A 47-year-old misunderstanding: Indomethacin Polymorph δ revealed to be two plastically bendable crystal forms by 3D electron diffraction

17 August 2021, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


Indomethacin is a clinically classical non-steroidal anti-inflammatory drug that has been marketed since 1965. The third polymorph, Form δ, was discovered by both melt and solution crystallization in 1974. δ-indomethacin cannot be cultivated as a large single crystal suitable for X-ray crystallography and, therefore, its crystal structure has not yet been determined. Here, we report the structure elucidation of δ-indomethacin by 3D electron diffraction and reveal the truth that melt-crystallized and solution-crystallized δ-indomethacin are in fact two polymorphs with different crystal structures. Intriguingly, both structures display plastic flexibility based on a slippage mechanism, making indomethacin the first drug to have two plastic polymorphs. This discovery and correction of a 47-year-old misunderstanding signify that 3D electron diffraction has become a powerful tool for polymorphic structural studies.


electron diffraction
structure determination
melt crystallization

Supplementary materials

Supplementary Information
Methodology and supplementary figures and tables


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