Biologists have discovered that the incredibly stretchy nature of spider silk is due to the silk protein's amino acid content. Spider silks with high proline contents act like elastic rubber bands, while spider silks with low proline content behave like stretchy springs.
Spider silk is the super tough material that is stronger than steel and nylon and more stretchable than Kevlar. Spider silks and their remarkable toughness have greatly fascinated Prof John Gosline from the University of British Columbia, Canada.
Prof Gosline notes that if we are to learn how to manufacture spider silk, we must first understand the relationship between the components and the spun fiber's mechanical properties. For this reason he is focusing on major ampullate silk, one of the many silks that spiders spin.
According to Gosline, spiders use major ampullate silk for draglines and to build the frame and radial structures in webs. The entire process involves deforming and absorbing enormous amounts of energy without fracturing.
Comparing the amino acid sequences of major ampullate silk proteins from the species Araneus diadematus and Nephila clavipes, Gosline has deduced that the sequences differed on one count— Araneus silk is comparatively rich in the amino acid proline, while proline levels in Nephila silk are very low.
In order to explore further the link between proline and the silks, Gosline and his student, Ken Savage, started comparing the silks' mechanical properties to find out how the amino acid affects spider silk toughness.
Gosline discovered that spiders adjust the way they manufacture their silks depending on their circumstances, so he and Savage allowed the spiders to roam freely so that the strands of dragline silk that they dropped were as uniform as possible.
Once a reliable silk supply was established, Savage set about testing the silks' mechanical properties. The team stretched the dry silk while measuring the force on it and realized that the silks behaved almost identically. They concluded that the presence of proline had little or no effect on dry silk.
On the other hand when Savage began investigating the hydrated silk, the wet Araneus silk shrank and swelled much more than the proline deficient Nephila silk. The team tested the silk's stiffness, and found that the Nephila silk was almost ten times stiffer than the Araneus silk. They came to know that regions of the silk proteins club together to form microscopic crystals in a fiber. Savage measured the fiber's birefringence to see how the two silks compared and if the organization of the proteins in the silk fiber changed when they were damp.
It was also observed that the proteins in the Nephila silk were always more organized than the proteins in the Araneus silk, irrespective of whether they were wet or dry.
Gosline concluded that the different mechanical properties could be accounted for by the silk proteins' amino acid composition. According to Gosline, proline amino acids are famed for breaking up the organized three-dimensional structures that protein chains fold into, so protein structures with high proline content would be poorly organized in comparison to proteins with little or no proline.
Araneus silk contains 16% proline, and it is found mostly in linker regions between the protein's crystalline structures, which would make the linkers flexible and randomly arranged.
In the case of Nephila silk the force remained essentially the same in different temperatures, which is a clear indication of spring-like elasticity. But, for the proline-rich Araneus silk the force varied in direct proportion to the temperature, behaving like a rubber band. So proline-rich spider silks extend like floppy rubber bands, while spider silks with low proline levels behave more like rigid springs.
Having found that proline amino acids have a dramatic effect on the mechanical behavior of hydrated spider silks, as a next step, Gosline and Savage are keen to find out why the behavior of the dry silks is almost indistinguishable and what the functional significance is of the different proline contents.