Microtesla SABRE enables 10% nitrogen-15 nuclear spin polarization

Development of a fully automated workstation for conducting routine SABRE hyperpolarization

SABRE is emerging as a fast, simple and low-cost hyperpolarization method because of its ability to regenerate enhanced NMR signals. Generally, SABRE hyperpolarization has been performed predominantly manually, leading to variations in reproducibility and efficiency. Recent advances in SABRE include the development of automated shuttling systems to address previous inconsistencies. However, the operational complexity of such systems and the challenges of integration with existing workflows hinder their widespread adoption. This work presents a fully automated lab workstation based on a benchtop NMR spectrometer, specifically designed to facilitate SABRE of different nuclei across different polarization fields. We demonstrated the capability of this system through a series of routine SABRE experimental protocols, including consecutive SABRE hyperpolarization with high reproducibility (average standard deviation of 1.03%), optimization polarization of 13C nuclei respect to the polarization transfer field, and measurement of polarization buildup rate or decay time across a wide range of magnetic fields. Furthermore, we have iteratively optimized the durations for pulsed SABRE-SHEATH 13C pyruvate. The constructed SABRE workstation offers full automation, high reproducibility, and functional diversification, making it a practical tool for conducting routine SABRE hyperpolarization experiments. It provides a robust platform for high-throughput and reliable SABRE and X-SABRE hyperpolarization studies.

Detecting biomarkers by dynamic nuclear polarization enhanced magnetic resonance

Hyperpolarization stands out as a technique capable of significantly enhancing the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). Dynamic nuclear polarization (DNP), among various hyperpolarization methods, has gained prominence for its efficacy in real-time monitoring of metabolism and physiology. By administering a hyperpolarized substrate through dissolution DNP (dDNP), the biodistribution and metabolic changes of the DNP agent can be visualized spatiotemporally. This approach proves to be a distinctive and invaluable tool for non-invasively studying cellular metabolism in vivo, particularly in animal models. Biomarkers play a pivotal role in influencing the growth and metastasis of tumor cells by closely interacting with them, and accordingly detecting pathological alterations of these biomarkers is crucial for disease diagnosis and therapy. In recent years, a range of hyperpolarized DNP molecular bioresponsive agents utilizing various nuclei, such as 13C, 15N, 31P, 89Y, etc., have been developed. In this context, we explore how these magnetic resonance signals of nuclear spins enhanced by DNP respond to biomarkers, including pH, metal ions, enzymes, or redox processes. This review aims to offer insights into the design principles of responsive DNP agents, target selection, and the mechanisms of action for imaging. Such discussions aim to propel the future development and application of DNP-based biomedical imaging agents.

Hyperpolarization of 15N-Pyridinium by Using Parahydrogen Enables Access to Reactive Oxygen Sensors and Pilot In Vivo Studies

Magnetic resonance with hyperpolarized contrast agents is one of the most powerful and noninvasive imaging platforms capable for investigating in vivo metabolism. While most of the utilized hyperpolarized agents are based on 13C nuclei, a milestone advance in this area is the emergence of 15N hyperpolarized contrast agents. Currently, the reported 15N hyperpolarized agents mainly utilize the dissolution dynamic nuclear polarization (d-DNP) protocol. The parahydrogen enhanced 15N probes have proven to be elusive and have been tested almost exclusively in organic solvents. Herein, we designed a reaction based reactive oxygen sensor 15N-boronobenzyl-2-styrylpyridinium (15N-BBSP) which can be hyperpolarized with para-hydrogen. Reactive oxygen species plays a vital role as one of the essential intracellular signalling molecules. Disturbance of the H2O2 level usually represents a hallmark of pathophysiological conditions. This H2O2 probe exhibited rapid responsiveness toward H2O2 and offered spectrally resolvable chemical shifts. We also provide strategies to bring the newly developed probe from the organic reaction solution into a biocompatible injection buffer and demonstrate the feasibility of in vivo 15N signal detection. The present work manifests its great potential not only for reaction based reactive sensing probes but also promises to serve as a platform to develop other contrast agents.

Oxidative Nitrogen Insertion into Silyl Enol Ether C═C Bonds

Here, we demonstrate a fundamentally new reactivity of the silyl enol ether functionality utilizing an in situ-generated iodonitrene-like species. The present transformation inserts a nitrogen atom between the silyl enol ether olefinic carbons with the concomitant cleavage of the C═C bond. Overall, this facile transformation converts a C-nucleophilic silyl enol ether to the corresponding C-electrophilic N-acyl-N,O-acetal. This unprecedented access to α-amido alkylating agents enables modular derivatization with carbon and heteroatom nucleophiles and the unique late-stage editing of carbon frameworks. The reaction efficiency of this transformation is well correlated with enol ether nucleophilicity as described by the Mayr N scale. Applications presented herein include late-stage nitrogen insertion into carbon skeletons of natural products with previously unattainable regioselectivity as well as modified conditions for 15N labeling of amides and lactams.

Toward Next-Generation Molecular Imaging with a Clinical Low-Field (0.064 T) Point-of-Care MRI Scanner

Low-field (LF) MRI promises soft-tissue imaging without the expensive, immobile magnets of clinical scanners but generally suffers from limited detection sensitivity and contrast. The sensitivity boost provided by hyperpolarization can thus be highly synergistic with LF MRI. Initial efforts to integrate a continuous-bubbling SABRE (signal amplification by reversible exchange) hyperpolarization setup with a portable, point-of-care 64 mT clinical MRI scanner are reported. Results from 1H SABRE MRI of pyrazine and nicotinamide are compared with those of benchtop NMR spectroscopy. Comparison with MRI signals from samples with known H2O/D2O ratios allowed quantification of the SABRE enhancements of imaged samples with various substrate concentrations (down to 3 mM). Respective limits of detection and quantification of 3.3 and 10.1 mM were determined with pyrazine 1H polarization (PH) enhancements of ∼1900 (PH ∼0.04%), supporting ongoing and envisioned efforts to realize SABRE-enabled MRI-based molecular imaging.

In vivo Metabolic Sensing of Hyperpolarized [1-13C]Pyruvate in Mice Using a Recyclable Perfluorinated Iridium Signal Amplification by Reversible Exchange Catalyst

Real-time visualization of metabolic processes in vivo provides crucial insights into conditions like cancer and metabolic disorders. Metabolic magnetic resonance imaging (MRI), by amplifying the signal of pyruvate molecules through hyperpolarization, enables non-invasive monitoring of metabolic fluxes, aiding in understanding disease progression and treatment response. Signal Amplification By Reversible Exchange (SABRE) presents a simpler, cost-effective alternative to dissolution dynamic nuclear polarization, eliminating the need for expensive equipment and complex procedures. We present the first in vivo demonstration of metabolic sensing in a human pancreatic cancer xenograft model compared to healthy mice. A novel perfluorinated Iridium SABRE catalyst in a fluorinated solvent and methanol blend facilitated this breakthrough with a 1.2-fold increase in [1-13C]pyruvate SABRE hyperpolarization. The perfluorinated moiety allowed easy separation of the heavy-metal-containing catalyst from the hyperpolarized [1-13C]pyruvate target. The perfluorinated catalyst exhibited recyclability, maintaining SABRE-SHEATH activity through subsequent hyperpolarization cycles with minimal activity loss after the initial two cycles. Remarkably, the catalyst retained activity for at least 10 cycles, with a 3.3-fold decrease in hyperpolarization potency. This proof-of-concept study encourages wider adoption of SABRE hyperpolarized [1-13C]pyruvate MR for studying in vivo metabolism, aiding in diagnosing stages and monitoring treatment responses in cancer and other diseases.

