Molecules for Dynamic Nuclear Polarization and NMR Structure Determination

This program is directed at generating methods to enhance the sensitivity of the NMR experiment by hyperpolarizing nuclei with high gyromagnetic ratio electrons and at generating probes that are capable of detecting biomedically significant molecules in complex environments. To accomplish our first goal, designs for new radicals will include systematic tuning of their magnetic transition energies (g-values), spectral linewidths and hyperfine couplings.

These advances can be applied to biomolecular structure determination, structural studies of solid drug formulations, and hyperpolarization agents for MRI imaging. Radicals specifically tuned to different mechanisms of hyperpolarization are required. For example, the Overhauser mechanism requires radicals with large hyperfine coupling to protons. Alternatively, the optimization of cross-effect hyperpolarization requires dipolar-coupled radicals that have magnetic transitions with energy differences that match the Larmor frequency of the nuclei of interest. These radicals will be most effective when they have narrow electron absorption line widths that provide the highest spectral overlap with the nuclear spin transitions.

This process requires the ability to systematically shift the g-values of radicals for proper pairing in biradicls, and we will make use of heavy atom effects to create a menu of radicals for this purpose. Biradicals can be assembled by coordination to metals, covalent bond construction, or using surfactant and nano-emulsions. The surfactant and nano-emulsion approaches have the advantage that the relative concentration of each radical can be variable for facile optimization. New methods for the selective detection of molecules in complex environments will be developed based upon 19F NMR fingerprinting. In this scheme, 19F group-containing probe molecules will be constructed which bind with molecules of interest to form complexes that are static on the NMR timescale. Initial results show this method to be extremely robust and compatible with complex environments. By creating probes with multiple 19F signals that vary independently with analyte binding we create a unique spectral 19F NMR fingerprint. Molecular constructs for the binding of biologically relevant heterocycles and carbohydrates will be produced that can be used to provide structural information that cannot be determined by conventional methods. It is anticipated that these species can be unambiguously detected in highly complex biological samples.