19F Hyperpolarization of 15N-3-19F-Pyridine Via Signal Amplification by Reversible Exchange.

We report synthesis of 15N-3-19F-pyridine via Zincke salt formation with the overall 35% yield and 84% 15N 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 19F nucleus in milli-Tesla and micro-Tesla magnetic field regimes in 15N-3-19F-pyridine and 14N-3-19F-pyridine. We found the mismatch between 15N and 19F magnetic field hyperpolarization profiles in the micro-Tesla regime indicating that the spontaneous hyperpolarization process likely happens directly from parahydrogen-derived hydride protons to 19F nucleus without spin-relaying via 15N site. In case of SABRE magnetic field regime (milli-Tesla magnetic field range), we found that magnetic field profiles for 1H and 19F hyperpolarization are very similar, and 19F polarization levels are significantly lower than 1H SABRE polarization levels and lower than 19F 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 19F 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.