Parahydrogen-Induced Polarization of 14N Nuclei

Hyperpolarization techniques provide a dramatic increase in sensitivity of nuclear magnetic resonance spectroscopy and imaging. In spite of the outstanding progress in solution-state hyperpolarization of spin-1/2 nuclei, hyperpolarization of quadrupolar nuclei remains challenging. Here, hyperpolarization of quadrupolar 14N nuclei with natural isotopic abundance of >99 % is demonstrated. This is achieved via pairwise addition of parahydrogen to tetraalkylammonium salts with vinyl or allyl unsaturated moieties followed by a subsequent polarization transfer from 1H to 14N nuclei at high magnetic field using PH-INEPT or PH-INEPT+ radiofrequency pulse sequence. Catalyst screening identified water-soluble rhodium complex [Rh(P(m-C6H4SO3Na)3)3Cl] as the most efficient catalyst for hyperpolarization of the substrates under study, providing up to 1.3 % and up to 6.6 % 1H polarization in the cases of vinyl and allyl precursors, respectively. The performance of PH-INEPT and PH-INEPT+ pulse sequences was optimized with respect to interpulse delays, and the resultant experimental dependences were in good agreement with simulations. As a result, 14N NMR signal enhancement of up to 760-fold at 7.05 T (corresponding to 0.15 % 14N polarization) was obtained.

15N Hyperpolarization of Metronidazole Antibiotic in Aqueous Media Using Phase-Separated Signal Amplification by Reversible Exchange with Parahydrogen

Metronidazole is a prospective hyperpolarized MRI contrast agent with potential hypoxia sensing utility for applications in cancer, stroke, neurodegenerative diseases, etc. We demonstrate a pilot procedure for production of ∼30 mM hyperpolarized [15N3]metronidazole in aqueous media by using a phase-separated SABRE-SHEATH hyperpolarization method, with nitrogen-15 polarization exceeding 2.2% on all three 15N sites achieved in less than 2 min. The 15N polarization T1 of ∼12 min is reported for the 15NO2 group at the clinically relevant field of 1.4 T in the aqueous phase, demonstrating a remarkably long lifetime of the hyperpolarized state. The produced aqueous solution of [15N3]metronidazole that contained only ∼100 μM of residual Ir was deemed biocompatible via validation through the MTT colorimetric test for assessing cell metabolic activity using human embryotic kidney HEK293T cells. This low-cost and ultrafast hyperpolarization procedure represents a major advance for the production of a biocompatible HP [15N3]metronidazole (and potentially other hyperpolarized drugs) formulation for MRI sensing applications.

NMRduino: A modular, open-source, low-field magnetic resonance platform

The NMRduino is a compact, cost-effective, sub-MHz NMR spectrometer that utilizes readily available open-source hardware and software components. One of its aims is to simplify the processes of instrument setup and data acquisition control to make experimental NMR spectroscopy accessible to a broader audience. In this introductory paper, the key features and potential applications of NMRduino are described to highlight its versatility both for research and education.

Two fields are better than one - A multifunctional (semi)automated setup for quantitative measurements of parahydrogen-induced signal enhancement at low and high fields

The rapid advancement of parahydrogen-induced hyperpolarization (PHIP) and its diverse array of applications highlights the critical need for enhanced signals in both 1H NMR and heteronuclear NMR spectroscopy. Simultaneously, there is an increasing interest in utilizing benchtop NMR analysis across various laboratory settings. However, due to their lower magnetic fields, benchtop NMR spectrometers inherently produce weaker signal intensities. Here, PHIP is a well-established solution to this challenge. Consequently, we are expanding our cost-effective PHIP setup from a high-field NMR spectrometer (11.7 T) to include an additional benchtop NMR spectrometer (1.4 T), thereby enabling concurrent execution of PHIP experiments and measurements. Through the implementation of automated experimental protocols, we aim to minimize experiment time while increasing reproducibility. In this work, a non-isotope labelled propargyl alcohol sample is used at low concentrations to demonstrate our setup's capabilities. It could be shown that single-scan PASADENA experiments can be run with comparable signal enhancements at the benchtop as well as the high-field spectrometer. At 1.4 T, fully automated PHIP pseudo-2D measurements will also be demonstrated. Additionally, two different field profiles for the spin-order transfer of p-H2 to 13C at zero- to ultralow fields are elaborated upon. The setup facilitates the measurement of carbon signal enhancement of more than 2000 on the benchtop NMR spectrometer, employing a straightforward one-pulse, one-scan experiment.

Multi-axis fields boost SABRE hyperpolarization

The inherently low signal-to-noise ratio of NMR and MRI is now being addressed by hyperpolarization methods. For example, iridium-based catalysts that reversibly bind both parahydrogen and ligands in solution can hyperpolarize protons (SABRE) or heteronuclei (X-SABRE) on a wide variety of ligands, using a complex interplay of spin dynamics and chemical exchange processes, with common signal enhancements between 103 and 104. This does not approach obvious theoretical limits, and further enhancement would be valuable in many applications (such as imaging mM concentration species in vivo). Most SABRE/X-SABRE implementations require far lower fields (μT-mT) than standard magnetic resonance (>1T), and this gives an additional degree of freedom: the ability to fully modulate fields in three dimensions. However, this has been underexplored because the standard simplifying theoretical assumptions in magnetic resonance need to be revisited. Here, we take a different approach, an evolutionary strategy algorithm for numerical optimization, multi-axis computer-aided heteronuclear transfer enhancement for SABRE (MACHETE-SABRE). We find nonintuitive but highly efficient multiaxial pulse sequences which experimentally can produce a sevenfold improvement in polarization over continuous excitation. This approach optimizes polarization differently than traditional methods, thus gaining extra efficiency.

MATRESHCA: Microtesla Apparatus for Transfer of Resonance Enhancement of Spin Hyperpolarization via Chemical Exchange and Addition

We present an integrated, open-source device for parahydrogen-based hyperpolarization processes in the microtesla field regime with a cost of components of less than $7000. The device is designed to produce a batch of 13C and 15N hyperpolarized (HP) compounds via hydrogenative or non-hydrogenative parahydrogen-induced polarization methods that employ microtesla magnetic fields for efficient polarization transfer of parahydrogen-derived spin order to X-nuclei (e.g., 13C and 15N). The apparatus employs a layered structure (reminiscent of a Russian doll "Matryoshka") that includes a nonmagnetic variable-temperature sample chamber, a microtesla magnetic field coil (operating in the range of 0.02-75 microtesla), a three-layered mu-metal shield (to attenuate the ambient magnetic field), and a magnetic shield degaussing coil placed in the overall device enclosure. The gas-handling manifold allows for parahydrogen-gas flow and pressure control (up to 9.2 bar of total parahydrogen pressure). The sample temperature can be varied either using a water bath or a PID-controlled heat exchanger in the range from -12 to 80 °C. This benchtop device measures 62 cm (length) × 47 cm (width) × 47 cm (height), weighs 30 kg, and requires only connections to a high-pressure parahydrogen gas supply and a single 110/220 VAC power source. The utility of the device has been demonstrated using an example of parahydrogen pairwise addition to form HP ethyl [1-13C]acetate (P13C = 7%, [c] = 1 M). Moreover, the Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) technique was employed to demonstrate efficient hyperpolarization of 13C and 15N spins in a wide range of biologically relevant molecules, including [1-13C]pyruvate (P13C = 14%, [c] = 27 mM), [1-13C]-α-ketoglutarate (P13C = 17%), [1-13C]ketoisocaproate (P13C = 18%), [15N3]metronidazole (P15N = 13%, [c] = 20 mM), and others. While the vast majority of the utility studies have been performed in standard 5 mm NMR tubes, the sample chamber of the device can accommodate a wide range of sample container sizes and geometries of up to 1 L sample volume. The device establishes an integrated, simple, inexpensive, and versatile equipment gateway needed to facilitate parahydrogen-based hyperpolarization experiments ranging from basic science to preclinical applications; indeed, detailed technical drawings and a bill of materials are provided to support the ready translation of this design to other laboratories.

