Microtesla Signal Amplification by Reversible Exchange Enables Simultaneous over 5% Polarization of ⁷⁷Se and ¹⁵N at Natural Abundance in a Selenium–Nitrogen Heterocycle

Signal amplification by reversible exchange (SABRE) increases NMR sensitivity through polarization transfer from parahydrogen, offering a cost-effective alternative to dissolution dynamic nuclear polarization (<i>d</i>-DNP). For the first time, this study demonstrates nuclear spin hyperpolarization of ⁷⁷Se nuclei by SABRE, achieved for a biologically important selenium-nitrogen heterocycle, 3-methyl-[1,2,4]-selenadiazolo-[4,5-<i>a</i>]-pyridine-4-ium chloride (SDAP), via polarization transfer at mictrotesla magnetic fields. Under microtesla SABRE conditions, polarization transfer on both ⁷⁷Se and ¹⁵N nuclei is achieved simultaneously, leading to the formation of multinuclear hyperpolarization within the time scale of several seconds. The resulting NMR signal enhancements achieved are 12,000-fold and 17,000-fold at a magnetic field of 9.4 T for ⁷⁷Se and ¹⁵N, respectively, corresponding to over 5% polarization for each nucleus. This enables fast single-scan detection at micromolar concentrations by heteronuclear NMR. Our work overcomes the sensitivity limitations of ¹⁵N and ⁷⁷Se NMR spectroscopy and provides new opportunities for studying selenium-containing compounds in organic synthesis and biological systems.