As with other such supramolecular structures being designed by a host of scientists, the non-covalent bonds that form the structure can be reversed to release the contents.
Feihe Huang, a doctoral student at Virginia Tech, will present his research in the organic chemistry section of the 225th national meeting of the American Chemical Society March 23-27 in New Orleans.
Crown ethers, used as host molecules, can either encircle (think donut) or fold around (think taco) a guest molecule. To make the fit between host and guest exact, the atoms on the host surface are preorganized* so that they recognize and latch onto the atoms on the surface of the guest. At that point, chemists covalently bond the edges of the host. A series of students in chemistry professor Harry Gibson's group at Virginia Tech have been improving these bonds.
Huang will give an oral presentation on a cryptand compound that is stronger than traditional host crown ethers because the new shape forms three bridges joined at two points (See www.research.vt.edu/resmag/Images/feihe_cryptand.jpg). The unit is also stronger than past rotaxanes because Huang has improved the association constants -- recognition and attraction between host and guest and at the clasps on the guest -- by 100 times over previous structures, as demonstrated by X-ray analysis and proton nuclear magnetic resonance (NMR) characterizations.
The paper, "Cryptands: Powerful hosts for paraquat derivatives (ORGN 671)," coauthored by Lev N. Zakharov and Arnold L. Rheingold of the University of Delaware Department of Chemistry and Biochemistry, Gibson and Jason W. Jones of the Virginia Tech chemistry department, and Frank R. Fronczek of the Louisiana State University chemistry department, will be presented at 8 a.m. Thursday, March 26, in section C of Convention Center Room 271.
NOTE TO MEDIA: The following may be released after 8 p.m. eastern U.S. time, Sunday March 23, 2003
Huang will also give a poster on the use of water molecules to bind two pseudorotaxanes. "There is improved complexation between the host and the guest," Huang explains. "For the first time it was found that water can enhance pseudorotaxane formation, specifically by acting as a key node to chelate the two OH moieties. Using the same host crown ether that likes to fold with a different guest, we use water as a clasp. Thus we bring the edges together through hydrogen bonding to form a dimer. Hydrogen bonds are the basic intermolecular force in biology, causing DNA to turn, for instance."
The poster, "Formation of a complex dimer assisted by water (ORGN 102)," coauthored by Zakharov, Rheingold, Gibson, and Jones, will be presented 8 to 10 p.m. Sunday, March 23, in section A of Convention Center Hall G.
Huang's research is funded by the National Science Foundation.
*The 1987 Nobel Prize in chemistry was awarded jointly to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their development and use of molecules with structure-specific interactions of high selectivity. A system used in nature -- for instance, that allows enzymes to bond to specific sites on DNA -- has been adapted for building molecules. Learn more at www.nobel.se/chemistry/laureates/1987/index.html