6 Cocoon Silk: From Mesoscopic Materials Design …
255
bonds, respectively) [42], numerous H-bondings accumulated between strands can
greatly reinforce the interactions and thus strengthen the α-helix and β-sheet. In
contrast, although adjacent β-strands can run either in the same direction (parallel
arrangement) or in the opposite direction (antiparallel arrangement), the β-sheets in
silk materials naturally choose an antiparallel β-strand arrangement (Fig. 6.10b) [37].
This is because the linear H-bonds (2.76 Å) within antiparallel β-strands are shorter
than the non-linear H-bonds (2.97 Å) between parallel β-strands, which consequently
results in greater β-sheet stability [22].
β-sheets are structurally more compact and stable in aqueous environments and
have stronger mechanical properties than random coils and α-helices. In fact, most
molecules within silk materials exhibiting β-sheet conformations (Silk II structure)
are more likely to be insoluble. However, when the α-helix conformation plays a
dominant role (Silk I structure), such materials can be dissolved easily. In addition,
because the α-helix consists of easily movable chains, it is reasonable that a higher
content of α-helix within SF materials can lead to greater flexibility.
6.3.3 Tertiary (Level Three) Structures of SF Materials
and Crystalline Binding Interaction
As mentioned at the beginning, SF materials, especially the silkworm cocoon fiber,
display a very high toughness in terms of breaking energy. This can be attributed to
the occurrence of β-crystallites and the crystal network. Typically, several secondary
structural elements might be used to create a highly compact and organized threedimensional (3D) protein structure, which is known as the tertiary structure. For
SF materials, the tertiary structure can be defined as intermolecular β-crystallites
(Fig. 6.10c) [16, 18]. Specifically, β-crystallites are comprised of several adjacent
β-sheets from different molecules, among which hydrogen bonds and hydrophobic
interactions/van der Waal’s interactions play a key role in the β-crystallite (Fig. 6.5d)
[16, 18, 22]. In this regard, these interactions can also be referred to as the crystalline
binding interaction/force. From the point of view of crystallography, β-sheets and βcrystallites can be considered as a type of polymorphism: if β-sheets are considered as
two-dimensional (2D) crystals, β-crystallites belong to a more stable form of 3D crystals. Although some β-sheets are capable of converting themselves into β-crystallites,
it is not necessary for all β-sheets to be converted into β-crystallites. In general, several
techniques, which will be introduced in the next section, can be applied to quantify
the total β-conformation and β-crystallite content. The β-conformation content is
always higher than the β-crystallite content because it consists of two types of polymorphism: the intramolecular 2D β-sheets and intermolecular 3D β-crystallites. The
intramolecular β-sheet content can be calculated simply by deducting the β-crystallite
content from the gross amount of β-conformation.
255
bonds, respectively) [42], numerous H-bondings accumulated between strands can
greatly reinforce the interactions and thus strengthen the α-helix and β-sheet. In
contrast, although adjacent β-strands can run either in the same direction (parallel
arrangement) or in the opposite direction (antiparallel arrangement), the β-sheets in
silk materials naturally choose an antiparallel β-strand arrangement (Fig. 6.10b) [37].
This is because the linear H-bonds (2.76 Å) within antiparallel β-strands are shorter
than the non-linear H-bonds (2.97 Å) between parallel β-strands, which consequently
results in greater β-sheet stability [22].
β-sheets are structurally more compact and stable in aqueous environments and
have stronger mechanical properties than random coils and α-helices. In fact, most
molecules within silk materials exhibiting β-sheet conformations (Silk II structure)
are more likely to be insoluble. However, when the α-helix conformation plays a
dominant role (Silk I structure), such materials can be dissolved easily. In addition,
because the α-helix consists of easily movable chains, it is reasonable that a higher
content of α-helix within SF materials can lead to greater flexibility.
6.3.3 Tertiary (Level Three) Structures of SF Materials
and Crystalline Binding Interaction
As mentioned at the beginning, SF materials, especially the silkworm cocoon fiber,
display a very high toughness in terms of breaking energy. This can be attributed to
the occurrence of β-crystallites and the crystal network. Typically, several secondary
structural elements might be used to create a highly compact and organized threedimensional (3D) protein structure, which is known as the tertiary structure. For
SF materials, the tertiary structure can be defined as intermolecular β-crystallites
(Fig. 6.10c) [16, 18]. Specifically, β-crystallites are comprised of several adjacent
β-sheets from different molecules, among which hydrogen bonds and hydrophobic
interactions/van der Waal’s interactions play a key role in the β-crystallite (Fig. 6.5d)
[16, 18, 22]. In this regard, these interactions can also be referred to as the crystalline
binding interaction/force. From the point of view of crystallography, β-sheets and βcrystallites can be considered as a type of polymorphism: if β-sheets are considered as
two-dimensional (2D) crystals, β-crystallites belong to a more stable form of 3D crystals. Although some β-sheets are capable of converting themselves into β-crystallites,
it is not necessary for all β-sheets to be converted into β-crystallites. In general, several
techniques, which will be introduced in the next section, can be applied to quantify
the total β-conformation and β-crystallite content. The β-conformation content is
always higher than the β-crystallite content because it consists of two types of polymorphism: the intramolecular 2D β-sheets and intermolecular 3D β-crystallites. The
intramolecular β-sheet content can be calculated simply by deducting the β-crystallite
content from the gross amount of β-conformation.