Highly Efficient Polarization of Spin-1/2 Insensitive NMR Nuclei by Adiabatic Passage through Level Anticrossings

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.

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.

Radio-Frequency Sweeps at Microtesla Fields for Parahydrogen-Induced Polarization of Biomolecules

Magnetic resonance imaging of ¹³C-labeled metabolites enhanced by parahydrogen-induced polarization (PHIP) enables real-time monitoring of processes within the body. We introduce a robust, easily implementable technique for transferring parahydrogen-derived singlet order into ¹³C magnetization using adiabatic radio frequency sweeps at microtesla fields. We experimentally demonstrate the applicability of this technique to several molecules, including some molecules relevant for metabolic imaging, where we show significant improvements in the achievable polarization, in some cases reaching above 60% nuclear spin polarization. Furthermore, we introduce a site-selective deuteration scheme, where deuterium is included in the coupling network of a pyruvate ester to enhance the efficiency of the polarization transfer. These improvements are enabled by the fact that the transfer protocol avoids relaxation induced by strongly coupled quadrupolar nuclei.

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.

Parahydrogen-induced polarization and spin order transfer in ethyl pyruvate at high magnetic fields

Nuclear magnetic resonance has experienced great advances in developing and translating hyperpolarization methods into procedures for fundamental and clinical studies. Here, we propose the use of a wide-bore NMR for large-scale (volume- and concentration-wise) production of hyperpolarized media using parahydrogen-induced polarization. We discuss the benefits of radio frequency-induced parahydrogen spin order transfer, we show that 100% polarization is theoretically expected for homogeneous B0 and B1 magnetic fields for a three-spin system. Moreover, we estimated that the efficiency of spin order transfer is not significantly reduced when the B1 inhomogeneity is below ± 5%; recommendations for the sample size and RF coils are also given. With the latest breakthrough in the high-yield synthesis of 1-13C-vinyl pyruvate and its deuterated isotopologues, the high-field PHIP-SAH will gain increased attention. Some remaining challenges will be addressed shortly.

Adiabatic Passage through Level Anticrossings in Systems of Chemically Inequivalent Protons Incorporating Parahydrogen: Theory, Experiment, and Prospective Applications

Level anticrossings (LACs) are ubiquitous in quantum systems and have been exploited for spin-order transfer in hyperpolarized nuclear magnetic resonance spectroscopy. This paper examines the manifestations of adiabatic passage through a specific type of LAC found in homonuclear systems of chemically inequivalent coupled protons incorporating parahydrogen (pH₂). Adiabatic passage through such a LAC is shown to elicit translation of the pH₂ spin order. As an example, with prospective applications in biomedicine, proton spin polarizations of at least 19.8 ± 2.6% on the methylene protons and 68.7 ± 0.5% on the vinylic protons of selectively deuterated allyl pyruvate ester are demonstrated experimentally. After ultrasonic spray injection of a precursor solution containing propargyl pyruvate and a dissolved Rh catalyst into a chamber pressurized with 99% para-enriched H₂, the products are collected and transported to a high magnetic field for NMR detection. The LAC-mediated hyperpolarization of the methylene protons is significant because of the stronger spin coupling to the pyruvate carbonyl ¹³C, setting up an ideal initial condition for subsequent coherence transfer by selective INEPT. Furthermore, the selective deuteration of the propargyl side arm increases the efficiency and polarization level. LAC-mediated translation of parahydrogen spin order completes the first step toward a new and highly efficient route for the ¹³C NMR signal enhancement of pyruvate via side-arm hydrogenation with parahydrogen.

