Whether and how intramolecular
crosslinks in polymeric materials
contribute to mechanical properties is debated in both experimental
and theoretical arenas. The tethering threads ofOctopus
bimaculoidesegg cases provide a rare window to investigate
this question in a biomaterial. The only detectable component of the
load-bearing fibers in octopus threads is a 135 kDa protein,octovafibrin, comprising 29 tandem repeats of epidermal
growth factor (EGF) each of which contains 3 intramolecular disulfide
linkages. The N- and C-terminal C-type lectins mediate linear end-to-end
octovafibrin self-assembly. Mechanical testing of threads shows that
the regularly spaced disulfide linkages result in improved stiffness,
toughness, and energy dissipation. In response to applied loads, molecular
dynamics and X-ray scattering show that EGF-like domains deform by
recruiting two hidden length β-sheet structures nested between
the disulfides. The results of this study further the understanding
of intramolecular crosslinking in polymers and provide a foundation
for the mechanical contributions of EGF domains to the extracellular
matrix.