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

We investigate solid-state dynamic nuclear polarization of ¹³ C and ¹⁵ N 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 ¹³ C and ¹⁵ N polarization for frozen [ ¹³ C, ¹⁵ N] urea and achieved maximum absolute enhancement factors of 240 and 470, respectively. The field profiles are consistent with polarization of ¹⁵ N spins via either the solid effect or the cross effect, and polarization of ¹³ C spins via a combination of cross effect and solid effect. For microcrystalline, ¹⁵ N-enriched tryptophan synthase sample containing trityl radical, a 1500-fold increase in ¹⁵ N 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.
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