Journal of Magnetic Resonance Open最新文献

Layered rare earth hydroxides (LREHs) are a new family of ion-exchangeable layered metal hydroxides, which have extensive applications in various fields due to the unique properties of rare earth cations in the layered structure and the anion exchange capacity. The transformation of layered metal hydroxides to new layered phases that can be restored through the memory effect is critical for their chemistry and applications. However, the structure details of these new phases such as the coordination environments of rare earth cations/counterions and their evolution as a function of calcination temperature remain unclear to date. Herein, a comprehensive ⁸⁹Y/³⁵Cl solid-state NMR (ssNMR) and theoretical modeling approach was used to reveal the structural evolution of a representative LREH, namely LYH-Cl, upon calcination. We first identified partial decomposition products of Y₃O(OH)₅Cl₂ and Y(OH)₃ during the dehydration stage, then uncovered the preferential removal of hydroxide ions on yttrium sites coordinated with chlorine during the dehydroxylation stage, and finally determined the preferential removal of chlorine exposed to the surface of layers during the dechlorination stage. The coordination environments of Y³⁺ and Cl⁻ undergo significant changes upon calcination, revealed by ssNMR experiments. These findings thus help us to overcome the obstacles impeding the rational design and synthesis of LREH-based functional materials via memory effect, underscoring the vast potential of ssNMR in deepening the understanding of layered metal hydroxides and related materials.

In this tutorial paper, we describe some basic principles and practical considerations for designing probe circuits for NMR or MRI. The goal is building a bridge from material that is familiar from undergraduate physics courses to more specialized information needed to put together and tune a resonant circuit for magnetic resonance. After a brief overview of DC and AC circuits, we discuss the properties of circuit elements used in an NMR probe and how they can be assembled into building blocks for multi-channel circuits. We also discuss the use of transmission lines as circuit elements as well as practical considerations for improving circuit stability and power handling.

A versatile strategy for synthesizing tailored peptide based biradicals is presented. By labeling the protected amino acid hydroxyproline with PROXYL via the OH functionality and using this building block in solid phase peptide synthesis (SPPS), the obtained peptides become polarization agents for DNP enhanced solid-state NMR in biotolerant media. To analyze the effect of the radical position on the enhancement factor, three different biradicals are synthesized. The PROXYL spin-label is inserted in a collagen inspired artificial peptide sequence by binding through the OH group of the hydroxyproline moieties at specific position in the chain. This labeling strategy is universally applicable for any hydroxyproline position in a peptide sequence since solid-phase peptide synthesis is used to insert the building block. High performance liquid chromatography (HPLC) and mass spectrometry (MS) analyses show the successful introduction of the spin label in the peptide chain and electron paramagnetic resonance (EPR) spectroscopy confirms its activity. Dynamic nuclear polarization (DNP) enhanced solid-state nuclear magnetic resonance (NMR) experiments performed on frozen aqueous glycerol-d₈ solutions containing these peptide radicals show significantly higher enhancement factors of up to 45 in ¹H→¹³C cross polarization magic angle spinning (CP MAS) experiments compared to an analogous mono-radical peptide including this building block (ε ≈ 14). Compared to commercial biradicals such as AMUPol for which enhancement factors > 100 have been obtained in the past and which have been optimized in their structure, the obtained enhancement up to 45 for our biradicals presents a significant progress in radical design.

The discovery of novel experimental techniques often lags behind contemporary theoretical understanding. In particular, it can be difficult to establish appropriate measurement protocols without analytic descriptions of the underlying system-of-interest. Here we propose a statistical learning framework that avoids the need for such descriptions for ergodic systems. We validate this framework by using Monte Carlo simulation and deep neural networks to learn a mapping between nuclear magnetic resonance spectra acquired on a novel low-field instrument and proton exchange rates in ethanol-water mixtures. We found that trained networks exhibited normalized-root-mean-square errors of less than 1 % for exchange rates under 150 s⁻¹ but performed poorly for rates above this range. This differential performance occurred because low-field measurements are indistinguishable from one another for fast exchange. Nonetheless, where a discoverable relationship between indirect measurements and emergent dynamics exists, we demonstrate the possibility of approximating it in an efficient, data-driven manner.

Benchtop NMR is enjoying a renaissance with numerous manufacturers bringing products to the market over the last decade. The improved accessibility, lower cost of ownership and ease of use (vs high field NMR), is attracting new users into NMR spectroscopy, which is highly beneficial for the field in general. As benchtop NMR systems seldom require deuterated solvents, this allows samples to be analyzed “as is”, without extraction or alteration. However, many interesting samples, be it an organic reaction mixture, beer, or a biofluid, contain one or more solvent/water signals, which often require suppression. Due to the lower spectral dispersion of benchtop NMR's (vs high field) the frequency of solvent/water is much closer to the analytes of interest, making solvent suppression more challenging. As such, there is a conundrum, where novel users wish to analyze unaltered samples but are quickly faced with the challenge of water suppression, and the wealth of options in the high field literature can be overwhelming. It is important to note that all manufacturers offer some sort of automated water suppression that can be performed with a “single click” that are sufficient for “walk-up” applications or occasional users. This primer is aimed as an accessible guide to those wishing to take the next step and is suitable for users who; 1) would like to pick the optimal water suppression approach for their sample type and 2) wish to understand how water suppression works. The guide focuses on water suppression approaches that are easy to apply, namely presaturation based sequences, binomial sequences for aggressive suppression, and WET for multiple signal suppression, across a range of samples including sucrose standards, espresso, human blood serum and wine. The primer finishes with a flow chart that can be used to guide users in choosing the optimal water suppression approach for their specific sample type, with considerations, including exchangeable signals and the preservation of macromolecular signals, amongst others. In addition, the primer includes 3 fully interactive videos based on H5P technology, focusing on how to acquire data using the approaches described here. The videos include quizzes, with a first-person-perspective of the spectrometer software with interactive elements, as if the users were acquiring the data themselves. In summary, the primer is aimed at advanced undergraduates, graduate students, new users, or users wishing to expand their water/solvent suppression skills/knowledge using benchtop NMR.

