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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.

NMR hyperpolarization techniques enhance nuclear spin polarization by several orders of magnitude resulting in corresponding sensitivity gains. This enormous sensitivity gain enables new applications ranging from studies of small molecules by using high-resolution NMR spectroscopy to real-time metabolic imaging in vivo. Several hyperpolarization techniques exist for hyperpolarization of a large repertoire of nuclear spins, although the 13 C and 15 N sites of biocompatible agents are the key targets due to their widespread use in biochemical pathways. Moreover, their long T1 allows hyperpolarized states to be retained for up to tens of minutes. Signal amplification by reversible exchange (SABRE) is a low-cost and ultrafast hyperpolarization technique that has been shown to be versatile for the hyperpolarization of 15 N nuclei. Although large sensitivity gains are enabled by hyperpolarization, 15 N natural abundance is only ∼0.4 %, so isotopic labeling of the molecules to be hyperpolarized is required in order to take full advantage of the hyperpolarized state. Herein, we describe selected advances in the preparation of 15 N-labeled compounds with the primary emphasis on using these compounds for SABRE polarization in microtesla magnetic fields through spontaneous polarization transfer from parahydrogen. Also, these principles can certainly be applied for hyperpolarization of these emerging contrast agents using dynamic nuclear polarization and other techniques.

Signal Amplification By Reversible Exchange (SABRE) is a particularly simple hyperpolarisation approach. However, compared to other hyperpolarisation methods, SABRE is more limited in substrate scope. Therefore, it is critical to understand and overcome the factors limiting generalization. Past developments in SABRE catalyst optimization have emphasized large enhancements in the canonical SABRE substrate: pyridine and structurally closely related motifs. However, the pyridine-optimized catalysts are not efficient at hyperpolarising more sterically demanding substrates, including 2-substituted pyridine derivatives. Here we report that modifications of the catalyst ligand sphere, using a chelating ligand in particular, can increase the volume fraction available for substrate coordination to the iridium catalyst, thus permitting significant signal enhancements on otherwise sterically hindered substrates. The system yields (1)H enhancements on the order of 100-fold over 8.5 T thermal measurements for 2-substituted pyridine derivatives, and smaller, yet significant (1)H enhancement for provitamin B(6) and caffeine. For the 2-substituted pyridine derivatives we further show (15)N enhancements on the order of 1000-fold and (19)F enhancements of 30-fold over 8.5 T thermal polarisations.

A study of ion-molecular reactions of fluoro-substituted phenyl cations with benzopyridine derivatives demonstrated the following: (a) at present only the nuclear-chemical method enables the reaction of a direct phenylation of the nitrogen atom in benzopyridine derivatives, which makes it possible to obtain previously unknown quaternized structures, and (b) generation of nucleogenic phenyl cations with various substituents leads to a simultaneous introduction of the required substituents into the aromatic ring at the nitrogen heteroatom. Quantum-chemical calculations additionally confirmed the assumption about the pathways of the ion-molecular reactions with nucleogenic phenyl cations.

Quinazoline derivatives are well known to have a diverse array of therapeutic activities. Unfortunately, “classic” chemical synthesis does not provide an opportunity for the formation of N-phenyl quaternary 1,3-diazinium compounds. A devised nuclear-chemical method of synthesis based on chemical effects of nuclear transformations enables a new way of the direct nitrogen atom phenylation by the nucleogenic (generated by tritium β-decay) phenyl cations in 1,3-diazines, furnishing, based on our prediction, formation of previously unknown derivatives with N-phenyl quaternary quinazolinium scaffold.

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.

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%.

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. Here 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/TiO(2) catalysts in methanol-d(4) and in D(2)O. The maximum obtained (1)H polarization was 0.6 ± 0.2% (for propyl acetate in CD(3)OD), while the highest (13)C polarization was 0.10 ± 0.03% (for ethyl acetate in CD(3)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.

NMR hyperpolarization techniques enhance nuclear spin polarization by several orders of magnitude resulting in corresponding sensitivity gains. This massive sensitivity gain enables new applications ranging from studies of small molecules using high-resolution NMR spectroscopy to real-time metabolic imaging in vivo. Several hyperpolarization techniques exist for hyperpolarization of a large repertoire of nuclear spins, although (13)C- and (15)N- sites of biocompatible agents are the key targets due to their widespread in biochemical pathways. Moreover, their long T(1) allows retaining hyperpolarized states for up to tens of minutes. Signal Amplification by Reversible Exchange (SABRE) is a low-cost and ultrafast hyperpolarization technique that has been shown to be versatile for hyperpolarization of (15)N nuclei. While large sensitivity gains are enabled by hyperpolarization, (15)N natural abundance is only ~0.4%, so that isotopic labeling of the to-be-hyperpolarized molecules is required in order to take full advantage of the hyperpolarized state. Here, we describe selected advances in the preparation of (15)N-labeled compounds with the primary emphasis on using these compounds for their SABRE polarization in microtesla magnetic fields via spontaneous polarization transfer from parahydrogen. Also, these principles can certainly be applied for hyperpolarization of these emerging contrast agents using Dynamic Nuclear Polarization and other techniques.

