Solubility of cyclomaltooligosaccharides (cyclodextrins) in H2O and D2O: a comparative study.

Cyclomaltooligosaccharides (cyclodextrins, CDs) are cyclic oligomers having six, seven, or eight units of alpha-D-glucose, named as cyclomaltohexaose (alpha-CD), cyclomaltoheptaose (beta-CD) and cyclomaltooctaose (gamma-CD), respectively. The molecule of CD has a cavity in which the interior is hydrophobic relative to its outer surface. The solubility of cyclodextrins in water is unusual, as an irregular trend is observed in the series of the cyclic oligomers of glucose. beta-CD is at least nine times less soluble than the others CDs. This intriguing behavior has been investigated, and some interesting explanations in terms of the effect caused by CD on the water lattice structure have been proposed. In this work a comparative study on the solubility of alpha, beta, and gamma-cyclodextrins was carried out in H2O and D2O and reveals a much lower solubility of the three CDs in D2O. The solid-phase structure of the CDs in equilibrium with the solution is quite similar with both solvents. The results are discussed in terms of the CD molecular structure and the differences in the hydrogen bonds formed between H2O and D2O.