Pulling out a peptide chain from β-sheet crystallite: Propagation of instability of H-bonds under shear force

Anti-parallel beta-sheet crystallite as the main component of silk fibroin has attracted much attention due to its superior mechanical properties. In this study, we examine the processes of pulling a peptide chain from beta-sheet crystallite using steered molecular dynamics simulations to investigate the rupture behavior of the crystallite. We show that the failure of beta-sheet crystallite was accompanied by a propagation of instability of hydrogen-bonds (H-bonds) in the crystallite. In addition, we find that there is an optimum size of the crystallite at which the H-bonds can work cooperatively to achieve the highest shear strength. In addition, we find that the stiffness of loading device and the loading rates have significant effects on the rupture behavior of beta-sheet crystallite. The stiff loading device facilitates the rebinding of the H-bond network in the stick-slip motion between the chains, while the soft one suppresses it. Moreover, the rupture force of beta-sheet crystallites decreases with loading rate. Particularly, when the loading rate decreases to a critical value, the rupture force of the beta-sheet crystallite becomes independent of the loading rates. This study provides atomistic details of rupture behaviors of beta-sheet crystallite, and, therefore, sheds valuable light on the underlying mechanism of the superior mechanical properties of silk fibroin.