Molecular origin of the sawtooth behavior and the toughness of nacre

We present in this paper a method to build a computer model that mimic the mineral-protein composite structure of a nacre tablet. Motivated by the interesting observations in AFM experiments of nacre, protein chains stretching out from grain boundaries are simulated by steered molecular dynamics (SMD) to gain an insight into the effect of protein-aragonite interaction on the mechanical properties of nacre and the molecular mechanisms of the sawtooth behavior. Force-extension curves are obtained and the key characteristics of sawtooth behavior are observed in SMD simulations in agreement with existing AFM experiments of nacre. The effect of water on protein-mineral interaction is investigated through including and excluding water molecules in the grain boundaries of the models. Different from the existing belief that protein unfolding is the origin of the "sawtooth" behavior, we have found that the electrostatic interactions between the protein and aragonite mineral are responsible for the sawtooth behavior and hence the high toughness of nacre.