High field hyperpolarization-EXSY experiment for fast determination of dissociation rates in SABRE complexes

Modeling Ligand Exchange Kinetics in Iridium Complexes Catalyzing SABRE Nuclear Spin Hyperpolarization

Large signal enhancements can be obtained for NMR analytes using the process of nuclear spin hyperpolarization. Organometallic complexes that bind parahydrogen can themselves become hyperpolarized. Moreover, if parahydrogen and a to-be-hyperpolarized analyte undergo chemical exchange with the organometallic complex it is possible to catalytically sensitize the detection of the analyte via hyperpolarization transfer through spin-spin coupling in this organometallic complex. This process is called Signal Amplification By Reversible Exchange (SABRE). Signal intensity gains of several orders of magnitude can thus be created for various compounds in seconds. The chemical exchange processes play a defining role in controlling the efficiency of SABRE because the lifetime of the complex must match the spin-spin couplings. Here, we show how analyte dissociation rates in the key model substrates pyridine (the simplest six-membered heterocycle), 4-aminopyridine (a drug), and nicotinamide (an essential vitamin biomolecule) can be examined. This is achieved for the most widely employed SABRE motif that is based on IrIMes-derived catalysts by 1H 1D and 2D exchange NMR spectroscopy techniques. Several kinetic models are evaluated for their accuracy and simplicity. By incorporating variable temperature analysis, the data yields key enthalpies and entropies of activation that are critical for understanding the underlying SABRE catalyst properties and subsequently optimizing behavior through rational chemical design. While several studies of chemical exchange in SABRE have been reported, this work also aims to establish a toolkit on how to quantify chemical exchange in SABRE and ensure that data can be compared reliably.

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.

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.

Analysis of Complex Mixtures by Chemosensing NMR Using para-Hydrogen-Induced Hyperpolarization

Nuclear magnetic resonance (NMR) is a powerful technique for chemical analysis. The use of NMR to investigate dilute analytes in complex systems is, however, hampered by its relatively low sensitivity. An additional obstacle is represented by the NMR signal overlap. Because solutes in a complex mixture are usually not isotopically labeled, NMR studies are often limited to ¹H measurements, which, because of the modest dispersion of the ¹H resonances (typically ∼10 ppm), can result in challenging signal crowding. The low NMR sensitivity issue can be alleviated by nuclear spin hyperpolarization (i.e., transiently increasing the differences in nuclear spin populations), which determines large NMR signal enhancements. This has been demonstrated for hyperpolarization methods such as dynamic nuclear polarization, spin-exchange optical pumping and para-hydrogen-induced polarization (PHIP). In particular, PHIP has grown into a fast, efficient, and versatile technique since the recent discovery of non-hydrogenative routes to achieve nuclear spin hyperpolarization.

For instance, signal amplification by reversible exchange (SABRE) can generate proton as well as heteronuclear spin hyperpolarization in a few seconds in compounds that are able to transiently bind to an iridium catalyst in the presence of para-hydrogen in solution. The hyperpolarization transfer catalyst acts as a chemosensor in the sense that it is selective for analytes that can coordinate to the metal center, such as nitrogen-containing aromatic heterocycles, sulfur heteroaromatic compounds, nitriles, Schiff bases, diaziridines, carboxylic acids, and amines. We have demonstrated that the signal enhancement achieved by SABRE allows rapid NMR detection and quantification of a mixture of substrates down to low-micromolar concentration. Furthermore, in the transient complex, the spin configuration of p-H₂ can be easily converted to spin hyperpolarization to produce up to 1000-fold enhanced NMR hydride signals. Because the hydrides’ chemical shifts are highly sensitive to the structure of the analyte associating with the iridium complex, they can be employed as hyperpolarized “probes” to signal the presence of specific compounds in the mixture. This indirect detection of the analytes in solution provides important benefits in the case of complex systems, as hydrides resonate in a region of the ¹H spectrum (at ca. −20 ppm) that is generally signal-free. The enhanced sensitivity provided by non-hydrogenative PHIP (nhPHIP), together with the absence of interference from the complex matrix (usually resonating between 0 and 10 ppm), set the detection limit for this NMR chemosensor down to sub-μM concentrations, approximately 3 orders of magnitude lower than for conventional NMR. This nhPHIP approach represents, therefore, a powerful tool for NMR analysis of dilute substrates in complex mixtures as it addresses at once the issues of signal crowding and NMR sensitivity. Importantly, being performed at high field inside the NMR spectrometer, the method allows for rapid acquisition of multiple scans, multidimensional hyperpolarized NMR spectra, in a fashion comparable to that of standard NMR measurements.

