Reaction media engineering organic solvents

Success stories can be found for all of these process options. The different concepts for reaction media engineering are briefly discussed and illustrated by case studies as representative examples. The major focus will be on the reaction medium engineering of aqueous media by adding organic solvents as well as solvent-free processes whereas biotransformaUons carried out in pure organic media will be the specific subject of another chapter of this book (Chapter 3). 

The specificity of enzyme reactions can be altered by varying the solvent system. For example, the addition of water-miscible organic co-solvents may improve the selectivity of hydrolase enzymes. Medium engineering is also important for synthetic reactions performed in pure organic solvents. In such cases, the selectivity of the reaction may depend on the organic solvent used. In non-aqueous solvents, hydrolytic enzymes catalyse the reverse reaction, ie the synthesis of esters and amides. The problem here is the low activity (catalytic power) of many hydrolases in organic solvents, and the unpredictable effects of the amount of water and type of solvent on the rate and selectivity. 

The term medium engineering , that is the possibility to affect enzyme selectivity simply by changing the solvent in which the reaction is carried out, was coined by Klibanov, who indicated it as an alternative or an integration to protein engineering [5aj. Indeed, several authors have confirmed that the enantio-, prochiral-, and even regioselectivity of enzymes can be influenced, sometimes very remarkably, by the nature of the organic solvent used. 

Numerous studies have demonstrated the solvent influence on enzyme enan-tioselectivity, and sometimes the enantiopreference may even be reversed by medium engineering. For instance, the enantioselectivity of asymmetric reduction of prochiral ketones catalyzed by T. ethanolicus ADH can be controlled by changing the reaction medium containing either organic solvents or ionic liquids [93]. Reversal of the enantioselectivity was reported for S. cerevisiae-catalyzed reduction of hydrophobic phenyl w-propyl ketone by means of the... 

---