Nico Kummer

In this thesis amyloid-like aggregates of hen egg white lysozyme (HEWL) were produced using different pathways and characterized regarding their structure and antimicrobial properties. Additionally, the self-assembly of sustainable biohybrid materials made from HEWL amyloids and nanocellulose as well as their antimicrobial potential was explored and assessed.

Self-Assembly. The self-assembly of HEWL amyloids as well as the co-assembly of amyloids and nanocellulose are governed by the balance of attractive and repulsive intermolecular forces. Due to the strongly pH-dependent charge of amyloids, variations of the pH offer the tunability of electrical double layer interactions. In the case of HEWL self-assembly into amyloid-like worms induced by disulfide bond reduction, the increase in double layer repulsion by lowering the pH led to stable aggregates, whereas higher pH resulted in flocculation and sedimentation of the HEWL.

Interfaces and Foams. The self-assembly at air-water interfaces of the amyloid fibrils system as dominated by unconverted peptides. Stable foams are obtained above pH 4, where the acidic amino acids were deprotonated, leading to a decreased adsorption barrier. The 2D foam lamellae created in-side a loop (similar to blowing soap bubbles) could be used to deposit homogenous layers of amyloid fibrils onto cellulose nanofiber based paper to introduce antimicrobial functionality. Lastly, polyelectrolyte complexation between oppositely charge amyloid fibrils and nanocellulose led to cross-linking and increased elasticity. An approach based on pH-mediated self-assembly was applied to produce homogenous amyloid-nanocellulose hybrid hydrogels. The amyloids and nanocellulose were mixed above the isoelectric point of the proteins, so they were both negatively charged and formed stable co-dispersion. Using dialysis, the pH was decreased to cross the isoelectric point and introduce positive charges on the amyloids leading to polyelectrolyte complexation and gelation.

Antimicrobial Activity. The positive charge of the HEWL amyloid fibrils at neutral pH also played an important role in their antimicrobial activity based on their interactions with the negatively charged cell walls and cell membranes of the microorganisms causing cell lysis. Suspensions of amyloid fibrils had a dose-dependent, enhanced and broad-spectrum antimicrobial effect against Gram-positive S. aureus, Gram-negative E. coli and the yeast C. albicans. A similar charge-mediated antimicrobial effect was seen for the HEWL amyloid fibril layer deposited onto cellulose nanofiber based paper using the 2D foam film coating method. However, the loss of surface area of the amyloid fibrils compressed into a 2D coating layer led to decreased bacterial inactivation compared to the suspensions.

In summary, this thesis offers new perspectives regarding the self-assembly of HEWL aggregates and their co-assembly with nanocellulose. It suggests bottom-up approaches resulting in hybrid materials with improved mechanical properties and added functionalities (antimicrobial activity), that could be used to develop sustainable bio-medical materials.