Structure of the Tannin Geraniin Based on Conventional X-ray Data at 295 K and on Synchrotron Data at 293 and 120 K

Geraniin, C₄₁H₂₈O₂₇.7H₂O, is the main tannin from Geranium thunbergii Sieb. et Zucc., which is one of the most popular folk medicines and also an official antidiarrheic drug in Japan. Conventional X-ray diffractometer data at 295 K and synchrotron data at 293 and 120 K (at EMBL/DESY Hamburg with a MARRESEARCH imaging plate) were measured. The structure could not be determined with any of the currently distributed direct-methods programs, but was easily solved with DIRDIF using several conformers of the hexahydroxydiphenoyl group as input fragments into a vector-search procedure. A molecular structure was established where a cyclohexenetrione moiety attached to O4 of a central glucose unit was in a hydrated six-membered hemiacetal ring structure, as expected from the NMR study of crystalline geraniin. Owing to the high oligocyclic substitution the glucopyranosyl ring is in the unusual ¹C₄ conformation. Refinement of the low-temperature synchrotron data allowed identification of almost all the H atoms, even within the seven water molecules, so that a rather complex system of approximately 30 different hydrogen bonds can be studied in some detail.

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