Sampling rifamycin conformational variety by cruising through crystal forms: implications for polymorph screening and for biological models

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Abstract
In this work, we report on a non-solvate and on a new solvate forms of rifamycin S, demonstrating that another stable conformation of the active molecule exists and it may be isolated by planning the crystallization conditions. In fact, in these structures rifamycin S has a previously uncharacterised, bent conformation: implications of these findings on the models used to explain rifamycin activity are discussed. Polymorphism has also been reported for the rifamycin-based antibiotic rifaximin, but without single-crystal X-ray diffraction characterization. Here the structure of a tetrahydrate form of rifaximin is discussed; confirming the flexibility of the drug, the two independent molecules in the unit cell are an example of conformational isomorphism.

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