POLYMORPHISM INDUCED BY THE REARRANGEMENT OF HYDROGEN ATOMS

L. Fábián, Gy. Argay, A. Kálmán

Institute of Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, POB 17, 1525 Budapest, Hungary; E-mail: lfabian@cric.chemres.hu

Keywords: Polymorphism, Hydrogen-Bonds

The steroidal sapogenin 1b,3b,11a-trihydroxy-spirosta-5,25(27)-diene was isolated from Helleborus serbicus Adam 1906 and the structure of its form A was reported in 1985 [1]. Recently, a chromatographic fraction was isolated from Helleborus multifidus Vis. subsp. serbicus (Adamovic) and suspected to contain the same compound. However, melting points did not agree, so X-ray structure analysis was performed and a new form (B) was identified [2].

Both polymorphs crystallized from ethanol in space group P 2₁ with two steroid and two water molecules in the asymmetric unit. The two forms were compared on the basis of their respective hydrogen bonding networks. The symmetry independent molecules form discrete layers in both modifications. One of the layers is common to both structures while the others are different.

The relative arrangement of the molecules is approximately the same in all layers; only the positions of some water and hydroxyl hydrogen atoms are altered. The two unique layers can be deduced from the common layer by altering the position of a single hydrogen atom. All the other rearrangements are inevitable consequences of this change. According to these intra-layer transformations the connection between the adjacent layers is also altered. Thus a minor change in the layer structure results in a major change in the crystal structure.

A possible explanation of the polymorphism can be the presence of a dihydroxy derivative that was also isolated from Helleborus serbicus Adam 1906 [3]. The assumed appearance of this derivative in a growing plane is consistent with the formation of the unique layer of the lower melting form A. Co-crystallization experiments are planned to judge this hypothesis.

(Sponsor: Hungarian Research Fund Grant No. OTKA T023212)

1. A. Kálmán, Gy. Argay, B. Ribár, D. Zivanov-Stakic, S. Vladimirov: Acta Cryst., C41 (1985), 1645-1647.
2. Gy. Argay, A. Kálmán, S. Vladimirov, D. Ivanov-Stakic, B. Ribár: Acta Chim. Hung., submitted
3. B. Ribár, A. Kapor, S. Vladimirov, D. Zivanov-Stakic, Gy. Argay, A. Kálmán: Acta Cryst., C42 (1986), 1780-1782.