Self-Assembly of an Amphiphilic Designer-Peptide into Double Helical Superstructures

Self-assembling amphiphilic designer-peptides are used as building blocks for the development of controllable, tailor-made biomaterials. In solution, they self-assemble above a critical aggregation concentration into supramolecular structures, like vesicles, bilayers, twisted tapes, fibers or tubes. In this study we investigated the concentration- and time-dependent self-assembly of an 8-residue amphiphilic designer-peptide. We observed structural transitions from peptide monomers to elongated pairwise aligned tapes. Highly concentrated samples assembled into the first double helix superstructure that was observed within the class of amphiphilic designer-peptides so far. Synchrotron small angle X-ray scattering provided a detailed insight into the internal organization of the double helix. The obtained electron-density-profile suggested a 3-shell-model, mirrored at the central axis. Shell 1 (∼1 nm) and shell 2 (∼3.5 nm) together account for the peptide containing region, where the hydrophobic parts of the peptide monomers were interdigitated and tightly packed. In the innermost region we confirmed antiparallel stacking due to intermolecular hydrogen bonding by circular dichroism and infrared spectroscopy. Shell 3 spans around 12 nm and was assigned to a hydration shell where negatively charged trifluoroacetate counter ions preferably attach to the positively charged peptide headgroups. The total diameter of the double helix was 24 nm, with a repeating pitch distance of ∼60 nm. Cryo transmission electron microscopy supported the double helix morphology and revealed that their lengths extended to several hundreds of nanometers. The double helices were intertwined into a tight network. The resulting hydrogel properties may lead to promising future applications.