NMOHIIilHulWEEg SCIENCE
REACHING OUT Extended polymers have dramatic effect on molecular recognition
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OLYMERS D O N T USUALLY
like to stretch out into highenergy, highly extended configurations. Nevertheless, they do from time to time and, as a new study shows, those excur sions can influence the course of molecular recognition events. Consider a surface to which polymers bearing a biotin mole cule at the tip are attached. Imag ine bringing that surface close to another lined with the biotin re ceptor streptavidin. At some finite separation, binding occurs. That distance, however, turns out to be far longer than the average reach of polymers at equilibrium. The phenomenon was first ob served in 1997 by chemical en gineering professor Jacob N. Israelachvili at the University of California, Santa Barbara, and others. Now, he and numerous collaborators have applied com putational methods and polymer diffusion reaction theory to the phenomenon. The results show that the dynamics of full polymer extension and its consequences can be predicted by theory [Sci ence, 293,465 (2001)]. "The work says you can get recognition of a surface through the rare excursion ofeven just one molecule to a high-energy config uration," says Thomas Ρ Russell, a professor ofpolymer science and engineering at the University of Massachusetts, Amherst. "It gives direct evidence that these rare events occur and can have signif icant consequence." If something out there can be grabbed, the polymer will latch on and pull it back, like a harpoon. The findings indicate that ligand-receptor interactions of tethered systems cannot be based only on equilibrium considera tions, because with flexible teth HTTP://PUBS.ACS.ORG/CEN
ers, dynamic fluctuations affect how ligands are presented and the range at which recognition occurs. Ligands tethered to the cell surface through proteins and car bohydrates are ubiquitous in biol ogy. However, it is not clear to what extent thefindingswill bear on biological systems. It depends on how flexible these proteins and carbohydrates are. The theoretical predictions are having an immediate impact on the work of team member Joyce Y Wong, an assistant professor of chemistry at Boston University For example, the effect of tether length on the timescale of inter actions will help her design teth ers for drug carriers depending on the target—arteries or capillaries. "Blood vessels of different diam eters will have different flow rates and different reaction timescales," she says. Deborah E. Leckband, a pro CHEMISTRY
STRETCH fessor of chemical engineering at Measurements of the University of Illinois, Urbanathe minimum Champaign, notes that studies of distance Id) receptor-ligand interactions have between the two focused mainly on dissociation dy surfaces shown namics. The new work shows that equal attention must be paid to the suggest that polylethylene dynamics of association events. glycol) tethers "Bond formation is not simply bearing biotin at two things coming together," Israelachvili says. "In fact, the two their tips extend beyond their things are quite apart, and sud average reach [Re], denly there's this instantaneous snap. The tether stretches, the li- enabling biotin to gand grabs the receptor, and pulls bind to streptavidin when d > Re. it. It's totally unexpected because stretching is rare. \ e t that's how it happens. The system relies on something unreliable."— M AU R Ε Ε Ν ROUHI
OLYMPIAD
Three Teams Tie For Best In The Field hina, Korea, and the Russian Federa tion turned in top performances at the 33rd annual International Chemistry Olympiad, held July 6-15 in Mumbai, India. The competition drew four-member teams of high school students from 54 nations for lab oratory and written exams in addition to a round of sightseeing and cultural activities. The top teams all earned three gold medals and a silver. The U.S. and Iranian teams each won two golds and two silvers. The only other team to win two gold medals M U M B A I M A G I C U.S. team members was Australia, which also garnered a silver Kedrowski (left), Martin, Wang, and Ling and a bronze. The host country's team won a enjoyed their stay in India. gold and three silvers. The U.S. team included four recent graduates: Sean M. Kedrowski, Baylor High School, Chattanooga, Tenn.; Binghai Ling, Brighton High School, Rochester, N.Y.; Collin H. Martin, Oklahoma School of Science & Math, Oklahoma City; and Albert Wang, Bellaire High School, Bellaire, Texas. Kedrowski and Ling won the golds.—SOPHIE WILKINSON
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