Dynamic nuclear polarization of small labelled molecules in frozen water-alcohol solutions

In recent years methods of creating 'hyperpolarized' substances have gained considerable attention in biomedical magnetic resonance and dynamic nuclear polarization (DNP) is one of the most promising, especially for imaging applications. Here we present results of DNP studies on protons and (13)C nuclei in frozen solutions of sodium acetate and glycine, dissolved in water-ethanol and water-glycerol, doped with TEMPO free radicals and EHBA-Cr(V) complexes. Up to 14% (13)C polarization and close to 50% proton polarization were achieved at similar to 1.2K in a magnetic field of similar to 3.5 T under irradiation with similar to 97 GHz microwaves, which corresponds to an enhancement of more than 15 000 with respect to thermal equilibrium polarization in a 9.4 T magnet at room temperature. For all investigated samples the main DNP mechanism was found to be thermal mixing. The absolute polarization values achieved are mainly depending on the type of solvent, the water-alcohol ratio, its degree of deuteration and the concentration of paramagnetic centres. This allows application of the so-established sample preparation and DNP procedure to other molecules in future experiments. Two further examples of DNP of molecules in solution underline the general applicability of the method to a wide variety of organic compounds.