In this Account, we focus on our chemosensing NMR technology, detailing its principles, advantages, and limitations and presenting a number of applications to real systems such as biofluids, beverages, and natural extracts.

Understanding Parahydrogen Hyperpolarized Urine Spectra: The Case of Adenosine Derivatives

Parahydrogen hyperpolarization has emerged as a promising tool for sensitivity-enhanced NMR metabolomics. It allows resolution and quantification of NMR signals of certain classes of low-abundance metabolites that would otherwise be undetectable. Applications have been implemented in pharmacokinetics and doping drug detection, demonstrating the versatility of the technique. Yet, in order for the method to be adopted by the analytical community, certain limitations have to be understood and overcome. One such question is NMR signal assignment. At present, the only reliable way to establish the identity of an analyte that gives rise to certain parahydrogen hyperpolarized NMR signals is internal standard addition, which can be laborious. Herein we show that analogously to regular NMR metabolomics, generating libraries of hyperpolarized analyte signals is a viable way to address this limitation. We present hyperpolarized spectral data of adenosines and give an early example of identifying them from a urine sample with the small library. Doing so, we verify the detectability of a class of diagnostically valuable metabolites: adenosine and its derivatives, some of which are cancer biomarkers, and some are central to cellular energy management (e.g., ATP).

Temperature Cycling Enables Efficient 13C SABRE-SHEATH Hyperpolarization and Imaging of [1-13C]-Pyruvate

Molecular metabolic imaging in humans is dominated by positron emission tomography (PET). An emerging nonionizing alternative is hyperpolarized MRI of 13C-pyruvate, which is innocuous and has a central role in metabolism. However, similar to PET, hyperpolarized MRI with dissolution dynamic nuclear polarization (d-DNP) is complex costly, and requires significant infrastructure. In contrast, Signal Amplification By Reversible Exchange (SABRE) is a fast, cheap, and scalable hyperpolarization technique. SABRE in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) can transfer polarization from parahydrogen to 13C in pyruvate; however, polarization levels remained low relative to d-DNP (1.7% with SABRE-SHEATH versus ≈60% with DNP). Here we introduce a temperature cycling method for SABRE-SHEATH that enables >10% polarization on [1-13C]-pyruvate, sufficient for successful in vivo experiments. First, at lower temperatures, ≈20% polarization is accumulated on SABRE catalyst-bound pyruvate, which is released into free pyruvate at elevated temperatures. A kinetic model of differential equations is developed that explains this effect and characterizes critical relaxation and buildup parameters. With the large polarization, we demonstrate the first 13C pyruvate images with a cryogen-free MRI system operated at 1.5 T, illustrating that inexpensive hyperpolarization methods can be combined with low-cost MRI systems to obtain a broadly available, yet highly sensitive metabolic imaging platform.

Parahydrogen hyperpolarization of minimally altered urine samples for sensitivity enhanced NMR metabolomics

Parahydrogen hyperpolarization has been shown to enhance NMR sensitivity in urine analysis by several orders of magnitude if urine samples are prepared by solid phase extraction (SPE). We present a different approach, developed for minimal sample alteration before analysis. Removing SPE from the workflow allows to retain a wider range of metabolites and paves the way towards more universal hyperpolarized NMR metabolomics of low abundance metabolites.

Hyperpolarization of common antifungal agents with SABRE

Signal amplification by reversible exchange (SABRE) is a robust and inexpensive hyperpolarization (HP) technique to enhance nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) signals using parahydrogen (pH2 ). The substrate scope of SABRE is continually expanding. Here, we present the polarization of three antifungal drugs (voriconazole, clotrimazole, and fluconazole) and elicit the detailed HP mechanisms for 1 H and 15 N nuclei. In this exploratory work, 15 N polarization values of ~1% were achieved using 50% pH2 in solution of 3-mM catalyst and 60-mM substrate in perdeuterated methanol. All hyperpolarized 15 N sites exhibited long T1 in excess of 1 min at a clinically relevant field of 1 T. Hyperpolarizing common drugs is of interest due to their potential biomedical applications as MRI contrast agents or to enable studies on protein dynamics at physiological concentrations. We optimize the polarization with respect to temperature and the polarization transfer field (PTF) for 1 H nuclei in the millitesla regime and for 15 N nuclei in the microtesla regime, which provides detailed insights into exchange kinetics and spin evolution. This work broadens the SABRE substrate scope and provides mechanistic and kinetic insights into the HP process.

