This talk will describe recent results from my research group showing how dynamic nuclear polarization (DNP) can be applied to study (i) microcrystalline active pharmaceutical ingredients (APIs), and (ii) commercial toothpastes. It has previously been shown that DNP experiments can be performed on microcrystalline APIs by impregnating the powdered solids with solutions containing dissolved polarizing agent solutions. The radical solution coats the outside of the API particles, and 1H spin diffusion is able to relay the DNP enhanced 1H magnetization from the surface of the particle into the subsurface and core regions. Relayed DNP experiments have routinely provided order of magnitude gains in sensitivity. I will give a couple examples of the successful application of relayed DNP to determine molecular structures of solid APIs and to detect dilute impurities in model APIs. However, relayed DNP experiments may be ineffective if the particles have large diameters (> 20 micrometers) and if the 1H longitudinal relaxation times are on the order of seconds. To address these problems we will show that in gamma irradiation can be used to create significant concentrations of free radicals that are homogeneously distributed throughout the solid particles. In favorable cases, the radicals created by gamma irradiation can provide better DNP performance than molecular polarizing agents used in relayed DNP experiments. In the final part of the talk I will describe our recent collaboration with Colgate-Palmolive, Inc. where we have used DNP enhanced 119Sn solid-state NMR to determine the oxidation state of tin ions in commercial toothpaste products that contain 0.45wt% of SnF2 as an active ingredient. SnF2 is widely used in commercial toothpastes as a fluoride source and Sn2+ ions have been shown to have favorable antimicrobial properties. However, tin ions with +2 oxidation state are unstable and will readily oxidize to the +4 oxidation state in the presence of atmospheric oxygen. Therefore, commercial toothpastes are formulated with various additives to prevent tin oxidation. But, it is challenging to measure and quantify the oxidation state of Sn ions in the toothpastes with existing analytical techniques. We will show that DNP enables detection of 119Sn NMR spectra of commercial toothpastes. Measured 119Sn chemical shift anisotropies (CSA) can be used to differentiate 119Sn NMR signals from Sn2+ and Sn4+. Hence, DNP enhanced 119Sn NMR is shown enable quantification of the ratio of Sn2+ to Sn4+ ions within the commercial formulations.
