Schedule Oct 15, 2008
The Elusiveness of Polymer Knots
Mehran Kardar, MIT & KITP

The abundance of knots in human art and culture indicates a long history of fascination and befuddlement. The scientific classification of knots, however, begins with the work of three Scottish physicists, Thompson, Maxwell, and Tait. It is thus fitting that the 2008 IgNobel prize in physics acknowledges research on knots in strings! This talk will examine knots in polymers, synthetic or biological. Somewhat contrary to intuition, physical considerations (energy or entropy) are found to favor tight knots, with the entanglements localized on relatively small segments. Knots are also statistically rare in swollen (coil) polymers, but abundant in compact (globular) configurations. Yet, our investigation of the Protein Data Bank reveals very few knotted structures in globular proteins. I shall discuss some intriguing examples of this set, including the most complicated protein knot (circa 2007) appearing in Human ubiquitin hydrolase, and new surprises from the 2008 additions.

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