On the polymorphism of a sapogenin monohydrate induced by different rotations of water molecules

The structure of 1β,3β,11α-trihydroxyspirosta-5,25(27)-diene (C₂₇H₄₀O₅; a steroidal sapogenin) isolated from Helleborus serbicus Adam 1906 (Ranunculaceae) and crystallized from absolute ethanol as a monohydrate (melting point 519–522 K) had been characterized by two symmetry-independent binary (steroid–water) layers, cross-linked by hydrogen bonds [Kálmán et al. (1985). Acta Cryst. C41, 1645–1647]. Recently, a novel monohydrate was crystallized again from absolute ethanol (source: Helleborus multifidus subspecies serbicus) with a somewhat higher melting point of 525–526 K. X-ray analysis of these crystals [Argay et al. (1998). Acta Chim. Hung. 135, 449–456] revealed a novel polymorph (hereinafter denoted polymorph B), which is also built up by two binary layers of C₂₇H₄₀O₅ and H₂O, but in which the relative position of these layers differs from that found in the first modification (polymorph A). Comparing the two polymorphs, layers of one type are found to be similar, displaying identical hydrogen bonding, whereas layers of the second type differ with respect to the orientations adopted by the water molecules; these orientations also differ from those in the layers of the first type. Consequently, by these water rotations, hydrogen bonds, at least partly, are reversed. This leads to two different close packings: in form A four consecutive layers are cross-linked by two homomolecular (hydroxyl...hydroxyl and water...water) hydrogen-bond pairs, while in B there are only heteromolecular hydroxyl...water bonds. These hydrogen-bond dissimilarities together with the differences in the weak CH...X etc. interactions explain the greater stability of the higher melting-point form B.