Metastable crystals of β -cyclodextrin complexes and the membrane diffusion model

WE have previously suggested1 that dimers of β-cyclodextrin (β-CD) may mimic membrane diffusion transport phenomena. This proposal was based on the orientation of guest molecules in the crystalline 3:2 p-iodophenol complex with β-CD, with the assumption that complex formation with guest molecules capable of hydrogen bond formation occurs through an initial interaction with one of the hydroxyl groups of β-CD. The 7:14 distribution of hydroxyl groups on the primary and secondary sides of the β-CD torus, respectively, was interpreted as favouring formation of a complex with the hydrogen bonding moiety on the secondary side of the torus. The fact that the guest molecules in the above complex were orientated in the opposite direction was taken as an indication that exchange of guest molecules between a dimeric complex and the solution had occurred. We report here that β-CD inclusion complexes with various guest molecules produce metastable crystals (designated CII) when growth is achieved by slow cooling, and we have determined the crystal structure of an n-propanol complex of this phase. We find that comparison of the structure of this metastable phase and that of the thermodynamically more stable phase1, CI, provides information relevant to the question of the presence of β-CD dimers in solution; this information is relevant to the suggestion that such dimers mimic bilayer membrane diffusion transport.