Spider Silk: Structure, Engineering, and Applications - Soft Fibrillar Materials: Fabrication and Applications - page 182

6.2
Mechanical Design of Spider Silk

6.2.1
Hierarchical Structure of Spider Silk

Before one can biotechnologically mimic silk or any complex protein structure,
the first step is to understand the structure underlying the function. Spider
dragline is a composite material with a hierarchical structure, composed mainly
of two proteins, Spidroins I and II [3]. The molecular structure that leads to the
functional properties of the material, however, is far from being understood. There
is increasing evidence that the structure higher up in the hierarchy is not solely
determined by the primary structure: beyond the secondary structure there is also a
tertiary structure as well as the nanocrystals and a nanofibrillar organization of the
silk thread [9]. The poly-Ala domains in Spidroin I (SPI) have been characterized by
(nuclear magnetic resonance) NMR and X-ray diffraction (XRD). They were found
to be predominantly in -sheet conformation [10] and to organized into crystallites

[11]. The crystalline portion of the fibroins consists of antiparallel pleated sheets
of polypeptide chains packed into an orthorhombic unit cell [12]. These crystallites
are interconnected in an amorphous glycine-rich matrix [13]. The intramolecular
as well as the intermolecular organization of the proteins of the dragline silk thread
are critical for the performance of spider silk [14]. The spinning process has a
significant impact on silk structure and the mechanical properties of the silk [15].
It has been found that the mechanical properties of the Nephila spider dragline silk
vary considerably with the speed of drawing [2, 16]. Based on the structural features
of spider dragline silk obtained from X-ray scattering and atomic force microscope
(AFM) measurements, a model has been set up to describe the structure of spider
dragline silk at the nanoscale (Figure 6.2).
The fibril structure of spider dragline silk revealed by Scanning Electron Micro-
scope (SEM) and AFM (Figure 6.2a,b) shows that the silk thread, having a diameter
of 4-5

β

m, consists of a number of silk fibrils with a diameter of 40-80 nm [17-19].
It is not surprising to find that a silk thread is composed of so many fibrils along
the thread axis, considering that the structure of the spider spinneret has hundreds
of tubes coming from the silk glands [20]. According to AFM images, a silk fibril is
not cylindrical. Some segments of a fibril are interlinked with each other. Inside the
''segment,'' several

μ

-structure
(noncrystalline domains). Many of these segments are interlinked with each other,
so as to constitute one single silk fibril along the silk thread axis. The size
[17, 21-23] and orientation [17, 24] of the crystallites as well as the intercrystallite
distance [17] within the silk fibril are found to be important parameters that control
the mechanical properties of silk fibers.
The hierarchically built fiber is produced by the assembly of silk proteins through
the spider's sophisticated spinning spinneret [9]. The silk assembly process occurs
via a lamellar liquid crystalline assembly into nano fibrils to form the final silk
thread (Figure 6.3). A diagrammatic optical section of the secretory part (A- and

β -sheets (crystalline domains) are connected by non- β