A study of the self-assembling morphology in peptide nanorings and nanotubes

Peptide nanotubes of cyclo[-(D-Ala-L-Gln)₄], cyclo[-(D-Ala-L-Gln)₃], cyclo[-(D-Cys-L-Gln)₃], and cyclo[-(L-Gln)₅] have been synthesized and their morphologies have been studied by scanning probe microscopy. The atomic force microscopy displayed several interesting self-assembling forms of these peptide nanotubes. The three D,L-peptides showed not only the straight tubular forms but also the self-assembled bundles. The number and size of observed nanotubes and their bundles were different among those three and also among employed solvents and substrates. On the contrary, the homo-L-peptide of cyclo[-(L-Gln)₅] provided meandering nanotubes, which were caused by the symmetry lowering due to the odd number of amino acids. The meandering characteristics also resulted peculiar super-ring forms of the homo-L-peptide nanotubes. Additionally, the scanning tunneling microscopy was performed and high-resolution images of D,L-peptide nanotubes as well as their bundles were obtained.