Synthesis of 13 C and 2 H Labeled Vinyl Pyruvate and Hyperpolarization of Pyruvate

The hyperpolarization of nuclear spins has enabled unique applications in chemistry, biophysics, and particularly metabolic imaging. Parahydrogen-induced polarization (PHIP) offers a fast and cost-efficient way of hyperpolarization. Nevertheless, PHIP lags behind dynamic nuclear polarization (DNP), which is already being evaluated in clinical studies. This shortcoming is mainly due to problems in the synthesis of the corresponding PHIP precursor molecules. The most widely used DNP tracer in clinical studies, particularly for the detection of prostate cancer, is 1-13 C-pyruvate. The ideal derivative for PHIP is the deuterated vinyl ester because the spin physics allows for 100 % polarization. Unfortunately, there is no efficient synthesis for vinyl esters of β-ketocarboxylic acids in general and pyruvate in particular. Here, we present an efficient new method for the preparation of vinyl esters, including 13 C labeled, fully deuterated vinyl pyruvate using a palladium-catalyzed procedure. Using 50 % enriched parahydrogen and mild reaction conditions, a 13 C polarization of 12 % was readily achieved; 36 % are expected with 100 % pH2 . Higher polarization values can be potentially achieved with optimized reaction conditions.

Analytical solution for the inverting pulses with constant adiabaticity

The exact solution was found for inverting pulses with constant adiabaticity for spin ½. The analytical relationship between the time-varying frequency of the microwave resonant field (or RF field in the case of NMR) and its amplitude time dependence such that the adiabaticity parameter remains constant for the single isochromat throughout the pulse is found. Comparison with EPR (hyperbolic tangent)-(hyperbolic secant) pulse method was carried out. On the basis of the analytical solution the pulses with different dependences of the microwave field amplitude conserving the constant adiabaticity have been constructed. The pulses exhibit rather sharp inversion selectivity that can be used in the field of EPR, NMR and MRI.

Recent advances in the application of parahydrogen in catalysis and biochemistry

Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) are analytical and diagnostic tools that are essential for a very broad field of applications, ranging from chemical analytics, to non-destructive testing of materials and the investigation of molecular dynamics, to in vivo medical diagnostics and drug research. One of the major challenges in their application to many problems is the inherent low sensitivity of magnetic resonance, which results from the small energy-differences of the nuclear spin-states. At thermal equilibrium at room temperature the normalized population difference of the spin-states, called the Boltzmann polarization, is only on the order of 10-5. Parahydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, which has widespread applications in Chemistry, Physics, Biochemistry, Biophysics, and Medical Imaging. PHIP creates its signal-enhancements by means of a reversible (SABRE) or irreversible (classic PHIP) chemical reaction between the parahydrogen, a catalyst, and a substrate. Here, we first give a short overview about parahydrogen-based hyperpolarization techniques and then review the current literature on method developments and applications of various flavors of the PHIP experiment.

Hyperpolarization read-out through rapidly rotating fields in the zero- and low-field regime

An integral part of para-hydrogen induced polarization (PHIP) methods is the conversion of nuclear singlet order into observable magnetization. In this study polarization transfer to a heteronucleus is achieved through a selective rotation of the proton singlet-triplet states driven by a combination of a rotating magnetic field and a weak bias field. Surprisingly we find that efficient polarization transfer driven by a STORM (Singlet-Triplet Oscillations through Rotating Magnetic fields) pulse in the presence of sub-μT bias fields requires rotation frequencies on the order of several kHz. The rotation frequencies therefore greatly exceed any of the internal frequencies of typical zero- to ultralow field experiments. We further show that the rotational direction of the rotating field is not arbitrary and greatly influences the final transfer efficiency. Some of these aspects are demonstrated experimentally by considering hyperpolarized (1-13C)fumarate. In contrast to most of the existing methods, the STORM procedure therefore represents a promising candidate for quadrupolar decoupled polarization transfer in PHIP experiments.

Symmetry Constraints on Spin Order Transfer in Parahydrogen-Induced Polarization (PHIP)

