Determining the binding and intracellular transporting abilities of a host-[3]rotaxane

The cellular permeability of compounds can be enhanced in the presence of a host-[2]rotaxane (HR). The effective concentration of an HR is limited by the stoichiometry of the complex formation of the HR and the delivered compound. We speculate that a complex forms between the HR and a guest during membrane passage. To further explore the relationship between guest binding and guest delivery and to obtain more efficient delivery devices, we present, in this report, the first example of a cyclophane-[3]rotaxane (Cy3R), which has two wheels and a cyclophane as a blocking group. The properties of Cy3R were compared to a new cyclophane-[2]rotaxane (Cy2R) that has the same cyclophane pocket as Cy3R but only a single wheel. The second wheel of Cy3R can form additional noncovalent bonds, e.g., salt bridges, cation-pi interactions or aromatic-aromatic interactions, with appropriately functionalized guests. We show by flow cytometric analysis that Cy3R transfers Fl-AVWAL (76%) and to a lesser degree Fl-QEAVD (26%) into live cells. The level of Fl-peptide within a cell is concentration dependent and largely temperature and ATP independent, suggesting that a Cy3R(.)Fl-peptide complex passes through the cellular membrane without requiring active cell-mediated processes. Cy2R, on the other hand, forms weaker complexes and requires a higher concentration to transfer materials into cells. These results demonstrate that the addition of a second wheel on a rotaxane can improve guest binding in various solvents and hence delivery through cellular membranes.