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You cannot fight the pressure: Structural rearrangements of active pharmaceutical ingredients under magic angle spinning
Authors:Sebastian Scheidel  Laurina Östreicher  Isabelle Mark  Ann-Christin Pöppler
Affiliation:Institute of Organic Chemistry, University of Würzburg, Würzburg, Germany
Abstract:Although solid-state nuclear magnetic resonance (NMR) is a versatile analytical tool to study polymorphs and phase transitions of pharmaceutical molecules and products, this work summarizes examples of spontaneous and unexpected (and unwanted) structural rearrangements and phase transitions (amorphous-to-crystalline and crystalline-to-crystalline) under magic angle spinning (MAS) conditions, some of them clearly being due to the pressure experienced by the samples. It is widely known that such changes can often be detected by X-ray powder diffraction (XRPD); here, the capability of solid-state NMR experiments with a special focus on 1H-13C frequency-switched Lee–Goldburg heteronuclear correlation (FSLG HETCOR)/MAS NMR experiments to detect even subtle changes on a molecular level not observable by conventional 1D NMR experiments or XRPD is presented. Furthermore, it is shown that a polymorphic impurity combined with MAS can induce a crystalline-to-crystalline phase transition. This showcases that solid-state NMR is not always noninvasive and such changes upon MAS should be considered in particular when compounds are studied over longer time spans.
Keywords:19F  1H-13C HETCOR  API  MAS  solid-state NMR  structural changes  XRPD
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