DNP requires a paramagnetic polarizing agent. This is great for DNP but not so great for the NMR experiment, since the paramagnetic species often causes line broadening due to increase nuclear relaxation. To decrease the unwanted relaxation enhancement some researchers suggested to remove (e.g. filter out) the paramagnetic species after the dissolution step.
The article describes an elegant method, using UV generated radicals for polarization at low temperatures, which recombine once the sample is heated up during the dissolution process, effectively removing the paramagnetic enhanced relaxation process.
Capozzi, Andrea, Saket Patel, Christine Pepke Gunnarsson, Irene Marco-Rius, Arnaud Comment, Magnus Karlsson, Mathilde H. Lerche, Olivier Ouari, and Jan Henrik Ardenkjaer-Larsen. “Efficient Hyperpolarization of U- 13C-Glucose Using Narrow-Line UV-Generated Labile Free Radicals.” Angewandte Chemie, December 20, 2018.
https://doi.org/10.1002/ange.201810522.
Free radicals generated via irradiation with UV-light of a frozen solution containing a fraction of pyruvic acid (PA), have demonstrated their dissolution Dynamic Nuclear Polarization (dDNP) potential providing up to 30% [1-13C]PA liquid-state polarization. Moreover, their labile nature has proven to pave a way to nuclear polarization storage and transport. Herein, differently from the case of PA, we tackled the issue of providing dDNP UV-radical precursors, trimethylpyruvic acid (TriPA) and its methyl-deuterated form d9-TriPA, not involved in any metabolic pathway. The 13C dDNP performance was evaluated for hyperpolarization of [U-13C6,1,2,3,4,5,6,6-d7]-Dglucose. The generated UV-radical proved to be a versatile and highly efficient polarizing agent providing, after dissolution and transfer (10 s), a 13C liquid-state polarization up to 32%.