Hyperpolarization of nitrogen-15 nuclei by cross polarization and dissolution dynamic nuclear polarization

Dynamic Nuclear Polarization (DNP) is often achieved by the direct transfer of polarization from electrons to nuclei such as ¹³C, induced by microwave saturation of the wings of narrow EPR lines of radicals like trityl. In the indirect approach on the other hand, DNP is used to transfer the polarization from the electrons of radicals such as nitroxides that have broad EPR lines to nuclear spins I = ¹H, followed by cross-polarization (CP) from I = ¹H to S = ¹³C or other nuclei with low gyromagnetic ratios. This approach is particularly attractive for S = ¹⁵N, since direct DNP yields modest polarizations P(¹⁵N) < 4% with build-up times that can be as long as τ_DNP(¹⁵N) > 2 h. In this paper, we show that CP from ¹H to ¹⁵N at 1.2 K can yield P(¹⁵N) = 25% with τ_CP-DNP(¹⁵N) = 10-15 min. After rapid dissolution and transfer to a solution-state NMR spectrometer, a polarization P(¹⁵N) = 20% was observed at 300 K. The longitudinal relaxation times in solution can be as long as T₁(¹⁵N) > 800 s in favorable cases.