We report synthesis of (15)N-3-(19)F-pyridine via Zincke salt formation with the overall 35% yield and 84% (15)N isotopic purity. Hyperpolarization studies of Signal Amplification by Reversible Exchange (SABRE) and SABRE in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) were performed to investigate the mechanism of polarization transfer from parahydrogen-derived hydride protons to (19)F nucleus in milli-Tesla and micro-Tesla magnetic field regimes in (15)N-3-(19)F-pyridine and (14)N-3-(19)F-pyridine. We found the mismatch between (15)N and (19)F magnetic field hyperpolarization profiles in the micro-Tesla regime indicating that the spontaneous hyperpolarization process likely happens directly from parahydrogen-derived hydride protons to (19)F nucleus without spin-relaying via (15)N site. In case of SABRE magnetic field regime (milli-Tesla magnetic field range), we found that magnetic field profiles for (1)H and (19)F hyperpolarization are very similar, and (19)F polarization levels are significantly lower than (1)H SABRE polarization levels and lower than (19)F SABRE-SHEATH (i.e. obtained at micro-Tesla magnetic field) polarization levels. Our findings support the hypothesis that in milli-Tesla magnetic field regime, the process of (19)F nuclei hyperpolarization is relayed via protons of substrate, and therefore is very inefficient. These findings are important in the context of improvement of the hyperpolarization hardware and rational design of the hyperpolarized molecular probes.

Die Magnetresonanz (MR) von Kernspins ist einer der nützlichsten physikalischen Effekte für die humane Bildgebung, ebenso wie für die Aufklärung von molekularen Strukturen und die chemische Analyse. Das volle Potential der MR kann jedoch kaum genutzt werden, da nur ein geringer Bruchteil aller Kernspins polarisiert, sprich in einem äußeren magnetischen Feld ausgerichtet werden. Hyperpolarisierungsmethoden versuchen, die Polarisierung und damit das MR-Signal auf anderem Wege zu erhöhen. Eine einzigartige Quelle reiner Spinordnung ist das langlebige, verschränkte Kernspin-Singulett-Isomer des Wasserstoffmoleküls, genannt Parawasserstoff (pH2). Mithilfe dieser “Spinordnung auf Abruf” kann das MR-Signal um mehrere Größenordnungen verstärkt werden, wenn pH2 in Kontakt mit einem anderen Molekül gebracht wird. In Bezug auf eine biomedizinische Anwendung von pH2-hyperpolarisierten Kontrastmitteln haben in den letzten zehn Jahren beeindruckende Fortschritte stattgefunden. Das Ziel dieses Aufsatzes ist es, diese Entwicklungen in den Bereichen der Spinphysik, Katalyse, Instrumentierung, Kontrastmittel und Anwendungen gebündelt und verständlich darzustellen.

We report a simple and effective method to remove IrIMes homogeneous polarization transfer catalysts from solutions where NMR Signal Amplification By Reversible Exchange (SABRE) has been performed, while leaving intact the substrate’s hyperpolarized state. Following microTesla SABRE hyperpolarization of (15)N spins in metronidazole, addition of SiO(2) microparticles functionalized with 3-mercaptopropyl or 2-mercaptoethyl ethyl sulfide moieties provides removal of the catalyst from solution well within the hyperpolarization decay time at 0.3 T (T(1)>3 mins)—and enabling transfer to 9.4 T for detection of enhanced (15)N signals in the absence of catalyst within the NMR-detection region. Successful catalyst removal from solution is supported by the inability to “re-hyperpolarize” (15)N spins in subsequent attempts, as well as by (1)H NMR and ICP-MS. Record-high (15)N nuclear polarization of up to ~34% was achieved, corresponding to >100,000-fold enhancement at 9.4 T, and approximately 5/6(th) of the (15)N hyperpolarization is retained after ~20-second-long purification procedure. Taken together, these results help pave the way for future studies involving in vivo molecular imaging using agents hyperpolarized via rapid and inexpensive parahydrogen-based methods.

Hyperpolarized forms of 1-13C-acetates and 1-13C-pyruvates are used as diagnostic contrast agents for molecular imaging of many diseases and disorders. Here, we report the synthetic preparation of 1-13C isotopically enriched and pure from solvent acetates and pyruvates derivatized with unsaturated ester moiety. The reported unsaturated precursors can be employed for NMR hyperpolarization of 1-13C-acetates and 1-13C-pyruvates via parahydrogen-induced polarization (PHIP). In this PHIP variant, Side arm hydrogenation (SAH) of unsaturated ester moiety is followed by the polarization transfer from nascent parahydrogen protons to 13C nucleus via magnetic field cycling procedure to achieve hyperpolarization of 13C nuclear spins. This work reports the synthesis of PHIP-SAH precursors: vinyl 1-13C-acetate (55% yield), allyl 1-13C-acetate (70% yield), propargyl 1-13C-acetate (45% yield), allyl 1-13C-pyruvate (60% yield), and propargyl 1-13C-pyruvate (35% yield). Feasibility of PHIP-SAH 13C hyperpolarization was ver...

