Cryogenic and Dissolution DNP NMR on γ-Irradiated Organic Molecules

초록

Nuclear magnetic resonance (NMR) plays a central role in the elucidation of chemical structures but is often limited by low sensitivity. Dissolution dynamic nuclear polarization (dDNP) emerges as a transformative methodology for both solution-state NMR and metabolic NMR imaging, which could overcome this limitation. Typically, dDNP relies on combining a stable radical with the analyte within a uniform glass under cryogenic conditions. The electron polarization is then transferred through microwave irradiation to the nuclei. The present study explores the use of radicals introduced via gamma-irradiation, as bearers of the electron spins that will enhance H-1 or C-13 nuclides. H-1 solid-state NMR spectra of gamma-irradiated powders at 1-5 K revealed, upon microwave irradiation, signal enhancements that, in general, were higher than those achieved through conventional glass-based DNP. Transfer of these samples to a solution-state NMR spectrometer via a rapid dissolution driven by a superheated water provided significant enhancements of solution-state H-1 NMR signals. Enhancements of C-13 signals in the gamma-irradiated solids were more modest, as a combined consequence of a low radical concentration and of the dilute concentration of C-13 in the natural abundant samples examined. Nevertheless, ca. 700-800-fold enhancements in C-13 solution NMR spectra of certain sites recorded at 11.7 T could still be achieved. A total disappearance of the radicals upon performing a dDNP-like aqueous dissolution and a high stability of the samples were found. Overall, the study showcases the advantages and limitations of gamma-irradiated radicals as candidates for advancing spectroscopic dDNP-enhanced NMR.

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