The bismuth(III) salt of gallic acid (free acid pictured right), or bismuth subgallate, has been used as a pharmaceutically active compound for over three centuries. It has been used to treat wounds, stomach and intestinal problems, and syphilis. Today, it shows promise for the treatment of resistant Helicobacter pylori infections, which can cause ulcers and stomach cancer.

However, in all this time, the exact structure of the compound has remained elusive because it forms nanocrystals that are not well suited for single-crystal X-ray crystallography. The formation of complex species by oligomerization also makes X-ray powder diffraction difficult.

A. Ken Inge, Stockholm University, Sweden, and colleagues have used electron diffraction to finally find the structure of bismuth subgallate. Usually, the high-energy electron beam needed for this approach destroys such samples quickly. However, the team combined the continuous rotation of the sample with cooling to 173 K to find the crystal structure.

The compound was found to form a coordination polymer in which all three phenolate groups of the gallic acid coordinate to bismuth, and the carboxylic acid groups remain free. The center phenolate forms a bridge between to Bi³⁺-ions, which results in the polymeric structure. This result stands in contrast to the molecular formulas that had been suggested until now. According to the researchers, the analysis method used could also be applicable to other electron-beam-sensitive crystals.

• Elucidation of the Elusive Structure and Formula of the Active Pharmaceutical Ingredient Bismuth Subgallate by Continuous Rotation Electron Diffraction,
  Yunchen Wang, Sofia Takki, Ocean Cheung, Hongyi Xu, Wei Wan, Lars Öhrström, A. Ken Inge,
  Chem. Commun. 2017.
  DOI: 10.1039/c7cc03180g