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63
globular structures of 1-15 µm in diameter.
The presence of shear
force, resulting from the elongational flow within the spinning duct,
further leads to a stretching and elongation of the globules. An
interaction of these “elongated globules” is proposed to be the basis
for fibril formation upon further alignment of the molecules.
63
4
is based on the
consideration of the spinning dope as a crystalline liquid, which
is able to flow while displaying a high orientational order. Typical
arrangements of biopolymers at sufficiently high concentration
include nematic and hexagonal columnar phases.
The theory presented by Vollrath
et al
.
4,65,66
In the major
ampulla gland, the long axes of the spider silk proteins have been
described to be oriented perpendicular to the epithelial wall right
after secretion.
4
Approaching the centre of the ampulla the long
axes of the molecules are aligned with the gland's wall (Fig. 5.4b).
This arrangement, called an escaped nematic texture, is proposed to
change again upon narrowing of the duct. Bending of the molecules
backwards and forwards would lead to the formation of a bi-
layered disc. Further application of shear forces and an accelerating
elongational flow finally leads to an alignment of these structures.
Upon changing the chemical parameters and water content, as
described earlier, thread formation is completed.
In a recent review it was suggested that these two theories
might not exclude each other, but rather reflect different systems
investigated involving different protein concentrations.
While
amphiphilic molecules will spontaneously form micelles at lower
protein concentrations, they will arrange into hexagonal columns at
higher concentrations, thereby representing a general characteristic
behaviour of lyotropic liquid crystal systems involving amphiphilic
molecules.
14
14,67
A third theory is based on a nucleation-aggregation process
similar to amyloid fibril formation.
68
It implies the formation of
β-sheet aggregates as a result of the stress-induced conformational
transition of some molecules into a β-sheet-rich structure followed
by further self-assembly. These assemblies then act as nuclei for
further unstructured chains. A recent study revealed a structural
composition of the stored spider silk proteins typical for natively
unfolded proteins, where the proteins adopt random coil or
polyproline II-helical structures.
69
Such structure would indeed
allow a rapid and energetically favoured conversion to β-pleated
sheets. Supporting this theory, nanofibrils similar to amyloid fibrils
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