How spiders spin their silk

May 13th, 2010 - 2:46 pm ICT by ANI  

London, May 13 (ANI): Spider silk is a fascinating material. It is stronger than steel and any available man-made fiber, and scientists have long puzzled over how to develop a material with such strength and flexibility. They might be one step closer.

Researchers have just figured out one step in the silk-making process: how the liquid proteins the eight-legged creatures carry onboard get spun into webs at a moment’s notice.

Specifically, spider silk has five times the tensile strength (a measure of how much something can be stretched before it breaks) of steel, and triple that of the best artificial fibers available today.

“The high elasticity and extreme tensile strength of natural spider silk are unmatched, even by fibers produced from pure spider silk proteins,” said Professor Horst Kessler, a professor at the Institute for Advanced Study at the Technische Universitaet Muenchen in Germany.

Kessler and colleagues wanted to pursue a particularly puzzling question: How do spiders keep the ingredients for silk at hand in such high concentrations, ready to be spun into webs at a moment’s notice.

Spider silk consists of protein molecules, long chains comprising thousands of amino-acid elements. X-ray structure analyses show that the finished fiber has areas in which several protein chains are interlinked via stable physical connections. These connections provide the high stability. Between these connections are unlinked areas that give the fibers their great elasticity.

The situation within the silk gland is, however, very different: The silk proteins are stored in high concentrations in an aqueous environment, awaiting deployment. The areas responsible for interlinking may not approach each other too closely; otherwise the proteins would clump up instantaneously. Hence, these molecules must have some kind of special storage configuration.

X-ray structure analysis, which is so successful in other domains, was of little help here, since it can only be used to analyze crystals. And up to the instant in which the solid silk fiber is formed, everything takes place in solution. The method of choice was therefore nuclear magnetic resonance spectroscopy (NMR).

Using the equipment of the Bavarian NMR Center, Franz Hagn, a biochemist from Horst Kessler’s work group at the Institute for Advanced Study (TUM-IAS) at the TU Muenchen, managed to unravel the structure of a control element responsible for the formation of the solid fiber. Now the researchers could shed light on this control element’s mode of operation.

“Under storage conditions in the silk gland these control domains are connected pair-wise in such a way that the interlinking areas of both chains can not lie parallel to each other. Interlinking is thus effectively prevented,” Thomas Scheibel said.

The protein chains are stored with the polar areas on the outside and the hydrophobic parts of the chain on the inside, ensuring good solubility in the aqueous environment.

When the protected proteins enter the spinning duct, they encounter an environment with an entirely different salt concentration and composition. This renders two salt bridges of the control domain unstable, and the chain can unfold.

Furthermore, the flow in the narrow spinning duct results in strong shear forces. The long protein chains are aligned in parallel, thus placing the areas responsible for interlinking side by side. The stable spider silk fiber is formed.

The results have been published in the current issue of the prestigious scientific journal Nature. (ANI)

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