13C MRI of hyperpolarized pyruvate at 120 µT

Nuclear spin hyperpolarization increases the sensitivity of magnetic resonance dramatically, enabling many new applications, including real-time metabolic imaging. Parahydrogen-based signal amplification by reversible exchange (SABRE) was employed to hyperpolarize [1-13C]pyruvate and demonstrate 13C imaging in situ at 120 µT, about twice Earth's magnetic field, with two different signal amplification by reversible exchange variants: SABRE in shield enables alignment transfer to heteronuclei (SABRE-SHEATH), where hyperpolarization is transferred from parahydrogen to [1-13C]pyruvate at a magnetic field below 1 µT, and low-irradiation generates high tesla (LIGHT-SABRE), where hyperpolarization was prepared at 120 µT, avoiding magnetic field cycling. The 3-dimensional images of a phantom were obtained using a superconducting quantum interference device (SQUID) based magnetic field detector with submillimeter resolution. These 13C images demonstrate the feasibility of low-field 13C metabolic magnetic resonance imaging (MRI) of 50 mM [1-13C]pyruvate hyperpolarized by parahydrogen in reversible exchange imaged at about twice Earth's magnetic field. Using thermal 13C polarization available at 120 µT, the same experiment would have taken about 300 billion years.

15NRORC: An Azine Labeling Protocol

A practical method for the synthesis of 15N-labeled azines with a high degree of isotopic enrichment is described. Activation of azine heterocycles with an electron-deficient arene allows for the facile substitution of the nitrogen atom with a specifically designed 15N-labeled reagent that undergoes a canonical ANRORC-type mechanism. A wide range of azines can be converted to their corresponding 15N isotopologs using this method, and it also allows for dearomative access to reduced heterocyclic congeners. A short dearomative formal synthesis of 15N-solifenacin is accomplished as well to demonstrate a practical application of this method for generating labeled pharmaceuticals.

LC-Photo-CIDNP hyperpolarization of biomolecules bearing a quasi-isolated spin pair: Magnetic-Field dependence via a rapid-shuttling device

Liquid-state low-concentration photochemically induced dynamic nuclear polarization (LC-photo-CIDNP) is an emerging technology tailored to enhance the sensitivity of NMR spectroscopy via LED- or laser-mediated optical irradiation. LC-photo-CIDNP is particularly useful to detect solvent-exposed aromatic residues (Trp, Tyr), either in isolation or within polypeptides and proteins. This study investigates the magnetic-field dependence of the LC-photo-CIDNP of Trp-α-13C-β,β,2,4,5,6,7-d7, a Trp isotopolog bearing a quasi-isolated 1Hα-13Cαspin pair (QISP). We employed a new rapid-shuttling side-illumination field-cycling device that enables ultra-fast (90-120 ms) vertical movements of NMR samples within the bore of a superconducting magnet. Thus, LC-photo-CIDNP hyperpolarization occurs at low field, while hyperpolarized signals are detected at high field (700 MHz). Resonance lineshapes were excellent, and the effect of several fields (1.18-7.08 T range) on hyperpolarization efficiency could be readily explored. Remarkably, unprecedented LC-photo-CIDNP enhancements ε ≅ 1,200 were obtained at 50 MHz (1.18 T), suggesting exciting avenues to hypersensitive LED-enhanced NMR in liquids at low field.

Metal-Mediated Catalytic Polarization Transfer from para Hydrogen to 3,5-Dihalogenated Pyridines

The neutral catalysts [IrCl(H)2(NHC)(substrate)2] or [IrCl(H)2(NHC)(substrate)(sulfoxide)] are used to transfer polarization from para hydrogen (pH2) to 3,5-dichloropyridine and 3,5-dibromopyridine substrates. This is achieved in a rapid, reversible, and low-cost process that relies on ligand exchange within the active catalyst. Notably, the sulfoxide-containing catalyst systems produced NMR signal enhancements between 1 and 2 orders of magnitude larger than its unmodified counterpart. Consequently, this signal amplification by reversible exchange hyperpolarization method can boost the 1H, 13C, and 15N nuclear magnetic resonance (NMR) signal intensities by factors up to 4350, 1550, and 46,600, respectively (14.0, 1.3, and 15.4% polarization). In this paper, NMR and X-ray crystallography are used to map the evolution of catalytically important species and provide mechanistic rational for catalytic efficiency. Furthermore, applications in spontaneous radiofrequency amplification by stimulated emission and NMR reaction monitoring are also shown.

Facile hyperpolarization chemistry for molecular imaging and metabolic tracking of [1-13C]pyruvate in vivo

Hyperpolarization chemistry based on reversible exchange of parahydrogen, also known as Signal Amplification By Reversible Exchange (SABRE), is a particularly simple approach to attain high levels of nuclear spin hyperpolarization, which can enhance NMR and MRI signals by many orders of magnitude. SABRE has received significant attention in the scientific community since its inception because of its relative experimental simplicity and its broad applicability to a wide range of molecules, however in vivo detection of molecular probes hyperpolarized by SABRE has remained elusive. Here we describe a first demonstration of SABRE-hyperpolarized contrast detected in vivo, specifically using hyperpolarized [1-13C]pyruvate. Biocompatible formulations of hyperpolarized [1-13C]pyruvate in, both, methanol-water mixtures, and ethanol-water mixtures followed by dilution with saline and catalyst filtration were prepared and injected into healthy Sprague Dawley and Wistar rats. Effective hyperpolarization-catalyst removal was performed with silica filters without major losses in hyperpolarization. Metabolic conversion of pyruvate to lactate, alanine, and bicarbonate was detected in vivo. Pyruvate-hydrate was also observed as minor byproduct. Measurements were performed on the liver and kidney at 4.7 T via time-resolved spectroscopy and chemical-shift-resolved MRI. In addition, whole-body metabolic measurements were obtained using a cryogen-free 1.5 T MRI system, illustrating the utility of combining lower-cost MRI systems with simple, low-cost hyperpolarization chemistry to develop safe, and scalable molecular imaging.

Molecular precursors to produce para-hydrogen enhanced metabolites at any field

Enhancing magnetic resonance signal via hyperpolarization techniques enables the real-time detection of metabolic transformations even in vivo. The use of para-hydrogen to enhance 13 C-enriched metabolites has opened a rapid pathway for the production of hyperpolarized metabolites, which usually requires specialized equipment. Metabolite precursors that can be hyperpolarized and converted into metabolites at any given field would open up opportunities for many labs to make use of this technology because already existing hardware could be used. We report here on the complete synthesis and hyperpolarization of suitable precursor molecules of the side-arm hydrogenation approach. The better accessibility to such side-arms promises that the para-hydrogen approach can be implemented in every lab with existing two channel NMR spectrometers for 1 H and 13 C independent of the magnetic field.

