Peptide nanotube formation: a crystal growth process

Above its critical aggregation concentration the tfa salt of the peptide A(6)K self-assembles into micron long, hollow nanotubes with uniform diameters of 52 nm and crystalline order. Here we combine the use of SAXS with cryoTEM and cryo-electron tomography (cryoET, 3D cryoTEM) to study the formation process of the 2D crystalline A(6)K nanotubes. This study reveals that the formation of these tubes in fact is a crystal growth process, involving different mechanisms depending on the conditions used. Inorganic crystals have been demonstrated to form not only through ion-by-ion addition but also through non-classical mechanisms including oriented attachment. Here we show an organic crystalline material that can form through molecule-by-molecule growth as well as through oriented attachment. We discuss the mechanisms in relation to the supersaturation levels of the peptide solutions and the molecular interactions between the peptide molecules in the tubular assemblies. The proposed mechanisms are supported by semi-empirical molecular orbital calculations and time resolved dissolution experiments.