THERMODYNAMICS OF MOLECULAR RECOGNITION BY CYCLODEXTRINS .1. CALORIMETRIC TITRATION OF INCLUSION COMPLEXATION OF NAPHTHALENESULFONATES WITH ALPHA-CYCLODEXTRIN, BETA-CYCLODEXTRIN, AND GAMMA-CYCLODEXTRIN - ENTHALPY ENTROPY COMPENSATION

Calorimetric titrations have been performed at 25-degrees-C in buffered aqueous solution (pH 7.20) to give the complex stability constants and the thermodynamic parameters for the inclusion complexation of naphthalenesulfonates 1-6 and naphthaleneacetate 7 with alpha-, beta-, and gamma-cyclodextrins (CDs). Data analyses assuming 1:1 stoichiometry were successfully applied to all of the host-guest combinations employed, except for the inclusion of 2-naphthalenesulfonate 2 with gamma-CD, where both 1:1 and 1:2 host-guest complex formations were observed. The thermodynamic parameters obtained are critical functions of the position, number, and type of the anionic substituent(s) introduced to the guest molecule. The inclusion complexation is mostly enthalpy-driven with a minor or major positive entropic contribution, but in some cases a substantial positive entropic contribution determines the complex stability. Furthermore, the general validity of the enthalpy-entropy compensation effect, originally proposed for the cation binding by acyclic and macro(bi)cyclic ligands, was tested for the inclusion complexation by CD. Using all the thermodynamic data obtained here and reported elsewhere, the DELTAH-TDELTAS plot for CD gave a good straight line. On the basis of our explanation proposed previously, the slope very close to unity (alpha 0.90) indicates that, despite the apparently rigid skeleton of CD, the inclusion complexation causes substantial conformational changes involving the reorganization of the original hydrogen bond network, while the intermediate intercept (TDELTAS(o) 3.1) means fairly extensive dehydration occurring upon inclusion. It is thus demonstrated that, beyond the major driving forces operating in both types of complexation, i.e., ion-dipole and van der Waals interaction, the host-guest complexation phenomena involving the weak interactions may be understood in the general context of the enthalpy-entropy compensation effect.