TEMPERATURE EFFECTS ON CD SPECTRA OF BETA-CYCLODEXTRIN COMPLEXES WITH 2-SUBSTITUTED NAPHTHALENES

CD spectra of β-cyclodextrin complexes with 2-naphthol, 2-naphthyloxyacetic acid, potassium 2-naphthoate, and potassium 2-naphthylacetate were measured at various concentrations of β-cyclodextrin and at temperatures ranging from 10 to 70 °C. The temperature-dependent CD spectra of the 2-naphthyloxyacetic acid complex clearly show the 1: 1 complex formation, but the other three complexes give CD intensity changes which are inexplicable in terms of the 1: 1 stoichiometry. The molecular ellipticity and thermodynamic parameters were determined by the least-squares method. Enthalpy and entropy are in the ranges from-22.2 to-26.8 kJ · mol-1 and from-24 to-38 J · K-1.mol-1 respectively. They are strongly correlated to the volume of the substituent of the guest molecule, although the free energy is found in a quite narrow region (from-14.3 to-15.4 kj-mol-1). The correlation is explained on the basis of the β-cyclodextrin-guest interaction and the solvation around the guest molecule.