Polarizing Agents: Evolution and Outlook in Free Radical Development for DNP

In this article, we describe an in-depth overview of the development of paramagnetic polarizing agents for dynamic nuclear polarization (DNP)enhanced solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR). In DNP experiments, the large polarization of unpaired electrons is transferred to surrounding nuclei, which provides a maximum theoretical DNP enhancement of 660 for 1H NMR. The article includes a description of the different polarizing mechanisms and outlines key structural and magnetic parameters that contributed to the rational design of improved polarizing sources. The application of (di)nitroxides, heterobiradicals, narrow-line radicals, paramagnetic metal ions as well as site-specific polarizing agents is discussed. With the best polarizing agents, ssNMR MAS NMR/DNP enhances sensitivity by a factor of up to 200, providing decreased experiment time by five orders of magnitude and opening new avenues for NMR.