Spider Silk - The Functional Fold: Amyloid Structures in Nature - page 78

similar to amyloid fibrils, are straight, rigid, unbranched, and up to
several micrometers long.

21
Like amyloids, spider silk nanofibrils bind the dyes Congo red
and thioflavin T, further indicating that silk fibrils show certain
amyloidogenic characteristics.

21

Not only silk nanofibrils bind Congo
red, but spider dragline silk derived from Nephila edulis was also

22

found to bind this dye. Since the specificity of dye binding depends

on solution and staining conditions

23,24

(there are some examples
of cross-reactions of Congo red with non-amyloidogenic aggregates,
globular proteins, and even non-proteinaceous materials), Congo
red binding has to be considered with caution.

25-27

A more detailed
analysis of secondary structural features, especially of antiparallel or
parallel β-sheets, can be obtained by Fourier transformed infrared
(FTIR) spectroscopy, and the orientation of the strands forming
β-sheets (cross-β) is typically monitored by X-ray fibre diffraction
studies (see the following text).

5.2.1 Specific Structural Features of Silks

As mentioned earlier, silks of insects and spiders are classified
according to their predominant secondary structure.

5,28

α-silks show
a high content of α-helical structure, with a X-ray fibre diffraction
(XRD) pattern similar to that obtained for α-keratin. Their amino
acid composition differs significantly from that of other silks with a
much lower content of glycine and an elaborated amount of charged
residues.

5,28,29
β-Silks are characterized by their high proportion of β-sheet
structure as detected by methods like FTIR or Raman micro-
spectroscopy. As an example, Bombyx mori cocoon silk contains 50%

30

and Analysing the

maxima of the amide I band of FTIR spectra of dragline spider silk
revealed peaks at 1637 cm

Nephila dragline silk 37 % of β-sheet structure.

−

1

,

31

of silk produced by the green lacewing

− 1 29

C. flava at 1625 cm , and of nanofibrils formed by recombinant

− 1 21

spider silk at 1623 cm all of which are representative for β-sheet

rich structures. Interestingly, the later wave numbers are within a
range specific for amyloid fibrils.

,

21,32

The arrangements of β-pleated sheets in β-silks can also be
determined by XRD. XRD spectra of egg stalk silk reveal a

series of layer lines corresponding to a (pseudo)repeat of 9.48 Å
on the meridian and a strong equatorial reflection signal at 5.45 Å

C. flava