Self-Assembling Nanotubes Consisting of Rigid Cyclic γ-Peptides

Self-assembling cyclic peptide nanotubes (SPNs) have been extensively studied due to their potential applications in biology and material sciences. Cyclic ?-peptides, which have a larger conformational space, have received less attention than the cyclic a- and beta-peptides. The self-assembly of cyclic homo-?-tetrapeptide based on cis-3-aminocyclohexanecarboxylic acid (?-Ach) residues, which can be easily synthesized by a one-pot process is investigated. Fourier transform infrared (FTIR) and NMR analysis along with density functional theory (DFT) calculations indicate that the cyclic homo-?-tetrapeptide, with a non-planar conformation, can self-assemble into nanotubes through hydrogen-bond-mediated parallel stacking. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) experiments reveal the formation of bundles of nanotubes in CH2Cl2/hexane, but individual nanotubes and bundles of only two nanotubes are obtained in water. The integration of TEG (triethylene glycol) monomethyl ether chains and cyclopeptide backbones may allow the control of width of single nanotubes.