The Absence of Quadrupolar Nuclei Facilitates Efficient 13 C Hyperpolarization via Reversible Exchange with Parahydrogen

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.

Carbon-13 Radiofrequency Amplification by Stimulated Emission of Radiation of the Hyperpolarized Ketone and Hemiketal Forms of Allyl [1-13C]Pyruvate

13C hyperpolarized pyruvate is an emerging MRI contrast agent for sensing molecular events in cancer and other diseases with aberrant metabolic pathways. This metabolic contrast agent can be produced via several hyperpolarization techniques. Despite remarkable success in research settings, widespread clinical adoption faces substantial roadblocks because the current sensing technology utilized to sense this contrast agent requires the excitation of 13C nuclear spins that also need to be synchronized with MRI field gradient pulses. Here, we demonstrate sensing of hyperpolarized allyl [1-13C]pyruvate via the stimulated emission of radiation that mitigates the requirements currently blocking broader adoption. Specifically, 13C Radiofrequency Amplification by Stimulated Emission of Radiation (13C RASER) was obtained after pairwise addition of parahydrogen to a pyruvate precursor, detected in a commercial inductive detector with a quality factor (Q) of 32 for sample concentrations as low as 0.125 M with 13C polarization of 4%. Moreover, parahydrogen-induced polarization allowed for the preparation of a mixture of ketone and hemiketal forms of hyperpolarized allyl [1-13C]pyruvate, which are separated by 10 ppm in 13C NMR spectra. This is a good model system to study the simultaneous 13C RASER signals of multiple 13C species. This system models the metabolic production of hyperpolarized [1-13C]lactate from hyperpolarized [1-13C]pyruvate, which has a similar chemical shift difference. Our results show that 13C RASER signals can be obtained from both species simultaneously when the emission threshold is exceeded for both species. On the other hand, when the emission threshold is exceeded only for one of the hyperpolarized species, 13C stimulated emission is confined to this species only, therefore enabling the background-free detection of individual hyperpolarized 13C signals. The reported results pave the way to novel sensing approaches of 13C hyperpolarized pyruvate, potentially unlocking hyperpolarized 13C MRI on virtually any MRI system─an attractive vision for the future molecular imaging and diagnostics.

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.

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.

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.

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.

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.

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.

Real-Time High-Sensitivity Reaction Monitoring of Important Nitrogen-Cycle Synthons by 15N Hyperpolarized Nuclear Magnetic Resonance

Here, we show how signal amplification by reversible exchange hyperpolarization of a range of ¹⁵N-containing synthons can be used to enable studies of their reactivity by ¹⁵N nuclear magnetic resonance (NO₂^– (28% polarization), ND₃ (3%), PhCH₂NH₂ (5%), NaN₃ (3%), and NO₃^– (0.1%)). A range of iridium-based spin-polarization transfer catalysts are used, which for NO₂^– work optimally as an amino-derived carbene-containing complex with a DMAP-d₂ coligand. We harness long ¹⁵N spin-order lifetimes to probe in situ reactivity out to 3 × T₁. In the case of NO₂^– (T₁ 17.7 s at 9.4 T), we monitor PhNH₂ diazotization in acidic solution. The resulting diazonium salt (¹⁵N-T₁ 38 s) forms within 30 s, and its subsequent reaction with NaN₃ leads to the detection of hyperpolarized PhN₃ (T₁ 192 s) in a second step via the formation of an identified cyclic pentazole intermediate. The role of PhN₃ and NaN₃ in copper-free click chemistry is exemplified for hyperpolarized triazole (T₁ < 10 s) formation when they react with a strained alkyne. We also demonstrate simple routes to hyperpolarized N₂ in addition to showing how utilization of ¹⁵N-polarized PhCH₂NH₂ enables the probing of amidation, sulfonamidation, and imine formation. Hyperpolarized ND₃ is used to probe imine and ND₄⁺ (T₁ 33.6 s) formation. Furthermore, for NO₂^–, we also demonstrate how the ¹⁵N-magnetic resonance imaging monitoring of biphasic catalysis confirms the successful preparation of an aqueous bolus of hyperpolarized ¹⁵NO₂^– in seconds with 8% polarization. Hence, we create a versatile tool to probe organic transformations that has significant relevance for the synthesis of future hyperpolarized pharmaceuticals.

Phase coherent excitation of SABRE permits simultaneous hyperpolarization of multiple targets at high magnetic field

Hyperpolarization methods in magnetic resonance overcome sensitivity limitations, especially for low-γ nuclei such as 13C and 15N. Signal Amplification By Reversible Exchange (SABRE) and extended SABRE (X-SABRE) are efficient and low-cost methods for generating large polarizations on a variety of nuclei, but they most commonly use low magnetic fields (μT-mT). High field approaches, where hyperpolarization is generated directly in the spectrometer, are potentially much more convenient but have been limited to selectively hyperpolarize single targets. Here we introduce a new pulse sequence-based approach that affords broadband excitation of SABRE hyperpolarization at high magnetic fields without having to tailor pulse sequence parameters to specific targets. This permits simultaneous hyperpolarization of multiple targets for the first time at high field and offers a direct approach to integration of high-field SABRE hyperpolarization into routine NMR applications, such as NMR-based metabonomics and biomolecular NMR.