Delivering Robust Proton-Only Sensing of Hyperpolarized [1,2-13C2]-Pyruvate Using Broad-Spectral-Range Nuclear Magnetic Resonance Pulse Sequences

Hyperpolarized [1-13C]pyruvate is the leading hyperpolarized injectable contrast agent and is currently under evaluation in clinical trials for molecular imaging of metabolic diseases, including cardiovascular disease and cancer. One aspect limiting broad scalability of the technique is that hyperpolarized 13C MRI requires specialized 13C hardware and software that are not generally available on clinical MRI scanners, which employ proton-only detection. Here, we present an approach that uses pulse sequences to transfer 13C hyperpolarization to methyl protons for detection of the 13C-13C pyruvate singlet, employing proton-only excitation and detection only. The new pulse sequences are robust to the B1 and B0 magnetic field inhomogeneities. The work focuses on singlet-to-magnetization (S2M) and rotor-synchronized (R) pulses, both relying on trains of hard pulses with broad spectral width coverage designed to effectively transform hyperpolarized 13C2-singlet hyperpolarization to 1H polarization on the CH3 group of [1,2-13C2]pyruvate. This approach may enable a broader adoption of hyperpolarized MRI as a molecular imaging technique.

R2 Relaxometry of SABRE-Hyperpolarized Substrates at a Low Magnetic Field

Nuclear magnetic resonance (NMR) relaxometry at a low magnetic field, in the milli-Tesla range or less, is enabled by signal enhancements through hyperpolarization. The parahydrogen-based method of signal amplification by reversible exchange (SABRE) provides large signals in a dilute liquid for the measurement of R2 relaxation using a single-scan Carr-Purcell-Meiboom-Gill (CPMG) experiment. A comparison of relaxation rates obtained at high and low fields indicates that an otherwise dominant contribution from chemical exchange is excluded in this low-field range. The SABRE process itself is based on exchange between the free and polarization transfer catalyst-bound forms of the substrate. At a high magnetic field of 9.4 T, typical conditions for producing hyperpolarization including 5 mM 5-fluoropyridine-3-carboximidamide as a substrate and 0.5 mM chloro(1,5-cyclooctadiene)[4,5-dimethyl-1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]iridium(I) as a polarization transfer catalyst precursor resulted in an R2 relaxation rate as high as 3.38 s-1. This relaxation was reduced to 1.19 s-1 at 0.85 mT. A quantitative analysis of relaxation rates and line shapes indicates that milli-Tesla or lower magnetic fields are required to eliminate the exchange contribution. At this magnetic field strength, R2 relaxation rates are indicative primarily of molecular properties. R2 relaxometry may be used for investigating molecular interactions and dynamics. The SABRE hyperpolarization, which provides signal enhancements without requiring a high magnetic field or large instrumentation, is ideally suited to enable these applications.

The experimental approach for the interleaved joint modulation of PHIP and NMR

Nuclear spin hyperpolarization derived from parahydrogen is a technique for enhancing nuclear magnetic resonance (NMR) sensitivity. The key to hyperpolarization experiments is to achieve rapid transfer and detection to minimize relaxation losses, while also avoiding bubbles or turbulence to guarantee high spectral resolution. In this article, we describe an experimental approach for the interleaved joint modulation of parahydrogen-induced polarization and NMR. We provide schematic diagrams of parahydrogen-based polarizer with in situ high-pressure detection capability and low-field polarization transfer. This approach can help to control the experimental process and acquire experimental information, one example of which is the attainment of the highest hyperpolarization signal intensity at 3.6 s after closing the valve. The polarizer demonstrates in situ detection capability, allowing sample to be restabilized within 0.3 ± 0.1 s and high-resolution NMR sampling under a pressure of 3 bars. Moreover, it can transfer polarized samples from the polarization transfer field to the detection region of NMR within 1 ± 0.3 s for completing signal amplification by reversible exchange experiments.

Enhancing the NMR signals of plant oil components using hyperpolarisation relayed via proton exchange

In this work, the limited sensitivity of magnetic resonance is addressed by using the hyperpolarisation method relayed signal amplification by reversible exchange (SABRE-Relay) to transfer latent magnetism from para-hydrogen, a readily isolated spin isomer of hydrogen gas, to components of key plant oils such as citronellol, geraniol, and nerol. This is achieved via relayed polarisation transfer in which an [Ir(H)2(IMes)(NH2R)3]Cl type complex produces hyperpolarised NH2R free in solution, before labile proton exchange between the hyperpolarisation carrier (NH2R) and the OH-containing plant oil component generates enhanced NMR signals for the latter. Consequently, up to ca. 200-fold 1H (0.65% 1H polarisation) and 800-fold 13C NMR signal enhancements (0.65% 13C polarisation) are recorded for these essential oils in seconds. Remarkably, the resulting NMR signals are not only diagnostic, but prove to propagate over large spin systems via a suitable coupling network. A route to optimise the enhancement process by varying the identity of the carrier NH2R, and its concentration is demonstrated. In order to prove utility, these pilot measurements are extended to study a much wider range of plant-derived molecules including rhodinol, verbenol, (1R)-endo-(+)-fenchyl alcohol, (-)-carveol, and linalool. Further measurements are then described which demonstrate citronellol and geraniol can be detected in an off-the-shelf healthcare product rose geranium oil at concentrations of just a few tens of μM in single scan 1H NMR measurements, which are not visible in comparable thermally polarised NMR experiments. This work therefore presents a significant expansion of the types of molecules amenable to hyperpolarisation using para-hydrogen and illustrates a real-world application in the diagnostic detection of low concentration analytes in mixtures.

In Vivo Metabolic Imaging of [1-13 C]Pyruvate-d3 Hyperpolarized By Reversible Exchange With Parahydrogen

Metabolic magnetic resonance imaging (MRI) using hyperpolarized (HP) pyruvate is becoming a non-invasive technique for diagnosing, staging, and monitoring response to treatment in cancer and other diseases. The clinically established method for producing HP pyruvate, dissolution dynamic nuclear polarization, however, is rather complex and slow. Signal Amplification By Reversible Exchange (SABRE) is an ultra-fast and low-cost method based on fast chemical exchange. Here, for the first time, we demonstrate not only in vivo utility, but also metabolic MRI with SABRE. We present a novel routine to produce aqueous HP [1-13 C]pyruvate-d3 for injection in 6 minutes. The injected solution was sterile, non-toxic, pH neutral and contained ≈30 mM [1-13 C]pyruvate-d3 polarized to ≈11 % (residual 250 mM methanol and 20 μM catalyst). It was obtained by rapid solvent evaporation and metal filtering, which we detail in this manuscript. This achievement makes HP pyruvate MRI available to a wide biomedical community for fast metabolic imaging of living organisms.

Reversible Parahydrogen Induced Hyperpolarization of 15 N in Unmodified Amino Acids Unraveled at High Magnetic Field

Amino acids (AAs) and ammonia are metabolic markers essential for nitrogen metabolism and cell regulation in both plants and humans. NMR provides interesting opportunities to investigate these metabolic pathways, yet lacks sensitivity, especially in case of 15 N. In this study, spin order embedded in p-H2 is used to produce on-demand reversible hyperpolarization in 15 N of pristine alanine and ammonia under ambient protic conditions directly in the NMR spectrometer. This is made possible by designing a mixed-ligand Ir-catalyst, selectively ligating the amino group of AA by exploiting ammonia as a strongly competitive co-ligand and preventing deactivation of Ir by bidentate ligation of AA. The stereoisomerism of the catalyst complexes is determined by hydride fingerprinting using 1 H/D scrambling of the associated N-functional groups on the catalyst (i.e., isotopological fingerprinting), and unravelled by 2D-ZQ-NMR. Monitoring the transfer of spin order from p-H2 to 15 N nuclei of ligated and free alanine and ammonia targets using SABRE-INEPT with variable exchange delays pinpoints the monodentate elucidated catalyst complexes to be most SABRE active. Also RF-spin locking (SABRE-SLIC) enables transfer of hyperpolarization to 15 N. The presented high-field approach can be a valuable alternative to SABRE-SHEATH techniques since the obtained catalytic insights (stereochemistry and kinetics) will remain valid at ultra-low magnetic fields.

