Reaction media engineering

Several types of solvents have been used in the reaction media engineering with the purpose of enhancing the enzyme activity, selectivity, and stability. Among them organic media,ionic liquids,and supercritical fluids are well described in the literature. 

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). 

Water activity of the reaction medium plays a central role in lipase catalyzed reactions (Berglund 2001). Different authors have described and demonstrated the usefulness of controlling water activity on lipase performance. In esterification reactions on cyclohexane media, the reaction rate increased with water activity in the low activity range however, it reached a maximum at a value of 0.84 and a subsequent increase in water activity led to a decrease in the reaction rate (Mat-sumoto et al. 2001). Results on the effect of water activity on enantioselectivity of lipases are rather contradictory (Berglund 2001). However, very good papers have been published in tuning lipase enantioselectivity by reaction medium engineering (Wehtje and Adlercreutz 1997 Matsumoto et al. 2001 Bomscheuer 2002). 

Despite the various uses for lipases, the availability of lipases with specific characteristics, their stability, and operational properties limits their uses in many applications, especially in the biosynthesis of molecules in organic media. Researchers have managed to overcome some of these shortcomings through reaction medium engineering and lipase engineering physically, chemically, or genetically. 

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. 

The aspects of medium engineering summarized so far were a hot topic in biocatalysis research during the 1980s and 1990s [5]. Nowadays, all of them constitute a well-established methodology that is successfully employed by chemists in synthetic applications, both in academia and industry. In turn, the main research interests of medium engineering have moved toward the use of ionic liquids as reaction media and the employment of additives. 

Solvent as a Parameter for Reaction Optimization ( Medium Engineering )... 

The question of reproducibility and scale-up will always imply the question about reaction conditions. In addition, the reaction medium (phase) plays a much more important role for this kind of power input compared with classical reactions. Besides the molecular mass, reaction mixture polarity is essential for absorption of microwave power. Because dielectric constants are known for a few compounds only and, moreover, at near room temperature, more problems are predictable and require dose contact with neighboring disciplines, for example with electrical engineering. The primary literature reflects the incomplete nature of results from microwave-assisted reactions and processes, as it does for conventional syntheses. The dependence of reaction engineering on technical considerations is, however, greater for microwave-assisted reactions, so improved description of reaction conditions is crucial. 

Despite their good catalytic properties, proteases are not ideal catalysts for the synthesis of peptides. Its specificity and selectivity might limit their potential, particularly in the case of rather large peptides where unwanted hydrolytic reactions will occur over the formed product and the substrates. Besides, the use of non-conventional reaction media and the conditions of temperature and pH required for synthesis can be detrimental both for protease activity and stability (Barberis et al. 2002 Bordusa 2002 Quiroga et al. 2005, 2006). However, there are different strategies to overcome such problems, which comprise the engineering of the reaction medium, the biocatalyst and the substrate (Lombard et al. 2005). 

---