It is well known that the association of parahydrogen (pH2) with an unsaturated molecule or a transient metalorganic complex can enhance the intensity of NMR signals; the effect is known as parahydrogen-induced polarization (PHIP). During recent decades, numerous methods were proposed for converting pH2-derived nuclear spin order to the observable magnetization of protons or other nuclei of interest, usually 13C or 15N. Here, we analyze the constraints imposed by the topological symmetry of the spin systems on the amplitude of transferred polarization. We find that in asymmetric systems, heteronuclei can be polarized to 100%. However, the amplitude drops to 75% in A2BX systems and further to 50% in A3B2X systems. The latter case is of primary importance for biological applications of PHIP using sidearm hydrogenation (PHIP-SAH). If the polarization is transferred to the same type of nuclei, i.e., 1H, symmetry constraints impose significant boundaries on the spin-order distribution. For AB, A2B, A3B, A2B2, AA’(AA’) systems, the maximum average polarization for each spin is 100%, 50%, 33.3%, 25%, and 0, respectively, (where A and B (or A’) came from pH2). Remarkably, if the polarization of all spins in a molecule is summed up, the total polarization grows asymptotically with ~1.27 and can exceed 2 in the absence of symmetry constraints (where is the number of spins). We also discuss the effect of dipole–dipole-induced pH2 spin-order distribution in heterogeneous catalysis or nematic liquid crystals. Practical examples from the literature illustrate our theoretical analysis.

Frequency-Selective Manipulations of Spins allow Effective and Robust Transfer of Spin Order from Parahydrogen to Heteronuclei in Weakly-Coupled Spin Systems

We present a selectively pulsed (SP) generation of sequences to transfer the spin order of parahydrogen (pH2 ) to heteronuclei in weakly coupled spin systems. We analyze and discuss the mechanism and efficiency of SP spin order transfer (SOT) and derive sequence parameters. These new sequences are most promising for the hyperpolarization of molecules at high magnetic fields. SP-SOT is effective and robust despite the symmetry of the 1 H-13 C J-couplings even when precursor molecules are not completely labeled with deuterium. As only one broadband 1 H pulse is needed per sequence, which can be replaced for instance by a frequency-modulated pulse, lower radiofrequency (RF) power is required. This development will be useful to hyperpolarize (new) agents and to perform the hyperpolarization within the bore of an MRI system, where the limited RF power has been a persistent problem.

Selective excitation of hydrogen doubles the yield and improves the robustness of parahydrogen-induced polarization of low-γ nuclei

We describe a new method for pulsed spin order transfer of parahydrogen-induced polarization (PHIP) that enables high polarization in incompletely ²H-labeled molecules by exciting only the desired protons in a frequency-selective manner. This way, the effect of selected J-couplings is suspended. Experimentally 1.25% ¹³C polarization were obtained for 1-¹³C-ethyl pyruvate and 50% pH₂ at 9.4 Tesla.

Coherent Evolution of Signal Amplification by Reversible Exchange in Two Alternating Fields (alt-SABRE)

Parahydrogen (pH2 ) is a convenient and cost-efficient source of spin order to enhance the magnetic resonance signal. Previous work showed that transient interaction of pH2 with a metal organic complex in a signal amplification by reversible exchange (SABRE) experiment enabled more than 10 % polarization for some 15 N molecules. Here, we analyzed a variant of SABRE, consisting of a magnetic field alternating between a low field of ∼1 μT, where polarization transfer is expected to take place, and a higher field >50 μT (alt-SABRE). These magnetic fields affected the amplitude and frequency of polarization transfer. Deviation of a lower magnetic field from a "perfect" condition of level anti-crossing increases the frequency of polarization transfer that can be exploited for polarization of short-lived transient SABRE complexes. Moreover, the coherences responsible for polarization transfer at a lower field persisted during magnetic field variation and continued their spin evolution at higher field with a frequency of 2.5 kHz at 54 μT. The latter should be taken into consideration for an efficient alt-SABRE. Theoretical and experimental findings were exemplified with Iridium N-heterocyclic carbene SABRE complex and 15 N-acetonitrole, where a 30 % higher 15 N polarization with alt-SABRE compared to common SABRE was reached.

Low-frequency excitation of singlet-triplet transitions. Application to nuclear hyperpolarization

Coupled pairs of nuclear spin-1/2 support one singlet state and three triplet states. Transitions between the singlet state and one of the triplet states may be driven by an oscillating low-frequency magnetic field, in the presence of couplings to a third nuclear spin, and a weak bias magnetic field. The oscillating field is in the same direction as the bias field and is called a WOLF (Weak Oscillating Low Field) pulse. Application of a WOLF pulse allows for the generation of strong nuclear hyperpolarization of ¹³C nuclei, starting from the nuclear singlet polarization of a ¹H spin pair, associated with the enriched para-spin isomer of hydrogen gas. Hyperpolarization is demonstrated for two molecular systems.

