Covalently Linked Hexakis(m-Phenylene Ethynylene) Macrocycles as Molecular Nanotubes

The construction of nanotubular structures with non-deformable inner pores is of both fundamental and practical significance. Herein we report a strategy for creating molecular nanotubes with defined lengths. Macrocyclic (MC) units based on shape-persistent hexakis(m-phenylene ethynylene) (m-PE) macrocycle MC-1, which are known to stack into hydrogen-bonded tubular assemblies, are tethered by oligo(beta-alanine) linkers to give tubular stacks MC-2 and MC-4 that have two and four MC units, respectively. The covalently linked MC units in MC-2 and MC-4 undergo face-to-face stacking through intramolecular non-covalent interactions that further results in the helical stacks of these compounds. Oligomer MC-4 can form potassium and proton channels across lipid bilayers, with the channels being open continuously for over 60 seconds, which is among the longest open durations for synthetic ion channels and indicates that the thermodynamic stability of the self-assembling channels can be drastically enhanced by reducing the number of molecular components involved. This study demonstrates that covalently tethering shape-persistent macrocyclic units is a feasible and reliable approach for building molecular nanotubes that otherwise are difficult to create de novo. The extraordinarily long lifetimes of the ion channels formed by MC-2 and MC-4 suggest the likelihood of constructing the next-generation synthetic ion channels with unprecedented stability.