Detection of pyridine derivatives by SABRE hyperpolarization at zero field

Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical tool used in modern science and technology. Its novel incarnation, based on measurements of NMR signals without external magnetic fields, provides direct access to intramolecular interactions based on heteronuclear scalar J-coupling. The uniqueness of these interactions makes each zero-field NMR spectrum distinct and useful in chemical fingerprinting. However, the necessity of heteronuclear coupling often results in weak signals due to the low abundance of certain nuclei (e.g., ¹⁵N). Hyperpolarization of such compounds may solve the problem. In this work, we investigate molecules with natural isotopic abundance that are polarized using non-hydrogenative parahydrogen-induced polarization. We demonstrate that spectra of hyperpolarized naturally abundant pyridine derivatives can be observed and uniquely identified whether the same substituent is placed at a different position of the pyridine ring or different constituents are placed at the same position. To do so, we constructed an experimental system using a home-built nitrogen vapor condenser, which allows for consistent long-term measurements, crucial for identifying naturally abundant hyperpolarized molecules at a concentration level of ~1 mM. This opens avenues for future chemical detection of naturally abundant compounds using zero-field NMR.

Spin dynamics of [1,2-13C2]pyruvate hyperpolarization by parahydrogen in reversible exchange at micro Tesla fields

Hyperpolarization of 13C-pyruvate via Signal Amplificaton By Reversibble Exchange (SABRE) is an important recent discovery because of both the relative simplicity of hyperpolarization and the central biological relevance of pyruvate as a biomolecular probe for in vitro or in vivo studies. Here, we analyze the [1,2-13C2]pyruvate-SABRE spin system and its field dependence theoretically and experimentally. We provide first-principles analysis of the governing 4-spin dihydride-13C2 Hamiltonian and numerical spin dynamics simulations of the 7-spin dihydride-13C2-CH3 system. The analytical and the numerical results are compared to matching systematic experiments. With these methods we unravel the observed spin state mixing of singlet states and triplet states at microTesla fields and we also analyze the dynamics during transfer from micro-Tesla field to high field for detection to understand the resulting spectra from the [1,2-13C2]pyruvate-SABRE system.

Over 20% Carbon-13 Polarization of Perdeuterated Pyruvate Using Reversible Exchange with Parahydrogen and Spin-Lock Induced Crossing at 50 μT

Carbon-13 hyperpolarized pyruvate is about to become the next-generation contrast agent for molecular magnetic resonance imaging of cancer and other diseases. Here, efficient and rapid pyruvate hyperpolarization is achieved via signal amplification by reversible exchange (SABRE) with parahydrogen through synergistic use of substrate deuteration, alternating, and static microtesla magnetic fields. Up to 22 and 6% long-lasting ¹³C polarization (T₁ = 3.7 ± 0.25 and 1.7 ± 0.1 min) is demonstrated for the C1 and C2 nuclear sites, respectively. The remarkable polarization levels become possible as a result of favorable relaxation dynamics at the microtesla fields. The ultralong polarization lifetimes will be conducive to yielding high polarization after purification, quality assurance, and injection of the hyperpolarized molecular imaging probes. These results pave the way to future in vivo translation of carbon-13 hyperpolarized molecular imaging probes prepared by this approach.

Parahydrogen in Reversible Exchange Induces Long-Lived 15N Hyperpolarization of Anticancer Drugs Anastrozole and Letrozole

Hyperpolarization modalities overcome the sensitivity limitations of NMR and unlock new applications. Signal amplification by reversible exchange (SABRE) is a particularly cheap, quick, and robust hyperpolarization modality. Here, we employ SABRE for simultaneous chemical exchange of parahydrogen and nitrile-containing anticancer drugs (letrozole or anastrozole) to enhance 15N polarization. Distinct substrates require unique optimal parameter sets, including temperature, magnetic field, or a shaped magnetic field profile. The fine tuning of these parameters for individual substrates is demonstrated here to maximize 15N polarization. After optimization, including the usage of pulsed μT fields, the 15N nuclei on common anticancer drugs, letrozole and anastrozole, can be polarized within 1-2 min. The hyperpolarization can exceed 10%, corresponding to 15N signal enhancement of over 280,000-fold at a clinically relevant magnetic field of 1 T. This sensitivity gain enables polarization studies at naturally abundant 15N enrichment level (0.4%). Moreover, the nitrile 15N sites enable long-lasting polarization storage with [15N]T1 over 9 min, enabling signal detection from a single hyperpolarization cycle for over 30 min.

LIGHT-SABRE Hyperpolarizes 1-13C-Pyruvate Continuously without Magnetic Field Cycling

Nuclear spin hyperpolarization enables real-time observation of metabolism and intermolecular interactions in vivo. 1-¹³C-pyruvate is the leading hyperpolarized tracer currently under evaluation in several clinical trials as a promising molecular imaging agent. Still, the quest for a simple, fast, and efficient hyperpolarization technique is ongoing. Here, we describe that continuous, weak irradiation in the audio-frequency range of the ¹³C spin at the 121 μT magnetic field (approximately twice Earth's field) enables spin order transfer from parahydrogen to ¹³C magnetization of 1-¹³C-pyruvate. These so-called LIGHT-SABRE pulses couple nuclear spin states of parahydrogen and pyruvate via the J-coupling network of reversibly exchanging Ir-complexes. Using ∼100% parahydrogen at ambient pressure, we polarized 51 mM 1-¹³C-pyruvate in the presence of 5.1 mM Ir-complex continuously and repeatedly to a polarization of 1.1% averaged over free and catalyst-bound pyruvate. The experiments were conducted at -8 °C, where almost exclusively bound pyruvate was observed, corresponding to an estimated 11% polarization on bound pyruvate. The obtained hyperpolarization levels closely match those obtained via SABRE-SHEATH under otherwise identical conditions. The creation of three different types of spin orders was observed: transverse ¹³C magnetization along the applied magnetic field, ¹³C z-magnetization along the main field B ₀, and ¹³C-¹H zz-spin order. With a superconducting quantum interference device (SQUID) for detection, we found that the generated spin orders result from ¹H-¹³C J-coupling interactions, which are not visible even with our narrow linewidth below 0.3 Hz and at -8 °C.

Competitive Displacement Restores the Hyperpolarized 15N NMR Signal in Blood Plasma

Hyperpolarized (HP) NMR can improve the sensitivity of conventional NMR experiments by several orders of magnitude, thereby making it feasible to detect the signal of low sensitivity nuclei such as 13C and 15N nuclei in vivo. Hyperpolarized substrates are usually administered by direct injection into the bloodstream, and interaction with serum albumin can cause rapid decay of the hyperpolarized signal due to the shortening of the spin-lattice (T1) relaxation time. Here we report that the 15N T1 of 15N labeled, partially deuterated tris(2-pyridylmethyl)amine decreases dramatically upon binding to albumin to such an extent that no HP-15 signal could be detected. We also demonstrate that the signal could be restored using a competitive displacer, iophenoxic acid, which binds stronger to albumin than tris(2-pyridylmethyl)amine. The methodology presented here eliminates the undesirable effect of albumin binding and should widen the range of hyperpolarized probes for in vivo studies.

