High-field NMR with dissolution triplet-DNP

Recent developments in materials and applications of triplet dynamic nuclear polarization

Dynamic nuclear polarization (DNP) is a method for achieving high levels of nuclear spin polarization by transferring spin polarization from electrons to nuclei by microwave irradiation, resulting in higher sensitivity in NMR/MRI. In particular, DNP using photoexcited triplet electron spins (triplet-DNP) can provide a hyperpolarized nuclear spin state at room temperature and in low magnetic field. In this review article, we highlight recent developments in materials and instrumentation for the application of triplet-DNP. First, a brief history and principles of triplet-DNP will be presented. Next, important advances in recent years will be outlined: new materials to hyperpolarize water and biomolecules; high-sensitivity solution NMR by dissolution triplet-DNP; and strategies for further improvement of the polarization. In view of these developments, future directions to widen the range of applications of triplet-DNP will be discussed.

LC-Photo-CIDNP hyperpolarization of biomolecules bearing a quasi-isolated spin pair: Magnetic-Field dependence via a rapid-shuttling device

Liquid-state low-concentration photochemically induced dynamic nuclear polarization (LC-photo-CIDNP) is an emerging technology tailored to enhance the sensitivity of NMR spectroscopy via LED- or laser-mediated optical irradiation. LC-photo-CIDNP is particularly useful to detect solvent-exposed aromatic residues (Trp, Tyr), either in isolation or within polypeptides and proteins. This study investigates the magnetic-field dependence of the LC-photo-CIDNP of Trp-α-13C-β,β,2,4,5,6,7-d7, a Trp isotopolog bearing a quasi-isolated 1Hα-13Cαspin pair (QISP). We employed a new rapid-shuttling side-illumination field-cycling device that enables ultra-fast (90-120 ms) vertical movements of NMR samples within the bore of a superconducting magnet. Thus, LC-photo-CIDNP hyperpolarization occurs at low field, while hyperpolarized signals are detected at high field (700 MHz). Resonance lineshapes were excellent, and the effect of several fields (1.18-7.08 T range) on hyperpolarization efficiency could be readily explored. Remarkably, unprecedented LC-photo-CIDNP enhancements ε ≅ 1,200 were obtained at 50 MHz (1.18 T), suggesting exciting avenues to hypersensitive LED-enhanced NMR in liquids at low field.

Real-time monitoring of enzyme-catalyzed phosphoribosylation of anti-influenza prodrug favipiravir by time-lapse nuclear magnetic resonance spectroscopy

Favipiravir (brand name Avigan), a widely known anti-influenza prodrug, is metabolized by endogenous enzymes of host cells to generate the active form, which exerts inhibition of viral RNA-dependent RNA polymerase activity; first, favipiravir is converted to its phosphoribosylated form, favipiravir-ribofuranosyl-5'-monophosphate (favipiravir-RMP), by hypoxanthine-guanine phosphoribosyltransferase (HGPRT). Because this phosphoribosylation reaction is the rate-determining step in the generation of the active metabolite, quantitative and real-time monitoring of the HGPRT-catalyzed reaction is essential to understanding the pharmacokinetics of favipiravir. However, assay methods enabling such monitoring have not been established. ¹⁹ F- or ³¹ P-based nuclear magnetic resonance (NMR) are powerful techniques for observation of intermolecular interactions, chemical reactions, and metabolism of molecules of interest, given that NMR signals of the heteronuclei sensitively reflect changes in the chemical environment of these moieties. Here, we demonstrated direct, sensitive, target-selective, nondestructive, and real-time observation of HGPRT-catalyzed conversion of favipiravir to favipiravir-RMP by performing time-lapse ¹⁹ F-NMR monitoring of the fluorine atom of favipiravir. In addition, we showed that ³¹ P-NMR can be used for real-time observation of the identical reaction by monitoring phosphorus atoms of the phosphoribosyl group of favipiravir-RMP and of the pyrophosphate product of that reaction. Furthermore, we demonstrated that NMR approaches permit the determination of general parameters of enzymatic activity such as V_max and K_m . This method not only can be widely employed in enzyme assays, but also may be of use in the screening and development of new favipiravir-analog antiviral prodrugs that can be phosphoribosylated more efficiently by HGPRT, which would increase the intracellular concentration of the drug's active form. The techniques demonstrated in this study would allow more detailed investigation of the pharmacokinetics of fluorinated drugs, and might significantly contribute to opening new avenues for widespread pharmaceutical studies.

