The Curious World of Polypseudorotaxanes: Cyclodextrins as Probes of Water Structure

Polymeric chains and cyclodextrins in water produce supramolecular host-guest assemblies called polypseudorotaxanes (PPR). The guest polymers thread the host cyclodextrin molecules such as beads on a string. Once formed, these molecular necklaces aggregate into larger assemblies that reveal themselves in a significant increase of turbidity. This astonishing phenomenon is enthalpically driven and its kinetics depends strongly on choice of reactants, concentration, solvent nature, addition of cosolutes, and temperature. We have studied the effects on PPR assembly induced by temperature, surface nature, and solute. These studies provide some insights into the cooperative mechanisms behind the formation of these organized structures. Hydrophilic surfaces (e.g., glass), low temperatures, and high concentrations of host molecules all favor the threading process. The addition of sugars to the solution promotes the assembly process in a way that provides strong hints on the nature of aqueous solution structure. The highly specific effects induced by sugars and electrolytes mimic precisely those seen in bubble-bubble coalescence phenomena. They reflect the capacity of solutes to modify bulk solvent and solution structure. In agreement with previous observations, the experimental results can be explained by invoking the formation of pre-assembled, wormlike aggregates of cyclodextrins in aqueous solutions. Once formed, these worms instantly swallow the polymer chain.