Binding equilibria between beta-cyclodextrin and p-nitro-aniline derivatives: the first systematic study in mixed water-methanol solvent systems.

Complexation equilibria, in mixed water-methanol solvent media, between native β-cyclodextrin and a set of suitably selected p-nitro-aniline derivatives were studied by means of polarimetry. The effects exerted by the organic co-solvent on the conditional inclusion free energies ΔGcond 0 and the differential molar optical rotations ΔΘ were thoroughly analyzed under the perspective of the enthalpy-entropy compensation effect. Experimental data suggest an intimate participation ('dynamic co-inclusion') of solvent molecules in the formation and in the conformational dynamics of the host-guest inclusion complex.