Order-Unity 13 C Nuclear Polarization of [1-13 C]Pyruvate in Seconds and the Interplay of Water and SABRE Enhancement

Signal Amplification By Reversible Exchange in SHield Enabled Alignment Transfer (SABRE-SHEATH) is investigated to achieve rapid hyperpolarization of 13 C1 spins of [1-13 C]pyruvate, using parahydrogen as the source of nuclear spin order. Pyruvate exchange with an iridium polarization transfer complex can be modulated via a sensitive interplay between temperature and co-ligation of DMSO and H2 O. Order-unity 13 C (>50 %) polarization of catalyst-bound [1-13 C]pyruvate is achieved in less than 30 s by restricting the chemical exchange of [1-13 C]pyruvate at lower temperatures. On the catalyst bound pyruvate, 39 % polarization is measured using a 1.4 T NMR spectrometer, and extrapolated to >50 % at the end of build-up in situ. The highest measured polarization of a 30-mM pyruvate sample, including free and bound pyruvate is 13 % when using 20 mM DMSO and 0.5 M water in CD3 OD. Efficient 13 C polarization is also enabled by favorable relaxation dynamics in sub-microtesla magnetic fields, as indicated by fast polarization buildup rates compared to the T1 spin-relaxation rates (e. g., ∼0.2 s-1 versus ∼0.1 s-1 , respectively, for a 6 mM catalyst-[1-13 C]pyruvate sample). Finally, the catalyst-bound hyperpolarized [1-13 C]pyruvate can be released rapidly by cycling the temperature and/or by optimizing the amount of water, paving the way to future biomedical applications of hyperpolarized [1-13 C]pyruvate produced via comparatively fast and simple SABRE-SHEATH-based approaches.

Decoupling of Spin Decoherence Paths near Zero Magnetic Field

We demonstrate a method to quantify and manipulate nuclear spin decoherence mechanisms that are active in zero to ultralow magnetic fields. These include (i) nonadiabatic switching of spin quantization axis due to residual background fields and (ii) scalar pathways due to through-bond couplings between ¹H and heteronuclear spin species, such as ²H used partially as an isotopic substitute for ¹H. Under conditions of free evolution, scalar relaxation due to ²H can significantly limit nuclear spin polarization lifetimes and thus the scope of magnetic resonance procedures near zero field. It is shown that robust trains of pulsed dc magnetic fields that apply π flip angles to one or multiple spin species may switch the effective symmetry of the nuclear spin Hamiltonian, imposing decoupled or coupled dynamic regimes on demand. The method should broaden the spectrum of hyperpolarized biomedical contrast-agent compounds and hyperpolarization procedures that are used near zero field.

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.

Hyperpolarised NMR to aid molecular profiling of electronic cigarette aerosols

Signal amplification by reversible exchange (SABRE) hyperpolarisation is used to enhance the NMR signals of nicotine and acrolein in methanol-d4 solutions of electronic cigarette aerosols. Consequently, detection of 74 μM nicotine is possible in just a single scan 1H NMR spectrum. The first example of an aldehyde hyperpolarised using SABRE is demonstrated and we work towards novel real-world applications of SABRE-hyperpolarised NMR for chemical analysis.

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.

15 N hyperpolarisation of the antiprotozoal drug ornidazole by Signal Amplification By Reversible Exchange in aqueous medium

Signal amplification by reversible exchange (SABRE) offers a cost-effective route to boost nuclear magnetic resonance (NMR) signal by several orders of magnitude by employing readily available para-hydrogen as a source of hyperpolarisation. Although ¹ H spins have been the natural choice of SABRE hyperpolarisation since its inception due to its simplicity and accessibility, limited spin lifetimes of ¹ H makes it harder to employ them in a range of time-dependent NMR experiments. Heteronuclear spins, for example, ¹³ C and ¹⁵ N, in general have much longer T₁ lifetimes and thereby are found to be more suitable for hyperpolarised biological applications as demonstrated previously by para-hydrogen induced polarisation (PHIP) and dynamic nuclear polarisation (DNP). In this study we demonstrate a simple procedure to enhance ¹⁵ N signal of an antibiotic drug ornidazole by up to 71,000-folds with net ¹⁵ N polarisation reaching ~23%. Further, the effect of co-ligand strategy is studied in conjunction with the optimum field transfer protocols and consequently achieving ¹⁵ N hyperpolarised spin lifetime of >3 min at low field. Finally, we present a convenient route to harness the hyperpolarised solution in aqueous medium free from catalyst contamination leading to a strong ¹⁵ N signal detection for an extended duration of time.

