Biology Reference
In-Depth Information
pH changes from 6.9 to 6.3, and the concentration of the proteins in
the spinning dope increases by the extraction of water.
4,60
To clarify the individual influence of these factors, the assembly
of recombinant spider silk proteins was investigated using a
microfluidic device.
61
Upon exposition to potassium phosphate,
the proteins formed spherical aggregates.
61,62
The aggregates could
be merged to finally form a fibre by simultaneously dropping the
pH value and employing an elongational flow.
62
These precisely
controlled triggers occurring in the distal part of the spinning duct
are thought to assist in the assembly and locking of the final state of
the protein molecules.
17-20
is completed by
stretching accomplished by pulling the silk. Recently, a salt-, pH-, and
shear-sensitive dimerization behaviour of the short non-repetitive
domains at the protein's termini has been demonstrated, stressing
their relevance for fibre assembly.
Thread formation
in
vivo
17-20
Finally, the pulling step reflects a fibre finishing procedure
that depends on several parameters such as reeling speed,
temperature, and relative humidity, which critically influence
mechanical properties of the final thread.
The fact, that silks
with various secondary structures, such as α-silks or cross-β silks,
can be transformed into parallel-β silks upon mechanical stress
15,22,54
5,33
underlines that the mode of processing has a great influence on silk
structure and its properties.
5.2.2.2 Models for silk assembly
It is assumed, that the amphiphilic nature of spider silk proteins
leads to a pre-orientation of the proteins inside the ampulla, thereby
supporting solubility and preventing premature aggregation. Three
mechanisms concerning this pre-orientation or pre-alignment have
been discussed in the literature.
The first mechanism involves the arrangement of the proteins
into micelle-like structures. Small droplets were found in solutions
collected directly from the spinning gland of
63
B. mori
.
In vitro
63
investigations on regenerated silk fibroin originating from
B. mori
17,19,61,64
and recombinantly produced spider silk proteins
support
the theory of micelle formation. It is suggested that the hydrophilic
non-repetitive regions of the amphiphilic proteins form the outer
shell of such structures, while the more hydrophobic parts are buried
inside. With increasing protein concentration, the observed micelles
in the gland of
B. mori
fuse via coalescence, forming observable
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