Sample Ripening through Nanophase Separation Influences the Performance of Dynamic Nuclear Polarization

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Sample Ripening through Nanophase Separation Influences the Performance of Dynamic Nuclear PolarizationAngew. Chem. Int. Ed. 2018, 57, 5171 –5175

 

Mixtures of water and organic solvents may feature unusual physicochemical events such as spontaneous nanophase separations (NPS) that occur in solutions despite their homogenous appearance on a macroscopic scale. Such NPS describe rather counterintuitive phenomena, in which the two components of a binary solvent mixture spontaneously form co-existing metastable transient nanophases. The intriguing nature of NPS recently fostered much academic interest. Herein, we show how such phenomena can influence the performance of dynamic nuclear polarization (DNP), a method to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy by boosting the nuclear spin polarization P, that is, the relative difference between the populations of the excited and ground spin states. DNP has recently witnessed significant developments, allowing everhigher levels of polarization to be achieved as a result of 1) instrumental developments and 2) novel polarization agents (PAs), such as bi-nitroxides or tri-aryl-methyl radicals, also known as “Trityls”. These developments provide access to systems that could not be studied before by NMR spectroscopy because of poor sensitivity.

 

 

 

 

Résumé: 

Angew. Chem. Int. Ed. 2018, 57, 5171 –5175

 

Mixtures of water and glycerol provide popular matrices for low-temperature spectroscopy of vitrified samples. However, they involve counterintuitive physicochemical properties, such as spontaneous nanoscopic phase separations (NPS) in solutions that appear macroscopically homogeneous. We demonstrate that such phenomena can substantially influence the efficiency of dynamic nuclear polarization (DNP) by factors up to 20% by causing fluctuations in local concentrations of polarization agents (radicals). Thus, a spontaneous NPS of water/glycerol mixtures that takes place on time scales on the order of 30–60 min results in a confinement of polarization agents in nanoscopic water-rich vesicles, which in return affects the DNP. Such effects were found for three common polarization agents, TEMPOL, AMUPol and Trityl.

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Sample Ripening through Nanophase Separation Influences the Performance of Dynamic Nuclear Polarization

 

Emmanuelle M. M. Weber, Giuseppe Sicoli, Hervé Vezin, Ghislaine Frébourg, Daniel Abergel, Geoffrey Bodenhausen, and Dennis Kurzbach

 

Angew. Chem. Int. Ed. 2018, 57, 5171 –5175

 

DOI : 10.1002/anie.201800493