Triplet-DNP in magnetically oriented microcrystal arrays

We explore dynamic nuclear polarization using electron spins in the photo-excited triplet state (Triplet-DNP) in magnetically oriented microcrystal arrays (MOMAs) of pentacene-doped p-terphenyl, in which the individual crystallites are magnetically aligned and UV-cured. In contrast to the conventional approach to Triplet-DNP in powder, which suffers from reduced nuclear polarization due to the averaged electron polarization and the broadening of electron-spin resonance, Triplet-DNP of the MOMAs offers as high dynamic polarization as that attainable in single-crystals. In the case of pentacene-doped p-terphenyl, the enhanced ¹H polarization in the one-dimensional MOMA, prepared simply by leaving the suspension in a stationary magnetic field before UV curation, can be higher than that attainable in the powder sample by an order of magnitude and comparable to that in single crystals and in the three-dimensional MOMA made using a modulational rotating field. Triplet-DNP of the MOMAs may find potential applications, such as the polarization of the co-doped target molecules and dissolution experiments.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Selective Isotope Labeling and LC-Photo-CIDNP Enable NMR Spectroscopy at Low-Nanomolar Concentration

NMR spectroscopy is a powerful tool to investigate molecular structure and dynamics. The poor sensitivity of this technique, however, limits its ability to tackle questions requiring dilute samples. Low-concentration photochemically induced dynamic nuclear polarization (LC-photo-CIDNP) is an optically enhanced NMR technology capable of addressing the above challenge by increasing the detection limit of aromatic amino acids in solution up to 1000-fold, either in isolation or within proteins. Here, we show that the absence of NMR-active nuclei close to a magnetically active site of interest (e.g., the structurally diagnostic 1Hα-13Cα pair of amino acids) is expected to significantly increase LC-photo-CIDNP hyperpolarization. Then, we exploit the spin-diluted tryptophan isotopolog Trp-α-13C-β,β,2,4,5,6,7-d7 and take advantage of the above prediction to experimentally achieve a ca 4-fold enhancement in NMR sensitivity over regular LC-photo-CIDNP. This advance enables the rapid (within seconds) detection of 20 nM concentrations or the molecule of interest, corresponding to a remarkable 3 ng detection limit. Finally, the above Trp isotopolog is amenable to incorporation within proteins and is readily detectable at a 1 μM concentration in complex cell-like media, including Escherichia coli cell-free extracts.

Challenges and Advances in the Application of Dynamic Nuclear Polarization to Liquid-State NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is a powerful method to study the molecular structure and dynamics of materials. The inherently low sensitivity of NMR spectroscopy is a consequence of low spin polarization. Hyperpolarization of a spin ensemble is defined as a population difference between spin states that far exceeds what is expected from the Boltzmann distribution for a given temperature. Dynamic nuclear polarization (DNP) can overcome the relatively low sensitivity of NMR spectroscopy by using a paramagnetic matrix to hyperpolarize a nuclear spin ensemble. Application of DNP to NMR can result in sensitivity gains of up to four orders of magnitude compared to NMR without DNP. Although DNP NMR is now more routinely utilized for solid-state (ss) NMR spectroscopy, it has not been exploited to the same degree for liquid-state samples. This Review will consider challenges and advances in the application of DNP NMR to liquid-state samples. The Review is organized into four sections: (i) mechanisms of DNP NMR relevant to hyperpolarization of liquid samples; (ii) applications of liquid-state DNP NMR; (iii) available detection schemes for liquid-state samples; and (iv) instrumental challenges and outlook for liquid-state DNP NMR.

