Direct dynamic nuclear polarization of ¹⁵N and ¹³C spins at 14.1 T using a trityl radical and magic angle spinning

We investigate solid-state dynamic nuclear polarization of ¹³C and ¹⁵N nuclei using monoradical trityl OX063 as a polarizing agent in a magnetic field of 14.1 T with magic angle spinning at ∼100 K. We monitored the field dependence of direct ¹³C and ¹⁵N polarization for frozen [¹³C, ¹⁵N] urea and achieved maximum absolute enhancement factors of 240 and 470, respectively. The field profiles are consistent with polarization of ¹⁵N spins via either the solid effect or the cross effect, and polarization of ¹³C spins via a combination of cross effect and solid effect. For microcrystalline, ¹⁵N-enriched tryptophan synthase sample containing trityl radical, a 1500-fold increase in ¹⁵N signal was observed under microwave irradiation. These results show the promise of trityl radicals and their derivatives for direct polarization of low gamma, spin-½ nuclei at high magnetic fields and suggest a novel approach for selectively polarizing specific moieties or for polarizing systems which have low levels of protonation.