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...

Fluorine-19 has high NMR detection sensitivity—similar to that of protons, owing to its large gyromagnetic ratio and high natural abundance (100%). Unlike protons, however, fluorine-19 (19F) has a negligible occurrence in biological objects, as well as a more sensitive chemical shift. As a result, in vivo 19F NMR spectroscopy and MR imaging offer advantages of negligible background signal and sensitive reporting of the local molecular environment. Here we report on NMR hyperpolarization of 19F nuclei using reversible exchange reactions with parahydrogen gas as the source of nuclear spin order. NMR signals of 3-fluoropyridine were enhanced by ~100 fold, corresponding to 0.3% 19F nuclear spin polarization (at 9.4 T) using ca. 50% parahydrogen. While future optimization efforts will likely significantly increase the hyperpolarization levels we already demonstrate the utility of 19F hyperpolarization for high-resolution hyperpolarized 19F imaging and hyperpolarized 19F pH sensing.

The successful transfer of para-hydrogen-induced polarization to 15N spins using heterogeneous catalysts in aqueous solutions was demonstrated. Hydrogenation of a synthesized unsaturated 15N-labeled precursor (neurine) with parahydrogen (p-H2) over Rh/TiO2 heterogeneous catalysts yielded a hyperpolarized structural analogue of choline. As a result, 15N polarization enhancements of over 2 orders of magnitude were achieved for the 15N-labeled ethyltrimethylammonium ion product in deuterated water at elevated temperatures. Enhanced 15N NMR spectra were successfully acquired at 9.4 and 0.05 T. Importantly, long hyperpolarization lifetimes were observed at 9.4 T, with a 15N T1 of ∼6 min for the product molecules, and the T1 of the deuterated form exceeded 8 min. Taken together, these results show that this approach for generating hyperpolarized species with extended lifetimes in aqueous, biologically compatible solutions is promising for various biomedical applications.

NMR hyperpolarization techniques have the potential to revolutionize the field of NMR spectroscopy and molecular MRI because they can transiently enhance nuclear spin polarization by 4–8 orders of magnitude, with corresponding gains in NMR signal-to-noise ratio (SNR). The SABRE-SHEATH (Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei) technique, first demonstrated in 2015, allows for direct, efficient (>20 % nuclear spin polarization), and fast (in under one minute) hyperpolarization of 15N sites. Several classes of biologically relevant 15N hyperpolarized contrast agents have been efficiently hyperpolarized to date including pH sensors, which can be potentially useful for non-invasive pH imaging of cancer and other diseases with altered metabolism. Here, we report the optimized 15N enrichment of imidazole-15N2 – a promising in vivo pH sensor with pKa ∼ 7.0. A hyperpolarized 0.1 M aqueous solution (ϵ15N ∼ 146,000 fold, P15N ∼ 0.24 %) was used to record 15N NMR spectra at 0.05 T, demonstrating the feasibility of high-resolution (full width at half maximum ∼ 1 Hz corresponding to 5 ppm at 0.05 T) NMR spectroscopy near its pKa (7.0) at ultra-low magnetic field. Given that proton-binding events modulate the chemical shift by ∼ 30 ppm for this pH-sensing probe, our results demonstrate the feasibility of ultra-low-field pH sensing near its pKa (7.0) with SNR approaching that of high-field (9.4 T) MR.

Nuclear spin hyperpolarization techniques are revolutionizing the field of 13C 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 13C 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 15N, 31P, and other heteronuclei using signal amplification by reversible exchange (SABRE) has been reported in 2015, but extension of this technique to 13C has proven to be challenging. Here, we present efficient hyperpolarization of 13C 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 13C polarization achieved via SABRE strongly depends not only upon spin–lattice relaxation, but also upon the presence of 15N (I=1/2) versus quadrupolar 14N (I=1) spins in the site binding the hexacoordinate Ir atom of the catalytic complex. We show that different 13C nuclei in the test molecular frameworks—pyridine and acetonitrile—can be hyperpolarized, including 13C 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.

The developed nuclear-chemical method allows direct phenylation of the nitrogen atom in the isoquinoline molecule and one-step synthesis of tritium-labeled biomarkers containing previously unknown N-(p-difluorophenyl)isoquinolinium fragment with high radiochemical yield.