Long-Term Generation of Longitudinal Spin Order Controlled by Ammonia Ligation Enables Rapid SABRE Hyperpolarized 2D NMR

Symmetry breaking of parahydrogen using iridium catalysts converts singlet spin order into observable hyperpolarization. In this contribution, iridium catalysts are designed to exhibit asymmetry in their hydrides, regulated by in situ generation of deuterated ammonia governed by ammonium buffers. The concentrations of ammonia (N) and pyridine (P) provide a handle to generate a variety of stereo-chemically asymmetric N-heterocyclic carbene iridium complexes, ligating either [3xP], [2xP;N], [P;2xN] or [3xN] in an octahedral SABRE type configuration. The non-equivalent hydride positions, in correspondence with the ammonium buffer solutions, enables to extend singlet-triplet or S ⟩ → T 0 ⟩ mixing at high magnetic field and experimentally induce prolonged generation of non-equilibrium longitudinal two-spin order. This long-lasting magnetization can be exploited in hyperpolarized 2D-OPSY-COSY experiments providing direct structural information on the catalyst using a single contact with parahydrogen. Separately, field cycling revealed hyperpolarization properties in low-field conditions. Controlling catalyst stereochemistry by introducing small and deuterated ligands, such as deuterated ammonia, simplifies the spin-system. This is shown to unify experimental and theoretically derived field-sweep experiments for four-spin systems.

Determination of hydrogen exchange and relaxation parameters in PHIP complexes at micromolar concentrations

Non-hydrogenative para-hydrogen-induced polarization (PHIP) is a fast, efficient and relatively inexpensive approach to enhance nuclear magnetic resonance (NMR) signals of small molecules in solution. The efficiency of this technique depends on the interplay of NMR relaxation and kinetic processes, which, at high concentrations, can be characterized by selective inversion experiments. However, in the case of dilute solutions this approach is clearly not viable. Here, we present alternative PHIP-based NMR experiments to determine hydrogen and hydride relaxation parameters as well as the rate constants for para-hydrogen association with and dissociation from asymmetric PHIP complexes at micromolar concentrations. Access to these parameters is necessary to understand and improve the PHIP enhancements of (dilute) substrates present in, for instance, biofluids and natural extracts.

Hyperpolarisation of weakly binding N-heterocycles using signal amplification by reversible exchange

Signal Amplification by Reversible Exchange (SABRE) is a catalytic method for improving the detection of molecules by magnetic resonance spectroscopy. It achieves this by simultaneously binding the target substrate (sub) and para-hydrogen to a metal centre. To date, sterically large substrates are relatively inaccessible to SABRE due to their weak binding leading to catalyst destabilisation. We overcome this problem here through a simple co-ligand strategy that allows the hyperpolarisation of a range of weakly binding and sterically encumbered N-heterocycles. The resulting 1H NMR signal size is increased by up to 1400 times relative to their more usual Boltzmann controlled levels at 400 MHz. Hence, a significant reduction in scan time is achieved. The SABRE catalyst in these systems takes the form [IrX(H)2(NHC)(sulfoxide)(sub)] where X = Cl, Br or I. These complexes are shown to undergo very rapid ligand exchange and lower temperatures dramatically improve the efficiency of these SABRE catalysts.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

High-throughput continuous-flow system for SABRE hyperpolarization

Signal Amplification By Reversible Exchange (SABRE) is a versatile method for hyperpolarizing small organic molecules that helps to overcome the inherent low signal-to-noise ratio of nuclear magnetic resonance (NMR) measurements. It offers orders of magnitude enhanced signal strength, but the obtained nuclear polarization usually rapidly relaxes, requiring a quick transport of the sample to the spectrometer. Here we report a new design of a polarizing system, which can be used to prepare a continuous flow of SABRE-hyperpolarized sample with a considerable throughput of several millilitres per second and a rapid delivery into an NMR instrument. The polarizer performance under different conditions such as flow rate of the hydrogen or liquid sample is tested by measuring a series of NMR spectra and magnetic resonance images (MRI) of hyperpolarized pyridine in methanol. Results show a capability to continuously produce sample with dramatically enhanced signal over two orders of magnitude. The constant supply of hyperpolarized sample can be exploited, e.g., in experiments requiring multiple repetitions, such as 2D- and 3D-NMR or MRI measurements, and also naturally allows measurements of flow maps, including systems with high flow rates, for which the level of achievable thermal polarization might not be usable any more. In addition, the experiments can be viably carried out in a non-deuterated solvent, due to the effective suppression of the thermal polarization by the fast sample flow. The presented system opens the possibilities for SABRE experiments requiring a long-term, stable and high level of nuclear polarization.

Parahydrogen induced hyperpolarization provides a tool for NMR metabolomics at nanomolar concentrations

Sensitivity enhancement by parahydrogen hyperpolarization allows NMR detection and quantification of hundreds of urinary metabolites at down to nanomolar concentrations.