Supramolecular adducts of negatively charged lanthanide(III) DOTP chelates and cyclodextrins functionalized with ammonium Groups: Mass spectrometry and nuclear magnetic resonance studies

Abstract

The interaction of the negatively charged Ln3+ chelate Ln-DOTP with β- and γ-cyclodextrins bearing ammonium groups at the upper rim (CD+s) was investigated using mass spectrometry and NMR spectroscopic techniques. Mass spectroscopy shows the presence of 1:1 adducts of Ln-DOTP and both β- or γ-CD+. The peak intensities increased upon increasing the pH of the samples from 7 to 9.0, suggesting an increase in the strength of the interaction. Lanthanide induced 1H NMR chemical shifts and relaxation ratesmeasured in aqueous solutions confirmed the presence of these adducts. The strength of the interactions appeared to be dependent on the pH, reflecting the strong electrostatic interactions between the oppositely charged host CD+ and guest Ln-DOTP chelate. Evaluation of the lanthanide induced relaxation rates showed that the Ln-DOTP does not enter the cavity of the CDs, but remains above it with a distance of 10-11 É between the Ln3+ ion and the centre of the CD. Molecular modelling indicated that hydrogen bonds between the functionalized groups participating in the interaction sites contribute to the adduct stabilization. The apparent binding constants at pH 7 and 9 were obtained by using relaxometric measurements at 9 MHz. Fitting the NMRD profiles showed an increase in the number of second-sphere water molecules surrounding the phosphonate pendant arms of the Ln-DOTP chelate upon its interaction with the CDs. A brief description of the PARACEST properties of the supramolecular systems formed by Tm-DOTP and the positively charged CDs is presented. Both CDs display a shift of the saturation transfer peaks of the ammonium functions by the Tm complex, with an accentuated effect observed for the γ-CD derivative. The negatively charged Ln-DOTP chelate forms relatively strong 1:1 supramolecular adducts with β- and γ-cyclodextrins bearing ammonium groups at the upper rim. The strength of the host-guest interaction depends on the pH. The geometry of the adducts, obtained from paramagnetic shift and relaxation effects, shows that the host is outside the guest cavity. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.