Nanomechanical properties and phase behavior of phenylalanine amyloid ribbon assemblies and amorphous self-healing hydrogels.

Phenylalanine was the minimalistic and first of numerous non-proteinaceous building blocks to be demonstrated to form amy-loid-like fibrils. This unexpected organization of such a simple building block into canonical architecture, which was previously observed only with proteins and peptides, has numerous implications for medicine and supramolecular chemistry. However, the morphology of phenylalanine fibrils and their mechanical properties was never characterized in solution. Here, using electron and atomic force microscopy, we analyze the morphological and mechanical properties of phenylalanine fibrils in both air and fluid. The fibrils demonstrate an exceptionally high Young's modulus (up to 30 GPa) and are found to be composed of inter-twined protofilaments in a helical or twisted ribbon morphology. In addition, X-ray scattering experiments provide convincing evidence of an amyloidal cross-beta-like secondary structure within the nanoassemblies. Furthermore, increasing phenylalanine concentration results in the formation of, highly homogenous, non-crystalline, self-healing hydrogels that display storage and loss moduli significantly higher than similar non-covalently cross-linked biomolecular nanofibrillar scaffolds. These remarkably stiff nanofibrillar hydrogels can be harnessed for various technological and biomedical applications, such as self-healing, printable, structural, load-bearing 3D scaffolds. The properties of this simple but quite remarkable hydrogel opens a possibility to utilize it in the biomaterial industry.