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

The structure of 1 beta,3 beta,11 alpha-trihydroxyspirosta-5,25 (27)diene ( C27H40O5; a steroidal sapogenin) isolated from Helleborus serbicus Adam 190 6 (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 [Kalman er n i. (1985). Acta Cryst. C41, 1645-1647]. Recently, a novel monohydrate was c rystallized again from absolute ethanol (source: Helleborus multifidus subs pecies serbicus) with a somewhat higher melting point of 525-526 K. X-ray a nalysis of these crystals [Argay ct nl. (1998). Acm Chim. Hung. 135, 449-45 6] revealed a novel polymorph thereinafter denoted polymorph B), which is a lso built up by two binary layers of C27H40O5 and H2O, but in which the rel ative position of these layers differs from that found in the first modific ation (polymorph A). Comparing the two polymorphs, layers of one type are f ound to be similar, displaying identical hydrogen bonding, whereas layers o f the second type differ with respect to the orientations adopted by the wa ter molecules; these orientations also differ from those in the layers of t he first type. Consequently, by these water rotations, hydrogen bonds, at l east partly, are reversed. This leads to two different close packings: in f orm A four consecutive layers are cross-linked by two homomolecular (hydrox yl ... hydroxyl and water water) hydrogen-bond pairs, while in B there are only heteromolecular hydroxyl water bonds. These hydrogen-bond dissimilarit ies together with the differences in the weak CH ... X etc. interactions ex plain the greater stability of the higher melting-point form B.