Download Types of Spider Silk The many unique characteristics of spider silk

Types of Spider Silk
The many unique characteristics of spider silk can be attributed from the
different types of spider silk. The variety of the silk comes from the ability of the
spider to produce different qualities of silks for different uses in their biological
environment. The common silks produced in most arachnids include, majorampullate silk, capture-spiral silk, tubuliform silk, aciniform silk and minor
ammuplate silk.
Though due to the extensive time it takes to produce mass amounts of this precious
silk, recent science has developed alternative methods to harvesting artificial spider
silk and other silk-like high performance materials.
Major-Ampullate Silk
Major-ampullate silk, commonly referred to as dragline silk, is a high
performance fiber with mechanical properties that carries unusual strength, about
50% stronger then the other silk fibers, however it lack extensibility.
The ecological function of dragline silk is to provides a dry frame for orb webs that
supports the sticky capture-spiral silk, and together they resist kinetic energy of
incoming flying insects i
Recent research has hypothesized the mechanical performance of dragline and
minor-ampullate silk are qualitatively more similar to one another than to the other
silks as they are both comprised of molecular models of silks fibroins that contain a
large number of poly-Ala or Gly-Ala amino acid sequence motifs ii. Blackledge et al.
(2006) published his hypothesis, that due to the higher frequency of poly-Ala motifs
in major-ampullate fibers, they form an exceptionally strong crystalline
secondary/tertiary structure, which provides insight on the greater strength and
decreased extensibility of dragline relative to minor-ampullate silkiii. They support
their assumptions with previous research that emphasizes the association of the
prevalence of poly-Ala motifs in major-ampullate silk with high tensile strengthiv.
Capture-Spiral Silk
Supported by the dry major-ampullate silk of orb webs, capture-spiral silk
performs together with dragline to absorb kinetic energy of flying insects.
Capture-spiral silk is coated with an aqueous glue which give the distinctive quality
of being sticky. It is stretchier than most silks; found to be an order of magnitude
stretchier and 1000 times more compliant than dragline silkv. Hayashi et al. (1998)
proclaims that the long uninterrupted runs of Gly-Pro-Gly containing motifs in
flagelliform fibroins may explain much of the extreme extensibility of the capture
spiral silk. They are hypothesized to form a successive -turns that perform as
molecular springs4. Tests have confirmed that the capture-spiral silk has shown
poor results at storing energy, while studying the viscoelastic behavior of polymers,
due to the weaker intermolecular interactions relative to dragline fibroinvi.
Denny et al. (1976) found that the extremely high compliance and extensibility of
capture-spiral silk aids in the gradual deceleration of impacting insects by “cradling”
so that the prey do not bounce out of the web. During this process, the kinetic
energy of the insect is absorbed through low initial resilience of both capture-spiral
and major-ampullate silk1.
Tubuliform Silk
Tubuliform silk is unique in the sense that it is only produced by the female
spider once in their lifespan. Also known as egg case silk, tubuliform silk possess the
highest stiffness out of all the dry spider silks, and is therefore the weakest. It’s
molecular structure is composed of very long, complex repeats instead of a short
simple repeat much like the other silks3. The fibroins in tubuliform consist of unique
high serine and low glycine content which formulates the structure into both an helix and disordered conformation as revealed by circular dichroism and infrared
spectroscopyvii. Good biocompatibility and low biodegradability of egg case silk are
an advantage for use in biomaterial applicationsviii.
Aciniform Silk
The biological function of aciniform silk is used to preserve prey by using the
silk to mummify the insect and well as providing an added layer of protection for the
egg sac3. The dry aciniform silk is the most resilient of all the spider silks due to its
high strength and extensibility. It’s molecular structure has been investigated by
Hayashi et al. (2004) and results exhibit that it is composed of highly homogenized
repeats that are 200 amino acids in length with a prevalence of poly-serine and
threonineix. However due to the little data obtained on the molecular structure of
aciniform, the secondary structure has still yet to be characterized. Mechanical data
however suggests that the greater performance of the aciniform fibroin must
indicate that the structure is not composed of simple repeats of alanine and/or
glycine rich motifs much like the other dry silks9.
Minor-Ampullate
Minor-ampullate silk share many similarities with dragline silk, both
containing glycine and alanine rich motifs that form crystalline -sheets. However
minor-ampullate possesses qualities of high modulus and extensibility, and
moderate tensile strength and toughness3. In biological settings the spiders use this
silk to temporarily build a structure of the orb web, as well it is added to dragline
silk to improve structural soundness of the orb web3.
Denny, M. (1976). Physical properties of spider silks and their role in design of orbwebs. J. Exp. Biol. 65,483 -506.
ii Blackledge, T. A., Swindeman, J. E. and Hayashi, C. Y. (2005c). Quasistatic and
continuous dynamic characterization of the mechanical properties of silk from the
cobweb of the black widow spider Latrodectus hesperus. J. Exp. Biol. 208,1937 1949.
i
Blackledge, T. A., Hasyashi, C. Y. (2006) Silken toolkits: biomechanics of silk fibers
spun by orb web spider Argiope Argentata (Fabricius 1775). J. Exp. Biol. 209, 24522461.
iv Hayashi, C. Y. and Lewis, R. V. (1998). Evidence from flagelliform silk cDNA for the
structural basis of elasticity and modular nature of spider silks. J. Mol. Biol. 275,773
-784.
v Köhler, T. and Vollrath, F. (1995). Thread biomechanics in the two orb-weaving
spiders Araneus diadematus (Araneae, Araneidae) and Uloborus walckenaerius
(Araneae, Uloboridae). J. Exp. Zool. 271, 1-17.
vi Dicko, C., Knight, D., Kenney, J. M. and Vollrath, F. (2004). Secondary structures and
conformational changes in flagelliform, cylindrical, major, and minor ampullate silk
proteins. Temperature and concentration effects. Biomacromolecules 5,2105 -2115.
vii Lin, Z., Huang, W., Zhang, J., Fan, J. S., Yang, D. (2009) Solution structure of eggcase
silk protein and its implications for silk fiber formation. PNAS. Vol 106 no. 22, 89068911.
viii Gellynck K, et al. (2008) Biocompatibility and biodegradability of spider egg sac
silk. J Mater Sci: Mater Med 19:2963–2970.
ix Hayashi, C. Y., Blackledge, T. A., Lewis, R. V. (2004). Molecular and Mechanical
Characterization of Aciniform Silk: Uniformity of Iterated Sequence Modules in a
Novel Member of the Spider Silk Fibroin Gene Family. Mol Biol Evol. 21(10):1950-9.
iii