Density Functional Theory Study of Reaction Equilibria in Signal Amplification by Reversible Exchange

An in-depth theoretical analysis of key chemical equilibria in Signal Amplification by Reversible Exchange (SABRE) is provided, employing density functional theory calculations to characterize the likely reaction network. For all reactions in the network, the potential energy surface is probed to identify minimum energy pathways. Energy barriers and transition states are calculated, and harmonic transition state theory is applied to calculate exchange rates that approximate experimental values. The reaction network energy surface can be modulated by chemical potentials that account for the dependence on concentration, temperature, and partial pressure of molecular constituents (hydrogen, methanol, pyridine) supplied to the experiment under equilibrium conditions. We show that, under typical experimental conditions, the Gibbs free energies of the two key states involved in pyridine-hydrogen exchange at the common Ir-IMes catalyst system in methanol are essentially the same, i. e., nearly optimal for SABRE. We also show that a methanol-containing intermediate is plausible as a transient species in the process.

Low-Cost High-Pressure Clinical-Scale 50% Parahydrogen Generator Using Liquid Nitrogen at 77 K

We report on a robust and low-cost parahydrogen generator design employing liquid nitrogen as a coolant. The core of the generator consists of catalyst-filled spiral copper tubing, which can be pressurized to 35 atm. Parahydrogen fraction >48% was obtained at 77 K with three nearly identical generators using paramagnetic hydrated iron oxide catalysts. Parahydrogen quantification was performed on the fly via benchtop NMR spectroscopy to monitor the signal from residual orthohydrogen-parahydrogen is NMR silent. This real-time quantification approach was also used to evaluate catalyst activation at up to 1.0 standard liter per minute flow rate. The reported inexpensive device can be employed for a wide range of studies employing parahydrogen as a source of nuclear spin hyperpolarization. To this end, we demonstrate the utility of this parahydrogen generator for hyperpolarization of concentrated sodium [1-13C]pyruvate, a metabolic contrast agent under investigation in numerous clinical trials. The reported pilot optimization of SABRE-SHEATH (signal amplification by reversible exchange-shield enables alignment transfer to heteronuclei) hyperpolarization yielded 13C signal enhancement of over 14,000-fold at a clinically relevant magnetic field of 1 T corresponding to approximately 1.2% 13C polarization-if near 100% parahydrogen would have been employed, the reported value would be tripled to 13C polarization of 3.5%.

Synthesis and 15 N NMR Signal Amplification by Reversible Exchange of [15 N]Dalfampridine at Microtesla Magnetic Fields

Signal Amplification by Reversible Exchange (SABRE) technique enables nuclear spin hyperpolarization of wide range of compounds using parahydrogen. Here we present the synthetic approach to prepare ¹⁵ N-labeled [¹⁵ N]dalfampridine (4-amino[¹⁵ N]pyridine) utilized as a drug to reduce the symptoms of multiple sclerosis. The synthesized compound was hyperpolarized using SABRE at microtesla magnetic fields (SABRE-SHEATH technique) with up to 2.0 % ¹⁵ N polarization. The 7-hour-long activation of SABRE pre-catalyst [Ir(IMes)(COD)Cl] in the presence of [¹⁵ N]dalfampridine can be remedied by the use of pyridine co-ligand for catalyst activation while retaining the ¹⁵ N polarization levels of [¹⁵ N]dalfampridine. The effects of experimental conditions such as polarization transfer magnetic field, temperature, concentration, parahydrogen flow rate and pressure on ¹⁵ N polarization levels of free and equatorial catalyst-bound [¹⁵ N]dalfampridine were investigated. Moreover, we studied ¹⁵ N polarization build-up and decay at magnetic field of less than 0.04 μT as well as ¹⁵ N polarization decay at the Earth's magnetic field and at 1.4 T.

Clinical-Scale Production of Nearly Pure (>98.5%) Parahydrogen and Quantification by Benchtop NMR Spectroscopy

Because of the extensive chemical, physical, and biomedical applications of parahydrogen, the need exists for the development of highly enriched parahydrogen in a robust and efficient manner. Herein, we present a parahydrogen enrichment equipment which substantially improves upon the previous generators with its ability to enrich parahydrogen to >98.5% and a production rate of up to 4 standard liters per minute with the added advantage of real-time quantification. Our generator employs a pulsed injection system with a 3/16 in. outside diameter copper spiral tubing filled with iron-oxide catalyst. This tubing is mated to a custom-made copper attachment to provide efficient thermal coupling to the cold head. This device allows for robust operation at high pressures up to 34 atm. Real-time quantification by benchtop NMR spectroscopy is made possible by direct coupling of the p-H2 outlet from the generator to a 1.4 T NMR spectrometer using a regular 5 mm NMR tube that is continuously refilled with the exiting parahydrogen gas at ∼8 atm pressure. The use of high hydrogen gas pressure offers two critical NMR signal detection benefits: increased concentration and line narrowing. Our work presents a comprehensive description of the apparatus for a convenient and robust parahydrogen production, distribution, and quantification system, especially for parahydrogen-based hyperpolarization NMR research.

