Nanomechanical properties of SSTSAA microcrystals are dominated by the inter-sheet packing

Amyloid fibrils have been associated with human disease for many decades, but it has also become apparent that they play a functional, non-disease-related role in e.g. bacteria and mammals. Moreover, they have been shown to possess interesting mechanical properties that can be harnessed for future man-made applications. Here, the mechanical behaviour of SSTSAA microcrystals has been investigated. The SSTSAA peptide organization in these microcrystals has been related to that in the corresponding amyloid fibrils. Using high-pressure X-ray diffraction experiments, the bulk modulus K, which is the reciprocal of the compressibility β, has been calculated to be 2.48 GPa. This indicates that the fibrils are tightly packed, although the packing of most native globular proteins is even better. It is shown that the value of the bulk modulus is mainly determined by the compression along the c-axis, that relates to the inter-sheet distance in the fibrils. These findings corroborate earlier data obtained by AFM and molecular dynamics simulations that showed that mechanical resistance varies according to the direction of the applied strain, which can be related to packing and hydrogen bond contributions. Pressure experiments provide complementary information to these techniques and help to acquire a full mechanical characterization of biomolecular assemblies. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.