Viscoelastic relaxation time and structural evolution during length contraction of spider silk protein nanostructures
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Viscoelastic relaxation time and structural evolution during length contraction of spider silk protein nanostructures Graham Bratzel, Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue Room 1-235A&B, Cambridge, Massachusetts; Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts Zhao Qin and Markus J. Buehler, Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue Room 1-235A&B, Cambridge, Massachusetts Address all correspondence to Markus J. Buehler at [email protected] (Received 12 June 2013; accepted 21 August 2013)
Abstract Spider dragline silk is a self-assembling protein that rivals many engineering fibers in strength, extensibility, and toughness, making it a versatile biocompatible material. Here, atomistic-level structures of wildtype MaSp1 protein from the Nephila clavipes spider dragline silk sequences, obtained using an in silico approach based on replica exchange molecular dynamics and explicit water, are subjected to nanomechanical testing and released preceding failure. We approximate the relaxation time from an exponential decay function, and identify permanent changes in secondary structure. Our work provides fundamental insights into the time-dependent properties of silk and possibly other protein materials.
Spider silk achieves remarkable properties due to specific secondary and tertiary structures of its repeating amino acid units, which self-assemble into a hierarchical fibrillar structure.[1–5] The webs of higher spiders, including the Golden Orb-Weaver Nephila clavipes, are composed of different kinds of silk, each with distinct mechanical properties that are adapted for the purpose of the respective part of the web.[6] Major ampullate silk, the strongest of N. clavipes silk, is used for the dragline as well as the spokes and outer frame of the web.[7,8] Two distinct proteins are typically found in dragline silks: major ampullate spidroin proteins (MaSp) MaSp1 and MaSp2, which have different repeating units and distinct mechanical functions.[6,9,10] MaSp1 contains poly-alanine (A)n and glycine-rich (GGX)n repeats, where X typically stands for alanine (A), tyrosine (Y), leucine (L), or glutamine (Q). Recent investigations have revealed that variations in crystallinity and alignment within the silk thread, for example, due to the reeling speed of the collected sample, have been mapped to macroscopic mechanical properties by affecting the formation of the β-sheet crystals.[11–17] These cross-linking β-sheet crystals employ a dense network of aligned hydrogen bonds and have dimensions of a few nanometers.[11–13] A previous study of dragline silk employing replica exchange molecular dynamics (REMD) has yielded the first atomistic results in comparison with experimental structure identification met
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