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 (19)F-spins. Polarization transfer from the parahydrogen-derived hydrides to the (19)F nucleus is accomplished via weak five-bond J-couplings using a shaped QUASi-Resonance (QUASR) radio-frequency pulse at a 0.05 T magnetic field. The net result is the direct generation of hyperpolarized (19)F z-magnetization, derived from the parahydrogen singlet order. An accumulation of (19)F polarization on the free ligand is achieved with subsequent repetition of this pulse sequence. The hyperpolarized (19)F signal exhibits clear dependence on the pulse length, irradiation frequency, and delay time in a manner similar to that reported for (15)N QUASR-SABRE. Moreover, the hyperpolarized (19)F signals of 3-(19)F-(14)N-pyridine and 3-(19)F-(15)N-pyridine isotopologues are similar, suggesting (i) that polarization transfer via QUASR-SABRE is irrespective of the nitrogen isotopologue, and (ii) the presence or absence of the spin-1/2 (15)N nucleus has no impact on the efficiency of QUASR-SABRE polarization transfer. Although optimization of polarization transfer efficiency to (19)F (P(19F)~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 (19)F spins.

(13)C-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 (1)H and (13)C 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 (1)H and (13)C nuclei, respectively, in CD(3)OD solutions. Allyl pyruvate and ethyl acetate were also hyperpolarized in aqueous phase using homogeneous hydrogenation with parahydrogen over water-soluble rhodium catalyst. (13)C polarization of 0.82% and 2.1% was obtained for allyl pyruvate and ethyl acetate, respectively. (13)C-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.

NMR hyperpolarization of uniformly (15)N-labeled metronidazole-(15)N(3) is demonstrated using SABRE-SHEATH (Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei). In this antibiotic, a (15)NO(2) group is hyperpolarized via spin relays created by (15)N spins in metronidazole-(15)N(3), and the polarization is transferred from parahydrogen-derived hydrides over six chemical bonds. This efficient spin-relayed mechanism of polarization transfer is supported by measurements of polarization dynamics and T(1) relaxation in micro-Tesla magnetic fields. In less than a minute of parahydrogen bubbling at ~0.4 μT, a high level of nuclear spin polarization P(15N) of ~16% is achieved on all three (15)N sites of metronidazole-(15)N(3) at up to ~41 mM concentration. This product of (15)N polarization and concentration of (15)N spins is ~6-fold better than any previous value for (15)N SABRE-derived hyperpolarization. At 1.4 T, the hyperpolarized state of the (15)NO(2) group of this antibiotic persists for tens of minutes (T(1)~10 min), in part because its (15)N spin has no detectable spin-spin couplings with protons of the molecule. A novel synthesis achieving robust uniform (15)N enrichment of metronidazole is reported with 15% yield over three steps. This synthetic approach should be suitable for preparation of other (15)N-labeled nitroimidazole derivatives, potentially enabling a wide range of in vivo metabolic probes from hypoxia sensing to theranostic imaging.

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 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.

Parahydrogen-induced polarization (PHIP) is an NMR hyperpolarization technique that increases nuclear spin polarization by orders of magnitude, and it is particularly well-suited to study hydrogenation reactions. However, the use of high-field NMR spectroscopy is not always possible, especially in the context of potential industrial-scale reactor applications. On the other hand, the direct low-field NMR detection of reaction products with enhanced nuclear spin polarization is challenging due to near complete signal cancellation from nascent parahydrogen protons. We show that hydrogenation products prepared by PHIP can be irradiated with weak (on the order of spin–spin couplings of a few hertz) alternating magnetic field (called Spin-Lock Induced Crossing or SLIC) and consequently efficiently detected at low magnetic field (e.g., 0.05 T used here) using examples of several types of organic molecules containing a vinyl moiety. The detected hyperpolarized signals from several reaction products at tens of millimolar concentrations were enhanced by 10000-fold, producing NMR signals an order of magnitude greater than the background signal from protonated solvents.

Supported metal catalyst was designed, characterized and tested for aqueous phase heterogeneous hydrogenation of vinyl acetate with parahydrogen to produce 13C-hyperpolarized ethyl acetate for potential biomedical applications. The Rh/TiO2 catalyst with 23.2 wt% loading produced strong hyperpolarized 13C-enriched ethyl acetate-1-13C detected at 9.4 T. Approximately 14-fold 13C signal enhancement was detected using ~50% parahydrogen gas without taking into account relaxation losses before and after polarization transfer via magnetic field cycling from nascent parahydrogen-derived protons to 13C nuclei. This first observation of 13C PHIP-hyperpolarized products over supported metal catalyst in the aqueous medium opens up new possibilities for production of catalyst-free aqueous solutions of nontoxic hyperpolarized contrast agents for a wide range of biomolecules amenable to Parahydrogen Induced Polarization via Side Arm Hydrogenation (PHIP-SAH) approach.

