Trityl-Nitroxide Triradicals for Efficient High-Field Dynamic Nuclear Polarization

Dynamic nuclear polarization (DNP) overcomes the inherent low sensitivity of solid-state NMR, but its advancement hinges on rational design of polarizing agents (PAs). Recent conflicting reports on tethering or mixing one trityl with two nitroxides motivated us to develop two series of multiradical-based PAs, named as MRPols and cMRPols, which feature one trityl OX063 tethered to 1-3 TEMPO or spirocyclohexanoyl nitroxides (SCHN), respectively. DNP results on [¹³C-¹⁵N] proline at 18.8 T showed that the linkage of two nitroxides into PAs (i.e., MRPol-2 and cMRPol-2) substantially shortened the polarization buildup times (T_B) compared to the single nitroxide-containing biradicals MRPol-1 and cMRPol-1. Notably, the SCHN-based triradical cMRPol-2 with a short T_B (3.24 s) and excellent ε_on/off value (160) provided 65% and 35% improvements in DNP efficiency (ε_on/off/√T_B) over the biradical cMRPol-1 and the benchmark PA SNAPol-1, respectively. The correlation of the excellent DNP performance of cMRPol-2 with the physicochemical properties of the two radical moieties (i.e., electron relaxation and hydrophilicity), as well as trityl-nitroxide and nitroxide-nitroxide interactions, was investigated. Furthermore, the biological application potential of cMRPol-2 was also demonstrated by its high DNP efficiency for cell membrane fractions, remarkable bioreduction resistance, and negligible protein binding. To the best of our knowledge, this work represents the first successful optimization of the DNP efficiency of PAs by balancing T_B and ε_on/off at high magnetic fields (e.g., 18.8 T), and the hetero triradical design will be an applicable strategy for improving DNP efficiency.