Approaches to NMR sensitivity enhancement based on theoretical analysis

Dynamic nuclear polarization (DNP) has significantly enhanced the sensitivity of nuclear magnetic resonance (NMR) experiments. While Overhauser DNP has a well-established nuclear counterpart, the nuclear Overhauser effect (NOE), the cross-effect DNP has traditionally been considered as an exclusively electron-mediated process. Here, we present the first evidence of a nuclear counterpart to the cross effect, termed the nuclear cross effect, which mediates polarization transfer in a three-spin nuclear system. The effect was experimentally verified using single-crystal 13C-15N-labeled glycine, with results consistent with both analytical theory and numerical simulations. The experimental realization of the pulsed nuclear cross-effect in this work provides new insights into its theoretical foundations, addressing challenges posed by hardware limitations and the absence of a suitable model system for pulsed cross-effect DNP. Furthermore, the pulsed cross-effect mechanism has the potential to advance enhanced NMR applications in various fields.

Outlining biological processes and disease mechanisms requires a real-time understanding of cellular metabolism. Mass spectrometry (MS) and Nuclear magnetic resonance (NMR) serve as potent analytical methods for metabolomics, leveraging their advantages in the identification and quantification of metabolites. In this review we have discussed MS and NMR based techniques, such as matrix-assisted laser desorption/ionization (MALDI), secondary ionization mass spectrometry (SIMS), desorption electrospray ionization (DESI), direct analysis in real time (DART), and nano electrospray ionization (nano ESI), to develop and implement a MS based technique in its live state for single cell and NMR strategies for thorough, real-time analysis of cell metabolism at bulk cellular level. Single-cell metabolomic investigations using mass spectrometry is a valuable tool for understanding cellular heterogeneity and cell-to-cell variation. However, they can reveal hidden processes and heterogeneity across cells that are often missed by bulk cell analysis. Overcoming the inherently low sensitivity of NMR is crucial for omics studies. This review examines hyperpolarization techniques, including dynamic nuclear polarization (DNP), parahydrogen-induced polarization, sample amplification by reversible exchange (SABER), high-resolution magic angle spinning (HRMAS) NMR, and in vivo magnetic resonance spectroscopy (MRS) for the analysis of live bulk cells. Our discussion encompasses the technological platforms and recent applications of these techniques, utilizing both NMR and MS, along with an overview of metabolomics data analysis tools, such as principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA).

The inherently low sensitivity of nuclear magnetic resonance(NMR)spectroscopy is the major limiting factor for its application to elucidatestructure and dynamics in solids. In the solid state, nuclear spinhyperpolarization methods based on microwave-induced dynamic nuclearpolarization (DNP) provide a versatile platform to enhance the bulkNMR signal of many different sample formulations, leading to significantsensitivity improvements. Here we show that 1H NMR hyperpolarizationcan also be generated in solids at high magnetic fields by opticalirradiation of the sample. We achieved this by exploiting a donor–chromophore–acceptormolecule with an excited state electron–electron interactionsimilar to the nuclear Larmor frequency, enabling solid-state 1H photochemically induced DNP (photo-CIDNP) at high magneticfields. Through hyperpolarization relay, we obtained bulk NMR signalenhancements εH by factors of ∼100 at both9.4 and 21.1 T for the 1H signal of o-terphenylin magic angle spinning (MAS) NMR experiments at 100 K. These findingsopen a pathway toward a general light-induced hyperpolarization approachfor dye-sensitized high-field NMR in solids.

Various hyperpolarization methods are able to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) by several orders of magnitude. Among these methods are para-hydrogen-induced polarization (PHIP) and signal amplification by reversible exchange (SABRE), which exploit the strong nuclear alignment of para-hydrogen. Several SABRE experiments have been reported but, so far, it has not been possible to account for the experimentally observed sign and magnetic-field dependence of substrate polarization. Herein, we present an analysis based on level anti-crossings (LACs), which provides a complete understanding of the SABRE effect. The field-dependence of both net and anti-phase polarization is measured for several ligands, which can be reproduced by the theory. The similar SABRE field-dependence for different ligands is also explained. In general, the LAC concept allows complex spin dynamics to be unraveled, and is crucial for optimizing the performance of novel hyperpolarization methods in NMR and MRI techniques.

Dynamic nuclear polarization (DNP) has become a very important hyperpolarization method because it can dramatically increase the sensitivity of nuclear magnetic resonance (NMR) of various molecules. Liquid-state DNP based on Overhauser effect is capable of directly enhancing polarization of all kinds of nuclei in the system. The combination of simultaneous Overhauser multi-nuclei enhancements with the multi-nuclei parallel acquisitions provides a variety of important applications in both MR spectroscopy (MRS) and image (MRI). Here we present two simple illustrative examples for simultaneously enhanced multi-nuclear spectra and images to demonstrate the principle and superiority. We have observed very large simultaneous DNP enhancements for different nuclei, such as 1H and 23Na, 1H and 31P, 19F and 31P, especially for the first time to report sodium ion enhancement in liquid. We have also obtained the simultaneous images of 19H and 31P, 19F and 31P at low field by solution-state DNP for the first time.

