Bismuth compounds have been used in medicines as active pharmaceutical ingredients (APIs) to treat wounds and gastrointestinal disorders for over two hundred years. However, structural characterization of these materials is often challenging and there is a general lack of understanding how these materials function. Bismuth subgallate is the API of many over-the-counter drugs such as Devrom®, Stryphnasal® N and Sulbogin®. Structural investigation of this compound has been proven difficult due its tendency to form as nanocrystals too small for conventional structure investigation techniques such as single crystal X-ray diffraction. Electron diffraction can be used in such cases but the high energy electron beam tends to destroy the samples during the experiment especially if the materials are built of organic molecules. These are some of the reasons why the structure of bismuth subgallate was not assertively known to scientists previously.
In this study, A. Ken Inge and his team utilized a combination of fast data acquisition and sample cooling to avoid sample damage. They were able to obtain electron diffraction data of sufficient quality to finally determine the structure of bismuth subgallate. In contrast to previous suggestions, bismuth subgallate forms a coordination polymer with microporosity within the structure. The researchers found that the pores of bismuth subgallate are small enough to allow carbon dioxide gas to be adsorbed within, but leaving other gases such as nitrogen out. This opens up new potential applications of bismuth subgallate in areas such as carbon capture and storage (CCS).
“Porous coordination polymers have only been of significant research interest for approximately the past 25 years, so it was rather surprising to discover that a compound used in medicine for over a century belongs to this relatively younger class of porous materials”, says A. Ken Inge. “Knowing the structure of bismuth subgallate has allowed us to better understand some of its properties and has led us to discover new potential applications. The structures of many other APIs still remain unknown, but by determining their structures we hope to establish a better understanding of their properties and functions.”  
The results of this work are published in the journal Chemical Communications.
Wang, Y.; Takki, S.; Cheung, O.; Xu, H.; Wan, W.; Öhrström, L.; Inge, A.K. Elucidation of the elusive structure and formula of the active pharmaceutical ingredient bismuth subgallate by continuous rotation electron diffraction. Chem. Commun. (2017) DOI: 10.1039/c7cc03180g!divAbstract

Researcher A. Ken Inge
Department of Materials and Environmental Chemistry