| Arachnids are perhaps the most ecologically pervasive predators on earth. Through over 125 million years of evolution, orb-web weavers have developed an arsenal of silken tools for the aerial arms race. The intricate multi-fiber design of the aerial orb-web has the strength to restrain a struggling insect and the proper elasticity to dissipate the energy of a prey that has the capability to jump and fly.; Orb-web weaving spiders can produce up to six fibers: major ampullate, minor ampullate, flagelliform, tubuliform, aciniform, pyriform, and one specialized glue: aggregate silk, from discrete glands. Each of these silks is designed to have a combination of properties specific to its function. Major ampullate silk, which is a composite of two proteins, MaSp1 and MaSp2, is a high-performance fiber with a blend of strength and elasticity. Surprisingly, this complex combination of material properties that is unrivaled by synthetic polymers, results from the molecular architecture and the spinning conditions. A set of four amino acid motifs: GAn, An, GGX, and GPGXX, each of which has been correlated with a secondary structure and a material property, has been identified. Crystalline beta-sheets from GAn, An, motifs confer strength to the fiber. The GPGXX motif is associated with elasticity while the GGX region is thought to stabilize the beta-crystallites. To directly confirm the role of the GPGXX motif as well as to determine the purpose for the different amino acid motifs of the Argiope aurantia MaSp2 repeat unit, which includes a specialized GPGXX motif, three constructs were engineered to alter the number of concatenated GPGXX regions (from 1 to 3). As a result of the engineering, the secondary structure showed an increase in type II beta-turns while the synthetic fibers had increased extensibility, confirming the proposed structure/function relationship. In addition to new mechanical testing methods developed to investigate the material properties of natural silk and to give a complete understanding of the design, synthetic and natural spinning conditions were investigated. The structure/function relationship of the GPGXX motif, and a more complete knowledge of natural and synthetic spinning will facilitate creating new silk-based designer biomaterials. |
