Structures and Inclusion Phenomena of Modified Cyclodextrins

Abstract

Crystal structures of modified cyclodextrins and inclusion complexes have been investigated by the X-ray method. The round macrocyclic conformation of native cyclodextrins is maintained by intramolecular hydrogen bonds formed between 2-OH and 3-OH of an adjacent glucose unit. The cylindrical cavity of cyclodextrin is defined by the diameter and depth and accommodates guest molecules having suitable size and shape. The macrocyclic conformation is not significantly affected by the modification of a primary hydroxy group. In the crystal, the substituent group is found to be inserted into the adjacent cyclodextrin ring. Even when all 2-OH and 6-OH hydroxy groups are methylated, cyclodextrins still maintain the round structure because of the formation of intramolecular 3-OH…O-2hydrogen bonds. However, the cavity is extended by the methyl groups attached at both edges of the cylindrical molecule. As a result, the disposition of the guest molecule in the host cavity is not the same as that in the corresponding native cyclodextrin complexes. In contrast, when all the hydroxy groups are methylated, the macrocyclic ring is free from the restraint imposed by the intramolecular hydrogen bonds, and therefore becomes more flexible. The macrocyclic ring changes its conformation to obtain better fit to the shape of the guest molecule. The effect is prominent in the recognition of chiral molec ules in the crystalline complex.