Real-Time Pyruvate Chemical Conversion Monitoring Enabled by PHIP

In recent years, parahydrogen-induced polarization side arm hydrogenation (PHIP-SAH) has been applied to hyperpolarize [1-¹³C]pyruvate and map its metabolic conversion to [1-¹³C]lactate in cancer cells. Developing on our recent MINERVA pulse sequence protocol, in which we have achieved 27% [1-¹³C]pyruvate carbon polarization, we demonstrate the hyperpolarization of [1,2-¹³C]pyruvate (∼7% polarization on each ¹³C spin) via PHIP-SAH. By altering a single parameter in the pulse sequence, MINERVA enables the signal enhancement of C1 and/or C2 in [1,2-¹³C]pyruvate with the opposite phase, which allows for the simultaneous monitoring of different chemical reactions with enhanced spectral contrast or for the same reaction via different carbon sites. We first demonstrate the ability to monitor the same enzymatic pyruvate to lactate conversion at 7T in an aqueous solution, in vitro, and in-cell (HeLa cells) via different carbon sites. In a second set of experiments, we use the C1 and C2 carbon positions as spectral probes for simultaneous chemical reactions: the production of acetate, carbon dioxide, bicarbonate, and carbonate by reacting [1,2-¹³C]pyruvate with H₂O₂ at a high temperature (55 °C). Importantly, we detect and characterize the intermediate 2-hydroperoxy-2-hydroxypropanoate in real time and at high temperature.

Missing Pieces in Structure Puzzles: How Hyperpolarized NMR Spectroscopy Can Complement Structural Biology and Biochemistry

Structure determination lies at the heart of many biochemical research programs. However, the "giants": X-ray diffraction, electron microscopy, molecular dynamics simulations, and nuclear magnetic resonance, among others, leave quite a few dark spots on the structural pictures drawn of proteins, nucleic acids, membranes, and other biomacromolecules. For example, structural models under physiological conditions or of short-lived intermediates often remain out of reach of the established experimental methods. This account frames the possibility of including hyperpolarized, that is, dramatically signal-enhanced NMR in existing workflows to fill these spots with detailed depictions. We highlight how integrating methods based on dissolution dynamic nuclear polarization can provide valuable complementary information about formerly inaccessible conformational spaces for many systems. A particular focus will be on hyperpolarized buffers to facilitate the NMR structure determination of challenging systems.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Enhancing 19F Benchtop NMR Spectroscopy by Combining para-Hydrogen Hyperpolarization and Multiplet Refocusing

Benchtop NMR spectrometers provide a promising alternative to high-field NMR for applications that are limited by instrument size and/or cost. ¹⁹F benchtop NMR is attractive due to the larger chemical shift range of ¹⁹F relative to ¹H and the lack of background signal in most applications. However, practical applications of benchtop ¹⁹F NMR are limited by its low sensitivity due to the relatively weak field strengths of benchtop NMR spectrometers. Here we present a sensitivity-enhancement strategy that combines SABRE (Signal Amplification By Reversible Exchange) hyperpolarization with the multiplet refocusing method SHARPER (Sensitive, Homogeneous, And Resolved PEaks in Real time). When applied to a range of fluoropyridines, SABRE-SHARPER achieves overall signal enhancements of up to 5700-fold through the combined effects of hyperpolarization and line-narrowing. This approach can be generalized to the analysis of mixtures through the use of a selective variant of the SHARPER sequence, selSHARPER. The ability of SABRE-selSHARPER to simultaneously boost sensitivity and discriminate between two components of a mixture is demonstrated, where selectivity is achieved through a combination of selective excitation and the choice of polarization transfer field during the SABRE step.

Nanomaterials for hyperpolarized nuclear magnetic resonance and magnetic resonance imaging

Nanomaterials play an important role in the development and application of hyperpolarized materials for magnetic resonance imaging (MRI). In this context they can not only act as hyperpolarized materials which are directly imaged but also play a role as carriers for hyperpolarized gases and catalysts for para-hydrogen induced polarization (PHIP) to generate hyperpolarized substrates for metabolic imaging. Those three application possibilities are discussed, focusing on carbon-based materials for the directly imaged particles. An overview over recent developments in all three fields is given, including the early developments in each field as well as important steps towards applications in MRI, such as making the initially developed methods more biocompatible and first imaging experiments with spatial resolution in either phantoms or in vivo studies. Focusing on the important features nanomaterials need to display to be applicable in the MRI context, a wide range of different approaches to that extent is covered, giving the reader a general idea of different possibilities as well as recent developments in those different fields of hyperpolarized magnetic resonance. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Efficient SABRE-SHEATH Hyperpolarization of Potent Branched-Chain-Amino-Acid Metabolic Probe [1-13C]ketoisocaproate

Efficient 13C hyperpolarization of ketoisocaproate is demonstrated in natural isotopic abundance and [1-13C]enriched forms via SABRE-SHEATH (Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei). Parahydrogen, as the source of nuclear spin order, and ketoisocaproate undergo simultaneous chemical exchange with an Ir-IMes-based hexacoordinate complex in CD3OD. SABRE-SHEATH enables spontaneous polarization transfer from parahydrogen-derived hydrides to the 13C nucleus of transiently bound ketoisocaproate. 13C polarization values of up to 18% are achieved at the 1-13C site in 1 min in the liquid state at 30 mM substrate concentration. The efficient polarization build-up becomes possible due to favorable relaxation dynamics. Specifically, the exponential build-up time constant (14.3 ± 0.6 s) is substantially lower than the corresponding polarization decay time constant (22.8 ± 1.2 s) at the optimum polarization transfer field (0.4 microtesla) and temperature (10 °C). The experiments with natural abundance ketoisocaproate revealed polarization level on the 13C-2 site of less than 1%-i.e., one order of magnitude lower than that of the 1-13C site-which is only partially due to more-efficient relaxation dynamics in sub-microtesla fields. We rationalize the overall much lower 13C-2 polarization efficiency in part by less favorable catalyst-binding dynamics of the C-2 site. Pilot SABRE experiments at pH 4.0 (acidified sample) versus pH 6.1 (unaltered sodium [1-13C]ketoisocaproate) reveal substantial modulation of SABRE-SHEATH processes by pH, warranting future systematic pH titration studies of ketoisocaproate, as well as other structurally similar ketocarboxylate motifs including pyruvate and alpha-ketoglutarate, with the overarching goal of maximizing 13C polarization levels in these potent molecular probes. Finally, we also report on the pilot post-mortem use of HP [1-13C]ketoisocaproate in a euthanized mouse, demonstrating that SABRE-hyperpolarized 13C contrast agents hold promise for future metabolic studies.

Hyperpolarizing DNA Nucleobases via NMR Signal Amplification by Reversible Exchange

The present work investigates the potential for enhancing the NMR signals of DNA nucleobases by parahydrogen-based hyperpolarization. Signal amplification by reversible exchange (SABRE) and SABRE in Shield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) of selected DNA nucleobases is demonstrated with the enhancement (ε) of 1H, 15N, and/or 13C spins in 3-methyladenine, cytosine, and 6-O-guanine. Solutions of the standard SABRE homogenous catalyst Ir(1,5-cyclooctadeine)(1,3-bis(2,4,6-trimethylphenyl)imidazolium)Cl ("IrIMes") and a given nucleobase in deuterated ethanol/water solutions yielded low 1H ε values (≤10), likely reflecting weak catalyst binding. However, we achieved natural-abundance enhancement of 15N signals for 3-methyladenine of ~3300 and ~1900 for the imidazole ring nitrogen atoms. 1H and 15N 3-methyladenine studies revealed that methylation of adenine affords preferential binding of the imidazole ring over the pyrimidine ring. Interestingly, signal enhancements (ε~240) of both 15N atoms for doubly labelled cytosine reveal the preferential binding of specific tautomer(s), thus giving insight into the matching of polarization-transfer and tautomerization time scales. 13C enhancements of up to nearly 50-fold were also obtained for this cytosine isotopomer. These efforts may enable the future investigation of processes underlying cellular function and/or dysfunction, including how DNA nucleobase tautomerization influences mismatching in base-pairing.

