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sing polypeptides consisting of the amino acid motif GAGAGS of
B. mori
30,41,42
cocoon silk
and of polyalanine segments of spider
36,43,44
draglines.
Recent studies revealed that beneath polyalanine
stretches also GGA motifs are likely to be incorporated into
β-sheet structures of dragline silk,
30
even though glycine-rich
motifs such as GGA or GPGQQ preferentially do not form crystalline
β-sheet structures.
43
-helical structures (e.g., polyglycine
II-helices), β-turns (type I and III), and some unordered structures
were found in dragline silk, which are thought to build an amorphous
matrix for the crystalline-like β-sheet structures.
Further, 3
1
15,30,45,46
Typical
features of the amorphous matrix are a high content of glycine and,
in
B. mori
, tyrosine residues, arranged in amino acid motifs like
47
GAGAGY.
However, NMR studies indicated the existence of regions
with different degree of orientation in the amorphous matrix,
including less ordered/isolated β-sheets as well as aligned and pre-
stressed chains.
43,44,48
Recent FTIR studies suggest a model where the
highly oriented β-sheet crystals are interconnected by amorphous
chains displaying a Gaussian distribution of pre-strain.
48-50
This hierarchical composition also bears the explanation for
the observed outstanding mechanical properties of spider silks.
In general, the β-sheet crystallites are thought to account for the
mechanical strength of the final thread and to be incorporated like
rigid fillers in the amorphous region,
51
which most likely accounts
for the elasticity. Indeed, silks with higher elasticity have been shown
to contain a larger fraction of flexible structures like β-spirals or
3
30,52
Further, the degree of pre-strain of the interconnecting
chains has been proposed to play a key role in controlling and fine-
tuning the final characteristics.
-helices.
1
53
Another parameter influencing
the mechanical stability is thought to be the orientation of the
crystallites, which can be triggered by the mode of spinning, with
increasing spinning speed leading to a higher orientation within the
thread.
54,55
During supercontraction of the fibre — a phenomenon
which is observed upon wetting leading to shrinkage of the fibre
accompanied by an increase in diameter — the thread is plasticized
and some orientation is lost.
36,56
Supercontraction is followed by a
more rubber-like behaviour of the fibre. However, after re-stretching,
the crystals are thought to be re-aligned, resulting in the original
degree of orientation.
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