Anisotropic Amorphous X-ray Diffraction Attributed to the Orientation of Cyclodextrin.

The beauty of cyclic molecules is reflected in their host-guest complexation reactions, as well as their unique X-ray diffraction patterns. Cyclodextrins, the longest known host molecules with rigid ring structures, show anisotropic X-ray diffraction characteristic of their single-molecule structure, rather than their intermolecular relationships. Amorphous derivatives of α-cyclodextrin exhibit broad and strong halo diffractions in the solid, melted, and dilute solution states. The diffraction angle corresponds to the intramolecular distance between neighboring glycosidic oxygen atoms located at the vertices of a regular hexagonal array. Because the hexagon is parallel to the aperture plane of the rigid cyclic molecule, the diffraction appears only in the direction parallel to this plane. The anisotropy was confirmed by stretching an amorphous thermoplastic polymer threaded through the inclusion cavities of a sequence of cyclodextrins. The resultant unique anisotropic X-ray diffraction suggests the possible use of rigid cyclic molecules as molecular orientation probes.