Washington, May 31 : Scientists probing what makes spider silk stretchy have found that the presence of the amino acide proline has a dramatic effect on the mechanical behaviour of hydrated spider silks.
A researcher named John Gosline focussed on major ampullate silk, one of the many silks that spiders spin.
He compared the amino acid sequences of major ampullate silk proteins from Araneus diadematus and Nephila clavipes, and found that the former was relatively rich in the proline and that the levels of the amino acid were very low in the latter.
With his student Ken Savage, Gosline set about comparing the silks' mechanical properties to discern how proline's presence affects spider silk toughness.
The researchers left the spiders roaming free so that the strands of dragline silk that they dropped were as uniform as possible, and upon establishing a reliable silk supply, they set about testing the silks' mechanical properties.
When the researchers gently stretched the dry silk while measuring the force on it, they found that the presence of proline had little or no effect on dry silk.
However, when they began investigating the hydrated silk, it was a completely different story.
The researchers observed that the wet Araneus silk shrank and swelled much more than the proline deficient Nephila silk.
They also observed that the Nephila silk was almost ten times stiffer than the Araneus silk.
With knowledge at hand that regions of the silk proteins stack to form microscopic crystals in a fibre, the researchers set about to see how the two silks compared, and if the organisation of the proteins in the silk fibre changed when they were damp.
They noted that the proteins in the Nephila silk were always more organised than the proteins in the Araneus silk whether wet or dry, and as the silks were stretched, the degree of organisation in the hydrated Nephila silk increased much more than the Araneus silk.
Gosline pointed out that the different mechanical properties could be due to the difference in the silk proteins' amino acid composition, for proline amino acids are famed for breaking up the organised three-dimensional structures that protein chains fold into.
He said that protein structures with high proline content would be poorly organised in comparison to proteins with little or no proline.
The researcher noted that Araneus silk contains 16% proline, found mostly in linker regions between the protein's crystalline structures, which would make the linkers flexible and randomly arranged, causing the hydrated silk to behave like an elastic band.
On the other hand, he added, Nephila silk has a very low proline content in the linker regions, allowing the linkers to form a relatively well-organised crystalline structure and behave more like a stiff spring.
The researchers now want to find out why the behaviour of the dry silks is almost indistinguishable, and what is the functional significance of the different proline contents.