Enzymes in non-conventional environments

Enzymes are Nature’s catalysts. They have evolved over millions of years to perform their tasks, often with exquisite precision, in water at physiological pH and ambient temperature and pressure. While these conditions are ideal for many of the natural reactions that are catalysed by enzymes in vivo, they are not always favourable for many synthetic biotransformations in vitro. Many organic substrates are sparingly soluble or insoluble in water and some processes, e.g. esterification, are not feasible in water (at least in the dissolved state) owing to unfavourable equilibria.

Although it has been known for more than a century that some enzymes could perform their tasks in non-aqueous media, it was the pioneering work of Klibanov in the mid-1980s that drew attention to the use of enzymes for performing a variety of biotransformations in essentially anhydrous organic solvents. This stimulated a surge in research on the use of enzymes in organic synthesis which continues to this day and has resulted in numerous commercial applications. Many enzymes are not only stable in the non-natural environment of organic solvents they are more stable than in water. On the other hand, rates are generally much lower compared to those in aqueous media. Furthermore, the use of many volatile organic solvents is under considerable environmental pressure which has stimulated research on the use of non-conventional media for performing atom efficient catalytic conversions, including bioconversions. These alternative solvents should be relatively non-toxic and non-hazardous, readily separated from the biocatalyst and the product, and recyclable. Added benefits could be enhanced activity, selectivity and stability compared to conventional organic solvents. Hence, there is currently much attention being focused on the use of e.g. ionic liquids and supercritical carbon dioxide and mixtures thereof, for conducting biotransformations. This special issue of Green Chemistry is devoted to various aspects of this important topic.

The use of enzymes in supercritical carbon dioxide is reviewed by Nakamura and coworkers.¹ Barreiros and coworkers compare the results of conducting an enzymatic transesterification in various non-conventional media: ionic liquids, supercritical ethane, supercritical carbon dioxide and hexane.² Other reports concern enhanced enantioselectivities and rates of lipase-catalysed transesterifications in some novel ionic liquids (Itoh et al.³) and chemoenzymatic dynamic kinetic resolution of alcohols in ionic liquids (Kim et al.⁴). An investigation of the lipase-catalysed synthesis of a key intermediate for the drug, Lotrafiban, in ionic liquids is reported by Lye and coworkers.⁵ The results compared favourably with those obtained in the industrial process in tert-butanol. Sometimes enzymes dissolve in ionic liquids, generally with loss of activity. The dissolution of Candida antarctica lipase B in a variety of ionic liquids was followed by FT-IR spectroscopy, showing that loss of activity correlated with denaturation of the enzyme. In contrast, when the enzyme retained its activity on dissolution denaturation had not occurred (Sheldon et al.⁶).

The use of enzymes or whole cells at solid–gas interfaces offers interesting benefits, such as high rates and minimum plant sizes, compared to the standard liquid systems. An overview of solid–gas biocatalysis is presented by Legoy and coworkers.⁷ Another interesting approach to obtaining high volumetric yields is by using solid-to-solid biocatalysis, which is presented by Ulijn and Halling.⁸

Finally, a wide variety of hyperthermophilic enzymes are now available. Their potential advantages for application in biotransformations are discussed by Kelly and coworkers.⁹

In short, biocatalysis in non-conventional media is scientifically interesting and challenging and has significant commercial potential. We hope that the papers in this special issue of Green Chemistry will stimulate further discoveries in this fascinating area.

Roger Sheldon

Gillian Stephens

Ken Seddon

References

1. T. Matsuda, T. Harada and K. Nakamura, Green Chem., 2004, 6 10.1039/b404564p , this issue.
2. S. Garcia, N. M. T. Lourenço, D. Lousa, A. F. Sequeira, P. Mimoso, J. M. S Cabral, C. A. M. Afonso and S. Barreiros, Green Chem., 2004, 6 10.1039/b405614k , this issue.
3. T. Itoh, S Han, Y. Matsushita and S. Hayase, Green Chem., 2004, 6 10.1039/b405396f , this issue.
4. M.-J. Kim, H. M. Kim, D. Kim, Y. Ahn and J. Park, Green Chem., 2004, 6 10.1039/b405651e , this issue.
5. N. J. Roberts, A. Seago, J. S. Carey, R. Freer, C. Preston and G. J. Lye, Green Chem., 2004, 6 10.1039/b405712k , this issue.
6. R. Madeira Lau, M. J. Sorgedrager, G. Carrea, F. van Rantwijk, F. Secundo and R. A. Sheldon, Green Chem., 2004, 6 10.1039/b405693k , this issue.
7. S. Lamare, M.-D. Legoy and M. Graber, Green Chem., 2004, 6 10.1039/b405869k , this issue.
8. R. V. Ulijn and P. J. Halling, Green Chem., 2004, 6 10.1039/b406267a , this issue.
9. D. A. Comfort, S. R. Chhabra, S. B. Conners, C.-J. Chou, K. L. Epting, M. R. Johnson, K. L. Jones, A. C. Sehgal and R. M. Kelly, Green Chem., 2004, 6 10.1039/b406297c , this issue.

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