Electron diffraction determines molecular absolute configuration in a pharmaceutical nanocrystal

Abstract

Dynamical refinement spots a difference For chiral molecules used in drugs, one isomer can have beneficial bioactivity, whereas the others are useless or even harmful. Determining the absolute configuration of molecules with chiral centers is often achieved through x-ray crystallography, but this requires relatively large crystals and high-quality data. Brázda et al. used electron diffraction to determine the absolute structure of an extremely radiation-sensitive crystal with micrometer dimensions (see the Perspective by Xu and Zou). In a strategy analogous to serial crystallography methods, many frames were combined to generate a complete dataset. Refinement incorporating dynamical effects differentiated the correct and incorrect molecular configuration. Science, this issue p. 667; see also p. 632 Absolute stereochemistry in an organic crystal can be determined by electron diffraction despite limited crystal stability. Determination of the absolute configuration of organic molecules is essential in drug development and the subsequent approval process. We show that this determination is possible through electron diffraction using nanocrystalline material. Ab initio structure determination by electron diffraction has so far been limited to compounds that maintain their crystallinity after a dose of one electron per square angstrom or more. We present a complete structure analysis of a pharmaceutical cocrystal of sofosbuvir and l-proline, which is about one order of magnitude less stable. Data collection on multiple positions of a crystal and an advanced-intensity extraction procedure enabled us to solve the structure ab initio. We further show that dynamical diffraction effects are strong enough to permit unambiguous determination of the absolute structure of material composed of light scatterers.