Unveiling the Structure of Anhydrous Sodium Valproate with 3D Electron Diffraction and a Facile Sample Preparation Workflow.

Understanding the structure of an active pharmaceutical ingredient is essential for gaining insights into its physicochemical properties. Sodium valproate, one of the most effective antiepileptic drugs, was first approved for medical use in 1967. However, the structure of its anhydrous form has remained unresolved. This is because it was difficult to grow crystals of sufficient size for single-crystal X-ray diffraction (SCXRD). Although 3D electron diffraction (3D ED) can be used for studying crystals that are too small for SCXRD, the crystals of anhydrous sodium valproate are extremely sensitive to both humidity and electron beams. They degrade quickly both in air and under an electron beam at room temperature. In this study, we developed a glovebox-assisted cryo-transfer workflow for the preparation of EM grids in a protected atmosphere to overcome the current challenges for studying air- and beam-sensitive samples using 3D ED. Using this technique, we successfully determined the structure of anhydrous sodium valproate, revealing the formation of Na-valproate polyhedral chains. Our results provide a robust framework for the 3D ED analysis of air-sensitive crystals, greatly enhancing its utility across various scientific disciplines.