Structural elucation of a novel impurity in rifaximin.

Rifaximin is a semi-synthetic rifamycin derivate. It acts on RNA polymerase of the subunit of DNA beta-2 in bacteria to restrain synthesis of RNA leading to antibacterial activity. The purpose of this study was to analyse the structure of the prepared impurity a. We found that the impurity a is the isomeride of impurity G defined in the European Pharmacopoeia.