Selective excitation doubles the transfer of parahydrogen-induced polarization to heteronuclei

In this work, we present a new pulse sequence to transform the spin order added to a molecule after the pairwise addition of parahydrogen into 13C polarization. Using a selective 90° preparation instead of a non-selective 45° excitation, the new variant performed twice as well as previous implementations in both simulations and experiments, exemplified with hyperpolarized ethyl acetate. This concept is expected to extend to other nuclei and other spin order transfer schemes that use non-selective excitation.

Heterogeneous 1 H and 13 C Parahydrogen-Induced Polarization of Acetate and Pyruvate Esters

Magnetic resonance imaging of [1-13 C]hyperpolarized carboxylates (most notably, [1-13 C]pyruvate) allows one to visualize abnormal metabolism in tumors and other pathologies. Herein, we investigate the efficiency of 1 H and 13 C hyperpolarization of acetate and pyruvate esters with ethyl, propyl and allyl alcoholic moieties using heterogeneous hydrogenation of corresponding vinyl, allyl and propargyl precursors in isotopically unlabeled and 1-13 C-enriched forms with parahydrogen over Rh/TiO2 catalysts in methanol-d4 and in D2 O. The maximum obtained 1 H polarization was 0.6±0.2 % (for propyl acetate in CD3 OD), while the highest 13 C polarization was 0.10±0.03 % (for ethyl acetate in CD3 OD). Hyperpolarization of acetate esters surpassed that of pyruvates, while esters with a triple carbon-carbon bond in unsaturated alcoholic moiety were less efficient as parahydrogen-induced polarization precursors than esters with a double bond. Among the compounds studied, the maximum 1 H and 13 C NMR signal intensities were observed for propyl acetate. Ethyl acetate yielded slightly less intense NMR signals which were dramatically greater than those of other esters under study.

Heterogeneous Catalysis and Parahydrogen-Induced Polarization

Parahydrogen-induced polarization with heterogeneous catalysts (HET-PHIP) has been a subject of extensive research in the last decade since its first observation in 2007. While NMR signal enhancements obtained with such catalysts are currently below those achieved with transition metal complexes in homogeneous hydrogenations in solution, this relatively new field demonstrates major prospects for a broad range of advanced fundamental and practical applications, from providing catalyst-free hyperpolarized fluids for biomedical magnetic resonance imaging (MRI) to exploring mechanisms of industrially important heterogeneous catalytic processes. This review covers the evolution of the heterogeneous catalysts used for PHIP observation, from metal complexes immobilized on solid supports to bulk metals and single-atom catalysts and discusses the general visions for maximizing the obtained NMR signal enhancements using HET-PHIP. Various practical applications of HET-PHIP, both for catalytic studies and for potential production of hyperpolarized contrast agents for MRI, are described.

Constant-adiabaticity pulse schemes for manipulating singlet order in 3-spin systems with weak magnetic non-equivalence

Parahydrogen-induced polarization (PHIP) is a source of nuclear spin hyperpolarization, and this technique allows for the preparation of biomolecules for in vivo metabolic imaging. PHIP delivers hyperpolarization in the form of proton singlet order to a molecule, but most applications require that a heteronuclear (e.g. ¹³C or ¹⁵N) spin in the molecule is hyperpolarized. Here we present high field pulse methods to manipulate proton singlet order in the [1-¹³C]fumarate, and in particular to transfer the proton singlet order into ¹³C magnetization. We exploit adiabatic pulses, i.e., pulses with slowly ramped amplitude, and use constant-adiabaticity variants: the spin Hamiltonian is varied in such a way that the generalized adiabaticity parameter is time-independent. This allows for faster polarization transfer, and we achieve 96.2% transfer efficiency in thermal equilibrium experiments. We demonstrate this in experiments using hyperpolarization, and obtain 6.8% ¹³C polarization. This work paves the way for efficient hyperpolarization of nuclear spins in a variety of biomolecules, since the high-field pulse sequences allow individual spins to be addressed.

