Studies of cyclodextrin inclusion complexes. I. Complex between cyclodextrins and bencyclane in aqueous solution.

Abstract

Bencyclane (Ben) inclusion complexes with α-, β-, and γ-cyclodextrins (CyD's) in aqueous solution were studied by ultraviolet (UV), circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopic methods. The composition ratio, the formation constants (K values), and the thermodynamic parameters for the three complexes were determined by analysis of the UV and CD spectra. The results indicated that CyD formed a 1 : 1 complex with Ben in all three cases, though the K values differed greatly (β>γ≫α). From the thermodynamic parameters obtained from the temperature dependence, it was concluded that the primary driving force for the complex formation is van der Waals interaction in the case of α- and β-CyD complexes, while it is hydrophobic interaction for the γ-CyD complex. Probable structures of the three complexes are discussed mainly on the basis of NMR data. It was concluded that the aromatic ring of Ben is inserted into the cavity of CyD from the small-diameter side in the case of β-or γ-CyD complexes, and from the opposite side in the case of α-CyD complex. It is highly probable that, in the case of β-CyD complex, binding is tight over the whole aromatic ring; in the case of γ-CyD complex, loose binding takes place over most of the aromatic ring and in the neighborhood of the benzyl carbon, with the para position of the aromatic ring extruding from the cavity; in the case of α-CyD complex, only partial inclusion occurs around the para position of the aromatic ring.