Direct evidence for symmetry control in cyclodextrin-water interactions

Abstract

Abstract The cyclodextrins, cyclic oligosaccharides possessing generally 6, 7, or 8 α, 1 –4 linked glucopyranose units, are widely used for the solubilisation and transport of organic molecules in aqueous media.3 Their solubilities 145 gL−1, α-Cd; 20 gL−1, β-CD; 220 gL−1, γ-CD, are considerably lower than those of simple saccharides and in particular that of β-CD, the most widely available compound, is anomalously low. We have previously shown that the solubilities arise from symmetry determined interactions of the CDs with the dynamic hexagonal structure of water. In the case of the unmodified cyclodextrins all three compounds exist in solution as large aggregates, thus the solubility of these systems is controlled by the interactions between these hydrogen bonded aggregates and the hydrogen bonding networks present in water. In the case of α- or γ-CD there are favorable overlaps with the hexagonal water structure, however for the seven-fold symmetry of β-CD no such favorable interactions can occur betwe...