Interactions of Cholesterol with Cyclodextrins in Aqueous Solution

The interaction of cholesterol with several cyclodextrins (CDs) was investigated in water using solubility method. It was found that heptakis (2, 6-di-O-methyl)-β-CD (DOM-β-CD) forms two types of soluble complex, with molar ratios of 1：1 and 1：2 (cholesterol：DOM-β-CD), and neither a soluble nor insoluble complex is formed between cholesterol and α-CD, β-CD, and γ-CD, although a minor soluble complex formation was observed between cholesterol and 2-hydroxylpropyl-β-CD. The thermodynamic parameters for 1：1 and 1：2 complex formation of cholesterol with DOM-β-CD obtained from the changes in K with temperature are as follows：ΔG°1：1 = -11.6 kJ/mol at 25 ℃ (K1：1 = 1.09 × 102 M-1) ; ΔH°1：1 = -3.38 kJ/mol ; TΔS°1：1 = 8.25 kJ/mol ; ΔG°1：2 = -27.1 kJ/mol at 25 ℃ (K1：2 = 5.68 × 104 M-1) ; ΔH°1：2 = -3.96 kJ/mol ; and TΔS°1：2 = 23.2 kJ/mol. The formation of the 1：2 complex occurred much more easily than that of the 1：1 complex. The driving force for 1：1 and 1：2 complex formation was considered to be mainly hydrophobic interaction. Also, based on the measurements of proton nuclear magnetic resonance spectra and studies with Corey-Pauling-Koltun atomic models, the probable structutures of the 1：2 complex were estimated.