Angewandte Chemie International Edition

Cover image for Vol. 56 Issue 31

Editor: Peter Gölitz, Deputy Editors: Neville Compton, Haymo Ross

Online ISSN: 1521-3773

Associated Title(s): Angewandte Chemie, Chemistry - A European Journal, Chemistry – An Asian Journal, ChemistryOpen, ChemPhotoChem, ChemPlusChem, Zeitschrift für Chemie

For full article and contact information, see Angew. Chem. Int. Ed. 2001, 40 (22), 4239 - 4242

No. 21/2001

Artificial Enzymes

Researchers develop "framed" catalysts
by copying natural enzymes

Enzymes are the perfect catalysts for chemical and biological reactions; they are very stable and highly selective, which means that they are very picky about which substrates they convert. Enzymatic reactions do not produce unwanted by-products. This is exactly what a chemist wants out of a synthetic catalyst as well. An American group working with SonBinh T. Nguyen and Joseph T. Hupp has now copied one of the enzymes’ essential tricks: the right "packaging".

Enzymes consist of a catalytic center surrounded by a larger protein structure. This superstructure is critical for the selectivity of enzymatic reactions. Also, the "packaging" shields the center from other reactive centers, thus increasing the stability of the enzyme.

Nguyen and his colleagues were able to borrow this principle. For their experiments they selected an important biochemical reaction, the epoxidation of olefins. In this reaction a carbon-carbon double bond is opened and an oxygen atom is inserted so as to form a three-membered ring. The catalyst for this is a complex molecule consisting of a flat ring system - a porphyrin framework - with a central manganese atom. A number of natural enzymes contain similar structures.

Next came the "packaging". For this, the chemists hooked together four more porphyrin rings - each with a zinc atom in the center, this time. This resulted in a structure that resembles a box without a lid or a bottom. A single catalyst molecule lodges within each of these "frames".

This "framing" does indeed stabilize the catalyst. Says Nguyen, "The lifetime of the catalyst increases from about ten minutes to over three hours. Function is also improved: depending on the substrate we see a ten- to a hundred-fold increase in the turnover compared with the "unframed" catalyst."

The "frame" also limits access to the catalyst. Only molecules smaller than a certain size fit inside and can be converted. By varying the "frame" the size-selectivity can easily be changed - a step toward tailored "artificial enzymes".