Constant-adiabaticity ultralow magnetic field manipulations of parahydrogen-induced polarization: application to an AA'X spin system

The field of magnetic resonance imaging with hyperpolarized contrast agents is rapidly expanding, and parahydrogen-induced polarization (PHIP) is emerging as an inexpensive and easy-to-implement method for generating the required hyperpolarized biomolecules. Hydrogenative PHIP delivers hyperpolarized proton spin order to a substrate via chemical addition of H₂ in the spin-singlet state, but it is typically necessary to transfer the proton polarization to a heteronucleus (usually ¹³C) which has a longer spin lifetime. Adiabatic ultralow magnetic field manipulations can be used to induce the polarization transfer, but this is necessarily a slow process, which is undesirable since the spins continually relax back to thermal equilibrium. Here we demonstrate two constant-adiabaticity field sweep methods, one in which the field passes through zero, and one in which the field is swept from zero, for optimal polarization transfer on a model AA'X spin system, [1-¹³C]fumarate. We introduce a method for calculating the constant-adiabaticity magnetic field sweeps, and demonstrate that they enable approximately one order of magnitude faster spin-order conversion compared to linear sweeps. The present method can thus be utilized to manipulate nonthermal order in heteronuclear spin systems.

Hydrogenative-PHIP polarized metabolites for biological studies

ParaHydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, but its application to biological investigations has been hampered, so far, due to chemical challenges. PHIP is obtained by means of the addition of hydrogen, enriched in the para-spin isomer, to an unsaturated substrate. Both hydrogen atoms must be transferred to the same substrate, in a pairwise manner, by a suitable hydrogenation catalyst; therefore, a de-hydrogenated precursor of the target molecule is necessary. This has strongly limited the number of parahydrogen polarized substrates. The non-hydrogenative approach brilliantly circumvents this central issue, but has not been translated to in-vivo yet. Recent advancements in hydrogenative PHIP (h-PHIP) considerably widened the possibility to hyperpolarize metabolites and, in this review, we will focus on substrates that have been obtained by means of this method and used in vivo. Attention will also be paid to the requirements that must be met and on the issues that have still to be tackled to obtain further improvements and to push PHIP substrates in biological applications.

Representation of population exchange at level anti-crossings

A theoretical framework is proposed to describe the spin dynamics driven by coherent spin mixing at level anti-crossings (LACs). We briefly introduce the LAC concept and propose to describe the spin dynamics using a vector of populations of the diabatic eigenstates. In this description, each LAC gives rise to a pairwise redistribution of eigenstate populations, allowing one to construct the total evolution operator of the spin system. Additionally, we take into account that in the course of spin evolution a "rotation" of the eigenstate basis case take place. The approach is illustrated by a number of examples, dealing with magnetic field inversion, cross-polarization, singlet-state nuclear magnetic resonance and parahydrogen-induced polarization.

Robust transformation of singlet order into heteronuclear magnetisation over an extended coupling range

Several important NMR procedures involve the conversion of nuclear singlet order into heteronuclear magnetisation, including some experiments involving long-lived spin states and parahydrogen-induced hyperpolarisation. However most existing sequences suffer from a limited range of validity or a lack of robustness against experimental imperfections. We present a new radio-frequency scheme for the transformation of the singlet order of a chemically-equivalent homonuclear spin pair into the magnetisation of a heteronuclear coupling partner. The proposed radio-frequency (RF) scheme is called gS2hM (generalized singlet-to-heteronuclear magnetisation) and has good compensation for common experimental errors such as RF and static field inhomogeneities. The sequence retains its robustness for homonuclear spin pairs in the intermediate coupling regime, characterised by the in-pair coupling being of the same order of magnitude as the difference between the out-of-pair couplings. This is a substantial improvement to the validity range of existing sequences. Analytical solutions for the pulse sequence parameters are provided. Experimental results are shown for two test cases.

