A spectroscopic study of the inclusion of azulene by β- and γ-cyclodextrins

The inclusion of azulene (AZ) inside the cavities of beta-cyclodextrin (beta-CD) and gamma-cyclodextrin (gamma-CD) was studied using absorption, fluorescence and induced-circular dichroism spectroscopy. The inclusion of AZ into the cavity of beta-CD has a stoichiometry of 1:1, whereas that of AZ/gamma-CD complex is 1:2. The equilibrium constants for the formation of the two complexes were calculated to be 780 +/- 150 M-1 for AZ: beta-CD and (4.5 +/- 0.86) x 10(5) M-2 for AZ:(gamma-CD)(2). The latter is due to a stepwise equilibrium mechanism in which a 1: 1 complex is formed with a binding constant of 775 M-1, followed by the formation of a 1:2 complex with a binding constant of 580 W. The difference between the two binding constant values is slight, indicating an almost equal contribution from each of the gamma-CD molecules to the overall binding in AZ:(gamma-CD)(2). From the induced-circular dichroism spectra, the inclusion of AZ was found to be axial in AZ:P-CD and nearly axial in AZ:(gamma-CD)(2). (c) 2004 Elsevier B.V. All rights reserved.