Second-generation rotaxane ion transporters: boosting transport activity via enhanced transport flux across lipid bilayers
Rotaxane-based synthetic ion transporters have demonstrated potential in mimicking natural transmembrane machinery, yet first-generation systems suffer from low transport activity, with EC50 values in the micromolar range. This study reports a second-generation rotaxane transporter design that addresses this limitation by modifying the ring component to a tricyclic architecture (TCE) incorporating two K+ recognition sites. The TCE ring enables the transport of two K+ ions per shuttle cycle, resulting in a tenfold reduction in EC50 compared to first-generation transporters that possess only one K+ recognition site. Further implementation of a cooperative shuttle-relay mechanism, using [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). This represents one of the highest K+ transport activities reported for molecular machine-based transporters. The findings provide a robust technical foundation for mimicking natural channel functions and support potential biomedical applications, including therapeutic agents and biosensing platforms.