Biopolymer self-assembly and aggregation at interfaces

Amyloid fibrils, typically form through a nucleation and growth mechanism, in a process analogous to polymer crystallization. The nucleation step typically requires a minimum concentration of peptides in the solution, thus preventing this one-dimensional crystallization process from moving forward in dilute solutions. However, under the right conditions, surfaces can promote rapid self-assembly of mono-layer thick amyloid fibrils within minutes. We demonstrate that this surface-enabled self-assembly process can proceed through diffusion-limited aggregation, without a nucleation barrier. On a rigid hydrophobic surface, the adsorbed peptides minimally disrupt the water structure and can as such rapidly diffuse, while roughness and softness can disrupt this process. The deposition rate of the peptides, as well as their orientation and diffusion on the surface, are also important factors in aggregation, which strongly depend on the effective surface energy. The self-assembly can be suppressed either by reducing the interaction such that the deposition rate is reduced or by increasing the interaction energy to a point where the peptide diffusion is substantially reduced.  However, the absorption of amphiphilic peptides themselves can disrupt this process, by changing the effective surface energy. Depending on the nature of the side chains, the surface can become more hydrophobic or hydrophilic and promote or prevent the further deposition of the peptides. We develop a new method to measure the dynamic change in the surface energy and its effect on the kinetics of adsorption/desorption and surface-mediated aggregation. We demonstrate how under the right conditions, the peptide absorption can be self-limiting, allowing control over two-dimensional self-assembly and the final effective surface energy. This process can be envisioned as a method for the directed assembly of amphiphilic polymers and biopolymers.