Crystalline host-guest assemblies of steroidal and related molecules: Diversity, hierarchy, and supramolecular chirality

Steroidal bile acids and over 50 of their derivatives serve as the hosts of inclusion crystals. These hosts each exhibit their own characteristic inclusion behaviors, which have been explored through more than 300 crystallographic data. The molecules with three-axial chirality combine in asymmetric fashion to form diverse assemblies, which have supramolecular properties, such as recognition and dynamics, through cooperative weak interactions. From an overview of these results, an analogy emerged: the steroidal assemblies may have hierarchical structures, such as primary, secondary, tertiary, and hostguest assemblies, similar to proteins. Accordingly, the assemblies with dimensionality bear supramolecular chirality, such as three-axial, tilt, helical, bundle, and complementary chirality. Such a concept can be extended to other organic substances, such as alkaloids and organic salts. These results move in the direction of supramolecular crystal engineering.