Hyperpolarization of Pyridyl Fentalogues by Signal Amplification By Reversible Exchange (SABRE)

Fentanyl, also known as 'jackpot', is a synthetic opiate that is 50-100 times more potent than morphine. Clandestine laboratories produce analogues of fentanyl, known as fentalogues to circumvent legislation regarding its production. Three pyridyl fentalogues were synthesized and then hyperpolarized by signal amplification by reversible exchange (SABRE) to appraise the forensic potential of the technique. A maximum enhancement of -168-fold at 1.4 T was recorded for the ortho pyridyl 1H nuclei. Studies of the activation parameters for the three fentalogues revealed that the ratio of ligand loss trans to hydride and hydride loss in the complex [Ir(IMes)(L)3(H)2]+ (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) ranged from 0.52 to 1.83. The fentalogue possessing the ratio closest to unity produced the largest enhancement subsequent to performing SABRE at earth's magnetic field. It was possible to hyperpolarize a pyridyl fentalogue selectively from a matrix that consisted largely of heroin (97 : 3 heroin:fentalogue) to validate the use of SABRE as a forensic tool.

Polarization of low-γ nuclei by transferring spin order of parahydrogen at high magnetic fields

In this work, we optimize the performance of a previously proposed method for transferring parahydrogen induced polarization to "insensitive" spin-1/2 NMR (Nuclear Magnetic Resonance) nuclei, which have low gyromagnetic ratio and low natural abundance. By optimizing the reaction conditions and pressure of the parahydrogen gas and using adiabatically switched radiofrequency fields we achieve high polarization transfer efficiency and report carbon spin polarization of dimethyl acetylene dicarboxylate reaching 35%, which corresponds to ¹³C NMR signal enhancements of about 43,000 at 9.4 Tesla. Such polarization levels allow one to work with mM concentrations at natural carbon abundance and to detect ¹³C NMR signal in single scan. In combination with a pseudo phase cycle, the polarization transfer method used here also enables efficient suppression of unwanted background signals.

Parahydrogen-Induced Polarization of 1-13C-Acetates and 1-13C-Pyruvates Using Sidearm Hydrogenation of Vinyl, Allyl, and Propargyl Esters

13C-hyperpolarized carboxylates, such as pyruvate and acetate, are emerging molecular contrast agents for MRI visualization of various diseases, including cancer. Here we present a systematic study of 1H and 13C parahydrogen-induced polarization of acetate and pyruvate esters with ethyl, propyl and allyl alcoholic moieties. It was found that allyl pyruvate is the most efficiently hyperpolarized compound from those under study, yielding 21% and 5.4% polarization of 1H and 13C nuclei, respectively, in CD3OD solutions. Allyl pyruvate and ethyl acetate were also hyperpolarized in aqueous phase using homogeneous hydrogenation with parahydrogen over water-soluble rhodium catalyst. 13C polarization of 0.82% and 2.1% was obtained for allyl pyruvate and ethyl acetate, respectively. 13C-hyperpolarized methanolic and aqueous solutions of allyl pyruvate and ethyl acetate were employed for in vitro MRI visualization, demonstrating the prospects for translation of the presented approach to biomedical in vivo studies.

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.

Only Para-Hydrogen Spectroscopy (OPSY) Revisited: In-Phase Spectra for Chemical Analysis and Imaging

We revisited only para-hydrogen spectroscopy (OPSY) for the analysis of para-hydrogen-enhanced NMR spectra at high magnetic fields. We found that the sign of the gradients and interpulse delays are pivotal for the performance of the sequence: the variant of double-quantum filter OPSY, where the second time interval is twice as long as the first one (OPSYd-12) converts the antiphase spectrum to in-phase and efficiently suppresses the background signal in a single scan better than the other variants. OPSYd-12 strongly facilitates the analysis of para-hydrogen-derived NMR spectra in homogeneous and inhomogeneous magnetic fields. Furthermore, the net magnetization produced is essential for subsequent applications such as imaging, e.g., in a reaction chamber or in vivo.

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.

