101805 - BE.342/442 Tuesday, October 18, 2005 Topic: Silk...

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BE.342/442 Tuesday, October 18, 2005 Topic: Silk Administrative: Take-home midterm is out! You have almost 1 month to turn it in. On question 10, you are asked to design a creative, novel material. In the past, students have designed ingenious materials. Graduate students are required to write a 5-page experimental design. Drawings are helpful! Undegrads don’t have to (but are encouraged to) produce an original design. They may, alternately, write a review paper. We have covered the material in the textbook. From now on, the course will rely on articles that will be posted on the course website. Spiders and silkworms can produce long continuous fibers. Silk had enormous impact on civilization. In China, people started spinning silk about 3,000 years ago, which traders distributed via the Silk Road toward Afghanistan, Iran, Turkey and on through Europe to Britain. China still produces enormous quantities of silk, ranging from blankets to works of art. Silkworm silk and spider silk both contains large amounts of Gly and Ala. Silkworm silk contains mostly alternating Ala and Gly and forms over 80% beta-sheets. The remaining 20% is mostly loops. One of the fibroins contains 5263 residues (391 kDa) – multiplying by 3.4 angstroms per residue in the beta-sheet structure, we get a length of approximately 15,000 angstroms, or 1.5 microns, per fibroin. These short silk fibroins overlap to form well-ordered silk fibers as long as 2 kilometers: that’s 1.3 billion times longer than the individual fibroin!When silk worms mature, the entire body becomes transparent: essentially, it is full of a gel. Once exposed to air, the silk becomes insoluble, and can never be returned to its
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101805 - BE.342/442 Tuesday, October 18, 2005 Topic: Silk...

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