Reversible guest-induced transformations between the tube and capsule isomers. Finally, having identified suitable guests for converting cage T into C as well as conditions for efficient guest removal, we hypothesized that repeated addition and removal of such guests might enable reversible transformations between the two cage isomers. To this end, we converted T into C using 8 as the guest, vigorously shook the aqueous solution of 8ÌC with EtOAc to generate guest-free C, allowed for the spontaneous regeneration of T at 60 ºC, and then repeated the cycle, as illustrated in Figure . The series of 1 H NMR spectra in Figure shows that the fraction of C (whose acidic imidazole proton peak is denoted with a red asterisk) could be reversibly toggled between <10% and ~90% for at least three cycles.