Jason W. Jones of Richmond, Va., a Ph.D. student in chemistry at Virginia Tech, will present his research at the 225th American Chemical Society national meeting March 23-27 in New Orleans.
"We design host and guest molecules with built-in recognition sites. Specifically, we prepare macrocyclic (donut-shaped) hosts and linear guests that self-assemble when mixed to form threaded complexes known as pseudorotaxanes. Because the complexes are pH, temperature, or solvent dependent, the supramolecular bonding can be reversed," Jones explains.
In 1999, the group led by Virginia Tech chemistry professor Harry Gibson proved that properly designed host and guest units will self-assemble into linear chains approximately nine units long, based on a pseudorotaxane interaction, which has an experimental association constant (Ka) of 750 M-1. The "association constant" represents how strong the supramolecular bonding interactions are and determines how long the self-assembled chain can grow. "In order to force the chain to grow to lengths more appropriate of polymers, we needed to dramatically increase the Ka value," says Jones. "In one of our pseudorotaxane systems, we knew that when the floppy macrocycle was mixed with the linear guest, a significant change in molecular conformation (shape) occurred to the host. So we decided to rigidify the macrocycle such that the starting and finishing conformations were the same, resulting in a 50-fold increase in Ka." A 'lock and key' is a frequently used illustration for the concept, known as pre-organization.
With such a large Ka value, the researchers knew that they could theoretically attain much longer self-assembled chains. The problem was that the pre-organized macrocycle was difficult to make. "It required multiple steps, which cost us a great deal of time and money," Jones says.
Jones and his colleagues have recently discovered a way to rigidify the molecule using hydrogen bonding instead of covalent chemistry. He points to DNA as a model. "The DNA helix is held together by hydrogen bonding, with specific receptor or donor groups precisely located along the lengths of the complimentary pair."
The researchers built a crown ether macrocycle with donor groups located at opposite ends of the crown. "When we add complimentary receptors to the crown solution, the otherwise floppy macrocycle is forced into the non-flexible binding conformation by the donor/receptor interaction," says Jones. "We can mix one compound with another in a matter of seconds and attain Ka values even larger than those found with the covalent pre-organized analog. It's easy and there are no further purification steps required."
Jones will present the paper, "Cooperative host/guest interactions via counterion-assisted chelation: Pseudorotaxanes from supramolecular cryptands (ORGN 723)," on March, 27, 2003 at 3:40 PM in room 271 of the New Orleans Convention Center. Co-authors are student Feihe Huang and Gibson at Virginia Tech and Lev N. Zakharov and Arnold L. Rheingold in chemistry and biochemistry at the University of California, San Diego.
Jones received his undergraduate degree at the College of William and Mary in 1998 and his master's degree at Virginia Tech.
Contact for more information: Jason W. Jones, jwjones@vt.edu, or Dr. Harry Gibson, hwgibson@vt.edu or 540-231-5902.
PR CONTACT: Susan Trulove 540-231-5646 strulove@vt.edu