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At the University of Massachusetts, Lowell, Ashok L. Cholli and coworkers--including Muthiah Thiyagarajan, who recently moved to the University of North Carolina, Charlotte--are using biocatalysts to create novel electrically conducting and optically active polymers that have potential applications in optoelectronics. A focus of the group's research is the enzymatic synthesis of polyaniline nanocomposites that are not only conducting and optically active but also chiral and water soluble. In recent work, the group synthesized a chiral conducting nanocomposite of poly(acrylic acid), polyaniline, and camphorsulfonic acid (CSA) using the enzyme horseradish peroxidase [J. Am. Chem. Soc., 125, 11502 (2003)] CSA acts as a chiral dopant. "We induce chirality in the achiral aniline monomer by complexing it with chiral molecules, such as camphorsulfonic acid," Cholli explains. "We create water-soluble nanocomposites by using the enzyme to polymerize the aniline-CSA monomers in the presence of a polyelectrolyte, such as poly(acrylic acid)." The polyaniline chains in the nanocomposites have a helical conformation. Cholli and coworkers discovered that the enzyme plays a dual role in the synthesis: It not only acts as a catalyst but also determines the conformation of the polymer chain. "The handedness of the helices is dictated by the enzyme," Cholli says. "Circular dichroism shows that the specificity of the helical handedness is the same regardless of whether the monomer entering the active site of the enzyme is (+) or (–). "We are now interested to know whether there are other naturally occurring enzymes that dictate the conformation in biocatalytically synthesized polymers," Cholli adds. "We are also focusing on the role of size and shape of dopants in the helical chain formation." |
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