Water-mediated association provides an ion pair receptor

In this paper, we report on the formation and properties of a water-stabilized dinner comprising calix[4]arene-guanosine conjugate cG 2. The 1,3-alternate calixarene cG 2 was poorly soluble in dry CDCl3 and gave an ill-resolved NMR spectrum, consistent with its nonspecific aggregation. The compound was much more soluble in water-saturated CDCl3. Two sets of well-resolved H-1 NMR signals for the guanosine residues in cG 2, present in a 1:1 ratio, indicated that the compound's D-2 symmetry had been broken and provided the first hint that cG 2 dimerizes in water-saturated CDCl3. The resulting dinner, (cG 2)(2).(H2O)(n), has a unique property: it extracts alkali halide salts from water into organic solution. This dimer is a rare example of a self-assembled ion pair receptor. The identity of the (cG 2)(2).NaCl.(H2O)(n) dimer was confirmed by comparing its self-diffusion coefficient in CDCl3, determined by pulsed-field gradient NMR, with that of control compound cA 3, an adenosine analogue. The dimer's stoichiometry was also confirmed by quantitative measurement of the cation and anion using ion chromatrography. Two-dimensional NMR and ion-induced NMR shifts indicated that the cation binding site is formed by an intermolecular G-quartet and the anion binding site is provided by the 5'-amide NH groups. Once bound, the salt increases the dimer's thermal stability. Both 1H NMR and ion chromatography measurements indicated that the cG 2 dimer has a modest selectivity for extracting K+ over Na+ and Br over Cl-. The anion's identity also influences the association process: NaCl gives a soluble, discrete dimer whereas addition of NaBPh4 to (cG 2)(2).(H2O)(n) leads to extensive aggregation and precipitation. This study suggests a new direction for ion pair receptors, namely, the use of molecular self-assembly. The study also highlights water's ability to stabilize a functional noncovalent assembly.