On the strength of β-sheet crystallites of Bombyx mori silk fibroin

Silk fibroin, a natural multi-domain protein, has attracted great attention due to its superior mechanical properties such as ultra-high strength and stretchability, biocompatibility, as well as its versatile biodegradability and processability. It is mainly composed of beta-sheet crystallites and amorphous domains. Although its strength is well known to be controlled by the dissociation of protein chains from beta-sheet crystallites, the way that water as the solvent affects its strength and the reason that its theoretically predicted strength is several times higher than experimental measurement remain unclear. We perform all-atom molecular dynamics simulations on a beta-sheet crystallite of Bombyx mori silk. We find that water solvent reduces the number and strength of hydrogen bonds between beta-chains, and thus greatly weakens the strength of silk fibroin. By dissociating protein chains at different locations from the crystallite, we also find that the pulling strength for the interior chains is several times higher than that for the surface/corner chains, with the former being consistent with the theoretically predicted value, while the latter on par with the experimental value. It is shown that the weakest rupture strength controls the failure strength of silk fibre. Hence, this work sheds light on the role of water in the strength of silk fibroin and also provides clues on the origin of the strength difference between theory and experiment.