Dynamic nuclear polarisation (DNP) is a process that transfers electron spin polarisation to nuclei by applying resonant microwave radiation, and has been widely used to improve the sensitivity of nuclear magnetic resonance (NMR). Here we demonstrate new levels of performance for static cross-effect proton DNP using high peak power chirped inversion pulses at 94 GHz to create a strong polarisation gradient across the inhomogeneously broadened line of the mono-radical 4-amino TEMPO. Enhancements of up to 340 are achieved at an average power of a few hundred mW, with fast build-up times (3 s). Experiments are performed using a home-built wideband kW pulsed electron paramagnetic resonance (EPR) spectrometer operating at 94 GHz, integrated with an NMR detection system. Simultaneous DNP and EPR characterisation of other mono-radicals and biradicals, as a function of temperature, leads to additional insights into limiting relaxation mechanisms and give further motivation for the development of wideband pulsed amplifiers for DNP at higher frequencies.

para-Hydrogen induced polarisation (PHIP) is an excellent tool for extracting mechanistic information in catalysis since it circumvents the intrinsic low sensitivity of nuclear magnetic resonance (NMR) spectroscopy. We report a class of iron complexes that are highly active in olefin hydrogenation catalysis and act as PHIP catalysts at 1.4 Tesla. Moreover, hyperpolarisation transfer to 19F is observed.

Nuclear magnetic resonance (NMR) has become the method of choice for many types of applications. Still, sensitivity is a limiting factor in the applicability of NMR, leading to long measurement times in advanced multidimensional experiments, and becoming prohibitive when very limited sample quantities are available. This low sensitivity is mostly an intrinsic consequence of the low energy scale of the nuclear moment in a static field, when compared to other thermodynamic energies like k B T. The commercial developments are mostly aimed at an increase in the static field and simultaneously a reduction of the noise using cryocooled detection coils. Current research shows a number of interesting developments toward the enhancement of the nuclear polarization by optical pumping or by transfer from the electronic bath in dynamic nuclear polarization (DNP) experiments. A more technological approach is based on the miniaturization of the RF coils. In the next decade, one may expect the advent of the “lab on a chip” with in situ chemical processing and NMR analysis capabilities. A brave new method to improve detection sensitivity is based on very sensitive micromechanical force detectors. Recently, it was demonstrated that the low-temperature force detection sensitivity is sufficient to detect the magnetic moment of a single (electron) spin. These developments show that the NMR detection limits in terms of absolute sensitivity or imaging resolution are still open to significant improvements.

Dynamic nuclear polarization with photo-excited triplet electrons (triplet-DNP) has the potential to enhance the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) at a moderate temperature. While many efforts have been devoted to achieving a large nuclear polarization based on triplet-DNP, the application of triplet-DNP has been limited to nuclear physics experiments. The recent introduction of materials chemistry into the field of triplet-DNP has achieved air-stable and water-soluble polarizing agents as well as the hyperpolarization of nanomaterials with a large surface area such as nanoporous metal-organic frameworks (MOFs) and nanocrystal dispersion in water. This Feature Article overviews the recently-emerged materials chemistry of triplet-DNP that paves new paths towards unprecedented biological and medical applications.

Gyrotrons have been developed as terahertz wave sources in dynamic nuclear polarisation (DNP) system to increase the sensitivity of nuclear magnetic resonance (NMR) in the high-magnetic field regime. However, the transmission of the Gaussian beam from the gyrotron output window to the NMR system proves to be a critical challenge. In this study, a terahertz transmission system, composed of a rectangular corrugated waveguide and a back-to-back rectangular corrugated horn for Gaussian mode filtering, is proposed. The attenuation in the optimised 0.33 THz corrugated waveguide remains lower than 0.01 dB/m in a broad frequency range from 280 GHz to 450 GHz. Besides, the coupling coefficient from the electric field in the 0.33 THz mode-filtering horn to the fundamental HE11 mode in the rectangular corrugated waveguide is obtained nearly 99.5%. This terahertz transmission system is potential to be applied in DNP-NMR spectrometer based on continuous wave gyrotrons.

Paul Christian Lauterbur

Nuclear Magnetic Resonance (NMR) spectroscopy would be a method of choice to follow biochemical events in cells because it can analyze molecules in complex environments. However, the intrinsically low sensitivity of NMR makes in-cell measurements challenging. Dynamic Nuclear Polarization (DNP) has emerged as a method to circumvent this limitation, but most polarizing agents developed for DNP are unstable in reducing cellular environments. Here, we introduce the use of Gd(III)-based DNP polarizing agents for in-cell NMR spectroscopy. Specifically, we show their persistent stability in cellular formulations, and we investigate the DNP performance of the Gd(III)-based complex [Gd(tpatcn)] in human embryonic kidney cell lysates and intact cells. For cell lysates, DNP enhancements up to -27 are obtained on the cellular signals, reproducible even after storage at room temperature for days. Mixing the [Gd(tpatcn)] solution with intact cells enables the observation of cellular signals with DNP, and DNP enhancement factors of about -40 are achieved.

J-driven dynamic nuclear polarization (JDNP) was recently proposed for enhancing the sensitivity of solution-state nuclear magnetic resonance (NMR), while bypassing the limitations faced by conventional (Overhauser) DNP at magnetic fields of interest in analytical applications. Like Overhauser DNP, JDNP also requires saturating the electronic polarization using high-frequency microwaves known to have poor penetration and associated heating effects in most liquids. The present microwave-free JDNP (MF-JDNP) proposal seeks to enhance solution NMR's sensitivity by shuttling the sample between higher and lower magnetic fields, with one of these fields providing an electron Larmor frequency that matches the interelectron exchange coupling J_{ex}. If spins cross this so-called JDNP condition sufficiently fast, we predict that a sizable nuclear polarization will be created without microwave irradiation. This MF-JDNP proposal requires radicals whose singlet-triplet self-relaxation rates are dominated by dipolar hyperfine relaxation, and shuttling times that can compete with these electron relaxation processes. This paper discusses the theory behind the MF-JDNP, as well as proposals for radicals and conditions that could enable this new approach to NMR sensitivity enhancement.

Review: 194 refs.

Solid-state MAS NMR at ultra low temperature of hydrated alanine doped with DNP radicals