Pseudopolymorphism in Tetradeca-2,6-<i>O</i>-methyl-β-cyclodextrin: The Crystal Structures for Two New HydratesConformational Variability in the Alkylated β-Cyclodextrin Molecule

Tetradeca-2,6-O-methyl-β-cyclodextrin, per-2,6-OMe-β-CD, is an unusual β-cyclodextrin, β-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-β-CD crystallized at different temperatures: per-2,6-OMe-β-CD·1.08H2O crystallized at 50 °C and per-2,6-OMe-β-CD·14.7H2O crystallized at 4 °C. The crystal structure for per-2,6-OMe-β-CD·1.08H2O reveals conformational flexibility in the methylated β-CD molecule; two adjacent glucose residues are disordered as a result of their adopting different tilts with respect to the O4 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.