Hyperpolarized [1–¹³C]pyruvate is the leading metabolite used in the emerging field of hyperpolarization-enhanced MRI. Signal amplification by reversible exchange (SABRE) is a straight forward hyperpolarization method that has recently been shown to hyperpolarize [1–¹³C]pyruvate at low (microtesla and below) magnetic fields. Here, we show that commercial optical magnetometers with Rb-vapor media can be used to readily monitor the build-up and decay of the hyperpolarized MR signal. In addition, we measure ZULF-NMR spectra in various conditions, ranging from a J-coupling-dominated regime transitioning into a Zeeman-dominated regime when going from a sub-nT field to a µT field. The experimentally acquired spectra are matched well by numerical simulations.

Though the Ernst angle concept presented in the original paper introducing Fourier NMR spectroscopy was developed for sensitivity optimization in a time-averaged single-pulse experiment it is shown here that its conclusions may be generalized to complex multidimensional experiments. The salient message is to explore (re-)design of NMR pulse sequences to return some of the magnetization to the z axis at the end, so that they can be performed without a relaxation delay. In favorable cases, such pulse sequences can be concatenated in a synergistic way to further enhance sensitivity.

Background and Significance

Achievable signal to noise ratios (SNR) in magnetic resonance microscopy images are limited by acquisition times and the decreasing number of spins in smaller voxels. A common method of enhancing SNR is to cool the RF receiver coil. Significant SNR gains are realized only when the Johnson noise generated within the RF hardware is large compared to the electromagnetic noise produced by the sample. Cryogenic cooling of imaging probes is in common use in high field systems, but it is difficult to insulate a sample from the extreme temperatures involved and in practice imaging cryoprobes have been limited to surface or partial volume designs only. In order to use a solenoid in which the windings were not directly cooled and in close proximity to the sample, we designed a chamber to cool only the tune and match circuitry and show experimentally it is possible to achieve much of the theoretically available SNR gain.

Methods

A microcoil circuit consisting of two tuning capacitors, one fixed capacitor, and SMB coaxial cable was designed to resonate at 650 MHz for imaging on a Bruker 15.2 T scanner. Sample noise increases with the sample diameter, so surface loops and solenoids of varying diameters were tested on the bench to determine the largest diameter coil that demonstrated significant SNR gains from cooling. A liquid N₂ cryochamber was designed to cool the tune and match circuit, coaxial cable, and connectors, while leaving the RF coil in ambient air. As the cryochamber was filled with liquid N₂, quality factors were measured on the bench while monitoring the coil's surface temperature. Improvements of SNR on images of ionic solutions were demonstrated via cooling the tune and match circuit in the magnet bore.

Results

At 650 MHz, loops and solenoids < 3 mm in diameter showed significant improvements in quality factor on the bench. The resistance of the variable capacitors and the coaxial cable were measured to be 45% and 32% of room temperature values near the Larmor frequency. Images obtained with a 2 turn, 3 mm diameter loop with the matching circuit at room temperature and then cooled with liquid nitrogen demonstrated SNR improvements of a factor of 2.

Conclusions

By cooling the tune and match circuit and leaving the surface loop in ambient air, SNR was improved by a factor of 2. The results are significant because it allows for more space to insulate the sample from the extreme temperatures used in imaging cryoprobes.

From complex-mixture analysis to in vivo molecular imaging, applications of liquid-state nuclear spin hyperpolarization have expanded widely over recent years. In most cases, hyperpolarized solutions are generated ex situ and transported from the polarization instrument to the measurement device. The sample hyperpolarization usually survives this transport, since the changes in magnetic fields that are external to the sample are typically adiabatic (slow) with respect to the internal nuclear spin dynamics. The passage of polarized samples through weakly magnetic components such as stainless steel syringe needles and ferrules is not always adiabatic, which can lead to near-complete destruction of the magnetization. To avoid this effect becoming “folklore” in the field of hyperpolarized NMR, we present a systematic investigation to highlight the problem and investigate possible solutions. Experiments were carried out on: (i) dissolution-DNP-polarized [1-¹³C]pyruvate with NMR detection at 1.4 T, and (ii) 1.5-T-polarized H${}_2$O with NMR detection at 2.5 $\mu$T. We show that the degree of adiabaticity of solutions passing through metal parts is intrinsically unpredictable, likely depending on many factors such as solution flow rate, degree of remanent ferromagnetism in the metal, and nuclear spin species. However, the magnetization destruction effects can be suppressed by application of an external field on the order of 0.1–10 mT.

A prototype scannable unilateral permanent (SUPER) magnet for use with the electron paramagnetic resonance (EPR) mobile universal surface explorer (MOUSE) is described. The unilateral magnetic field is scannable from -94 to 94 mT by changing the relative angles of two fixed position ring magnets. The angular dependence of the modeled and measured magnetic fields are in agreement. The SUPER magnet is demonstrated on both the narrow spectral line sample DPPH as well as the broad spectral line sample rhodochrosite using the EPR MOUSE.