An open-source hyperpolarizer producing (13)C hyperpolarized contrast agents using parahydrogen induced polarization (PHIP) for biomedical and other applications is presented. This PHIP hyperpolarizer utilizes an Arduino microcontroller in conjunction with a readily modified graphical user interface written in the open-source processing software environment to completely control the PHIP hyperpolarization process including remotely triggering an NMR spectrometer for efficient production of payloads of hyperpolarized contrast agent and in situ quality assurance of the produced hyperpolarization. Key advantages of this hyperpolarizer include: (i) use of open-source software and hardware seamlessly allowing for replication and further improvement as well as readily customizable integration with other NMR spectrometers or MRI scanners (i.e., this is a multiplatform design), (ii) relatively low cost and robustness, and (iii) in situ detection capability and complete automation. The device performance is demonstrated by production of a dose (∼2-3 mL) of hyperpolarized (13)C-succinate with %P13C ∼ 28% and 30 mM concentration and (13)C-phospholactate at %P13C ∼ 15% and 25 mM concentration in aqueous medium. These contrast agents are used for ultrafast molecular imaging and spectroscopy at 4.7 and 0.0475 T. In particular, the conversion of hyperpolarized (13)C-phospholactate to (13)C-lactate in vivo is used here to demonstrate the feasibility of ultrafast multislice (13)C MRI after tail vein injection of hyperpolarized (13)C-phospholactate in mice.

Direct NMR hyperpolarization of naturally abundant (15)N sites in metronidazole is demonstrated using SABRE-SHEATH (Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei). In only a few tens of seconds, nuclear spin polarization P(15)N of up to ∼24% is achieved using parahydrogen with 80% para fraction corresponding to P(15)N ≈ 32% if ∼100% parahydrogen were employed (which would translate to a signal enhancement of ∼0.1-million-fold at 9.4 T). In addition to this demonstration on the directly binding (15)N site (using J(2)H-(15)N), we also hyperpolarized more distant (15)N sites in metronidazole using longer-range spin-spin couplings (J(4)H-(15)N and J(5)H-(15)N). Taken together, these results significantly expand the range of molecular structures and sites amenable to hyperpolarization via low-cost parahydrogen-based methods. In particular, hyperpolarized nitroimidazole and its derivatives have powerful potential applications such as direct in vivo imaging of mechanisms of action or hypoxia sensing.

Two synthetic strategies are investigated for the preparation of water-soluble iridium-based catalysts for NMR signal amplification by reversible exchange (SABRE). In one approach, PEGylation of a variant N-heterocyclic carbene provided a novel catalyst with excellent water solubility. However, while SABRE-active in ethanol solutions, the catalyst lost activity in >50% water. In a second approach, synthesis of a novel di-iridium complex precursor where the cyclooctadiene (COD) rings have been replaced by CODDA (1,2-dihydroxy-3,7-cyclooctadiene) leads to the creation of a catalyst [IrCl(CODDA)IMes] that can be dissolved and activated in water—enabling aqueous SABRE in a single step, without need for either an organic cosolvent or solvent removal followed by aqueous reconstitution. The potential utility of the CODDA catalyst for aqueous SABRE is demonstrated with the ∼(−)32-fold enhancement of 1H signals of pyridine in water with only 1 atm of parahydrogen.

15N nuclear spins of imidazole-15N2 were hyperpolarized using NMR signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH). A 15N NMR signal enhancement of ∼2000-fold at 9.4 T is reported using parahydrogen gas (∼50% para-) and ∼0.1 M imidazole-15N2 in methanol:aqueous buffer (∼1:1). Proton binding to a 15N site of imidazole occurs at physiological pH (pKa ∼ 7.0), and the binding event changes the 15N isotropic chemical shift by ∼30 ppm. These properties are ideal for in vivo pH sensing. Additionally, imidazoles have low toxicity and are readily incorporated into a wide range of biomolecules. 15N-Imidazole SABRE-SHEATH hyperpolarization potentially enables pH sensing on scales ranging from peptide and protein molecules to living organisms.

Scalable and versatile methodology for production of vinylated carboxylic compounds with 13C isotopic label in C1 position is described. It afforded robust synthesis of vinyl acetate-1-13C, which is a precursor for preparation of 13C hyperpolarized ethyl acetate-1-13C, which provides a convenient vehicle for potential in vivo delivery of hyperpolarized acetate to probe metabolism in living organisms. Kinetics of vinyl acetate molecular hydrogenation and polarization transfer from parahydrogen to 13C via magnetic field cycling were investigated. Nascent proton nuclear spin polarization (%PH) of ~3.3% and carbon-13 polarization (%P13C) of ~1.8% were achieved in ethyl acetate utilizing 50% parahydrogen corresponding to ~50% polarization transfer efficiency. The use of nearly 100% parahydrogen and the improvements of %PH of parahydrogen-nascent protons will likely enable production of 13C hyperpolarized contrast agents with P13C of 20–50% in seconds using presented here chemistry for preparation of metabolically relevant precursors. Hyperpolarized in this fashion 13C contrast agents can be employed for ultra-fast molecular imaging, the feasibility of which is presented here. A series of 3D images was acquired using 13C hyperpolarized ethyl acetate-1-13C with high spatial (0.5 ×0.5×4 mm3) and temporal (~2.5 s) resolution.

