In spite of extensive research, it remains a formidable challenge to control the dimension of the nanostructures self-assembled from short designed peptides. In this work, we show that peptide bolaamphiphiles form monolayer wall nanotubes, facilitated by the interplay between the side chain structure and hydrophobicity of the central residues. The peptide KI4K self-assembles into nanotubes with a width of similar to 100 nm, but changes in the molecular structure of amino acid side chains could hugely impact the nanostructures formed. The three variants of KI4K, via the substitution of aromatic amino acids (F, Y, and Dopa) for the I residue closest to the C-terminus, could substantially reduce nanotube diameters, indicating a significant steric hindrance of the benzene rings on the lateral packing of 13-sheets. However, the introduction of hydroxyl groups on the benzene rings alleviates the steric effect, with nanotube diameter increasing in the order of KI3FK, KI3YK, and KI3DopaK, suggesting the formation of side chain H-bonds between 13-sheets in addition to hydrophobic contacts. Because the self-assembly process of KI3DopaK nanotubes is slow, key intermediates and their structural details are well characterized. With increasing incubation time, monolayered twisted ribbons and helical ribbons grow into mature KI3DopaK nanotubes via the pitch closing route. (c) 2020 Elsevier Inc. All rights reserved.