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Enzymes find pastures greener


Chemists put biological catalysts to work in clean industrial solvents.

In a move towards cleaner chemical processing, researchers in Spain and France have worked out how to use enzymes as catalysts using two ’green’ solvents: one to dissolve the enzyme, the other to dissolve the materials it transforms.

In some industrial processes chemists have replaced polluting organic solvents, such as chlorine and benzene, with supercritical carbon dioxide. This is the liquid-like fluid that is made by putting carbon dioxide under moderately high pressure and at temperatures equivalent to a hot bath. Supercritical carbon dioxide dissolves many organic compounds used for chemical synthesis. It decaffeinates coffee, for example.

Another, more recent, green option is the use of ionic liquids - these are salts that are molten at room temperature. They too dissolve many organic compounds, and don’t give off nasty fumes.

Jose Iborra of the University of Murcia in Spain and co-workers have used a combination of supercritical carbon dioxide and ionic liquids to help an enzyme transform some organic molecules1. This is an ideal form of green chemistry, as it uses natural catalysts in clean solvents.

Enzymes are designed to work in water inside cells. But water won’t dissolve many of the organic reagents used in industrial and pharmaceutical chemistry. So many industrial processes that use enzymes as catalysts need organic solvents.

Unfortunately, enzymes typically don’t work well in carbon dioxide. "It reacts with the enzyme," explains chemist Eric Beckman of the University of Pittsburgh. This and other complications stop the enzyme working as a catalyst. Some enzymes, though, work well in ionic liquids. So Iborra’s group devised a two-phase reactor in which the organic starting materials are dissolved in supercritical carbon dioxide and passed through a chamber containing a yeast enzyme dissolved in an ionic liquid.

The enzyme converts the reagents to the desired products, presumably by reactions occurring at the boundary between the two solvents. Product molecules dissolve in the carbon dioxide and are carried out of the reaction chamber. The enzyme, which stays in the ionic solvent, is protected from the worst of the deactivating influence of carbon dioxide.

It’s not a perfect solution - some carbon dioxide can dissolve in the ionic liquid and so can still get at the enzyme. But it’s a lot better than trying to carry out the reaction entirely in carbon dioxide, which deactivates the enzyme quickly.

"It’s an intriguing idea," says Beckman, as using enzymes in supercritical solvents has previously been fraught with difficulties.

  1. Lozano, P. , de Diego, T., Carrie, D., Vaultier, M. & Iborra, J. L. Continuous green biocatalytic processes using ionic liquids and supercritical carbon dioxide. Chemical Communications, 2002, 692 - 693, (2002).

PHILIP BALL | © Nature News Service

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