Thiourea-based Rotaxanes Transport Anions across Membranes of Different Fluidity

We report the synthesis of two rotaxanes (1 and 2) whose rings have appended thiourea units for the selective recognition of Cl− anions. Two-dimensional exchange spectroscopy indicates the ring can shuttle much faster within rotaxanes featuring an axle similar to the one in 1 than in 2 when investigated in solution. Furthermore, rotaxane 1 transports Cl− anions across lipid bilayers more efficiently than 2, exhibiting EC50 values of 0.76 µM versus 2.30 µM respectively, which might be related to the difference in the shuttling rate. Moreover, control rotaxane (3) without the thiourea units and the individual axle (4) also showed Cl− transport, although with a much lower efficiency (EC50 values of 12.64 µM and 15.58 µM). Thus, it is plausible that in addition to shuttling, other pathways, such as a relay process, might contribute to the observed anion transport of 1 and 2. Our studies also suggest that rotaxane 1 could behave as a Cl−/NO3− antiport. Additionally, the transport activity 1 is significantly influenced by the membrane composition, exhibiting higher transport in membranes with higher fluidity. Our study provides new insights into the structural features of rotaxanes required for efficient anion transport and a better understanding of their functioning in different membranes. This will help in developing future rotaxane-based transporters for applications in biomedicine and bioengineering, purification technologies, and energy storage and conversion devices.