Biocatalysis reaction media

The use of ionic liquids (ILs) to replace organic or aqueous solvents in biocatalysis processes has recently gained much attention and great progress has been accomplished in this area lipase-catalyzed reactions in an IL solvent system have now been established and several examples of biotransformation in this novel reaction medium have also been reported. Recent developments in the application of ILs as solvents in enzymatic reactions are reviewed. 

There are many factors that control and limit biocatalysis in carbon dioxide, including, but not limited to, pressure, water content, temperature, and mass transfer, and their influence on each enzyme s activity and stability. It is also important to understand whether one is operating in a one-phase or two-phase reaction medium. To this end, solubility studies should be carried out regularly to determine if the reaction is one or two phases. Only if the reaction is one-phase can one determine the kinetic and thermodynamic parameters with certainty. 

The use of water as the reaction medium for biocatalysis has long been advocated as one of the major advantages of the application of biocatalysts. This so-called... 

The combination of ILs and SCCO2 as a reaction medium has also attracted the attention of the biocatalysis community. In an interesting study the new concept of continuous biphasic biocatalysis was described. A homogeneous enzyme solution was immobilized in one liquid phase and the substrates and products resided largely... 

Although the main focus of the present volume is organometaUic catalysis, it has been noted on several occasions that the principles discussed here and the challenges outlined above are not restricted to this subdiscipline of catalysis. For example, it may be interesting to note that SCCO2 has found attention as a reaction medium for biocatalysis even before the first reports on organometaUic catalysis in... 

Biocatalysis in organic solvents has unique advantages compared to traditional aqueous enzymology/fermentation. Often times in nonaqueous media enzymes exhibit properties drastically different from those displayed in aqueous buffers. These novel properties are given in Table 4.3. In addition to those mentioned in Table 4.3, the solubility of hydrophobic substrates and/or products increases in organic solvents, which diminishes diffusional barriers for bioconversions, and thus speeds up the reactions and improves the potential for direct applications in industrial chemical processes. Once organic solvent becomes a reaction medium, there cannot be contamination, which thus precludes release of proteolytic enzymes by microbes and favors the direct application of the process in an industrial setting. Most proteins (enzymes) inherently function in an aqueous environment, and hence their behavior in nonaqueous solvents is completely different due to the loss in the three-dimensional structure. Thus, only polar solvents... 

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. 

The use of organic solvent in the medium is one stategy that has been proposed for biocatalysis [9-15]. In the organic phase, the reactant has a much greater solubility than in the aqueous phase. This could significantly reduce the volume of the reaction mixture. Enzymes and micro-organisms have been shown to be active in the presence of organic solvents [16-20]. 

In a biphasic medium, two situations are distinguished for the reaction. Biocatalysis occurs at the liquid-liquid interface [42,43] or in the bulk of the aqueous phase [25,27]. Models have been developed for both types, and interaction between mass transfer and enzyme-catalyzed reactions has been also studied. 

Biocatalysis localization in the biphasic medium depends on physicochemical properties of the reactants. When all the chemical species involved in the reaction are hydro-phobic, catalysis occurs at the liquid-liquid interface. However, when the substrate is hydrophobic (initially dissolved in the apolar phase) and the product is hydrophilic (remains in the aqueous phase), the reaction occurs in the aqueous phase [25]. The majority of biphasic systems use sparingly water-soluble substrates and yield hydrophobic products therefore, the aqueous phase serves as a biocatalyst container [34,35] [Fig. 2(a)]. Nevertheless, in some systems, one of the reactants (substrate or product) can be soluble in the aqueous phase [23,36-38] (Fig. 2(b), (c)). 

Over the last twenty years, biocatalysis in organic solvents has emerged as an important area of research and has led to the widespread industrial use of enzymes. In an aqueous medium, with which enzymes are naturally associated, most substrates are poorly soluble, and this leads to low reaction rates and high reaction volumes. In addition, in water, undesired side reactions and degradation of organic compounds often occur. Also, the thermodynamic equilibrium is unfavorable and product recovery can be difficult. Working in organic solvent avoids these problems, and enzymes often show beneficial behavior in such systems [12-14]. 

It thus appears that the enantioselectivity of CALB for this reaction in the solid/ gas bioreactor is similar to that in an organic liquid medium. Solid/gas biocatalysis therefore offers important potential for production of enantiomerically pure compounds, provided that these transformations involve components having a degree of volatility. Furthermore, as the addition of solvents is avoided in this system, separation and purification during downstream processing are simplified, and side reactions are suppressed. 

There are many factors that influence the outcome of enzymatic reactions in carbon dioxide. These include enzyme activity, enzyme stability, temperature, pH, pressure, diffusional limitations of a two-phase heterogeneous mixture, solubility of enzyme and/or substrates, water content of the reaction system, and flow rate of carbon dioxide (continuous and semibatch reactions). It is important to understand the aspects that control and limit biocatalysis in carbon dioxide if one wants to improve upon the process. This chapter serves as a brief introduction to enzyme chemistry in carbon dioxide. The advantages and disadvantages of running reactions in this medium, as well as the factors that influence reactions, are all presented. Many of the reactions studied in this area are summarized in a manner that is easy to read and referenced in Table 6.1. 

Martinek et al. [28] defined the apparent reaction equilibrium in a biphasic system by the constant Kbi. In their model, the ratio K /K represents the equilibrium change when using a biphasic or pure aqueous medium. When the biocatalysis occurring in the aqueous phase involves four chemical species ... 

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