(165i) Molecular Dynamic Simulation of Self-Assembly and Mechanical Deformation of Silk Fibroin

Silk fibroin has outstanding mechanical properties, such as high strength and toughness, which come from its unique nanoscale structure, where beta-sheet crystals are dispersed in an amorphous matrix. Natural silk fibroin can be extracted from organisms (spiders and silkworms), whereas synthetic silk fibroin with tailored sequence can be prepared recombinantly. Computational simulation can enable the rationale design of synthetic silk fibroins with targeted mechanical properties, reducing the time and effort involved with an experimental trial-and-error approach. Our work utilizes molecular dynamics (MD) simulation of the self-assembly and tensile behavior of silk fibroin to give us information on structure-property relationships. To investigate the relationship between the silk fibroin sequence, specifically the length ratio of “hard” beta-sheet forming segments and “soft” amorphous segments, model silk chains with different “hard/soft” ratios are created within a simulation box with periodic boundary conditions, where the simulation size, chain length, and chain number can be controlled. A Lennard-Jones potential is used to describe inter-particle interactions. After annealing and relaxation at high temperatures to generate uniformly distributed silk chains, we then simulate self-assembly of the silk by reducing simulation box size and removing solvent. Beta-sheet crystals formed during assembly can be observed by increasing the number of bonds representing hydrogen bonding and hydrophobic interactions. With the resulting silk structure, a uniaxial tensile test is carried out in silico. Our results show that as the hard/soft segment ratio increases, the stiffness of the silk increases. This trend comes from increased beta crystal size attributed to increased hard segment and decreased amorphous region resulting from soft segment. However, silk chains with a 1-to-1 hard/soft segment ratio exhibit the greatest tensile strength. Thus, our results demonstrate that MD simulations can provide a powerful computational tool for designing synthetic silk fibroin sequences with specific desired mechanical properties.