Surface-initiated ATRP of N-isopropylacrylamide from initiator-modified self-assembled peptide nanotubes

Peptide-polymer hybrid nanotubes (PPNT) were prepared by a combination of self-assembling functional cyclic peptides and in-situ surface-initiated atom transfer radical polymerization (ATRP). Cyclic peptides that consist of alternating D- and L-amino acids, carrying ATRP initiators in distinct side chains, were self-assembled into hollow nanotubes that expose all initiation moieties at the outer surface, thereby forming a cyclic peptide initiator nanotube (CP-ini). The CP-ini nanotubes were dispersed in 2-propanol, and a surface-initiated ATRP reaction has been performed using N-isopropylacrylamide (NIPAM) as monomer, tris[2(dimethylamino)ethyl] amine (Me6TREN) as ligand, and additional sacrificial (model) initiator. The molar mass of the resulting PNIPAM can be well controlled by adjusting the polymerization time (i.e., reaction conversion). The solvent-free height of the PNIPAM-PPNT, as measured by statistical analysis of cross sections of atomic force microscopy (AFM) height micrographs, increases with increasing molar mass of the attached PNIPAM chains in a well-controlled manner. The latter allows for the first time to tailor the outer diameter of self-assembled peptide nanotubes in a very precise way without changes to the primary sequence of the peptide ring. The length of the PNIPAM-PPNT remains almost constant with increasing polymerization time; however, at larger polymerization times, a decrease in absolute number of PPNT is observed, and smaller particles are increasingly present due to a breakup of the PNIPAM-PPNT into smaller peptide-polymer hybrid nanoobjects.