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Inorganic molecules achieve self-recognition

Liu’s group studied the structures of the two macroion clusters (A and B) and discovered that in mixed dilute aqueous solutions, the clusters (middle of C) self-assemble into distinct blackberry structures (C, right) and do not form mixed species (C, left).

Tianbo Liu, associate professor of chemistry, and his research group have discovered a high-level molecular self-recognition in dilute aqueous solutions, something that was previously considered achievable only by biological molecules.

The group’s results were published in the March 25 issue of Science, the nation’s premier science journal. Liu was lead author on the article, which was titled “Self-Recognition Among Different Polyprotic Macroions During Assembly Processes in Dilute Solution.”

“Publication of this work in Science is important recognition of the research being conducted in Tianbo’s lab,” said Robert Flowers, department chair and professor of chemistry. “His ability to succeed at such a high level shows that first-rate science is being done at Lehigh.”

Liu’s group has spent several years exploring the fascinating solutions of large, soluble ions called macroions. The behavior of these ions is completely different from the behavior of small ions, such as sodium chloride.

Despite being water-soluble and carrying the same type of charge, macroions tend to attract each other with surprising strength, says Liu, and to form very stable, uniform, single-layered hollow spheres known as “blackberry structures.” The structures are common when ions become large, and they mimic some biological processes such as the virus capsid shell formation.

Forming two distinct blackberry structures

Exciting discoveries have been generated from blackberry solutions. Liu’s group found that, when mixed into the same solution, two different types of 2.5-nm spherical macroions ({Mo72Fe30} and {Mo72Cr30}) with almost identical size, shape and molecular structures tend to form two types of individual blackberries instead of mixed ones.

The macroions—Bucky ball-shaped inorganic compounds—were synthesized by a research team led by Achim Müller, professor of chemistry at the University of Bielefeld, Germany. Müller was a coauthor on the Science article.

This result, says Liu, suggests that even in dilute solutions these two macroions can self-recognize during assembly.

This level of “intelligence,” he adds, is usually believed to be achievable only by complex biological molecules. Self-recognition by large inorganic ions could lead to more opportunities for understanding the nature of biological interactions.

Liu’s group believes the self-recognition results from the very slow formation of the dimers in the first step of the assembly. The slow speed ensures the formation of dimers with the lowest free energy, such as A-A and B-B dimers.

The differences in charge density between the two types of macroions play an important role in the recognition, says Liu, as does their surface water mobility difference.

The Science article was coauthored by one current and three former members of Liu’s research group. Dong Li is a Ph.D. candidate in the chemistry department. Melissa Langston earned a Ph.D. from Lehigh in 2009 and is now an assistant professor of chemistry at Delaware Valley College in Pennsylvania. Joseph M. Pigga earned a Ph.D. in chemistry from Lehigh in 2010 and is now a postdoctoral researcher at Lehigh. Celine Pichon was a postdoctoral researcher at Lehigh from 2008 to 2010 and is now a research scientist at the University of Bordeaux in France.

Liu’s work was funded by the U.S. National Science Foundation (NSF), the Alfred P. Sloan Foundation and Lehigh. The German collaborators were funded by Deutsche Forschungsgemeinschaft (DFG). Liu and Müller also have an NSF-DFG International Collaborative Grant.


Story by Tricia Long

Posted on Friday, April 22, 2011

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