Twisted Nanotubes Formed from Ultrashort Amphiphilic Peptide I₃K and Their Templating for the Fabrication of Silica Nanotubes

Abstract

Many de novo designed amphiphilic peptides capable of self-assembly and further structural templating into hierarchical organizations such as nanofibers and gels carry more than 10 amino acid residues. A curious question is now raised about the minimal size that is required for initiating amphiphilically driven nanostructuring. In this work, we show that ultrashort peptides I3K and L3K could readily self-assemble into stable nanostructures. While L3K formed spherical nanospheres with diameters of ∼10−15 nm, I3K self-assembled into nanotubes with diameters of ∼10 nm and lengths of >5 μm. I3K nanotubes were very smooth and carried defined pitches of twisting. The difference could arise from the different β-sheet promoting power between isoleucine and leucine, suggesting that while hydrophobic interaction was dominant in the formation of L3K nanospheres hydrogen bonding governed the templating of antiparallel β-sheets and the subsequent formation of I3K nanotubes. Because of their extreme stability against heating or exposure to organic solvents, I3K nanotubes were used as templates for silicification from the hydrolysis of organosilicate precursors using TEOS (tetraethoxysilane). The lysine groups on the inner and outer nanotube surfaces worked to catalyze silicification, leading to the formation of silica nanotubes, which is evident from both AFM and TEM imaging. The formation of interesting nanotubes and nanospheres as demonstrated from very short peptide amphiphiles is significant for further exploration of their use in technological applications.