BDPA radicals with improved persistence for dynamic nuclear polarization

Abstract

Dynamic nuclear polarization (DNP) has emerged as an important technique to enhance the signal intensity of nuclear magnetic resonance (NMR) spectroscopy by transferring spin polarization from unpaired electrons to nuclei of interest. Persistent organic radicals are a common source of unpaired electrons for DNP. Carbon-based radicals, such as trityl and 1,3-bisdiphenylene-2-phenylallyl (BDPA), are very promising polarizing agents at high magnetic fields. BDPA has the advantage that it is easier to synthesize than trityl, but its lack of solubility in aqueous media limits its application for structural studies of biomolecules by DNP NMR. This PhD thesis describes the design and synthesis of BDPA radicals that are soluble in aqueous media and have improved persistence. The first part revolves around the syntheses of phosphoester-derived BDPA radicals. Unexpectedly, these BDPA radicals, and BDPA radicals in general, were found to have limited persistence. Therefore, the effects of various factors on the persistence were investigated. Decomposition in the solid state was found to be due to oxidation while dimer formation was discovered to be a significant pathway of degradation in solution. In the second part, a new class of BDPA radicals is described, their synthesis and characterization. The radicals contain four alkyl/aryl-ammonium groups and are soluble in aqueous solution. More importantly, they are highly persistent in polar solvents due to reduced decomposition via dimerization. The third part describes the synthesis of three water-soluble BDPA-nitroxide biradicals. Different linkers were used to optimize the spin-spin interaction between the two radical centers. These biradicals are being investigated by our collaborators, as polarizing agents for DNP-NMR spectroscopy at high magnetic fields