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.

Geminal parahydrogen-induced polarization: accumulating long-lived singlet order on methylene proton pairs.

In the majority of hydrogenative parahydrogen-induced polarization (PHIP) experiments, the hydrogen molecule undergoes pairwise cis addition to an unsaturated precursor to occupy vicinal positions on the product molecule. However, some ruthenium-based hydrogenation catalysts induce geminal hydrogenation, leading to a reaction product in which the two hydrogen atoms are transferred to the same carbon centre, forming a methylene (CH2) group. The singlet order of parahydrogen is substantially retained over the geminal hydrogenation reaction, giving rise to a singlet-hyperpolarized CH2 group. Although the T1 relaxation times of the methylene protons are often short, the singlet order has a long lifetime, provided that singlet-triplet mixing is suppressed, either by chemical equivalence of the protons or by applying a resonant radiofrequency field. The long lifetime of the singlet order enables the accumulation of hyperpolarization during the slow hydrogenation reaction. We introduce a kinetic model for the behaviour of the observed hyperpolarized signals, including both the chemical kinetics and the spin dynamics of the reacting molecules. Our work demonstrates the feasibility of producing singlet-hyperpolarized methylene moieties by parahydrogen-induced polarization. This potentially extends the range of molecular agents which may be generated in a hyperpolarized state by chemical reactions of parahydrogen.

Iodonitrene in Action: Direct Transformation of Amino Acids into Terminal Diazirines and 15N2-Diazirines and Their Application as Hyperpolarized Markers.

A one-pot metal-free conversion of unprotected amino acids to terminal diazirines has been developed using phenyliodonium diacetate (PIDA) and ammonia. This PIDA-mediated transformation occurs via three consecutive reactions and involves an iodonitrene intermediate. This method is tolerant to most functional groups found on the lateral chain of amino acids, it is operationally simple, and it can be scaled up to provide multigram quantities of diazirine. Interestingly, we also demonstrated that this transformation could be applied to dipeptides without racemization. Furthermore, 14N2 and 15N2 isotopomers can be obtained, emphasizing a key trans-imination step when using 15NH3. In addition, we report the first experimental observation of 14N/15N isotopomers directly creating an asymmetric carbon. Finally, the 15N2-diazirine from l-tyrosine was hyperpolarized by a parahydrogen-based method (SABRE-SHEATH), demonstrating the products' utility as hyperpolarized molecular tag.

Selective hyperpolarization of heteronuclear singlet states via pulsed microtesla SABRE.

Signal Amplification By Reversible Exchange (SABRE) and its heteronuclear variant SABRE in SHield Enables Alignment Transfer to Heteronuclei create large nuclear magnetization in target ligands, exploiting level crossings in an iridium catalyst that transiently binds both the ligands and parahydrogen. This requires a specific, small magnetic field to match Zeeman splittings to scalar couplings. Here, we explore a different strategy, direct creation of heteronuclear singlet states in the target ligands, which produces enhanced signals at other field strengths, including zero field. We also show that pulsed methods (including pulsed field nulling) coherently and selectively pump such singlets, affording a significant enhancement on the resulting hyperpolarization.

Terminal Diazirines Enable Reverse Polarization Transfer from 15 N2 Singlets.

Diazirine moieties are chemically stable and have been incorporated into biomolecules without impediment of biological activity. The 15 N2 labeled diazirines are appealing motifs for hyperpolarization supporting relaxation protected states with long-lived lifetimes. The (-CH15 N2 ) diazirine groups investigated here are analogues to methyl groups, which provides the opportunity to transfer polarization stored on a relaxation protected (-CH15 N2 ) moiety to 1 H, thus combining the advantages of long lifetimes of 15 N polarization with superior sensitivity of 1 H detection. Despite the proximity of 1 H to 15 N nuclei in the diazirine moiety, 15 N T1 times of up to (4.6±0.4) min and singlet lifetimes Ts of up to (17.5±3.8) min are observed. Furthermore, we found terminal diazirines to support hyperpolarized 1 H2 singlet states in CH2 groups of chiral molecules. The singlet lifetime of 1 H singlets is up to (9.2±1.8) min, thus exceeding 1 H T1 relaxation time (at 8.45 T) by a factor of ≈100.

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.

Diazirines as Potential Molecular Imaging Tags: Probing the Requirements for Efficient and Long-Lived SABRE-Induced Hyperpolarization.

Diazirines are an attractive class of potential molecular tags for magnetic resonance imaging owing to their biocompatibility and ease of incorporation into a large variety of molecules. As recently reported, 15 N2 -diazirine can be hyperpolarized by the SABRE-SHEATH method, sustaining both singlet and magnetization states, thus offering a path to long-lived polarization storage. Herein, we show the generality of this approach by illustrating that the diazirine tag alone is sufficient for achieving excellent signal enhancements with long-lasting polarization. Our investigations reveal the critical role of Lewis basic additives, including water, on achieving SABRE-promoted hyperpolarization. The application of this strategy to a 15 N2 -diazirine-containing choline derivative demonstrates the potential of 15 N2 -diazirines as molecular imaging tags for biomedical applications.

