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IMITATING NATURE
Supramolecular structure functions as an artificial enzyme
CELIA HENRY
A manganese porphyrin epoxidation catalyst, when placed in the cavity formed by the directed assembly of zinc porphyrin molecules, acts as an artificial enzyme [Angew. Chem. Int. Ed., 40, 4239 (2001)].
"We have replicated the topology that nature intended for a metalloenzyme: an active metal site encapsulated by a cavity that protects the active center, defines the size/shape selectivity, and allows substrate binding," says SonBinh T. Nguyen, a chemistry professor at Northwestern University and one of the group leaders along with Joseph T. Hupp, another Northwestern chemistry professor.
The cavity protects the catalyst but is large enough to allow substrate molecules to reach the catalyst. The increased stability allows the catalyst to last longer, with the lifetime increasing from 10 minutes to more than three hours. The bare manganese porphyrin generally loses its activity after 50 cycles, but in the supramolecular structure, one catalyst reached turnover numbers as high as 21,000.
The selectivity of the catalyst can be tuned by adding up to two ligands, which change the size of the cavity. The resulting size selectivity allows significant differentiation between substrates, even those that are similar to one another.
"One of the most impressive aspects of Nguyen and Hupp's work is that they were able to demonstrate that the use of a nanostructured system affords something special in terms of properties and/or performance compared to conventional materials," says Douglas Gin, a professor in the departments of chemistry and chemical engineering at the University of Colorado, Boulder. Gin is also working on nanostructured materials to do size-selective catalysis.
"The results in this paper beautifully demonstrate how the stability and selectivity of catalysts can be increased by incorporation into a well-defined supramolecular structure," says Patrick J. Walsh, a professor of chemistry at the University of Pennsylvania. "The modular nature of the encapsulated catalysts and ligand additives will permit synthesis of new catalysts employing the same supramolecular porphyrin structure."
CATALYST IN A BOX The artificial enzyme consists of a manganese porphyrin in a cavity created by zinc porphyrin molecules. The assembly catalyzes olefin epoxidation.
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