Cooperative self-assembly and molecular binding behavior of cyclodextrin–crown ether conjugates mediated by alkali metal ions

Abstract

In order to quantitatively investigate their molecular binding ability, a series of cyclodextrin–crown ether conjugates containing β-cyclodextrin (β-CyD) and crown ether units, i.e.N-(benzoaza-15-crown-5)acylaminomethylene tethered 6-diethylenetriamino-6-deoxy-β-CyD (1), N-(benzoaza-15-crown-5)acylaminomethylene tethered 6-triethylenetetraamino-6-deoxy-β-CyD (2) and 4′,5′-dimethylene-benzo-15-crown-5 tethered 6-diethylenetriamino-6-deoxy-β-CyD (3), have been prepared as ditopic molecular receptors. Their inclusion complexation behavior with four representative fluorescent dyes, i.e. ammonium 8-anilino-1-naphthalenesulfonate (ANS), sodium 6-toluidino-2-naphthalenesulfonate (TNS), acridine red (AR) and rhodamine B (RhB), has been comprehensively investigated in aqueous NaH₂PO₄/Na₂HPO₄ or KH₂PO₄/K₂HPO₄ buffer solution (pH 7.20) by means of circular dichroism, fluorescence, and 2D NMR spectra. The results indicate that the self-assembly of crown ether modified β-CyD mediated by potassium ion exhibits a dimeric structure, which significantly enhances the original binding ability and molecular selectivity of parent β-CyD and its derivatives towards guest molecules through the cooperative binding of two hydrophobic CyD cavities with one guest. This cooperative binding mode of K⁺/CyD–crown ether systems are further confirmed by Job's experiments and 2D NMR investigations. Attributed to the positive contributions from the metal-ligated crown ether cap and K⁺-mediated dimerization of CyDs, the binding constant (Ks) values of CyD–crown ether conjugates 1–3 toward ANS are 10–83 times higher than that of β-CyD. The increased binding ability and molecular selectivity of CyD–crown ether conjugates are discussed from the viewpoints of size/shape-fit and multiple recognition mechanism.