Exploring the hyperpolarisation of EGTA-based ligands using SABRE

The design of molecules whose magnetic resonance (MR) signals report on their biological environment is receiving attention as a route to non-invasive functional MR. Hyperpolarisation techniques improve the sensitivity of MR and enable real time low concentration MR imaging, allowing for the development of novel functional imaging methodologies. In this work, we report on the synthesis of a series of EGTA-derived molecules (EGTA - ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid), whose core structures are known to bind biologically relevant metal ions in vivo, in addition to pyridyl rings that allow reversible ligation to an iridium dihydride complex. Consequently, they are amenable to hyperpolarisation through the parahydrogen-based signal amplification by reversible exchange (SABRE) process. We investigate how the proximity of EGTA and pyridine units, and the identity of the linker group, affect the SABRE hyperpolarisation attained for each agent. We also describe the effect of catalyst identity and co-ligand presence on these measurements and can achieve ¹H NMR signal enhancements of up to 160-fold. We rationalise these results to suggest the design elements needed for probes amenable to SABRE hyperpolarisation whose MR signals might in the future report on the presence of metal ions.

Reversible Hyperpolarization of Ketoisocaproate Using Sulfoxide-containing Polarization Transfer Catalysts

The substrate scope of sulfoxide-containing magnetisation transfer catalysts is extended to hyperpolarize α-ketoisocaproate and α-ketoisocaproate-1-[13 C]. This is achieved by forming [Ir(H)2 (κ2 -ketoisocaproate)(N-heterocyclic carbene)(sulfoxide)] which transfers latent magnetism from p-H2 via the signal amplification by reversible exchange (SABRE) process. The effect of polarization transfer field on the formation of enhanced 13 C magnetization is evaluated. Consequently, performing SABRE in a 0.5 μT field enabled most efficient magnetisation transfer. 13 C NMR signals for α-ketoisocaproate-1-[13 C] in methanol-d4 are up to 985-fold more intense than their traditional Boltzmann derived signal intensity (0.8 % 13 C polarisation). Single crystal X-ray diffraction reveals the formation of the novel catalyst decomposition products [Ir(μ-H)(H)2 (IMes)(SO(Ph)(Me)2 )]2 and [(Ir(H)2 (IMes)(SO(Me)2 ))2 (μ-S)] when the sulfoxides methylphenylsulfoxide and dimethylsulfoxide are used respectively.

Benchtop NMR analysis of piperazine-based drugs hyperpolarised by SABRE

Piperazine-based drugs, such as N-benzylpiperazine (BZP), became attractive in the 2000s due to possessing effects similar to amphetamines. Herein, BZP, in addition to its pyridyl analogues, 2-, 3-, and 4-pyridylmethylpiperidine (2-PMP, 3-PMP, and 4-PMP respectively) was subjected to the hyperpolarisation technique Signal Amplification By Reversible Exchange (SABRE) in order to demonstrate the use of this technique to detect these piperazine-based drugs. Although BZP was not hyperpolarised via SABRE, 2-PMP, 3-PMP, and 4-PMP were, with the ortho- and meta-pyridyl protons of 4-PMP showing the largest enhancement of 313-fold and 267-fold, respectively, in a 1.4-T detection field, following polarisation transfer at Earth's magnetic field. In addition to the freebase, 4-PMP.3HCl was also appraised by SABRE and was found not to polarise, however, the addition of increasing equivalents of triethylamine (TEA) produced the freebase, with a maximum enhancement observed upon the addition of 3 equivalents of TEA. Further addition of TEA led to a reduction in the observed enhancement. SABRE was also employed to polarise 4-PMP.3HCl (~20% w/w) in a simulated tablet to demonstrate the forensic application of the technique (138-fold enhancement for the ortho-pyridyl protons). The amount of 4-PMP.3HCl present in the simulated tablet was quantified via NMR using D₂ O as a solvent and compared well to complimentary gas chromatography-mass spectrometry data. Exchanging D₂ O for CD₃ OD as the solvent utilised for analysis resulted in a significantly lower amount of 4-PMP.3HCl being determined, thus highlighting safeguarding issues linked to drug abuse in relation to determining the amount of active pharmaceutical ingredient present.

Rational ligand choice extends the SABRE substrate scope

Here we report on chelating ligands for Signal Amplification By Reversible Exchange (SABRE) catalysts that permit hyperpolarisation on otherwise sterically hindered substrates. We demonstrate 1H enhancements of ∼100-fold over 8.5 T thermal for 2-substituted pyridines, and smaller, yet significant enhancements for provitamin B6 and caffeine. We also show 15N-enhancements of ∼1000-fold and 19F-enhancements of 30-fold.

Quantifying the effects of quadrupolar sinks via15N relaxation dynamics in metronidazoles hyperpolarized via SABRE-SHEATH

15N spin-lattice relaxation dynamics in metronidazole-15N3 and metronidazole-15N2 isotopologues are studied for rational design of 15N-enriched biomolecules for signal amplification by reversible exchange in microtesla fields. 15N relaxation dynamics mapping reveals the deleterious effects of interactions with the polarization transfer catalyst and a quadrupolar 14N nucleus within the spin-relayed 15N-15N network.

