Direct Observation of the “End-Capping Effect” of a PEG@α-CD Polypseudorotaxane in Aqueous Media

This study demonstrates that the shuttling of α-cyclodextrin (α-CD) rings in a poly(ethylene glycol) (PEG) polypseudorotaxane drives the shrinkage and extension of the molecule and that a strong end-capping effect stabilizes the structure at the solid–liquid interface. We performed single-molecule imaging of PEG100k@α-CD polypseudorotaxane under aqueous solution using fast-scanning atomic force microscopy. The polypseudorotaxane displayed shrinking and extending motions driven by the α-CD rings shuttling along the polymer chain. These motions mainly occurred in exposed, self-shrinking PEG segments, where repeated expansion and reshrinking were observed as α-CD moved. All-atom molecular dynamics simulations corroborated these findings, confirming dynamic structural changes linked to α-CD shuttling. Additionally, globules formed at both ends of the PEG chain caused a strong end-capping effect that stabilized the structure in solution. These insights into the localized structural changes in PEG@α-CD polypseudorotaxane improve our understanding of supramolecular functionality and could inform the design of advanced materials.