Bugs speed green chemistry

Bacterial catalysis goes supercritical.

Bacteria could clean up industrial chemistry. Chemists in Japan have found that the bugs speed up an industrially useful reaction in the environmentally friendly solvent supercritical carbon dioxide¹.

Tomoko Matsuda of Ryukoku University and colleagues investigated a reaction in which carbon dioxide is added to the organic molecule pyrrole. Such ’carboxylation’ reactions often transform simple molecules into the starting materials for more complex syntheses.

Bacteria have previously performed carboxylations in water. But many industrial reactions need organic solvents if the starting materials are insoluble in water. Unfortunately these are often toxic. Supercritical carbon dioxide is now the premier ’green’ alternative.

It is called supercritical because it exists as a hybrid state, displaying the properties of a gas and a liquid. It is made by liquefying carbon dioxide at pressures of about 100 atmospheres and then warming the liquid to 31 ºC. At this point the carbon dioxide no longer forms distinct liquid and gas states, but exists as a single fluid.

Chemists are also keen to use nature’s catalysts – enzymes – in industrial processes. Enzymes work in mild conditions, such as low temperatures, and are often more selective than artificial catalysts about the kind of reactions they speed up.

Some industrial processes use particular enzymes extracted from bacterial cells, but it is often more convenient to use whole, living cells – just as yeast has been used for centuries to turn sugar into alcohol.

Supercritical carbon dioxide has been used as the solvent for such biocatalysis reactions – but only with isolated enzymes, not whole cells.

Matsuda’s team has now found that the bacterium Bacillus megaterium can survive in supercritical carbon dioxide at temperatures of 40 ºC and pressures of 100 atmospheres – and that they can carboxylate pyrrole.

The reaction works in water at atmospheric pressure, but it is much faster in the supercritical solvent. This speed-up is crucially important if the process is to scaled up for use in industry.

The work „sounds very nice in principle“, says biocatalysis specialist Bernard Witholt of the Swiss Federal Institute of Technology in Zürich, but he cautions that the cost of using supercritical solvents in biocatalysis may hinder large-scale applications.

References

1. Matsuda, T. et al. Conversion of pyrrole to pyrrole-2-carboxylate by cells of Bacillus megaterium in supercritical CO2. Chemical Communications, (2001).