A cationic host displaying positive cooperativity in water.

The affinity of guanidinium and Cu(II) containing hosts for polycarboxylate guests is studied in water by using UV-visible spectroscopy and isothermal titration calorimetry. By combining a Cu(II) coordination site and a diguanidinium moiety around a single scaffold it is found that the Gibbs free-energy release upon binding is greater than could be expected based on the sum of the free energy released by the binding functional groups when operating in isolation. This effect is known as positive cooperativity and has rarely been observed with synthetic hosts in water. The isothermal titration calorimetry data suggest that the source of this positive cooperativity is enthalpic in origin. We propose that covalently positioning the cationic binding moieties in close proximity destabilizes the unbound host, and thereby the stabilizing effect of binding the anionic carboxylates is exothermic and also, the source of positive cooperativity observed herein.