Effects of Deuteration of 13C-Enriched Phospholactate on Efficiency of Parahydrogen-Induced Polarization by Magnetic Field Cycling

We report herein a large-scale (>10 g) synthesis of isotopically enriched 1-¹³C-phosphoenolpyruvate and 1-¹³C-phosphoenolpyruvate-d₂ for application in hyperpolarized imaging technology. The 1-¹³C-phosphoenolpyruvate-d₂ was synthesized with 57% overall yield (over two steps), and >98% ²H isotopic purity, representing an improvement over the previous report. The same outcome was achieved for 1-¹³C-phosphoenolpyruvate. These two unsaturated compounds with C=C bonds were employed for parahydrogen-induced polarization via pairwise parahydrogen addition in aqueous medium. We find that deuteration of 1-¹³C-phosphoenolpyruvate resulted in overall increase of ¹H T₁ of nascent hyperpolarized protons (4.30 ± 0.04 s versus 2.06 ± 0.01 s) and ¹H polarization (~2.5% versus ~0.7%) of the resulting hyperpolarized 1-¹³C-phospholactate. The nuclear spin polarization of nascent parahydrogen-derived protons was transferred to 1-¹³C nucleus via magnetic field cycling procedure. The proton T₁ increase in hyperpolarized deuterated 1-¹³C-phospholactate yielded approximately 30% better ¹³C polarization compared to non-deuterated hyperpolarized 1-¹³C-phospholactate. Analysis of T₁ relaxation revealed that deuteration of 1-¹³C-phospholactate may have resulted in approximately 3-fold worse H→¹³C polarization transfer efficiency via magnetic field cycling. Since magnetic field cycling is a key polarization transfer step in the Side-Arm Hydrogenation approach, the presented findings may guide more rationale design of contrast agents using parahydrogen polarization of a broad range of ¹³C hyperpolarized contrast agents for molecular imaging employing ¹³C MRI. The hyperpolarized 1-¹³C-phospholactate-d₂ is of biomedical imaging relevance because it undergoes in vivo dephosphorylation and becomes ¹³C hyperpolarized lactate, which as we show can be detected in the brain using ¹³C hyperpolarized MRI; an implication for future imaging of neurodegenerative diseases and dementia.

Parahydrogen-Based Hyperpolarization for Biomedicine

Magnetic resonance (MR) is one of the most versatile and useful physical effects used for human imaging, chemical analysis, and the elucidation of molecular structures. However, its full potential is rarely used, because only a small fraction of the nuclear spin ensemble is polarized, that is, aligned with the applied static magnetic field. Hyperpolarization methods seek other means to increase the polarization and thus the MR signal. A unique source of pure spin order is the entangled singlet spin state of dihydrogen, parahydrogen (pH2 ), which is inherently stable and long-lived. When brought into contact with another molecule, this "spin order on demand" allows the MR signal to be enhanced by several orders of magnitude. Considerable progress has been made in the past decade in the area of pH2 -based hyperpolarization techniques for biomedical applications. It is the goal of this Review to provide a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agents, and applications.

Spin Relays Enable Efficient Long-Range Heteronuclear Signal Amplification By Reversible Exchange

A systematic experimental study is reported on the polarization transfer to distant spins, which do not directly bind to the polarization transfer complexes employed in Signal Amplification By Reversible Exchange (SABRE) experiments. Both, long-range transfer to protons and long-range transfer to heteronuclei i.e. 13C and 15N are examined. Selective destruction of hyperpolarization on 1H, 13C, and 15N sites is employed, followed by their re-hyperpolarization from neighboring spins within the molecules of interest (pyridine for 1H studies and metronidazole-15N2-13C2 for 13C and 15N studies). We conclude that long-range sites can be efficiently hyperpolarized when a network of spin-½ nuclei enables relayed polarization transfer (i.e. via short-range interactions between sites). In case of proton SABRE in the milli-Tesla regime, a relay network consisting of protons only is sufficient. However, in case 13C and 15N are targeted (i.e. via SABRE in SHield Enables Alignment Transfer to Heteronuclei or SABRE-SHEATH experiment), the presence of a heteronuclear network (e.g. consisting of 15N) enables a relay mechanism that is significantly more efficient than the direct transfer of spin order from para-H2-derived hydrides.

