Thomas Theis

Microtesla SABRE Enables 10% Nitrogen-15 Nuclear Spin Polarization

Parahydrogen is demonstrated to efficiently transfer its nuclear spin hyperpolarization to nitrogen-15 in pyridine and nicotinamide (vitamin B3 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 15N 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.

Parahydrogen-enhanced zero-field nuclear magnetic resonance

NMR is typically carried out in strong magnetic fields, but recent technological developments have enabled the development of different methods for creating and detecting nuclear magnetization that do not depend on the use of strong magnets. A study that combines such methods demonstrates now that high-resolution NMR spectra with chemically relevant information can be obtained at zero magnetic field.

Direct and cost-efficient hyperpolarization of long-lived nuclear spin states on universal 15N2-diazirine molecular tags

More than 10,000-fold enhanced magnetic resonance signals with >20-min signal lifetimes on universal biomolecular markers. Conventional magnetic resonance (MR) faces serious sensitivity limitations which can be overcome by hyperpolarization methods, but the most common method (dynamic nuclear polarization) is complex and expensive, and applications are limited by short spin lifetimes (typically seconds) of biologically relevant molecules. We use a recently developed method, SABRE-SHEATH, to directly hyperpolarize 15N2 magnetization and long-lived 15N2 singlet spin order, with signal decay time constants of 5.8 and 23 minutes, respectively. We find >10,000-fold enhancements generating detectable nuclear MR signals that last for over an hour. 15N2-diazirines represent a class of particularly promising and versatile molecular tags, and can be incorporated into a wide range of biomolecules without significantly altering molecular function.

Zero-field NMR enhanced by parahydrogen in reversible exchange.

We have recently demonstrated that sensitive and chemically specific NMR spectra can be recorded in the absence of a magnetic field using hydrogenative parahydrogen induced polarization (PHIP) (1-3) and detection with an optical atomic magnetometer. Here, we show that non-hydrogenative parahydrogen-induced polarization (4-6) (NH-PHIP) can also dramatically enhance the sensitivity of zero-field NMR. We demonstrate the detection of pyridine, at concentrations as low as 6 mM in a sample volume of 250 μL, with sufficient sensitivity to resolve all identifying spectral features, as supported by numerical simulations. Because the NH-PHIP mechanism is nonreactive, operates in situ, and eliminates the need for a prepolarizing magnet, its combination with optical atomic magnetometry will greatly broaden the analytical capabilities of zero-field and low-field NMR.

15N Hyperpolarization of Imidazole-15N2 for Magnetic Resonance pH Sensing via SABRE-SHEATH

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.

Over 20% 15N Hyperpolarization in Under One Minute for Metronidazole, an Antibiotic and Hypoxia Probe

Direct NMR hyperpolarization of naturally abundant 15N 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 P15N of up to ∼24% is achieved using parahydrogen with 80% para fraction corresponding to P15N ≈ 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 15N site (using J2H-15N), we also hyperpolarized more distant 15N sites in metronidazole using longer-range spin–spin couplings (J4H-15N and J5H-15N). 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.

Hyperpolarization of “Neat” Liquids by NMR Signal Amplification by Reversible Exchange

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 P15N ≈ 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.

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.

The Absence of Quadrupolar Nuclei Facilitates Efficient 13C 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 (≈

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

USD106 ). As a consequence of carbons 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.