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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.

Dissolution-dynamic nuclear polarization (d-DNP) is a state-of-the-art technology that can dramatically enhance the detection sensitivity of nuclear magnetic resonance (NMR). DNP NMR has been applied to small molecules with stable isotopes and has been used to obtain metabolic and physiological information in vivo. However, the hyperpolarized state exponentially decays back to the thermal equilibrium state, depending on the spin-lattice relaxation time (T1). This signal decay has remained a major problem associated with this technology. Therefore, DNP NMR molecular probes useful for in vivo analysis have been limited to naturally occurring small molecules that inherently show long T1. While peptides are promising targets for DNP NMR studies, because of the limitation in T1, DNP NMR molecular probes applicable in vivo have been limited to amino acids or dipeptides. Herein we propose a 13C-labeling strategy to utilize the C-terminal [1-13C]Gly-d2 residue for realizing long T1 in peptides. Structure-based T1 relaxation analysis of amino acids and peptides revealed that (1) T1 does not decrease monotonically with increasing molecular weight and (2) T1 is not significantly affected by a side chain on the neighboring amino acid residue. These findings suggest that the C-terminal [1-13C]Gly-d2 residue affords sufficiently long T1 for biological uses, even in oligopeptides, and allowed us to develop 13C-b- casomorphin-5 (Tyr-Pro-Phe-Pro-[1-13C]Gly-d2, T1 = 24 ± 4 s at 3 T in H2O) and 13C-glutathione (g-Glu-Cys-[1-13C]Gly-d2, T1 = 58 ± 3 s at 3 T in H2O) as DNP NMR probes with long T1. We succeeded in in vivo detection of enzymatic conversions of these two probes. These results demonstrate the utility of our strategy and would contribute to further expansion of the substrate scope for DNP applications.

Dissolution-dynamic nuclear polarization (d-DNP) is a state-of-the-art technology that can dramatically enhance the detection sensitivity of nuclear magnetic resonance (NMR). DNP NMR has been applied to small molecules with stable isotopes and has been used to obtain metabolic and physiological information in vivo. However, the hyperpolarized state exponentially decays back to the thermal equilibrium state, depending on the spin-lattice relaxation time (T1). This signal decay has remained a major problem associated with this technology. Therefore, DNP NMR molecular probes useful for in vivo analysis have been limited to naturally occurring small molecules that inherently show long T1. While peptides are promising targets for DNP NMR studies, because of the limitation in T1, DNP NMR molecular probes applicable in vivo have been limited to amino acids or dipeptides. Herein we propose a 13C-labeling strategy to utilize the C-terminal [1-13C]Gly-d2 residue for realizing long T1 in peptides. Structure-based T1 relaxation analysis of amino acids and peptides revealed that (1) T1 does not decrease monotonically with increasing molecular weight and (2) T1 is not significantly affected by a side chain on the neighboring amino acid residue. These findings suggest that the C-terminal [1-13C]Gly-d2 residue affords sufficiently long T1 for biological uses, even in oligopeptides, and allowed us to develop 13C-β-casomorphin-5 (Tyr-Pro-Phe-Pro-[1-13C]Gly-d2, T1 = 24 ± 4 s at 3 T in H2O) and 13C-glutathione (γ-Glu-Cys-[1-13C]Gly-d2, T1 = 58 ± 3 s at 3 T in H2O) as DNP NMR probes with long T1. We succeeded in in vivo detection of enzymatic conversions of these two probes. These results demonstrate the utility of our strategy and would contribute to further expansion of the substrate scope for DNP applications.

Dynamic nuclear polarization (DNP), a technique to transfer spin polarization from electrons to nuclei, has been studied since its early discovery [1] and has opened the way for high sensitive nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging. In "conventional" DNP, which uses thermally-polarized unpaired electrons as the source of polarization, the DNP enhancement factor is limited to γe/γn, where γe(n) are the gyromagnetic ratios of the electron (nuclear) spins. In DNP using the paramagnetic electrons in thermal equilibrium, experiments need to be performed at cryogenic temperature to increase the electron-spin polarization as much as possible. Conversely, the spins of optically created electrons in the triplet state can have much higher polarization than their thermal equilibrium value, DNP using the triplet state, referred to as triplet DNP, can lead to nuclear hyperpolarization beyond the limit of the conventional DNP using thermal electron polarization. Furthermore, experiments can be carried out at room temperature, by irradiating the sample with laser light. Recently, DNP of an ensemble of 13C nuclear spins using negatively charged nitrogen-vacancy (NV−) color centers in a bulk diamond single crystal has been demonstrated at room temperature and the 13C polarization of 6 % has been achieved via the combination of the thermal mixing and the solid effect [2]. DNP using NV− in powdered microdiamonds has been reported by Ajoy et al., who took advantage of the reduced width of the anisotropic electron spin resonance powder pattern of the NV− centers at the magnetic field of ca. 30 mT [3]. However, triplet DNP is much older and achieved a 1H polarization of 34 % at room temperature and a magnetic field of 0.4 T using pentacene in p-terphenyl crystal [4]. Even though triplet DNP in both systems, NV- centers and pentacene, relies on the transfer of spin polarization from optically hyperpolarized triplet electrons to nuclei , there are important differences. While the NV- center has an electronic triplet ground state and is therefore paramagnetic, pentacene has an electronic singlet ground state, is diamagnetic, and only becomes paramagnetic through optical excitation into a triplet state. On the other hand, the zero-field splitting parameter D for pentacene is only half as large as in NV- centers, which is an advantage when disordered powder is used as a sample. In this work, we compare triplet-DNP of NV− centers in diamond and pentacene doped in [carboxyl-13C] benzoic acid (PBA) in polycrystalline samples at room temperature [5]. In the DNP experiments, the integrated solid effect (ISE) was used to transfer the polarization from electrons to nuclei. The ISE employs microwave irradiation and external magnetic-field sweep, so that the Hartmann–Hahn matching is implemented between the electron spins in the rotating frame and the nuclear spins in the laboratory frame. We study the behavior of the 13C polarization buildup in terms of the polarization efficiency of the transfer from the electron to nuclei, exchange rate, and the 13C spin diffusion. As a result, we obtained the 13C polarization of 0.01 % in the microdiamonds, and of 0.12 % in PBA at room temperature in a magnetic field of 0.4 T by using the integrated solid effect and the obtained exchange rate was 0.87 % for microdiamonds and 3.5 % for PBA. The 13C polarization enhancements for the diamond and the PBA were 300 and 3600 compared to the thermal NMR polarization. Besides the initial polarization transfer from the triplet electron to the nuclei, we also shed light on the process of nuclear spin-spin diffusion, which distributes the hyperpolarization within the sample., 2021 MRS Fall Meeting & Exhibit