Fast <sup>19</sup>F Magic Angle Spinning NMR Crystallography for Structural Characterization of Fluorine-Containing Pharmaceutical Compounds
Changmiao Guo, Matthew Fritz, Jochem Struppe, Sebastian Wegner, John A. Stringer, Ivan V. Sergeyev, Caitlin M. Quinn, Angela M. Gronenborn, Tatyana Polenova
Abstract
Fluorine-containing compounds comprise 20 to 30 percent of all commercial drugs, and the proportion of fluorinated pharmaceuticals is rapidly growing. While magic angle spinning (MAS) NMR spectroscopy is a popular technique for analysis of solid pharmaceutical compounds, fluorine has been underutilized as a structural probe so far. Here, we report a fast (40–60 kHz) MAS 19F NMR approach for structural characterization of fluorine-containing crystalline pharmaceutical compounds at natural abundance, using the antimalarial fluorine-containing drug mefloquine as an example. We demonstrate the utility of 2D 19F–13C and 19F–19F dipolar-coupling-based correlation experiments for 19F and 13C resonance frequency assignment, which permit identification of crystallographically inequivalent sites. The efficiency of 19F–13C cross-polarization and the effect of 1H and 19F decoupling on spectral resolution and sensitivity were evaluated in a broad range of experimental conditions. We further demonstrate a protocol for measuring accurate interfluorine distances based on 1D DANTE-RFDR experiments combined with multispin numerical simulations.