Unified understanding of the breakdown of thermal mixing dynamic nuclear polarization: The role of temperature and radical concentration #DNPNMR

Published: Wednesday, 10 July 2024 - 10:00 -0400

Author: Thorsten Maly

Epasto, Ludovica M., Thibaud Maimbourg, Alberto Rosso, and Dennis Kurzbach. “Unified Understanding of the Breakdown of Thermal Mixing Dynamic Nuclear Polarization: The Role of Temperature and Radical Concentration.” Journal of Magnetic Resonance 362 (May 2024): 107670.


We reveal an interplay between temperature and radical concentration necessary to establish thermal mixing (TM) as an efficient dynamic nuclear polarization (DNP) mechanism. We conducted DNP experiments by hyperpolarizing widely used DNP samples, i.e., sodium pyruvate-1-13C in water/glycerol mixtures at varying nitroxide radical (TEMPOL) concentrations and microwave irradiation frequencies, measuring proton and carbon-13 spin temperatures. Using a cryogen consumption-free prototype-DNP apparatus, we could probe cryogenic temperatures between 1.5 and 6.5 K, i.e., below and above the boiling point of liquid helium. We identify two mechanisms for the breakdown of TM: (i) Anderson type of quantum localization for low radical concentration, or (ii) quantum Zeno localization occurring at high temperature. This observation allowed us to reconcile the recent diverging observations regarding the relevance of TM as a DNP mechanism by proposing a unifying picture and, consequently, to find a trade-off between radical concentration and electron relaxation times, which offers a pathway to improve experimental DNP performance based on TM.