(587f) Self-Assembly and Mechanical Response of a Model Silk Fibroin

Silk is a natural biopolymer with outstanding mechanical properties, such as high strength and toughness, due to its hierarchical structure. Synthesizing silk-inspired proteins by recombinant technology enables facile control of amino acid sequence and provides a large experimental space for obtaining targeted material properties. However, there remains a lack of understanding of the relationship between the primary silk chain structure (such as ratios between “hard” β-sheet forming segments and “soft” amorphous segments), resulting supramolecular structure (including the volume and percolation of β-sheet rich nanocrystalline domains within an amorphous matrix), and mechanical properties. Previous molecular dynamics (MD) strategies have not accurately described the two different orthogonal interactions in the β-sheet crystal structure (hydrogen bonding and stacking bonding), and they also have not effectively represented tensile failure at high strains due to the use of a non-reactive potentials as well as poor-choice of model geometry.

To address these challenges, we developed a reactive coarse-grained MD model mimicking the repetitive core domain of spider dragline silk fibroin (spidroin). The MD simulation was set up to accurately represent the formation of β-sheet nanocrystals in a silk-like material at a massive scale by enforcing the orthogonal directionality of hydrogen bonding between β-strands and Van der Waals stacking interactions between β-sheets. We observed that hard/soft segment ratio had a substantial impact on the degree of β-sheet nanocrystal formation. In addition, tensile deformation of the self-assembled silk was simulated to obtain bulk mechanical properties by describing bond breaking/forming with a reactive force field, with high aspect ratio samples. The resulting assembled systems showed silk-like tensile deformation behaviors with different Young’s modulus, strength, and toughness values. The semi-crystalline silk system consisting of both amorphous and β-crystal structure showed higher stiffness and strength values than those in amorphous silk system, which implied β-sheet nanocrystal domain contributed to an improvement in mechanical properties. As the hard segment content in a single silk chain increased, the stiffness and strength of the resulting systems increased alongside decreasing failure strain.