Spider Silk: Structure, Engineering, and Applications - Soft Fibrillar Materials: Fabrication and Applications - page 186

Since the above two types of silk fibers show distinct stress-strain profiles,
it is important to understand the structural origin of the stress-strain behavior
of spider dragline silk. Two types of -structures are identified in silks by XRD

and Fourier transform infrared spectroscopy (FTIR): the intra-molecular

β

β -sheets

(non-crystalline β -sheets) [13] and inter-molecular β -sheets ( β -crystallites). The

β -crystallites (orthogonal unit cell, a = 1.03, b = 0.944, c = 0.695 nm for spider

N. pilipes dragline silk [37] and a 0.698 nm for silkworm

B. mori silk [38]) constructed by several neighboring silk protein molecules are
the intermolecular

= 0.938, b = 0.949, c =

-sheets that crosslink individual silk protein molecules [39]
so as to form the molecular network [13]. The intra-molecular

β

β -sheets, on the

other hand, are merely normal -sheets folded within individual silk protein

molecules. To identify the proportion of these two

β

β -structures, XRD was used to

determine the content of -crystallites (crystallinity) [40], while FTIR was used to

examine the total amount of secondary structure in silk fiber [41]. Deducting the
content of

β

-structures
by FTIR, the percentage of intra-molecular β-sheets can be obtained. For the
dragline filaments of N. pilipes spiders, it was found that 57% of the total β-sheets
are in the amorphous region. The remaining 43% of β-sheets are β-crystallites
[30]. This is consistent with previous results from Raman and NMR studies of
Nephila spiders [13, 42], which showed the existence of unaggregated

β -crystallites measured by XRD from the total content of β

β -sheets

comprising 60% of the alanine residues (the major amino acid in the β -sheet

conformation of spidroin) [13, 43] and highly oriented rigid -crystallites comprising

40% of the alanines. In contrast, the amount of intra-molecular

β

-sheets in
silkworm ( B. mori ) silk is only 18%, which is much less than that in spider
draglines.
Figure 6.5 demonstrates a hypothesis of how the two

β

-structures contribute to
their stress-strain profiles. Upon stretching, the intra-molecular

β

-sheets within
the amorphous regions unfold first (since they are weaker) to release the length
of protein chains, while the

β

-crystallites remain unaffected. This will give rise to
a high extension of draglines, without breaking the inter-molecular linkages. This
occurs after the yield point S in Figure 6.4. At the initial stage of protein unfolding,
modulus drops to nearly zero as fiber extension is mainly caused by the breaking of
weak intra-molecular hydrogen bonds. As the progressive unfolding and alignment
of protein chains continue, protein backbones and nodes of the molecular network
are stretched to support the load. Consequently the dragline filaments become
stiffer due to the contribution from the enthalpic component [44]. This is how
strain hardening occurs in spider draglines. Single protein molecule stretching
with an AFM tip reveals similar unfolding behavior of recombinant spider silk
protein [14] and other elastic proteins [45]. The further stretching beyond the
inflection point H will cause the breaking of the

β

-crystallites, demolishing the
crosslinks of the molecular networks in the silk filaments, thus causing weakening
of the silk filaments (so-called strain-weakening). This model is consistent with the
X-ray data of deformed fiber, showing that the

β

-crystallinity of the dragline start
declining once a spider dragline is stretched beyond point H. On the other hand,
silkworm silk has far fewer intra-molecular β-sheets in the amorphous region;

β