Thermodynamics of interactions between organic ammonium ions and sulfonatocalixarenes

Calorimetric titration and NMR experiments in aqueous phosphate buffer (pH 7.2) at 298.15 K have been done to determine the binding mode, complex stability constants and thermodynamics (ΔG°, ΔH°, and TΔS°) for 1:1 inclusion complexation of water-soluble calix[n]arenesulfonates (CnAS, n = 4 and 6) and thiacalix[4]arene tetrasulfonate (TCAS) with acethylcholine, carnitine, betaine and benzyltrimethylammonium ion. The results show the inclusion complexations are driven by enthalpy (ΔH° < 0), accompanied by negative entropic changes (ΔS° < 0). The binding affinities (C4AS > C6AS > TCAS) are discussed from the viewpoint of CH-π/π-π interactions, electrostatic interactions and size/shape-fit relationship between host and guest.