Hyperpolarization of Nitrile Compounds Using Signal Amplification by Reversible Exchange

Signal Amplification by Reversible Exchange (SABRE), a hyperpolarization technique, has been harnessed as a powerful tool to achieve useful hyperpolarized materials by polarization transfer from parahydrogen. In this study, we systemically applied SABRE to a series of nitrile compounds, which have been rarely investigated. By performing SABRE in various magnetic fields and concentrations on nitrile compounds, we unveiled its hyperpolarization properties to maximize the spin polarization and its transfer to the next spins. Through this sequential study, we obtained a ~130-fold enhancement for several nitrile compounds, which is the highest number ever reported for the nitrile compounds. Our study revealed that the spin polarization on hydrogens decreases with longer distances from the nitrile group, and its maximum polarization is found to be approximately 70 G with 5 μL of substrates in all structures. Interestingly, more branched structures in the ligand showed less effective polarization transfer mechanisms than the structural isomers of butyronitrile and isobutyronitrile. These first systematic SABRE studies on a series of nitrile compounds will provide new opportunities for further research on the hyperpolarization of various useful nitrile materials.

Remarkable Levels of 15N Polarization Delivered through SABRE into Unlabeled Pyridine, Pyrazine, or Metronidazole Enable Single Scan NMR Quantification at the mM Level

While many drugs and metabolites contain nitrogen, harnessing their diagnostic ¹⁵N NMR signature for their characterization is underutilized because of inherent detection difficulties. Here, we demonstrate how precise ultralow field signal amplification by reversible exchange (±0.2 mG) in conjunction parahydrogen and an iridium precatalyst of the form IrCl(COD)(NHC) with the coligand d₉-benzylamine allows the naturally abundant ¹⁵N NMR signatures of pyridine, pyrazine, metronidazole, and acetonitrile to be readily detected at 9.4 T in single NMR observations through >50% ¹⁵N polarization levels. These signals allow for rapid and precise reagent quantification via a response that varies linearly over the 2-70 mM concentration range.

"Direct" 13 C Hyperpolarization of 13 C-Acetate by MicroTesla NMR Signal Amplification by Reversible Exchange (SABRE)

Herein, we demonstrate "direct" ¹³ C hyperpolarization of ¹³ C-acetate via signal amplification by reversible exchange (SABRE). The standard SABRE homogeneous catalyst [Ir-IMes; [IrCl(COD)(IMes)], (IMes=1,3-bis(2,4,6-trimethylphenyl), imidazole-2-ylidene; COD=cyclooctadiene)] was first activated in the presence of an auxiliary substrate (pyridine) in alcohol. Following addition of sodium 1-¹³ C-acetate, parahydrogen bubbling within a microtesla magnetic field (i.e. under conditions of SABRE in shield enables alignment transfer to heteronuclei, SABRE-SHEATH) resulted in positive enhancements of up to ≈100-fold in the ¹³ C NMR signal compared to thermal equilibrium at 9.4 T. The present results are consistent with a mechanism of "direct" transfer of spin order from parahydrogen to ¹³ C spins of acetate weakly bound to the catalyst, under conditions of fast exchange with respect to the ¹³ C acetate resonance, but we find that relaxation dynamics at microtesla fields alter the optimal matching from the traditional SABRE-SHEATH picture. Further development of this approach could lead to new ways to rapidly, cheaply, and simply hyperpolarize a broad range of substrates (e.g. metabolites with carboxyl groups) for various applications, including biomedical NMR and MRI of cellular and in vivo metabolism.

In Situ SABRE Hyperpolarization with Earth's Field NMR Detection

Hyperpolarization methods, which increase the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), have the potential to expand the range of applications of these powerful analytical techniques and to enable the use of smaller and cheaper devices. The signal amplification by reversible exchange (SABRE) method is of particular interest because it is relatively low-cost, straight-forward to implement, produces high-levels of renewable signal enhancement, and can be interfaced with low-cost and portable NMR detectors. In this work, we demonstrate an in situ approach to SABRE hyperpolarization that can be achieved using a simple, commercially-available Earth’s field NMR detector to provide ¹H polarization levels of up to 3.3%. This corresponds to a signal enhancement over the Earth’s magnetic field by a factor of ε > 2 × 10⁸. The key benefit of our approach is that it can be used to directly probe the polarization transfer process at the heart of the SABRE technique. In particular, we demonstrate the use of in situ hyperpolarization to observe the activation of the SABRE catalyst, the build-up of signal in the polarization transfer field (PTF), the dependence of the hyperpolarization level on the strength of the PTF, and the rate of decay of the hyperpolarization in the ultra-low-field regime.