Nicotinamide (a vitamin B3 amide) is one of the key vitamins as well as a drug for treatment of M. tuberculosis, HIV, cancer, and other diseases. Here, an improved Zincke reaction methodology is presented allowing for straightforward and scalable synthesis of nicotinamide-1-(15)N with an excellent isotopic purity (98%) and good yield (55%). (15)N nuclear spin label in nicotinamide-1-(15)N can be NMR hyperpolarized in seconds using parahydrogen gas. NMR hyperpolarization using the process of temporary conjugation between parahydrogen and to-be-hyperpolarized biomolecule on hexacoordinate iridium complex via the Signal Amplification By Reversible Exchange (SABRE) method significantly increases detection sensitivity (e.g.,20,000-fold for nicotinamide-1-(15)N at 9.4 T) as has been shown by Theis T. et al. (J. Am. Chem. Soc. 2015, 137, 1404), and hyperpolarized in this fashion, nicotinamide-1-(15)N can be potentially used to probe metabolic processes in vivo in future studies. Moreover, the presented synthetic methodology utilizes mild reaction conditions, and therefore can also be potentially applied to synthesis of a wide range of (15)N-enriched N-heterocycles that can be used as hyperpolarized contrast agents for future in vivo molecular imaging studies.

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.

Here we present the feasibility of NMR Signal Amplification by Reversible Exchange (SABRE) using radio-frequency irradiation at low magnetic field (0.05 T) in the regime, where the chemical shifts of free and catalyst bound species are similar. In SABRE, the (15)N-containing substrate and parahydrogen perform simultaneous chemical exchange on an Iridium hexacoordinate complex. Shaped Spin-Lock Induced Crossing (SLIC) radio-frequency pulse sequence followed by a delay is applied at QUASi-Resonance (QUASR) condition of (15)N spins of (15)N-enriched substrate. As a result of this pulse sequence application, (15)N z-magnetization is created from the spin-order of parahydrogen derived hyperpolarized hydrides. The repetition of the pulse-sequence block consisting of shaped radio-frequency pulse and the delay leads to the build-up of (15)N magnetization. The modulation of this effect by the irradiation frequency, pulse duration and amplitude, delay duration, and the number of pumping cycles was demonstrated. Pyridine-(15)N, acetonitrile-(15)N, metronidazole-(15)N(2)-(13)C(2) substrates were studied representing three classes of compounds (five- and six-membered heterocycles and nitrile) showing the wide applicability of the technique. Metronidazole-(15)N(2)-(13)C(2) is an FDA-approved antibiotic that can be injected in large quantities promising non-invasive and accurate hypoxia sensing. The (15)N hyperpolarization levels attained with QUASR-SABRE on metronidazole-(15)N(2)-(13)C(2) were more than two-fold greater than with SABRE-SHEATH (SABRE in SHield Enables Alignment Transfer to Heteronuclei) demonstrating that QUASR-SABRE can deliver significantly more efficient means of SABRE hyperpolarization.

We report herein a large-scale (>10 g) synthesis of isotopically enriched 1-(13)C-phosphoenolpyruvate and 1-(13)C-phosphoenolpyruvate-d(2) for application in hyperpolarized imaging technology. The 1-(13)C-phosphoenolpyruvate-d(2) was synthesized with 57% overall yield (over two steps), and >98% (2)H isotopic purity, representing an improvement over the previous report. The same outcome was achieved for 1-(13)C-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-(13)C-phosphoenolpyruvate resulted in overall increase of (1)H T(1) of nascent hyperpolarized protons (4.30 ± 0.04 s versus 2.06 ± 0.01 s) and (1)H polarization (~2.5% versus ~0.7%) of the resulting hyperpolarized 1-(13)C-phospholactate. The nuclear spin polarization of nascent parahydrogen-derived protons was transferred to 1-(13)C nucleus via magnetic field cycling procedure. The proton T(1) increase in hyperpolarized deuterated 1-(13)C-phospholactate yielded approximately 30% better (13)C polarization compared to non-deuterated hyperpolarized 1-(13)C-phospholactate. Analysis of T(1) relaxation revealed that deuteration of 1-(13)C-phospholactate may have resulted in approximately 3-fold worse H→(13)C 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 (13)C hyperpolarized contrast agents for molecular imaging employing (13)C MRI. The hyperpolarized 1-(13)C-phospholactate-d(2) is of biomedical imaging relevance because it undergoes in vivo dephosphorylation and becomes (13)C hyperpolarized lactate, which as we show can be detected in the brain using (13)C hyperpolarized MRI; an implication for future imaging of neurodegenerative diseases and dementia.

