Abstract
The effect of solution pH on the formation of an inclusion complex between (−)-epigallocatechin gallate (EGCg: pKa = 7.5) and β-cyclodextrin (β-CD) was investigated by isothermal titration calorimetry and 1H-NMR spectroscopy. The formation of an inclusion complex (EGCg-β-CD) depended on the solution pH; two different types of inclusion complexes were formed at 1:1 molar ratio in acid/neutral solutions, and only one type of complex was formed in the basic solution. The first type of EGCg-β-CD with larger association constant was formed independently of pH, with the AC-ring of EGCg being deeply inserted into the cavity of β-CD and the B-ring existing near the secondary hydroxyl group of β-CD. On the other hand, the formation of the second type depended on the solution pH. The B′-ring of EGCg was included in the case of acid and neutral solutions, but the formation of an inclusion complex in the basic solution was difficult due to the ionization of the 4″-OH on the B′-ring. 1H-NMR spectroscopy supported these results. These results suggested that when determining the structures of EGCg-β-CD in an aqueous solution, it is necessary to consider unionized and ionized forms of EGCg.
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