A versatile synthetic route to the preparation of 15 N heterocycles

A robust medium-scale (approximately 3 g) synthetic method for 15 N labeling of pyridine (15 N-Py) is reported based on the Zincke reaction. 15 N enrichment in excess of 81% was achieved with approximately 33% yield. 15 N-Py serves as a standard substrate in a wide range of studies employing a hyperpolarization technique for efficient polarization transfer from parahydrogen to heteronuclei; this technique, called SABRE (signal amplification by reversible exchange), employs a simultaneous chemical exchange of parahydrogen and a to-be-hyperpolarized substrate (e.g., pyridine) on metal centers. In studies aimed at the development of hyperpolarized contrast agents for in vivo molecular imaging, pyridine is often employed either as a model substrate (for hyperpolarization technique development, quality assurance, and phantom imaging studies) or as a co-substrate to facilitate more efficient hyperpolarization of a wide range of emerging contrast agents (e.g., nicotinamide). Here, the produced 15 N-Py was used for the feasibility study of spontaneous 15 N hyperpolarization at high magnetic (HF) fields (7 T and 9.4 T) of an NMR spectrometer and an MRI scanner. SABRE hyperpolarization enabled acquisition of 2D MRI imaging of catalyst-bound 15 N-pyridine with 75 × 75 mm2 field of view (FOV), 32 × 32 matrix size, demonstrating the feasibility of 15 N HF-SABRE molecular imaging with 2.4 × 2.4 mm2 spatial resolution.

SABRE: Chemical kinetics and spin dynamics of the formation of hyperpolarization

In this review, we present the physical principles of the SABRE (Signal Amplification By Reversible Exchange) method. SABRE is a promising hyperpolarization technique that enhances NMR signals by transferring spin order from parahydrogen (an isomer of the H₂ molecule that is in a singlet nuclear spin state) to a substrate that is to be polarized. Spin order transfer takes place in a transient organometallic complex which binds both parahydrogen and substrate molecules; after dissociation of the SABRE complex, free hyperpolarized substrate molecules are accumulated in solution. An advantage of this method is that the substrate is not modified chemically, and its polarization can be regenerated multiple times by bubbling fresh parahydrogen through the solution. Thus, SABRE requires two key ingredients: (i) polarization transfer and (ii) chemical exchange of both parahydrogen and substrate. While there are several excellent reviews on applications of SABRE, the background of the method is discussed less frequently. In this review we aim to explain in detail how SABRE hyperpolarization is formed, focusing on key aspects of both spin dynamics and chemical kinetics, as well as on the interplay between them. Hence, we first cover the known spin order transfer methods applicable to SABRE - cross-relaxation, coherent spin mixing at avoided level crossings, and coherence transfer - and discuss their practical implementation for obtaining SABRE polarization in the most efficient way. Second, we introduce and explain the principle of SABRE hyperpolarization techniques that operate at ultralow (<1 μT), at low (1μT to 0.1 T) and at high (>0.1 T) magnetic fields. Finally, chemical aspects of SABRE are discussed in detail, including chemical systems that are amenable to SABRE and the exchange processes that are required for polarization formation. A theoretical treatment of the spin dynamics and their interplay with chemical kinetics is also presented. This review outlines known aspects of SABRE and provides guidelines for the design of new SABRE experiments, with the goal of solving practical problems of enhancing weak NMR signals.

Rapid 13C NMR hyperpolarization delivered from para-hydrogen enables the low concentration detection and quantification of sugars

The monosaccharides glucose and fructose are rapidly detected and quantified by ¹³C NMR in conjunction with the hyperpolarisation method signal amplification by reversible exchange-relay.

Monosaccharides, such as glucose and fructose, are important to life. In this work we highlight how the rapid delivery of improved ¹³C detectability for sugars by nuclear magnetic resonance (NMR) can be achieved using the para-hydrogen based NMR hyperpolarization method SABRE-Relay (Signal Amplification by Reversible Exchange-Relay). The significant ¹³C signal enhancements of 250 at a high field of 9.4 T, and 3100 at a low field of 1 T, enable the detection of trace amounts of these materials as well as the quantification of their tautomeric makeup. Using studies on ¹³C and ²H isotopically labelled agents we demonstrate how hyperpolarization lifetime (T₁) values can be extended, and how singlet states with long lifetimes can be created. The precise quantification of d-glucose-¹³C₆-d₇ at the millimolar concentration level is shown to be possible within minutes in conjunction with a linear hyperpolarized response as a function of concentration. In addition to the measurements using labelled materials, low concentration detection is also illustrated for millimolar samples with natural abundance ¹³C where isomeric form quantification can be achieved with a single transient.

Understanding substrate substituent effects to improve catalytic efficiency in the SABRE hyperpolarisation process

The use of parahydrogen based hyperpolarisation in NMR is becoming more widespread due to the rapidly expanding range of suitable target molecules and low-cost of parahydrogen production. Hyperpolarisation via SABRE catalysis employs a metal complex to transfer polarisation from parahydrogen into a substrate whilst they are bound. In this paper we present a quantitative study of substrate-iridium ligation effects by reference to the substrates 4-chloropyridine (A), 4-pyridinecarboxaldehyde methyl hemiacetal (B), 4-methylpyridine (C) and 4-methoxypyridine (D), and evaluate the role they play in the SABRE catalysis. Substrates whose substituents enable stronger associations yield slower substrate dissociation rates (k d). A series of variable temperature studies link these exchange rates to optimal SABRE performance and reveal the critical impact of NMR relaxation times (T 1). Longer catalyst residence times are shown to result in shorter substrate T 1 values in solution as binding to iridium promotes relaxation thereby not only reducing SABRE efficiency but decreasing the overall level of achieved hyperpolarisation. Based on these data, a route to achieve more optimal SABRE performance is defined.

