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Fig. 10. Molecular model of yield and self-healing in the distal byssal thread. Our
data suggests that His–metal coordinate cross-links in the termini of
the preCols are bonds that are reversibly sacrificed in yield and
self-healing. A difference in healing rate between species, despite high
homology of the His-rich domains, suggests that there is a separate entropic
elastomeric component driving recovery. Since preCol D and preCol NG are
co-localized in the distal thread, and since preCols are believed to align in
register, we propose that the flanking domain of NG is playing this role. In
this model, the stiffness of the amorphous Gly-rich flanking domain of preCol
NG is roughly the same as the His-rich domain of preCol D, and the silk domain
and NG His-rich domain are somewhat stiffer. During yield, the His–metal
cross-links of preCol D begin to rupture as the Gly-rich domain begins to
unravel, whereas the stiffer silk domain, collagen domains, and NG His-rich
domain stay folded. When the tensile force is released, there is a
time-dependent entropic drive for the Gly-rich domain of preCol NG to recover
its initial length. In doing so, the histidine residues are brought back
within proximity of one another, allowing the metal coordination bonds to
reform and initial mechanical properties to be recovered. The covalent bonds
between the ends of preCols in series have been proposed to be diDOPA
cross-links.
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