From Micelles to Fibers: Balancing Self-Assembling and Random Coiling Domains in pH-Responsive Silk-Collagen-Like Protein-Based Polymers

We study the self-assembly of genetically engineered protein-based triblock copolymers consisting of a central pH-responsive silk-like middle block (Sn-H, where S-H is a silk-like octapeptide, (GA)(3)GH and n is the number of repeats) flanked by hydrophilic random coil outer blocks (C-2). Our previous work has already shown that triblocks with very long midblocks (n = 48) self-assemble into long, stiff protein filaments at pH values where the middle blocks are uncharged. Here we investigate the self-assembly behavior of the triblock copolymers for a range of midblock lengths, n = 8, 16, 24, 48. Upon charge neutralization of S-n(H) by adjusting the pH, we find that (C2S8C2)-C-H and (C2S16C2)-C-H form spherical micelles, whereas both (C2S24C2)-C-H and (C2S48C2)-C-H form protein filaments with a characteristic beta-roll secondary structure of the silk midblocks. Hydrogels formed by (C2S48C2)-C-H are much stronger and form much faster than those formed by (C2S24C2)-C-H. Enzymatic digestion of much of the hydrophilic outer blocks is used to show that with much of the hydrophilic outer blocks removed, all silk-midblocks are capable of self-assembling into stiff protein filaments. In that case, reduction of the steric repulsion by the hydrophilic outer blocks also leads to extensive fiber bundling. Our results highlight the opposing roles of the hydrophilic outer blocks and central silk-like midblocks in driving protein filament formation. They provide crucial information for future designs of triblock protein-based polymers that form stiff filaments with controlled bundling, that could mirnick properties of collagen in the extracellular matrix.