NMR is one of the most versatile and useful physical effects used for human imaging, chemical analysis and the elucidation of molecular structures. Yet, the full potential of NMR is hardly ever used, because only a small fraction of the nuclear spin ensemble is polarized - i.e. aligned with the applied static magnetic field. This fraction is termed nuclear spin polarization P. As a result, no more than a few parts per million of all nuclear spins effectively contribute to the signal in all magnetic fields (B(0)) available for NMR or MRI today. Because P is approximately linear with B(0), a stronger field offers some but limited improvements. Hyperpolarization methods seek other means to increase P and thus the MR signal. A unique source of pure spin order is the spin singlet state of dihydrogen, parahydrogen (pH(2)), which is inherently stable and long-lived. When brought into contact with another molecule, this “spin order on demand” allows enhancing the NMR signal by several orders of magnitude. In contrast to other methods, this process is very fast (seconds) and can take place in the liquid state. Nuclear spin polarization of the order of unity was demonstrated, manifesting as significant NMR and MRI signal enhancement by several orders of magnitude. Considerable progress was made in the past decade in the area of pH(2)-based hyperpolarization techniques for biomedical applications. It is the goal of this review to provide a comprehensive, selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, contrast agents’ preparation and application.

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.

Here, we report the production of 13C-hyperpolarized ethyl acetate via heterogeneously catalyzed pairwise addition of parahydrogen to vinyl acetate over TiO2-supported rhodium nanoparticles, followed by magnetic field cycling. Importantly, the hyperpolarization is demonstrated even at the natural abundance of 13C isotope (ca. 1.1%) along with the easiest separation of the catalyst from the hyperpolarized liquid.

We report NMR Signal Amplification by Reversible Exchange (SABRE) hyperpolarization of the rare isotopes in "neat" liquids, each composed only of an otherwise pure target compound with isotopic natural abundance (n.a.) and millimolar concentrations of dissolved catalyst. Pyridine (Py) or Py derivatives are studied at 0.4% isotopic natural abundance ¹⁵N, deuterated, ¹⁵N enriched, and in various combinations using the SABRE-SHEATH variant (microTesla magnetic fields to permit direct ¹⁵N polarization from parahydrogen via reversible binding and exchange with an Ir catalyst). We find that the dilute n.a. ¹⁵N spin bath in Py still channels spin order from parahydrogen to dilute ¹⁵N spins, without polarization losses due to the presence of ¹⁴N or ²H. We demonstrate P(15N) ≈ 1% (a gain of 2900 fold relative to thermal polarization at 9.4 T) at high substrate concentrations. This fundamental finding has a significant practical benefit for screening potentially hyperpolarizable contrast agents without labeling. The capability of screening at n.a. level of ¹⁵N is demonstrated on examples of mono- and dimethyl-substituted Py (picolines and lutidines previously identified as promising pH sensors), showing that the presence of a methyl group in the ortho position significantly decreases SABRE hyperpolarization.

NMR signal amplification by reversible exchange (SABRE) is a NMR hyperpolarization technique that enables nuclear spin polarization enhancement of molecules via concurrent chemical exchange of a target substrate and parahydrogen (the source of spin order) on an iridium catalyst. Recently, we demonstrated that conducting SABRE in microtesla fields provided by a magnetic shield enables up to 10% 15N-polarization (Theis, T.; et al. J. Am. Chem. Soc.2015, 137, 1404). Hyperpolarization on 15N (and heteronuclei in general) may be advantageous because of the long-lived nature of the hyperpolarization on 15N relative to the short-lived hyperpolarization of protons conventionally hyperpolarized by SABRE, in addition to wider chemical shift dispersion and absence of background signal. Here we show that these unprecedented polarization levels enable 15N magnetic resonance imaging. We also present a theoretical model for the hyperpolarization transfer to heteronuclei, and detail key parameters that should be optimized for efficient 15N-hyperpolarization. The effects of parahydrogen pressure, flow rate, sample temperature, catalyst-to-substrate ratio, relaxation time (T1), and reversible oxygen quenching are studied on a test system of 15N-pyridine in methanol-d4. Moreover, we demonstrate the first proof-of-principle 13C-hyperpolarization using this method. This simple hyperpolarization scheme only requires access to parahydrogen and a magnetic shield, and it provides large enough signal gains to enable one of the first 15N images (2 × 2 mm2 resolution). Importantly, this method enables hyperpolarization of molecular sites with NMR T1 relaxation times suitable for biomedical imaging and spectroscopy.

Simultaneous reversible chemical exchange of parahydrogen and to-be-hyperpolarized substrate on metal centers enables spontaneous transfer of spin order from parahydrogen singlet to nuclear spins of the substrate. When performed at sub-micro-Tesla magnetic field, this technique of NMR Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH). SABRE-SHEATH has been shown to hyperpolarize nitrogen-15 sites of a wide range of biologically interesting molecules to a high polarization level (P > 20%) in one minute. Here, we report on a systematic study of (1)H, (13)C and (15)N spin-lattice relaxation (T(1)) of metronidazole-(13)C(2)-(15)N(2) in SABRE-SHEATH hyperpolarization process. In micro-Tesla range, we find that all (1)H, (13)C and (15)N spins studied share approximately the same T(1) values (ca. 4 s at the conditions studied) due to mixing of their Zeeman levels, which is consistent with the model of relayed SABRE-SHEATH effect. These T(1) values are significantly lower than those at higher magnetic (i.e. the Earth’s magnetic field and above), which exceed 3 minutes in some cases. Moreover, these relatively short T(1) values observed below 1 micro-Tesla limit the polarization build-up process of SABRE-SHEATH– thereby, limiting maximum attainable (15)N polarization. The relatively short nature of T(1) values observed below 1 micro-Tesla is primarily caused by intermolecular interactions with quadrupolar iridium centers or dihydride protons of the employed polarization transfer catalyst, whereas intramolecular spin-spin interactions with (14)N quadrupolar centers have significantly smaller contribution. The presented experimental results and their analysis will be beneficial for more rational design of SABRE-SHEATH (i) polarization transfer catalyst, and (ii) hyperpolarized molecular probes in the context of biomedical imaging and other applications.