Quasi-Resonance Fluorine-19 Signal Amplification by Reversible Exchange

We report on an extension of the quasi-resonance (QUASR) pulse sequence used for signal amplification by reversible exchange (SABRE), showing that we may target distantly J-coupled 19F-spins. Polarization transfer from the parahydrogen-derived hydrides to the 19F nucleus is accomplished via weak five-bond J-couplings using a shaped QUASR radio frequency pulse at a 0.05 T magnetic field. The net result is the direct generation of hyperpolarized 19F z-magnetization, derived from the parahydrogen singlet order. An accumulation of 19F polarization on the free ligand is achieved with subsequent repetition of this pulse sequence. The hyperpolarized 19F signal exhibits clear dependence on the pulse length, irradiation frequency, and delay time in a manner similar to that reported for 15N QUASR-SABRE. Moreover, the hyperpolarized 19F signals of 3-19F-14N-pyridine and 3-19F-15N-pyridine isotopologues are similar, suggesting that (i) polarization transfer via QUASR-SABRE is irrespective of the nitrogen isotopologue and (ii) the presence or absence of the spin-1/2 15N nucleus has no impact on the efficiency of QUASR-SABRE polarization transfer. Although optimization of polarization transfer efficiency to 19F (P19F ≈ 0.1%) was not the goal of this study, we show that high-field SABRE can be efficient and broadly applicable for direct hyperpolarization of 19F spins.

Hyperpolarising Pyruvate through Signal Amplification by Reversible Exchange (SABRE)

Hyperpolarisation methods that premagnetise agents such as pyruvate are currently receiving significant attention because they produce sensitivity gains that allow disease tracking and interrogation of cellular metabolism by magnetic resonance. Here, we communicate how signal amplification by reversible exchange (SABRE) can provide strong 13 C pyruvate signal enhancements in seconds through the formation of the novel polarisation transfer catalyst [Ir(H)2 (η2 -pyruvate)(DMSO)(IMes)]. By harnessing SABRE, strong signals for [1-13 C]- and [2-13 C]pyruvate in addition to a long-lived singlet state in the [1,2-13 C2 ] form are readily created; the latter can be observed five minutes after the initial hyperpolarisation step. We also demonstrate how this development may help with future studies of chemical reactivity.

Hyperpolarizing Concentrated Metronidazole 15 NO2 Group over Six Chemical Bonds with More than 15 % Polarization and a 20 Minute Lifetime

The NMR hyperpolarization of uniformly 15 N-labeled [15 N3 ]metronidazole is demonstrated by using SABRE-SHEATH. In this antibiotic, the 15 NO2 group is hyperpolarized through spin relays created by 15 N spins in [15 N3 ]metronidazole, and the polarization is transferred from parahydrogen-derived hydrides over six chemical bonds. In less than a minute of parahydrogen bubbling at approximately 0.4 μT, a high level of nuclear spin polarization (P15N ) of around 16 % is achieved on all three 15 N sites. This product of 15 N polarization and concentration of 15 N spins is around six-fold better than any previous value determined for 15 N SABRE-derived hyperpolarization. At 1.4 T, the hyperpolarized state persists for tens of minutes (relaxation time, T1 ≈10 min). A novel synthesis of uniformly 15 N-enriched metronidazole is reported with a yield of 15 %. This approach can potentially be used for synthesis of a wide variety of in vivo metabolic probes with potential uses ranging from hypoxia sensing to theranostic imaging.

Using coligands to gain mechanistic insight into iridium complexes hyperpolarized with para-hydrogen

We report the formation of a series of novel [Ir(H)2(IMes)(α-13C2-carboxyimine)L] complexes in which the identity of the coligand L is varied. When examined with para-hydrogen, complexes in which L is benzylamine or phenethylamine show significant 1H hydride and 13C2 imine enhancements and may exist in 13C2 singlet spin order. Isotopic labeling techniques are used to double 13C2 enhancements (up to 750-fold) and singlet state lifetimes (up to 20 seconds) compared to those previously reported. Exchange spectroscopy and Density Functional Theory are used to investigate the stability and mechanism of rapid hydrogen exchange in these complexes, a process driven by dissociative coligand loss to form a key five coordinate intermediate. When L is pyridine or imidazole, competitive binding to such intermediates leads to novel complexes whose formation, kinetics, behaviour, structure, and hyperpolarization is investigated. The ratio of the observed PHIP enhancements were found to be affected not only by the hydrogen exchange rates but the identity of the coligands. This ligand reactivity is accompanied by decoherence of any 13C2 singlet order which can be preserved by isotopic labeling. Addition of a thiol coligand proved to yield a thiol oxidative addition product which is characterized by NMR and MS techniques. Significant 870-fold 13C enhancements of pyridine can be achieved using the Signal Amplification By Reversible Exchange (SABRE) process when α-carboxyimines are used to block active coordination sites. [Ir(H)2(IMes)(α-13C2-carboxyimine)L] therefore acts as unique sensors whose 1H hydride chemical shifts and corresponding hyperpolarization levels are indicative of the identity of a coligand and its binding strength.

