Effect of the acyl group and the dodecylsulfonate chain on the inclusion of pyrene inside the cavity of beta-cyclodextrin.

The solubilization of pyrene in aqueous solution of beta-cyclodextrin (beta-CD) or its derivatives such as beta-CD-hexanoyl, beta-CD-benzoyl and beta-CD-dodecylsulfonate was investigated by spectrophotometry. Linear and non-linear regression methods were used to estimate the association constants (K(1)). A 1:1 stoichiometric ratio and different effects of the hexanoyl, benzoyl and dodecylsulfonate groups on the association constant were observed for the binary inclusion complex between pyrene and beta-CD. The formation constant was shown to decrease when beta-CD was modified by a dodecylsulfonate chain. The value of K(1) was 190+/-10 L mol(-1) for the [pyrene/beta-CD] complex and 145 L mol(-1) for the [pyrene/beta-CD-dodecylsulfonate] complex. Partitioning of the pyrene molecules between the dodecylsulfonate chains and cyclodextrin cavities can explain the decrease in the association constant value. In the cases of beta-CD-hexanoyl and beta-CD-benzoyl derivatives, no association constants were detected. Results suggest that the high hydrophobicity of the hexanoyl and benzoyl groups prevents the inclusion of pyrene molecules inside the cyclodextrin cavity.