Long-Lived 13C2 Nuclear Spin States Hyperpolarized by Parahydrogen in Reversible Exchange at Microtesla Fields.

Parahydrogen is an inexpensive and readily available source of hyperpolarization used to enhance magnetic resonance signals by up to four orders of magnitude above thermal signals obtained at ∼10 T. A significant challenge for applications is fast signal decay after hyperpolarization. Here we use parahydrogen-based polarization transfer catalysis at microtesla fields (first introduced as SABRE-SHEATH) to hyperpolarize 13C2 spin pairs and find decay time constants of 12 s for magnetization at 0.3 mT, which are extended to 2 min at that same field, when long-lived singlet states are hyperpolarized instead. Enhancements over thermal at 8.5 T are between 30 and 170 fold (0.02 to 0.12% polarization). We control the spin dynamics of polarization transfer by choice of microtesla field, allowing for deliberate hyperpolarization of either magnetization or long-lived singlet states. Density functional theory calculations and experimental evidence identify two energetically close mechanisms for polarization transfer: First, a model that involves direct binding of the 13C2 pair to the polarization transfer catalyst and, second, a model transferring polarization through auxiliary protons in substrates.

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

Nuclear spin hyperpolarization techniques are revolutionizing the field of 13 C molecular MRI. While dissolution dynamic nuclear polarization (d-DNP) is currently the leading technique, it is generally slow (requiring ≈1 h) and costly (≈$USD106 ). As a consequence of carbon's central place in biochemistry, tremendous progress using 13 C d-DNP bioimaging has been demonstrated to date including a number of clinical trials. Despite numerous attempts to develop alternatives to d-DNP, the competing methods have faced significant translational challenges. Efficient hyperpolarization of 15 N, 31 P, and other heteronuclei using signal amplification by reversible exchange (SABRE) has been reported in 2015, but extension of this technique to 13 C has proven to be challenging. Here, we present efficient hyperpolarization of 13 C nuclei using micro-Tesla SABRE. Up to ca. 6700-fold enhancement of nuclear spin polarization at 8.45 T is achieved within seconds, corresponding to P13C ≈4.4 % using 50 % parahydrogen (P13C >14 % would be feasible using more potent ≈100 % parahydrogen). Importantly, the 13 C polarization achieved via SABRE strongly depends not only upon spin-lattice relaxation, but also upon the presence of 15 N (I=1/2) versus quadrupolar 14 N (I=1) spins in the site binding the hexacoordinate Ir atom of the catalytic complex. We show that different 13 C nuclei in the test molecular frameworks-pyridine and acetonitrile-can be hyperpolarized, including 13 C sites up to five chemical bonds away from the exchangeable hydrides. The presented approach is highly scalable and can be applied to a rapidly growing number of biomolecules amendable to micro-Tesla SABRE.

Direct Hyperpolarization of Nitrogen-15 in Aqueous Media with Parahydrogen in Reversible Exchange.

Signal amplification by reversible exchange (SABRE) is an inexpensive, fast, and even continuous hyperpolarization technique that uses para-hydrogen as hyperpolarization source. However, current SABRE faces a number of stumbling blocks for translation to biochemical and clinical settings. Difficulties include inefficient polarization in water, relatively short-lived 1H-polarization, and relatively limited substrate scope. Here we use a water-soluble polarization transfer catalyst to hyperpolarize nitrogen-15 in a variety of molecules with SABRE-SHEATH (SABRE in shield enables alignment transfer to heteronuclei). This strategy works in pure H2O or D2O solutions, on substrates that could not be hyperpolarized in traditional 1H-SABRE experiments, and we record 15N T1 relaxation times of up to 2 min.

Generalizing, Extending, and Maximizing Nitrogen-15 Hyperpolarization Induced by Parahydrogen in Reversible Exchange.

Signal Amplification by Reversible Exchange (SABRE) is a fast and convenient NMR hyperpolarization method that uses cheap and readily available para-hydrogen as a hyperpolarization source. SABRE can hyperpolarize protons and heteronuclei. Here we focus on the heteronuclear variant introduced as SABRE-SHEATH (SABRE in SHield Enables Alignment Transfer to Heteronuclei) and nitrogen-15 targets in particular. We show that 15N-SABRE works more efficiently and on a wider range of substrates than 1H-SABRE, greatly generalizing the SABRE approach. In addition, we show that nitrogen-15 offers significantly extended T1 times of up to 12 minutes. Long T1 times enable higher hyperpolarization levels but also hold the promise of hyperpolarized molecular imaging for several tens of minutes. Detailed characterization and optimization are presented, leading to nitrogen-15 polarization levels in excess of 10% on several compounds.

Hyperpolarization of Nitrogen-15 Schiff Bases by Reversible Exchange Catalysis with para-Hydrogen.

NMR with thermal polarization requires relatively concentrated samples, particularly for nuclei with low abundance and low gyromagnetic ratios, such as (15) N. We expand the substrate scope of SABRE, a recently introduced hyperpolarization method, to allow access to (15) N-enriched Schiff bases. These substrates show fractional (15) N polarization levels of up to 2 % while having only minimal (1) H enhancements.