Hyperpolarised 1H–13C Benchtop NMR Spectroscopy

Benchtop NMR spectrometers with sub-ppm spectral resolution have opened up new opportunities for performing NMR outside of the standard laboratory environment. However, the relatively weak magnetic fields of these devices (1–2 T) results in low sensitivity and significant peak overlap in 1H NMR spectra. Here, we use hyperpolarised 13C{1H} NMR to overcome these challenges. Specifically, we demonstrate the use of the signal amplification by reversible exchange (SABRE) parahydrogen-based hyperpolarisation technique to enhance the sensitivity of natural abundance 1D and 2D 13C{1H} benchtop NMR spectra. We compare two detection methods for SABRE-enhanced 13C NMR and observe an optimal 13C{1H} signal-to-noise ratio (SNR) for a refocused INEPT approach, where hyperpolarisation is transferred from 1H to 13C. In addition, we exemplify SABRE-enhanced 2D 13C benchtop NMR through the acquisition of a 2D HETCOR spectrum of 260 mM of 4-methylpyridine at natural isotopic abundance in a total experiment time of 69 min. In theory, signal averaging for over 300 days would be required to achieve a comparable SNR for a thermally polarised benchtop NMR spectrum acquired of a sample of the same concentration at natural abundance.

Unveiling coherently driven hyperpolarization dynamics in signal amplification by reversible exchange

Signal amplification by reversible exchange (SABRE) is an efficient method to hyperpolarize spin-1/2 nuclei and affords signals that are orders of magnitude larger than those obtained by thermal spin polarization. Direct polarization transfer to heteronuclei such as ¹³C or ¹⁵N has been optimized at static microTesla fields or using coherence transfer at high field, and relies on steady state exchange with the polarization transfer catalyst dictated by chemical kinetics. Here we demonstrate that pulsing the excitation field induces complex coherent polarization transfer dynamics, but in fact pulsing with a roughly 1% duty cycle on resonance produces more magnetization than constantly being on resonance. We develop a Monte Carlo simulation approach to unravel the coherent polarization dynamics, show that existing SABRE approaches are quite inefficient in use of para-hydrogen order, and present improved sequences for efficient hyperpolarization.

There is increasing effort to improve the signal sensitivity and explore the hyperpolarization dynamics. Here the authors demonstrate the parahydrogen spin transfer dynamics in compounds containing ¹⁵N using SABRE hyperpolarization technique with different strengths of the magnetic field.

Scalar relaxation of NMR transitions at ultralow magnetic field

Nuclear magnetic resonance signals for ¹H in simple chlorinated, brominated and deuterated liquids were detected at field strengths between 1 nT and a few μT to investigate the influence of scalar relaxation of the second kind (SR2K). SR2K describes the acceleration in magnetization decay rate for a spin-1/2 nucleus that is scalar coupled to a fast-relaxing quadrupolar nucleus. In agreement with simple theoretical models, the experimental data show that couplings to nuclei with small, nonzero quadrupole moments (²H) give rise to higher transverse relaxation rates at ultralow field than rapidly relaxing quadrupolar nuclei (Cl and Br). This behavior is opposite to the case normally encountered in high-field NMR, and demonstrates that certain nuclei in the spin system may be "weakly coupled" or even decoupled when the applied magnetic field is zero. The results show that the capability for precision determination of NMR frequencies and molecular structural information depends strongly on the composition and topology of the nuclear spin system.

Chemical Exchange Reaction Effect on Polarization Transfer Efficiency in SLIC-SABRE

Signal Amplification By Reversible Exchange (SABRE) is a new and rapidly developing hyperpolarization technique. The recent discovery of Spin-Lock Induced Crossing SABRE (SLIC-SABRE) showed that high field hyperpolarization transfer techniques developed so far were optimized for singlet spin order that does not coincide with the experimentally produced spin state. Here, we investigated the SLIC-SABRE approach and the most advanced quantitative theoretical SABRE model to date. Our goal is to achieve the highest possible polarization with SLIC-SABRE at high field using the standard SABRE system, IrIMes catalyst with pyridine. We demonstrated the accuracy of the SABRE model describing the effects of various physical parameters such as the amplitude and frequency of the radio frequency field, and the effects of chemical parameters such as the exchange rate constants. By fitting the model to the experimental data, the effective life time of the SABRE complex was estimated, as well as the entropy and enthalpy of the complex-dissociation reaction. We show, for the first time, that this SLIC-SABRE model can be useful for the evaluation of the chemical exchange parameters that are very important for the production of highly polarized contrast agents via SABRE.