Pseudopolymorphism in tetradeca-2,6-O-methyl-beta-cyclodextrin: the crystal structures for two new hydrates-conformational variability in the alkylated beta-cyclodextrin molecule.

Tetradeca-2,6-O-methyl-beta-cyclodextrin, per-2,6-OMe-beta-CD, is an unusual beta-cyclodextrin, beta-CD, derivative because it has a negative thermal solubility coefficient in aqueous solution. This report describes two new crystal structures for hydrates of per-2,6-OMe-beta-CD crystallized at different temperatures: per-2,6-OMe-beta-CD.1.08H(2)O crystallized at 50 degrees C and per-2,6-OMe-beta-CD.14.7H(2)O crystallized at 4 degrees C. The crystal structure for per-2,6-OMe-beta-CD.1.08H(2)O reveals conformational flexibility in the methylated beta-CD molecule; two adjacent glucose residues are disordered as a result of their adopting different tilts with respect to the O(4) plane characterizing the molecule. When one conformation is present there is a water molecule in the lattice (0.5 population parameter); said water is displaced by the methyl of a methoxy substituent when the other conformer is present.