Polarizing agents for high-frequency Dynamic Nuclear Polarization : development and applications

Abstract

Dynamic Nuclear Polarization (DNP) is utilized to enhance NMR signal intensities in systems including metals, ceramics, polymers and biological solids. The enhancement results from polarization transfers from unpaired electrons, usually carried by a polarizing agent such as TEMPO (2,2,6,6-tetramethylpiperydin-l-oxyl), to the target nuclei. In this thesis, polarizing agents were developed for efficient polarization mechanisms at 5 Tesla. DNP using biradicals yielded improvements of proton enhancement by about fourfold compared to an identical amount of monomeric TEMPO as used previously. The polarizing mechanism involved was the cross effect (CE), which relies on three-spin processes involving two electrons and one nucleus. Optimization of the CE requires the appropriate electron-electron interaction and the correct EPR frequency separation matching the nuclear Larmor frequency. Due to the relatively short inter-radical distance in interesting biradicals, multi-frequency EPR lineshape analyses are suitable to characterize, the distance and relative g-tensor orientations between electrons, revealing spectral parameters that explain the improvement of DNP efficiency. Alternatively, radical mixtures of TEMPO and Trityl, methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[ 1 ,2-d:4,5-d']bis( 1 ,3)dithiol-4-yl, improve the probability of the correct EPR frequency separation compared to TEMPO by itself.

(cont.) A 1:1 radical mixture produced a combined EPR spectrum with the required frequency separation and gave an improvement of the DNP enhancement by about threefold relative to TEMPO alone. In addition, a quantum mechanical theory of the CE was developed to provide sound explanations of the improved polarizing mechanism using the above polarizing agents. The soluble biradical-TOTAPOL, yielding proton enhancements of 160-290-was developed and applied to a wide range of aqueous systems from amyloid peptide nanocrystals to liquid samples. Polarizing nanocrystals relies on nuclear spin diffusion that transfers enhanced nuclear polarization from solvent into crystals that are isolated from paramagnetic species. This requires efficient polarizing agents that produce and maintain a high level of nuclear polarization surrounding the nanocrystals. In a second application, efficient polarizing agents that reduce the required radical concentration are important for temperature-jump DNP experiments involving a cycle of freezing, polarization, melting and observation of the liquid-state NMR spectrum of samples of interest. During melting, preservation of the nuclear polarization benefits from reduced paramagnetic relaxation at low radical concentrations.