Parahydrogen is demonstrated to efficiently transfer its nuclear spin hyperpolarization to nitrogen-15 in pyridine and nicotinamide (vitamin B(3) amide) by conducting "signal amplification by reversible exchange" (SABRE) at microtesla fields within a magnetic shield. Following transfer of the sample from the magnetic shield chamber to a conventional NMR spectrometer, the (15)N NMR signals for these molecules are enhanced by ∼30,000- and ∼20,000-fold at 9.4 T, corresponding to ∼10% and ∼7% nuclear spin polarization, respectively. This method, dubbed "SABRE in shield enables alignment transfer to heteronuclei" or "SABRE-SHEATH", promises to be a simple, cost-effective way to hyperpolarize heteronuclei. It may be particularly useful for in vivo applications because of longer hyperpolarization lifetimes, lack of background signal, and facile chemical-shift discrimination of different species.

The Signal Amplification by Reversible Exchange (SABRE) technique employs exchange with singlet-state parahydrogen to efficiently generate high levels of nuclear spin polarization. Spontaneous SABRE has been shown previously to be efficient in the milli-Tesla and micro-Tesla regimes. We have recently demonstrated that high-field SABRE is also possible, where proton sites of molecules that are able to reversibly coordinate to a metal center can be hyperpolarized directly within high-field magnets, potentially offering the convenience of in situ hyperpolarization-based spectroscopy and imaging without sample shuttling. Here, we show efficient polarization transfer from parahydrogen (para-H2) to the 15N atoms of imidazole-15N2 and nicotinamide-15N achieved via high-field SABRE (HF-SABRE). Spontaneous transfer of spin order from the para-H2 protons to 15N atoms at the high magnetic field of an MRI scanner allows one not only to record enhanced 15N NMR spectra of in situ hyperpolarized biomolecules, but also to perform imaging using conventional MRI sequences. 2D 15N MRI of high-field SABRE-hyperpolarized imidazole with spatial resolution of 0.3×0.3 mm2 at 9.4 T magnetic field and a high signal-to-noise ratio (SNR) of ~99 was demonstrated. We show that 1H MRI of in situ HF-SABRE hyperpolarized biomolecules (e.g. imidazole-15N2) is also feasible. Taken together, these results show that heteronuclear (15N) and 1H spectroscopic detection and imaging of high-field-SABRE-hyperpolarized molecules are promising tools for a number of emerging applications.

High-resolution 13C NMR spectroscopy of hyperpolarized succinate-1-13C is reported in vitro and in vivo using a clinical-scale, biplanar (80 cm-gap) 48.7 mT permanent magnet with a high homogeneity magnetic field. Non-localized 13C NMR spectra were recorded at 0.52 MHz resonance frequency over the torso of a tumor-bearing mouse every 2 seconds. Hyperpolarized 13C NMR signals with linewidths of ~3 Hz (corresponding to ~6 ppm) were recorded in vitro (2 mL in a syringe) and in vivo (over a mouse torso). Comparison of the full width at half maximum (FWHM) for 13C NMR spectra acquired at 48.7 mT and at 4.7 T in a small-animal MRI scanner demonstrates a factor of ~12 improvement for the 13C resonance linewidth attainable at 48.7 mT compared to that at 4.7 T in vitro. 13C hyperpolarized succinate-1-13C resonance linewidths in vivo are at least one order of magnitude narrower at 48.7 mT compared to those observed in high-field (≥ 3 T) studies employing HP contrast agents. The demonstrated high-resolution 13C in vivo spectroscopy could be useful for high-sensitivity spectroscopic studies involving monitoring HP agent uptake or detecting metabolism using HP contrast agents with sufficiently large 13C chemical shift differences.

Parahydrogen is an inexpensive and readily available source of hyperpolarization used to enhance magnetic resonance signals by up to 4 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 micro-Tesla 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 minutes 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 μT field allowing for deliberate hyperpolarization of either magnetization or long-lived singlet states. Density functional theory (DFT) 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 (PTC), and second, a model transferring polarization through auxiliary protons in substrates.

Fluorine-19 has high NMR detection sensitivity-similar to that of protons-owing to its large gyromagnetic ratio and high natural abundance (100 %). Unlike protons, however, fluorine-19 (