Na:Nb ratio. On the other hand, LiNbO₃ nanoparticles display the usual rhombohedral structure R3c. ⁷Li MAS NMR results reveal the presence of two types of Li species, with distinct dynamics. Highly mobile Li⁺ ions are found at the surface of the nanoparticles, while bulk Li⁺ show restricted movement. Finally, as a proof of principle, the photocatalytic activity of these niobates was tested for the degradation of methylene blue dye, a common organic-water contaminant.