Spider silk and silkworm silk are almost identical in chemical composition and microscopic structure. And yet spider silk is far tougher. “One strand of pencil thick spider silk can stop a Boeing 747 in flight,” say Xiang Wu and colleagues at the National University of Singapore. Whereas a pencil thick strand of silkworm silk couldn’t. Why the difference?
Xiang and co say they’ve worked out why by successfully simulating the structure of both silks for the first time. Both spider silk and silkworm silk are made up of long chains of amino acids called polypeptides and are particularly rich in the amino acids alanine and glycine.
Various imaging techniques have shown that the sequences of amino acids differ slightly between spide and silkworm silk but this alone cannot explain the huge difference in strength, says Xiang.
Instead, the secret appears to lie in the way polypeptide chains fold into larger structures. Xiang’s model shows that a subtle difference exists between the silks in structures called beta sheets and beta crystallites. This insight has allowed the team to model for the first time the way in which both silks break.
That’s important because being able to predict the mechancial properties of an organic material simply by studying its structure is going to be increasingly useful. It may even allow us to take a better stab at making spider-like silk synthetically for the the first time.
Anybody with a 747 watch out.
Ref: arxiv.org/abs/0902.3518: Molecular Spring: From Spider Silk to Silkworm Silk