Metabolic Tumor Imaging with Rapidly Signal-Enhanced 1-13 C-Pyruvate-d3

The metabolism of malignant cells differs significantly from that of healthy cells and thus, it is possible to perform metabolic imaging to reveal not only the exact location of a tumor, but also intratumoral areas of high metabolic activity. Herein, we demonstrate the feasibility of metabolic tumor imaging using signal-enhanced 1-¹³ C-pyruvate-d₃ , which is rapidly enhanced via para-hydrogen, and thus, the signal is amplified by several orders of magnitudes in less than a minute. Using as a model, human melanoma xenografts injected with signal-enhanced 1-¹³ C-pyruvate-d3, we show that the conversion of pyruvate into lactate can be monitored along with its kinetics, which could pave the way for rapidly detecting and monitoring changes in tumor metabolism.

NMR Discrimination of d- and l-α-Amino Acids at Submicromolar Concentration via Parahydrogen-Induced Hyperpolarization

Differentiation of enantiomers represents an important research area for pharmaceutical, chemical, and food industries. However, enantiomer separation is a laborious task that demands complex analytical techniques, specialized equipment, and expert personnel. In this respect, discrimination and quantification of d- and l-α-amino acids is no exception, generally requiring extensive sample manipulation, including isolation, functionalization, and chiral separation. This complex sample treatment results in high time costs and potential biases in the quantitative determination. Here, we present an approach based on the combination of non-hydrogenative parahydrogen-induced hyperpolarization and nuclear magnetic resonance that allows detection, discrimination, and quantification of d- and l-α-amino acids in complex mixtures such as biofluids and food extracts down to submicromolar concentrations. Importantly, this method can be directly applied to the system under investigation without any prior isolation, fractionation, or functionalization step.

Proton-Only Sensing of Hyperpolarized [1,2-13C2]Pyruvate

Hyperpolarized MRI is emerging as a next-generation molecular imaging modality that can detect metabolic transformations in real time deep inside tissue and organs. 13C-hyperpolarized pyruvate is the leading hyperpolarized contrast agent that can probe cellular energetics in real time. Currently, hyperpolarized MRI requires specialized "multinuclear" MRI scanners that have the ability to excite and detect 13C signals. The objective of this work is the development of an approach that works on conventional (i.e., proton-only) MRI systems while taking advantage of long-lived 13C hyperpolarization. The long-lived singlet state of [1,2-13C2]pyruvate is hyperpolarized with parahydrogen in reversible exchange, and subsequently, the polarization is transferred from the 13C2 spin pair to the methyl protons of pyruvate for detection. This polarization transfer is accomplished with spin-lock induced crossing pulses that are only applied to the methyl protons yet access the hyperpolarization stored in the 13C2 singlet state. Theory and first experimental demonstrations are provided for our method, which obviates 13C excitation and detection for proton sensing of 13C-hyperpolarized pyruvate with an overall experimental-polarization transfer efficiency of ∼22% versus a theoretically predicted polarization transfer efficiency of 25%.

Interplay of Near-Zero-Field Dephasing, Rephasing, and Relaxation Dynamics and [1-13C]Pyruvate Polarization Transfer Efficiency in Pulsed SABRE-SHEATH

Hyperpolarized [1-13C]pyruvate is a revolutionary molecular probe enabling ultrafast metabolic MRI scans in 1 min. This technology is now under evaluation in over 30 clinical trials, which employ dissolution Dynamic Nuclear Polarization (d-DNP) to prepare a batch of the contrast agent; however, d-DNP technology is slow and expensive. The emerging SABRE-SHEATH hyperpolarization technique enables fast (under 1 min) and robust production of hyperpolarized [1-13C]pyruvate via simultaneous chemical exchange of parahydrogen and pyruvate on IrIMes hexacoordinate complexes. Here, we study the application of microtesla pulses to investigate their effect on C-13 polarization efficiency, compared to that of conventional SABRE-SHEATH employing a static field (∼0.4 μT), to provide the matching conditions of polarization transfer from parahydrogen-derived hydrides to the 13C-1 nucleus. Our results demonstrate that using square-microtesla pulses with optimized parameters can produce 13C-1 polarization levels of up to 14.8% (when detected, averaging over all resonances), corresponding to signal enhancement by over 122,000-fold at the clinically relevant field of 1.4 T. We anticipate that our results can be directly translated to other structurally similar biomolecules such as [1-13C]α-ketoglutarate and [1-13C]α-ketoisocaproate. Moreover, other more advanced pulse shapes can potentially further boost heteronuclear polarization attainable via pulsed SABRE-SHEATH.

Geometrization for Energy Levels of Isotropic Hyperfine Hamiltonian Block and Related Central Spin Problems for an Arbitrarily Complex Set of Spin-1/2 Nuclei

Description of interacting spin systems relies on understanding the spectral properties of the corresponding spin Hamiltonians. However, the eigenvalue problems arising here lead to algebraic problems too complex to be analytically tractable. This is already the case for the simplest nontrivial (Kmax−1) block for an isotropic hyperfine Hamiltonian for a radical with spin-12 nuclei, where n nuclei produce an n-th order algebraic equation with n independent parameters. Systems described by such blocks are now physically realizable, e.g., as radicals or radical pairs with polarized nuclear spins, appear as closed subensembles in more general radical settings, and have numerous counterparts in related central spin problems. We provide a simple geometrization of energy levels in this case: given n spin-12 nuclei with arbitrary positive couplings ai, take an n-dimensional hyper-ellipsoid with semiaxes ai, stretch it by a factor of n+1 along the spatial diagonal (1, 1, …, 1), read off the semiaxes of thus produced new hyper-ellipsoid qi, augment the set {qi} with q0=0, and obtain the sought n+1 energies as Ek=−12qk2+14∑iai. This procedure provides a way of seeing things that can only be solved numerically, giving a useful tool to gain insights that complement the numeric simulations usually inevitable here, and shows an intriguing connection to discrete Fourier transform and spectral properties of standard graphs.

Catalyst-Free Aqueous Hyperpolarized [1-13C]Pyruvate Obtained by Re-Dissolution Signal Amplification by Reversible Exchange

Despite great successes in oncology, patient outcomes are often still discouraging, and hence the diagnostic imaging paradigm is increasingly shifting toward functional imaging of the pathology to better understand individual disease biology and to personalize therapies. The dissolution Dynamic Nuclear Polarization (d-DNP) hyperpolarization method has enabled unprecedented real-time MRI sensing of metabolism and tissue pH using hyperpolarized [1-13C]pyruvate as a biosensor with great potential for diagnosis and monitoring of cancer patients. However, current d-DNP is expensive and suffers from long hyperpolarization times, posing a substantial translational roadblock. Here, we report the development of Re-Dissolution Signal Amplification By Reversible Exchange (Re-D SABRE), which relies on fast and low-cost hyperpolarization of [1-13C]pyruvate by chemical exchange with parahydrogen at microtesla magnetic fields. [1-13C]pyruvate is precipitated from catalyst-containing methanol using ethyl acetate and rapidly reconstituted in aqueous media. 13C polarization of 9 ± 1% is demonstrated after redissolution in water with residual iridium mass fraction of 8.5 ± 1.5 ppm; further improvement is anticipated via process automation. Re-D SABRE makes hyperpolarized [1-13C]pyruvate biosensor available at a fraction of the cost (<$10,000) and production time (≈1 min) of currently used techniques and makes aqueous hyperpolarized [1-13C]pyruvate "ready" for in vivo applications.

