Proteins' Web of Intrigue
The latent strength of Miss Muffet's arachnoid friend may have been in sexual allegory, but the image of a spider's web as somehow weak, a glistening, gossamer netting for trapping only flies could not be further from the truth. In fact, kilo for kilo, spider silk has a breakage energy 100 times greater than steel and an elasticity much higher than rubber.
Add to that spider silk's relatively low density and stiffness and you have an almost ideal material for countless engineering and technological applications. If only materials scientists could understand its structure in fine detail and develop a way to design a system to mass produce this tough and elastic fiber.
Now, Thomas Scheibel's research group at the Technical University of Munich has inched across an analytical web towards a clearer understanding of the differences between the main forms of spider silk. They have discovered that key to the spider's spinning success is the interaction between the water friendly, hydrophilic, and fat friendly, lipophilic, properties of the silk proteins.
Technically speaking, the spinning of spider silk represents a phase change from a solution of protein molecules into a solid thread. Clues came from the way in which orb weaver spiders somehow manage to spin out two different types of thread from the same spinneret, their silk-making gland. They use the first form to spin the edges and spokes of their webs and the second to rapidly escape predators.
The Munich team has now successfully used genetic engineering to produce one of the spider silk proteins of the European garden spider (Araneus daidematus). The team used protein dialysis to purify the materials and noticed that the silk solution would separate into two different fluid layers. One fluid phase, they explain, consists of protein molecules joined together in pairs, while the second consists of bigger aggregates known as oligomers, in which several protein units are linked.
The researchers found that when they added potassium phosphate, a natural initiator of silk aggregation, to the solution, they could pull threads from the mixture. This property, they explain, suggests that the two different forms of silk really only differ very subtly. "It is clearly not a structural change in the protein," says Scheibel, "but rather the degree of oligomerization that is crucial for thread formation."
The solution within the spinneret has a very high protein concentration, add the researchers, but also contains a high concentration of common salt, sodium chloride, which suppresses oligomer formation. The team found that if they removed the sodium chloride, protein aggregation into oligomers occurs. It seems that common salt is not the only controlling factor, however. pH (the alkalinity of the solution) is also important. Within the spinneret, the solution has a high pH, it is alkaline, but at the exit point, the spinning duct the pH drops to a slightly acidic level.
Scheibel explains that they observed no separation of protein phases at alkaline pH because the normally electrically neutral tyrosine amino acid groups in the protein lose a hydrogen atom—they are deprotonated; leaving them with a negative charge. This charge, he says, weakens the interaction between the fat friendly parts of the proteins, allowing oligomerization to occur as the fat friendly parts of the protein can stick together to exclude water molecules. "Our insights form a foundation for the establishment of an effective spinning process for the production of genetically engineered spider silk," says Scheibel.
Angew Chem Int Edn, 2007, 46, in press; http://dx.doi.org/10.1002/anie.200604718
Appl Phys A: Mater Sci Process (multiple papers), 2006, 82, 191-273; http://tinyurl.com/yw2wtk
Today's Chemist at Work, 2001, 10, 23-27; http://pubs.acs.org/subscribe/journals/tcaw/10/i03/html/03inst.html