(337ae) Molecular Dynamics Simulation for the Rational Design of Silk-Mimetic Materials

Outstanding mechanical properties of silk protein-based materials, such as high strength and toughness, result from its protein structure including nanoscale beta-sheet crystals and the amorphous matrix, which is determined by its amino acid sequence. Recombinant silk fibroin synthesis enables rationale design of silk-mimetic material targeted for specific applications by tailoring the primary structure. Molecular dynamics (MD) simulation of the self-assembly and mechanical deformation behavior of silk-like molecules can facilitate understanding of the relationship between amino acid sequence, nanoscale protein structure, and mechanical properties and reduce the effort involved with experimental studies.

However, previous MD modeling strategies for silk materials have not been able to accurately simulate beta crystal/amorphous structure formation as they have not distinguished between two interactions (hydrogen bond and beta-sheet stacking bond). Furthermore, a common shortcoming of MD simulations is the lack of description of bond forming/breaking, which is crucial in predicting the tensile deformation behaviors.

To address those challenges, we explore a reactive coarse-grained MD model development to simulate the self-assembly and tensile deformation of the repetitive core domain of dragline spidroins. Our MD simulation describes hydrogen bonding and beta-sheet stacking interactions separately while maintaining directional orthogonality between the two interactions. In addition, we use reactive potential which enables both bond formation and bond breaking spontaneously. Using the developed model, we investigate the effect of primary silk structures, such as i) chain length and ii) the ratios between “hard” β-sheet forming segments and “soft” amorphous segments in a single chain on the mechanical properties of the resulting silk systems. Our study reveals that the stiffness and strength of the assembled silk system increase as the hard over soft segment ratio and the chain length increases. We are able to observe beta-sheet crystal formation during self-assembly simulations and the gradual orientation of the crystals until failure during uni-axial tensile simulations by molecular-level post-characterization in the simulation such as isotropy and bond count analysis. We are also examining the melt-processibility of our silk-like macromolecules at different temperatures, which will help suggest design parameters for obtaining thermoplastic silk-mimetic materials as sustainable plastics.