Difference in self-assembling morphology of peptide nanorings

We synthesized the peptide nanorings of cyclo[-(D-Ala-L-Gln)₃], cyclo[-(D-Cys-L-Gln)₃], cyclo[-D-Cys-L-His-D-Ala-L-Asn-Gly-L-Gln-] and cyclo[-(L-Gln)₃], and studied the way in which the difference in the type and/or number of component amino acid residues changes the self-assembling morphology of the nanorings on gold substrates by atomic force microscopy. The study revealed that cyclo[-(D-Ala-L-Gln)₃] formed nanotube bundles through inter-ring hydrogen bonds, while the nanorings of cyclo[-(D-Cys-L-Gln)₃] adhered to the gold surface directly due to the high affinity of thiol to gold. In contrast, a random amino acid sequence of cyclo[-D-Cys-L-His-D-Ala-L-Asn-Gly-L-Gln-] resulted in many isolated nanotubes, which were first observed in the present study. While the D,L-peptide nanotubes have very straight forms, the homo-L-peptide of cyclo[-(L-Gln)₅] formed interesting randomly branching nanotubes that were entwined and grew on the substrate. Scanning tunneling microscopy was also performed and high-resolution images of both the peptide nanotubes and the nanotube bundles were obtained.