Localised semicrystalline phases of MaSp1 proteins show high sensitivity to overshearing in β-sheet nanocrystals

We consider the effects of β-sheet over-shearing on the development of semi-crystallinity in clusters of MaSp1 spider dragline silk proteins. Using molecular dynamics methods, we find that the extent of crystalline matter in the localised region of an over-sheared nanocrystal can in fact increase through over-shearing β-sheets. This is due to both; the influence of β-sheets on surrounding amorphous matter and the influence of amorphous matter in destabilising β-sheets at their interfaces. In both cases the result is the development of increased molecular order. Beyond a critical distance of over-shearing, crystallinity decreases due to a decrease in the stability of both over-sheared β-sheets and amorphous matter. Crystallinity of a system as a function of overshearing of β-sheets follows a Gaussian form. Larger stacks of overshearing β-sheets are energetically more stable than smaller stacks and are consequently able to develop the highest levels of crystallinity within the model systems. The elastic moduli computed herein indicate that the most crystalline oversheared systems are heightened in stiffness through overshearing, though there is concurrently a decrease in extensibility.