X-Ray Structure of i-Cyclodextrin

Abstract

Crystal structure of i-cyclodextrin consisting of 14 glucose units were determined by the X-ray method. The crystal of 9 hydrate belongs to a monoclinic space group C2 with Z=2 and cell dimensions are a = 36.536(9), b = 10.085(4), c = 20.944(2) Å, and β = 114.97(2)°. The molecule with crystallographic twofold symmetry is in a saddle-like shape where the elliptical macrocyclic ring is bent by 75.2°. The seven glucose units in the asymmetric unit are arranged in a left-handed helical fashion to form a U-shape. Distances from the center of the molecule to each O4 atom are in the range from 7.6 to 9.7 Å indicating the presence of a large cavity in the molecule. Adjacent two glucose units in the asymmetric portion is in cis arrangement where the O2H hydroxyl group forms a hydrogen bond with O3H of next glucose unit. In contrast, symmetry-related glucose units are connected with trans arrangement and the O3H hydroxyl group is hydrogen-bonded to O6H of the adjacent glucose unit. The latter conformation of the glycosidic linkage is the first observation in cyclodextrins and α-1,4-linked glucose oligomers. The asymmetric portion of the molecule is arranged in a zigzag mode along the twofold screw axis and symmetry-related molecules are partly interdigitated. The vacant space in the molecule forms a continuous channel along the twofold axis, suggesting the possibility of the formation of crystalline inclusion complexes.