The outstanding mechanical properties of silk, such as high strength and toughness, is due to the exquisite hierarchical structure of silk fibroin, its primary protein component. The ability to synthesize recombinant silk fibroin enables a tailored sequence of amino acids with mechanical properties targeted for specific applications. Computational modeling can accelerate the process of designing recombinant silk fibroin by unraveling the relationship between chain structure, supramolecular assembly, and resulting tensile properties. In our work, molecular dynamic (MD) simulation was utilized to simulate the formation of crystalline beta-sheet domains by silk fibroin, as well as tensile deformation of the resulting material. Silk fibroin chains with different ratios of hydrophobic (“hard”, beta-sheet forming) and hydrophilic (“soft”, amorphous) blocks were created with periodic boundary conditions. A reactive Lennard-Jones potential at the level of united-atom method was used to govern the interactions between particles representing hydrogen bonding and hydrophobic stacking interactions. Such a reactive potential is needed to understand the self-assembly, plastic flow and fracture at large deformations, as most molecular mechanics models do not properly describe the bond formation and breaking process. After preparing uniformly distributed systems of silk fibroin chains, the self-assembly of these chains was initiated by reducing the simulation box volume and the solvent concentration to mimic compression and dehydration occurring during natural spinning. The formation of crystalline beta-sheet domains was checked by counting the number of hydrogen bonds and hydrophobic interactions. Then, a uniaxial tensile test was simulated to gain information such as Young’s modulus, strength, and toughness. We observed that the “hard”/”soft” block ratio of the silk chain has a significant effect on material stiffness and strength. Higher “hard” block content leads to stiffer materials, while higher “soft” block content yields more extensible materials. However, the per-residue efficiency of beta-sheet formation decreases with increased “hard” block content. Our results demonstrate that coarse-grained MD simulation modeling can be a powerful tool to predict mechanical properties of silk fibroin and provide a method for <i>de novo</i> design silk-like materials with targeted mechanical properties.

Understanding the structure-property relationships of silk fibroin though molecular dynamic simulation

ACS Spring 2022

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