Bottom-Up Multiscale Simulations of Silk Fiber Mechanics

Abstract

Silk is an intriguing protein-based material that combines elasticity and strength to an extent not yet reached by any synthetic material today. Silk fibers are composed of highly-ordered beta-sheet crystals and an amorphous peptide matrix that both contribute to its outstanding mechanical properties. However, how the way of organization of these two components affects silk fiber mechanics has remained unclear. To quantify the mechanics of silk fibers and its components, we here in a bottom-up approach extract elastic and rupture parameters of silk composite units from all-atom molecular simulations and a novel force distribution analysis serve as input for finite element analysis [1,2]. By doing so, we can derive macroscopic fiber mechanics from the nano-structure, in quantitative agreement with experiments. One of our most striking predictions is that a serial arrangement of silk crystals in the fiber, as commonly observed in form of lamellae for other block copolymers, outperforms a random distribution, in sharp contrast to the current view of silk fiber organization [3]. We also show why the typical beta-strand length of eight residues in silk crystals is mechanically optimal [2]. Finally, a smaller cross-sectional area of silk crystals (∼1nm2) in fibers provides a better reinforcement of the amorphous phase than larger ones. [3] We expect our straightforward multiscale approach to serve as a guideline for the design of silk-like synthetic materials. 1. S Xiao, W Stacklies, M Cetinkaya, B Markert, and F Graeter. Biophys J, 96(10):3997–4005, May 2009. 2. S Xiao, W Stacklies, C Debes, and F Graeter. Soft Matter, 2010. 3. M Cetinkaya, S Xiao, B Markert, W Stacklies, and F Graeter. in review, 2010.