Porphyrins as Versatile, Aggregation-Tolerant, and Biocompatible Polarizing Agents for Triplet Dynamic Nuclear Polarization of Biomolecules

Triplet dynamic nuclear polarization (triplet-DNP) achieves nuclear spin polarization at moderate temperatures by using spin polarization of photoexcited triplet electrons. The applications of triplet-DNP for biomolecules have been hampered because acenes, the only polarizing agents used so far, tend to aggregate and lose their polarization in biomolecular matrices. Here, we report for the first time use of porphyrins as polarizing agents of triplet-DNP and propose a new concept of aggregation-tolerant polarizing agents. Sodium salts of tetrakis(4-carboxyphenyl)porphyrin (TCPPNa) can be dispersed in amorphous as well as crystalline biomolecular matrices, and importantly, it can generate polarized triplet electrons even in a slightly aggregated state. Triplet-DNP of crystalline erythritol containing slightly aggregated TCPPNa can achieve more than 120-fold signal enhancement. Because TCPPNa is also the first biocompatible triplet-DNP polarizing agent, this work provides a crucial step forward for the biological and medical applications of triplet-DNP.

Room-temperature hyperpolarization of polycrystalline samples with optically polarized triplet electrons: pentacene or nitrogen-vacancy center in diamond?

We demonstrate room-temperature 13 C hyperpolarization by dynamic nuclear polarization (DNP) using optically polarized triplet electron spins in two polycrystalline systems: pentacene-doped [carboxyl-13 C] benzoic acid and microdiamonds containing nitrogen-vacancy (NV- ) centers. For both samples, the integrated solid effect (ISE) is used to polarize the 13 C spin system in magnetic fields of 350-400 mT. In the benzoic acid sample, the 13 C spin polarization is enhanced by up to 0.12 % through direct electron-to-13 C polarization transfer without performing dynamic 1 H polarization followed by 1 H - 13 C cross-polarization. In addition, the ISE has been successfully applied to polarize naturally abundant 13 C spins in a microdiamond sample to 0.01 %. To characterize the buildup of the 13 C polarization, we discuss the efficiencies of direct polarization transfer between the electron and 13 C spins as well as that of 13 C - 13 C spin diffusion, examining various parameters which are beneficial or detrimental for successful bulk dynamic 13 C polarization.

Invasive optical pumping for room-temperature masers, time-resolved EPR, triplet-DNP, and quantum engines exploiting strong coupling

We explore an approach for optically pumping a body of optically dense magnetic material. This challenge arises in time-resolved electron paramagnetic resonance (TREPR), triplet-based dynamic nuclear polarisation (DNP), and cavity QED. Crystals of pentacene-doped p-terphenyl were grown around variously shaped ends of optical waveguides, through which pump light could be injected deeply into the crystal. When incorporated into a maser as the gain medium, we found that, compared to conventional side-pumping, 11 times less pump beam intensity was needed to reach the masing threshold and 54 times more pulse energy could be absorbed by the gain medium without damage, resulting in a record peak output power of -5 dBm.

Materials chemistry of triplet dynamic nuclear polarization

Dynamic nuclear polarization with photo-excited triplet electrons (triplet-DNP) has the potential to enhance the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) at a moderate temperature. While many efforts have been devoted to achieving a large nuclear polarization based on triplet-DNP, the application of triplet-DNP has been limited to nuclear physics experiments. The recent introduction of materials chemistry into the field of triplet-DNP has achieved air-stable and water-soluble polarizing agents as well as the hyperpolarization of nanomaterials with a large surface area such as nanoporous metal-organic frameworks (MOFs) and nanocrystal dispersion in water. This Feature Article overviews the recently-emerged materials chemistry of triplet-DNP that paves new paths towards unprecedented biological and medical applications.