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.
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