Subsecond Three-Dimensional Nitrogen-15 Magnetic Resonance Imaging Facilitated by Parahydrogen-Based Hyperpolarization

Magnetic resonance imaging (MRI) provides unique information about the internal structure and function of living organisms in a non-invasive way. The use of conventional proton MRI for the observation of real-time metabolism is hampered by the dominant signals of water and fat, which are abundant in living organisms. Heteronuclear MRI in conjunction with the hyperpolarization methods does not encounter this issue. In this work, we polarized 15N nuclei of [15N1]fampridine (a drug used for the treatment of multiple sclerosis) to the level of 4% in nuclear magnetic resonance (NMR) experiments and 0.7% in MRI studies using spin-lock-induced crossing combined with signal amplification by reversible exchange. Consequently, three-dimensional 15N MRI of the hyperpolarized 15N-labeled drug was acquired in 0.1 s with a signal-to-noise ratio of 70. In addition, the NMR signal enhancements for 15N-enriched fampridine and fampridine with a natural abundance of 15N nuclei were compared and an explanation for their difference was proposed.

Design, synthesis and evaluation of 15N- and 13C-labeled molecular probes as hyperpolarized nitric oxide sensors

Nitric oxide (NO) is an important signaling molecule involved in a wide range of biological processes. Development of non-invasive, real-time detection of NO is greatly desired yet remains challenging. Here we report the design and development of novel 15N- and 13C-labeled NO-sensing probes for hyperpolarized nuclear magnetic resonance (HP-NMR) studies. These probes undergo selective and rapid reaction with NO to generate in situ AZO-products that can be monitored with distinguishable NMR signals as a read-out. This study also allows for a direct comparison of the 15N and 13C nuclei performances in hyperpolarized reaction-based probes. The simple and general SABRE-SHEATH hyperpolarization method works on the 15N- and 13C-NO-sensing probes. Measured long spin-lattice relaxation (T1) values, especially for 15N-NO probes, will allow for real-time reaction-based imaging of NO.

In Situ Ternary Adduct Formation of Yttrium Polyaminocarboxylates Leads to Small Molecule Capture and Activation

In this work the chemistry of yttrium complexes is exploited for small molecule capture and activation. Nuclear magnetic resonance (NMR) and density functional theory (DFT) studies were used to investigate the in situ formation of solution state ternary yttrium-acetate, yttrium-bicarbonate, and yttrium-pyruvate adducts with a range of polyaminocarboxylate chelates. These studies reveal that [Y(DO3A)(H₂ O)₂ ] (H₃ DO3A - 1,4,7,10-tetraazacyclododecane-1,4,7-tricarboxylic acid) and [Y(EDTA)(H₂ O)_q ]^- (H₄ EDTA - ethylenediaminetetraacetic acid, q = 2 and 3) are able to form ternary adducts with bicarbonate and pyruvate. In the latter, unusual decarboxylation of pyruvate to form acetic acid and CO₂ was observed and further studied using SABRE-hyperpolarised ¹³ C NMR (SABRE - signal amplification by reversible exchange) to provide information about the reaction timescale and lifetime of intermediates involved in this conversion. The work presented demonstrates that yttrium complexes can capture and activate small molecules, which may lead to novel and useful applications of this metal in catalysis and medical imaging.

Rapid 13C Hyperpolarization of the TCA Cycle Intermediate α-Ketoglutarate via SABRE-SHEATH

α-Ketoglutarate is a key biomolecule involved in a number of metabolic pathways─most notably the TCA cycle. Abnormal α-ketoglutarate metabolism has also been linked with cancer. Here, isotopic labeling was employed to synthesize [1-13C,5-12C,D4]α-ketoglutarate with the future goal of utilizing its [1-13C]-hyperpolarized state for real-time metabolic imaging of α-ketoglutarate analytes and its downstream metabolites in vivo. The signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH) hyperpolarization technique was used to create 9.7% [1-13C] polarization in 1 minute in this isotopologue. The efficient 13C hyperpolarization, which utilizes parahydrogen as the source of nuclear spin order, is also supported by favorable relaxation dynamics at 0.4 μT field (the optimal polarization transfer field): the exponential 13C polarization buildup constant Tb is 11.0 ± 0.4 s whereas the 13C polarization decay constant T1 is 18.5 ± 0.7 s. An even higher 13C polarization value of 17.3% was achieved using natural-abundance α-ketoglutarate disodium salt, with overall similar relaxation dynamics at 0.4 μT field, indicating that substrate deuteration leads only to a slight increase (∼1.2-fold) in the relaxation rates for 13C nuclei separated by three chemical bonds. Instead, the gain in polarization (natural abundance versus [1-13C]-labeled) is rationalized through the smaller heat capacity of the "spin bath" comprising available 13C spins that must be hyperpolarized by the same number of parahydrogen present in each sample, in line with previous 15N SABRE-SHEATH studies. Remarkably, the C-2 carbon was not hyperpolarized in both α-ketoglutarate isotopologues studied; this observation is in sharp contrast with previously reported SABRE-SHEATH pyruvate studies, indicating that the catalyst-binding dynamics of C-2 in α-ketoglutarate differ from that in pyruvate. We also demonstrate that 13C spectroscopic characterization of α-ketoglutarate and pyruvate analytes can be performed at natural 13C abundance with an estimated detection limit of 80 micromolar concentration × *%P13C. All in all, the fundamental studies reported here enable a wide range of research communities with a new hyperpolarized contrast agent potentially useful for metabolic imaging of brain function, cancer, and other metabolically challenging diseases.

Multiaxial fields improve SABRE efficiency by preserving hydride order

Signal Amplification By Reversible Exchange (SABRE) and the heteronuclear variant, X-SABRE, increase the sensitivity of magnetic resonance techniques using order derived from reversible binding of para-hydrogen. One current limitation of SABRE is suboptimal polarization transfer over the lifetime of the complex. Here, we demonstrate a multiaxial low-field pulse sequence which allows optimal polarization build-up during a low-field "evolution" pulse, followed by a high-field "mixing" pulse which permits proton decoupling along an orthogonal axis. This preserves the singlet character of the hydrides while allowing exchange to replenish the ligands on the iridium catalyst. This strategy leads to a 2.5-fold improvement over continuous field SABRE SHEATH experimentally which was confirmed with numerical simulation.

Screening of Protein-Ligand Binding Using a SABRE Hyperpolarized Reporter

Hyperpolarization through signal amplification by reversible exchange (SABRE) provides a facile means to enhance nuclear magnetic resonance (NMR) signals of small molecules containing an N-heterocycle or other binding site for a polarization transfer catalyst. A purpose-designed reporter ligand, which is capable of binding both to a target protein and to the catalyst, makes the sensitivity enhancement by this technique compatible with the measurement of a range of biomolecular interactions. The ¹H polarization of the reporter ligand 4-amidinopyridine, which is targeting trypsin, is used to screen ligands that are not themselves hyperpolarizable by SABRE. The respective protein-ligand dissociation constants (K_D) are determined by an observed change in the R₂ relaxation rate of the reporter. A calculation of expected signal changes indicates that the accessible ligand K_D values extend over several orders of magnitude, while the concentrations of target proteins and ligands can be reduced considering the sensitivity gains from hyperpolarization. In general, the design of a single, weakly binding ligand for a target protein enables the use of SABRE hyperpolarization for ligand screening or other biophysical studies involving macromolecular interactions.