Solubilization of polycyclic aromatics in water by γ-cyclodextrin derivatives.

A series of hydrophilic per-6-thio-6-deoxy-γ-cyclodextrins (CDs) were synthesized from per-6-iodo-6-deoxy-γ-CD. These new hosts are able to solubilize polycyclic aromatic guests in aqueous solution to much higher extents than native CDs. Phase-solubility diagrams were mostly linear in accordance with both 1:1 and 1:2 CD-guest complexes in aqueous solution. The stoichiometry of the inclusion complexes was further investigated by fluorescence spectroscopy, which revealed very pronounced Stokes shifts typical for 1:2 complexes. This finding was further consolidated by quantum mechanical calculations of dimer formation of the guests and space-filling considerations by using the cross-sectional areas of the CDs and guests. The calculated dimerization energies correlated well with the binding free energies measured for the 1:2 complexes, and provided the main contribution to the driving force of complexation in the γ-CD cavity.