Structure elucidation and control of cyclic peptide-derived nanotube assemblies in solution

Abstract

We describe the solution assembly of polymer–cyclic peptide conjugates into nanotubes, by direct in situ measurements. The conjugates were assembled by exploiting the β-sheet assembly of the alt(D,L) cyclic octapeptide core. The conjugated polymer permits solubilization of the peptide and the resulting nanotubes, thus allowing for the first time the direct study of the assembly mechanism of this system. The resulting materials present unique properties for a wide range of applications. We find that the polymer can act to both shield the peptide core from the solvent and to put strain on the peptide core through steric repulsions. By controlling both the solvent mixture and the length of the polymer, control over the length of the resulting nanotubes can be obtained. In addition, monitoring the assembly with temperature allows the strength of the assembly to be probed, adding evidence for a co-operative mechanism of assembly. Finally, selective deuteration of the polymer component in SANS analyses leads to the precise measurement of the nanotubes core dimensions, and by cross-linking the polymeric shell, the structure of the nanotubes in solution are confirmed by transmission electron microscopy (TEM). This study establishes the fundamentals needed for the design and control of these new materials.