Properties+of+Spider+Silk

Dragline spider silk is a phenomenal substance. Its numerous uses are just beginning to be discovered. Dragline silk is classified as such because it is the silk that is used along the radii and the outside of the spider`s web, as well as the dragline that the spider uses to lower itself. Despite how thin the material is, this type of silk is exceptionally strong and elastic. The reasons surrounding this strength and elasticity are attributed to the evolutionary purpose of the silk itself. The spider’s silk needs to be thin in order to increase efficiency and to conserve energy while spinning the web. Since the web is the main source of food for the spider, it needs to have the proper amount of elasticity so a flying insect does not simply bounce off. The web also needs to be incredibly strong so the flying insect does not simply fly through [1]. Because of the dragline silks elasticity and strength, companies are very interested in using dragline spider silk as a new textile or substance.

flat

=**Protein Structure/Protein Folding** =

The general structure of the dragline silk is a “semicrystalline polymer that has crystalline regions that are embedded in the general matrix of the silk” (Huemmerich et al, 2004) [5]. Dragline spider silk is also called major ampullate (MA) silk. It has been found that this MA silk of the spider called //Nephila clavipes// is made of two types of proteins: MaSp1 and MaSp2 (major ampullate Spidroins). These two proteins have been found to have masses between 180kDa and 720kDa. The main difference found between these two proteins is that MaSp1 is proline deficient, whereas MaSp2 is proline rich [3]. It is suggested that the MaSp1 forms the crystallized regions, where MaSp2 forms the matrix surrounding the crystals[5].

DNA analysis has shown that the MaSp proteins have over 100 different amino acids, and can be isolated into 4 different motifs: GPGXX (where X often equals Q), An stretches, GGX, and spacers. The GPGXX is thought to be β-turn spirals, which cause the elasticity in the silk. As dragline silk is not as elastic as other silks, it is hypothesised that there are at most 9 GPGXX motifs before an interruption in dragline silks as compared to the other silks in the web. The An motifs are 6-9 alanine residues that forms the β-sheets in the protein. These long alanine residues are thought to be the cause of dragline silk’s exceptional strength [3]. These An motifs also form secondary β structures that form the cross links that result once the protein is spun by the spider [4]. The GGX motif forms the matrix that is used in the connection of the crystalline regions. Finally, the spacers contain charged amino acids, and are used to separate the peptides into clusters [3]. Evidence of a glycine-rich 31-helix secondary structure has also been hypothesised [5]. An example of the protein is “(GGAGQGGYGGLGSQGAGRGGLGGQGAG**AAAAAA**GGAGQGGYGGLGSQGAGRGGLGGQG AG)N where the bolded ‘**A**’ identifies the region that forms β-sheet nanocrystals and the rest forms semiamorphous domains” (Giesa et al, 2011)[8].

=Mechanical Properties =

Dragline spider silk is especially adept at absorbing the kinetic energy of a flying insect. The energy to break this silk is approximately 1x105 J Kg-1. As seen in the Table 1, this is larger than Kevlar, high tensile steel and bone. Kevlar is stronger, but spider silk can extend much farther before breaking than these other materials [1]. Dragline silk can therefore be classified as a rubber. A classified “is composed of a large number of kinetically free, random polymer chains that are cross-linked to form a macroscopic network” (Gosline et at, 1984). Spider silk does not have this specific structure, however when it is contracted (has absorbed water), it shows the same ratio of enthalpy and internal energy as other rubbers [2].

Although dragline spider silk can be thought of as a new ‘super-substance’, it begins to change its behaviors at different temperatures. At very low temperatures, it was found that dragline silk had an increase of strength, showing that is was more energy absorbent than synthetic polymer fibres. The strength decreased, however, with an increase of temperature over 60°C. There are a two main temperatures where the protein begins to breakdown. The first, at 198°C, is attributed to the breakdown of the crystal phase of the protein into a liquid. The second, at 309°C, is attributed to the partial re-crystallization of the silk [7].

**Table 1 Approximate Mechanical Properties for Some Structural Materials [1]**
 * = **Material** ||= **Modulus (N m-2)** ||= **Strength (N m-2)** ||= **Energy to break (J kg-1)** ||
 * = Spider frame silk ||= 1 x 1010 ||= 1 x 109 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">1 x 105 ||
 * = <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">KEVLAR ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">1 x 1011 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">4 x 109 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">3 x 104 ||
 * = <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Cellulose fibres ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">3 x 1010 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">8 x 108 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">9 x 103 ||
 * = <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">High tensile steel ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">2 x 1011 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">2 x 109 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">1 x 103 ||
 * = <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Tendon ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">1 x 109 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">1 x 108 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">5 x 103 ||
 * = <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Bone ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">2 x 1010 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">2 x 108 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">3 x 103 ||
 * = <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Rubber ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">ca. 106 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">1 x 108 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">8 x 104 ||
 * = <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">Viscid silk ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">3 x 106 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">5 x 108 ||= <span style="display: block; font-family: 'Times New Roman',Times,serif; font-size: 120%; text-align: center;">1 x 105 ||

<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**References**
 * 1) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> Gosline JM, Demont ME, Denny MW. 1986. The structure and properties of spider silk. Endeavour. 10(1):37-43.
 * 2) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> Gosline JM, Denny MW, Demont ME. 1984. Spider silk as rubber. Nature. 309:551-552.
 * 3) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> Scheibel T. 2004. Spider silks: recombinant syntheses, assembly, spinning, and engineering of synthetic proteins. Microbial Cell Factories.3:14.
 * 4) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> Bini E, Foo CWP, Huang J, Karageorgiou V, Kitchel B, Kaplan DL. 2006. RGD-functionalized bioengineered spider dragline silk biomaterial. Biomacromolecues. 7:3139-3145.
 * 5) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> Huemmerich D, Scheibel T, Vollrath F, Cohen S, Gat U, Ittah S. 2004. Novel assembly properties of recombinant spider dragline silk proteins. Current Biology. 14:2070-2074.
 * 6) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> Du N, Liu XY, Narayanan J, Li L, Lim MLM, Li D. 2006. Design of superior spider silk: from nanostructure to mechanical properties. Biophysics Journal. 91:4528-4535
 * 7) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> Yang Y, Chen X, Shao Z, Zhou P, Porter D, Knight DP, Vollrath F. 2005. Toughness of spider silk at high and low temperatures. Adv. Mater. 17(1):84-88.
 * 8) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> Giesa T, Arslan M, Pugno NM, Buehler MJ. 2011. Nanoconfinement of spider silk fibrils begets superior strength, extensibility, and toughness. Nano lett. 11(11):5038-5046.