6 Cocoon Silk: From Mesoscopic Materials Design …
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the form of glycine-X (GX), where X can be alanine, serine, tyrosine, valine, or threonine. Specifically, the hydrophobic domains can be divided into four distinct subdomains, including the GAGAGS repeats, GAGAGY repeats, GAAS tetrapeptides,
and irregular sequences [40]. Statistically, the hexapeptide GAGAGS and GAGAGY
repeats are the most abundant; they are present in 433 and 120 copies, respectively,
accounting for 72% of all the repeat dipeptides.
It is reported that these four subdomains play distinct roles. For instance, the
(GAGAGS) n repeats are the main units constituting β-sheets [41] and are the
only units that can organize themselves into well-defined β-sheet crystallites (βcrystallites). However, no well-defined crystalline structures have yet been formed by
GAGAGY repeats. Although some evidence indicates that GAGAGY repeats might
form β-crystallites, the structural units (especially the inter-sheet distances) within
such β-crystallites are significantly different from those formed by (GAGAGS) n
repeats. The GAAS tetrapeptides, which are located between the (GAGAGS) n repeats
and serve as β-strand turns, can limit the size of (GAGAGS) n β-crystallites. Similarly, by forming a loop structure with a distorted Ω shape, irregular sequences can
reverse the direction of β-strand backbones, which control the crystallite dimensions. In addition, a reversion of the backbone direction might also enable adjacent
β-strands to be in contact with each other to facilitate the occurrence of inter-strand
interactions.
Apart from domestic mulberry B. mori silkworms, many other arachnids or insects
can also produce silk. Among these, the fibers from non-mulberry A. mylitta and
A. pernyi silkworms are two typical examples and have been extensively studied.
Irrespective of the sources, all these silk fibers are primarily composed of proteins
and have a hierarchical structure similar to that of B. mori fibers. However, the
amino acid sequences of these silk proteins can vary significantly from one species
to another, which results in silk proteins having distinct mechanical properties. For
instance, A. pernyi silk fibers reportedly present significant differences in both the
amino acid compositions and amino acid sequences compared to those of B. mori
silk fibers (Fig. 6.9d). The primary repetitive domains of A. pernyi silk fibers are the
alternating tandem repeats of the poly-alanine motif. In addition, it was confirmed
that such poly-alanine motifs serve as structural units for the construction of β-sheets
and higher-level structures.
6.3.2 Secondary (Level Two) Structures of SF Materials
Secondary structure refers to the symmetrical structures formed by folding peptide
chains, which are stabilized by hydrogen bonding (H-bonding) between the amine
and carboxyl groups (the backbone-backbone and sidechain-sidechain hydrogen
bonds are non-relevant). The most common secondary structural elements are αhelices and β-sheets (Fig. 6.10a) [37]. Random coils refer to the multi-conformational
state of peptides, unfolded proteins, and polypeptide chains. Sometimes, random coil
is also adopted to describe unstructured conformations. It should be noted that random
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the form of glycine-X (GX), where X can be alanine, serine, tyrosine, valine, or threonine. Specifically, the hydrophobic domains can be divided into four distinct subdomains, including the GAGAGS repeats, GAGAGY repeats, GAAS tetrapeptides,
and irregular sequences [40]. Statistically, the hexapeptide GAGAGS and GAGAGY
repeats are the most abundant; they are present in 433 and 120 copies, respectively,
accounting for 72% of all the repeat dipeptides.
It is reported that these four subdomains play distinct roles. For instance, the
(GAGAGS) n repeats are the main units constituting β-sheets [41] and are the
only units that can organize themselves into well-defined β-sheet crystallites (βcrystallites). However, no well-defined crystalline structures have yet been formed by
GAGAGY repeats. Although some evidence indicates that GAGAGY repeats might
form β-crystallites, the structural units (especially the inter-sheet distances) within
such β-crystallites are significantly different from those formed by (GAGAGS) n
repeats. The GAAS tetrapeptides, which are located between the (GAGAGS) n repeats
and serve as β-strand turns, can limit the size of (GAGAGS) n β-crystallites. Similarly, by forming a loop structure with a distorted Ω shape, irregular sequences can
reverse the direction of β-strand backbones, which control the crystallite dimensions. In addition, a reversion of the backbone direction might also enable adjacent
β-strands to be in contact with each other to facilitate the occurrence of inter-strand
interactions.
Apart from domestic mulberry B. mori silkworms, many other arachnids or insects
can also produce silk. Among these, the fibers from non-mulberry A. mylitta and
A. pernyi silkworms are two typical examples and have been extensively studied.
Irrespective of the sources, all these silk fibers are primarily composed of proteins
and have a hierarchical structure similar to that of B. mori fibers. However, the
amino acid sequences of these silk proteins can vary significantly from one species
to another, which results in silk proteins having distinct mechanical properties. For
instance, A. pernyi silk fibers reportedly present significant differences in both the
amino acid compositions and amino acid sequences compared to those of B. mori
silk fibers (Fig. 6.9d). The primary repetitive domains of A. pernyi silk fibers are the
alternating tandem repeats of the poly-alanine motif. In addition, it was confirmed
that such poly-alanine motifs serve as structural units for the construction of β-sheets
and higher-level structures.
6.3.2 Secondary (Level Two) Structures of SF Materials
Secondary structure refers to the symmetrical structures formed by folding peptide
chains, which are stabilized by hydrogen bonding (H-bonding) between the amine
and carboxyl groups (the backbone-backbone and sidechain-sidechain hydrogen
bonds are non-relevant). The most common secondary structural elements are αhelices and β-sheets (Fig. 6.10a) [37]. Random coils refer to the multi-conformational
state of peptides, unfolded proteins, and polypeptide chains. Sometimes, random coil
is also adopted to describe unstructured conformations. It should be noted that random