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

Abstract

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 1 C 4 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.