Transition Metal Doping Reveals Link between Electron T Reduction and C Dynamic Nuclear Polarization Efficiency.

Optimal efficiency of dissolution dynamic nuclear polarization (DNP) is essential to provide the required high sensitivity enhancements for in vitro and in vivo hyperpolarized C nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI). At the nexus of the DNP process are the free electrons, which provide the high spin alignment that is transferred to the nuclear spins. Without changing DNP instrumental conditions, one way to improve C DNP efficiency is by adding trace amounts of paramagnetic additives such as lanthanide (e.g., Gd, Ho, Dy, Tb) complexes to the DNP sample, which has been observed to increase solid-state C DNP signals by 100-250%. Herein, we have investigated the effects of paramagnetic transition metal complex R-NOTA (R = Mn, Cu, Co) doping on the efficiency of C DNP using trityl OX063 as the polarizing agent. Our DNP results at 3.35 T and 1.2 K show that doping the C sample with 3 mM Mn-NOTA led to a substantial improvement of the solid-state C DNP signal by a factor of nearly 3. However, the other transition metal complexes Cu-NOTA and Co-NOTA complexes, despite their paramagnetic nature, had essentially no impact on solid-state C DNP enhancement. W-band electron paramagnetic resonance (EPR) measurements reveal that the trityl OX063 electron T was significantly reduced in Mn-doped samples but not in Cu- and Co-doped DNP samples. This work demonstrates, for the first time, that not all paramagnetic additives are beneficial to DNP. In particular, our work provides a direct evidence that electron T reduction of the polarizing agent by a paramagnetic additive is an essential requirement for the improvement seen in solid-state C DNP signal.