Second-generation rotaxane ion transporters: boosting transport activity via enhanced transport flux across lipid bilayers

Inspired by natural transmembrane molecular machines, rotaxane-based synthetic molecules have demonstrated significant potential in constructing ion transporters capable of performing complex tasks akin to their biological counterparts. Addressing the need to enhance the ion transport activity of rotaxane transporters, we herein report a new strategy for developing second-generation rotaxane transporters by modifying the ring structure to boost transport flux, in which the ring component TCE features a tricyclic architecture incorporating two K⁺ recognition sites. This innovative design allows the rotaxanes to transport two K⁺ ions in a single shuttle-mediated transport cycle, leading to a tenfold reduction in EC₅₀ values compared to first-generation rotaxane transporters, which possess only one K⁺ recognition site. By further implementing a cooperative shuttle-relay mechanism, [3]R-TCE2—where two rings traverse the thread within the lipid membrane—achieved an EC₅₀ value as low as 60 nM (0.18 mol%, relative to lipid). It represents one of the highest K⁺ transport activities reported to date for molecular machine-based transporters. This work marks a significant advancement in improving the ion transport performance of rotaxane-based systems, offering robust technical support for their ability to mimic natural channel functions and paving the way for potential biomedical applications.