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Fig. 2. A conceptual model representing the distribution of hydrogen bond energies
contained within proline-rich (+Pro) Araneus and proline-deficient
(–Pro) Nephila MA silk networks. Green peaks represent bond
energies in poly-alanine β-sheet crystals; black peaks represent bond
energies in the network chains in Araneus silk, and red peaks
represent bond energies in the network chains in Nephila MA silk.
(A,B) Two possible scenarios for the bond energy distributions of the silks in
the dry state. In A the distribution of hydrogen bond energies in +Pro and
–Pro networks are not sufficiently different to be detected in the dry
mechanics. In B the difference in hydrogen bond energies between +Pro and
–Pro networks is sufficiently large that there are significant
differences in the dry mechanics. According to both dry models, the peaks
associated with hydrogen-bonded structures are well above the energy
associated with Brownian motion, kT, and thus represent stable structures. (C)
Hydration weakens the hydrogen bond strengths of the +Pro networks, and
consequently the +Pro peak shifts to levels that are near or below kT. It is
unclear where on this scale the –Pro peak might shift; however, if it
remains above kT, we hypothesize that there would be a difference in the
properties of hydrated MA silks. The poly-alanine β-sheet crystals are
stable both in the dry and hydrated state, and it is unclear if the crystals
`soften' in the presence of water. We arbitrarily shifted the poly-alanine
peak to the left slightly, but the position well to the right of kT indicates
that the poly-alanine crystals are quite stable in water.
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