Photoinduced Nonpersistent Radicals as Polarizing Agents for X-Nuclei Dissolution Dynamic Nuclear Polarization

Hyperpolarization via dissolution dynamic nuclear polarization (DNP) is a versatile method to dramatically enhance the liquid-state NMR signal of X-nuclei and can be used for performing metabolic and molecular imaging. It was recently demonstrated that instead of incorporating persistent radicals as source of unpaired electron spins, required for DNP, nonpersistent radicals can be photoinduced in frozen beads of neat pyruvic acid (PA), the most common substrate for metabolic imaging. In the present work, it is shown that the same radicals can be created in frozen solutions containing a fraction of PA in addition to C-13- or Li-6-labeled salts or Xe-129 nuclei. The use of these nonpersistent radicals prevents the loss of a substantial part of the polarization during the transfer of hyperpolarized solutions into iron-shielded high-field MRI scanners. It is also demonstrated that UV-irradiated d(4)-PA yields nonpersistent radicals exhibiting similarities with the most efficient and widely used persistent trityl radicals.