A pulse programmable parahydrogen polarizer using a tunable electromagnet and dual channel NMR spectrometer

Applications of parahydrogen induced polarization (PHIP) often warrant conversion of the chemically-synthesized singlet-state spin order into net heteronuclear magnetization. In order to obtain optimal yields from the overall hyperpolarization process, catalytic hydrogenation must be tightly synchronized to subsequent radiofrequency (RF) transformations of spin order. Commercial NMR consoles are designed to synchronize applied waves on multiple channels and consequently are well-suited as controllers for these types of hyperpolarization experiments that require tight coordination of RF and non-RF events. Described here is a PHIP instrument interfaced to a portable NMR console operating with a static field electromagnet in the milliTesla regime. In addition to providing comprehensive control over chemistry and RF events, this setup condenses the PHIP protocol into a pulse-program that in turn can be readily shared in the manner of traditional pulse sequences. In this device, a TTL multiplexer was constructed to convert spectrometer TTL outputs into 24 VDC signals. These signals then activated solenoid valves to control chemical shuttling and reactivity in PHIP experiments. Consolidating these steps in a pulse-programming environment speeded calibration and improved quality assurance by enabling the B₀/B₁ fields to be tuned based on the direct acquisition of thermally polarized and hyperpolarized NMR signals. Performance was tested on the parahydrogen addition product of 2-hydroxyethyl propionate-1-¹³C-d₃, where the ¹³C polarization was estimated to be P_13C=20±2.5% corresponding to ¹³C signal enhancement approximately 25 million-fold at 9.1 mT or approximately 77,000-fold ¹³C enhancement at 3 T with respect to thermally induced polarization at room temperature.

Singlet order conversion and parahydrogen-induced hyperpolarization of 13C nuclei in near-equivalent spin systems

We have demonstrated two radiofrequency pulse methods which convert the nuclear singlet order of proton spin pairs into the magnetisation of nearby ¹³C nuclei. These irradiation schemes work well in the near-equivalence regime of the three-spin system, which applies when the difference in the two ¹H-¹³C couplings is much smaller than the ¹H-¹H coupling. We use pulse sequences to generate thermally polarized singlet states in a reproducible manner, and study the singlet-to-magnetisation transfer step. Preliminary results demonstrate a parahydrogen-enhanced ¹³C polarization level of at least 9%, providing a signal enhancement factor of more than 9000, using 50% enriched parahydrogen.

Symmetry constraints on spin dynamics: Application to hyperpolarized NMR

Spin dynamical evolution is constrained by the symmetries of the spin Hamiltonians that generate the quantum dynamics. The consequences of symmetry-induced constraints are examined for some common hyperpolarized NMR experiments, including the excitation of singlet order in spin-pair systems, and the transfer of parahydrogen-induced hyperpolarized singlet order to magnetization in systems displaying chemical and magnetic equivalence.

Effects of Magnetic Field Cycle on the Polarization Transfer from Parahydrogen to Heteronuclei through Long-Range J-Couplings

Hyperpolarization of (13)C carboxylate signals of metabolically relevant molecules, such as acetate and pyruvate, was recently obtained by means of ParaHydrogen Induced Polarization by Side Arm Hydrogenation (PHIP-SAH). This method relies on functionalization of the carboxylic acid with an unsaturated alcohol (side arm), hydrogenation of the unsaturated alcohol using parahydrogen, and polarization transfer to the target (13)C signal. In this case, parahydrogen protons are added three to four bonds away from the target (13)C nucleus, while biologically relevant molecules had been hyperpolarized, using parahydrogen, through hydrogenation of an unsaturated bond adjacent to the target (13)C signal. The herein reported results show that the same polarization level can be obtained on the (13)C carboxylate signal of an ester by means of addition of parahydrogen to the acidic or to the alcoholic moiety and successive application of magnetic field cycle (MFC). Experimental results are supported by calculations that allow one to predict that, upon accurate control of magnetic field strength and speed of the passages, more than 20% polarization can be achieved on the (13)C-carboxylate resonance of the esters by means of side arm hydrogenation and MFC.

Transfer of SABRE-derived hyperpolarization to spin-1/2 heteronuclei

Spin mixing at level anti-crossings (lacs) enables transfer of SABRE-derived spin polarization to spin-1/2 hetero-nuclei.