Thermodynamic and structural study of inclusion complexes between trivalent lanthanide ions and native cyclodextrins

The interaction of trivalent lanthanide ions (Ln(3+)) with native cyclodextrins (CDs) is investigated in acidic and basic aqueous media. At low pH, the association constants for 1:1 complexes are in the range log K = 2-4 (mu = 0.1 M, NaCl or TMACl), as determined by pH-potentiometric study of the hydrolysis of the Ln ions in absence and in presence of CDs. The thermodynamic parameters of the inclusion reaction show that the complexation of Ln(3+) ions inside the CD cavity is entropically driven and does not depend upon the host. H-1 and C-13 NMR spectra point to inclusion of La3+ cations in the cavity of alpha-CD occurring in the narrowest part of the host molecule, close to the C-5 carbon atoms. alpha-CD acts like a crown ether, its anomeric oxygen atoms being the donor atoms. Complexation of Tb3+ by partially deprotonated a-CD was investigated at pH 12.33: C-13 NMR measurements show that complexation occurs at the OH-3 secondary hydroxyl groups of alpha-CD while UV-Vis spectrophotometric titration leads to an apparent constant log K-app approximate to 4.2 (1:1 complex). Lifetime measurements of the Eu(D-5(0)) and Tb(D-5(4)) levels confirm these findings. (C) 1999 Elsevier Science S.A. All rights reserved.