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The diffraction pattern described earlier is consistent with that of
various denatured globular and fibrous proteins, and has been found
to be present in every analysed amyloid structure.
28,35
Therefore, it
is considered to be a common feature of the amyloid core structure.
Nanofibrils formed from recombinant spider silk display a similar
pattern (Fig. 5.3b), indicating the presence of a cross-β structure.
A sharp meridional reflection at 4.7 Å is observed, as well as an
equatorial spacing of 5.3 Å.
21
A specific feature of the silk diffraction
patterns is the quite narrow inter-sheet distance (when compared
to typical amyloid patterns), which is thought to result from less
bulky amino acid side chains (there is a high content of glycine and
alanine residues in silk proteins) in the β-strands forming the cross-β
structure.
5,21,28
The slightly higher inter-sheet distance observed
for
Chrysopa
silk can be explained by the higher content of serine
residues.
5
silk to its six-fold
length, a change in the diffraction pattern is observed.
Interestingly, upon stretching of the
Chrysopa
33
The stretched
silk gives rise to reflection signals in the equatorial direction, whereas
the meridional signals are gone (Fig. 5.3c), with the degree of the
effect being proportional to the degree of fibre extension. Likely, this
effect reflects the molecular transformation of the cross-β structure
to a parallel-β structure.
The majority of silks produced by insects and spiders re-
presents parallel-β silks with β-strands of individual sheets oriented
parallel to the direction of the fibre axis.
33
As a result, XRD patterns
consist only of equatorial reflections arising from both the main
chain-and inter-sheet spacing (Fig. 5.3d). Depending on the amino
acid residues involved in β-sheet formation and on the parallel or
antiparallel orientation of the β-strands, these spacings vary for
different parallel-β silks.
5,28
7
silk shows an inter-sheet spacing
of 9.3 Å, assigned to a composition of alternating Ala-Gly residues.
B. mori
5,37
Further analysis revealed that the methyl groups of the alanine
residues alternately point to opposite sides of the sheet, resulting
in an antipolar-antiparallel arrangement of the strands within a
β-sheet.
Silks with β-sheets consisting of mostly alanine
or serine residues consequently have bigger inter-sheet spacings
ranging from 10.6 Å up to 15 Å.
30,38-40
5,37
Solid state nuclear magnetic resonance (NMR) measurements and
Raman spectroscopy verified these structural features by analy-
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