Drastic Increase in the Flexibility of Open Host Frameworks of a Steroidal Host Compound upon Shortening Its Spacer
✍ Scribed by Kazuaki Kato; Michihiro Sugahara; Norimitsu Tohnai; Kazuki Sada; Mikiji Miyata
- Book ID
- 102827091
- Publisher
- John Wiley and Sons
- Year
- 2004
- Tongue
- English
- Weight
- 656 KB
- Volume
- 2004
- Category
- Article
- ISSN
- 1434-193X
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✦ Synopsis
Abstract
Nordeoxycholic acid (NDCA), which has a shorter side chain than the steroidal host compound, deoxycholic acid (DCA), forms inclusion crystals with various organic substances at 1:1 or 2:1 host−guest ratios. X‐ray crystallographic studies reveal that four types of host frameworks (bilayer, monolayer, tape, and hexagonal) are generated by NDCA, in stark contrast to the robust and dominant bilayer frameworks formed from DCA, although they have the same functional groups capable of hydrogen bonding. The first three host frameworks are further divided into three, two, and four sub‐types by arrangements of the hydrogen‐bonded host columns, tapes, or layers, respectively. A total of nine types of open host frameworks are observed depending on the nature of the guest compounds. The sizes of the guest compounds induce isomerization of these host frameworks having molecular cavities, which is rationalized by the range of values of PC~cavity~, which are ratios of the volumes between the guest compound and the host cavities. Such a drastic increase of flexibility of the open host framework by the slight chemical modification from DCA to NDCA deserves attention with respect to crystal engineering. Unique 2~1~ helical assemblies of DCA are linked by the carboxyl group at the side chain to form a robust sheet‐like structural motif. In NDCA, on the other hand, the linkage between the helices becomes impossible because of the shortening of the side chain’s length, which results in destruction of the sheet‐like motif to yield renewed molecular voids among the unique 2~1~ helical assemblies. These remarkable differences in the flexibility of the open host frameworks provide us with a possible strategy for designing of new host compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)