Geometric confinement governs the rupture strength of H-bond assemblies at a critical length scale
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1061-MM08-02
Geometric confinement governs the rupture strength of H-bond assemblies at a critical length scale Sinan Keten, and Markus J. Buehler Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139 ABSTRACT Using theoretical considerations and MD simulation techniques, we show that the rupture strength of H-bond assemblies is governed by geometric confinement effects, suggesting that clusters of at most 3-4 H-bonds break concurrently, even under uniform shear loading of a much larger number of H-bonds. This universally valid result leads to an intrinsic strength limitation that suggests that shorter strands with less H-bonds achieve the highest shear strength. Our finding explains how the intrinsic strength limitation of H-bonds is overcome by the formation of a nanocomposite structure of H-bond clusters, thereby enabling the formation larger, much stronger beta-sheet structures. Our results explain recent experimental proteomics data, suggesting a correlation between the shear strength and the prevalence of beta-strand lengths in biology. INTRODUCTION Mechanical behavior of biological structural protein materials under physiological or external stress conditions is exceedingly complex, partly due to the hierarchical architecture of these materials that extends from nano to macro [1-5]. Elasticity, strength and the extreme toughness of these lightweight materials can be attributed to the nanostructure of their building blocks, which are single protein biomolecules that create filaments, fibrils, and fibers through self-assembly. The strength of biological materials at the nano-scale is linked to protein unfolding, where the rupture of interstrand H-bonds primarily controls the mechanical resistance (Figure 1(a) illustrates the role of H-bonds in stabilizing key domains in the muscle tissue protein titin, Z1-Z2 telethonin complex and I27). Atomistic simulation [6, 7] and single-molecule force spectroscopy studies [8, 9] have shown that beta-sheet rich proteins exhibit particularly large rupture forces, since they employ parallel strands with numerous H-bonds that act as mechanical clamps under shear loading [10-13]. The unfolding behavior depends strongly on the pulling velocity, as discussed extensively in the literature [14-16]. Figure 1. Schematics. Beta-structures are commonly found in proteins with mechanical function (subplot (a), where hydrogen bonds behave cooperatively and act as mechanical clamps (subplot b). We consider a model system like shown in subplot (b) and study two deformation modes, referred to as SHEAR and TEAR (subplots (c) and (d)). In TEAR mode, middle strand is pulled perpendicular to the sheet plane, leading to sequential rupture of H-bonds. In SHEAR mode, the middle strand is pulled in the direction of the beta-strand, enabling uniform deformation of H-bonds.
Despite significant advancements in our understanding of the nanomechanics of biological materials [8-13, 17, 18], several key fundamental questio
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