Solvent systems, for biocatalytic

Solvent Systems for Biocatalytic Reductions 243 Table 9.3 Influence of (co)solvents in the bioreduction of 2 -chloroacetophenone. 

Thanks to their special properties and potential advantages, ionic liquids may be interesting solvents for biocatalytic reactions to solve some of the problems discussed above. After initial trials more than 15 years ago, in which ethylammonium nitrate was used in salt/water mixtures [29], results from the use of ionic liquids as pure solvent, as co-solvent, or for biphasic systems have recently been reported. The reaction systems are summarized in Tables 8.3-1 and 8.3-2, below. Table 8.3-1 compiles all biocatalytic systems except lipases, which are shown separately in 8.3-2. Some of the entries are discussed in more detail below. 

Semenov, A N., Khmelnitsky, Y.L., Berezin, I.V. and Martinek, K. (1987) Water-organic solvent systems as media for biocatalytic reactions the potential for shifting chemical equilibria towards higher yield of end products. Biocatalysis, 1, 3-8. 

The use of exotic media for biocatalytic polymerization has ranged from the extensive, and often fundamental, studies using supercritical C02, to more exploratory and recent reports for media such as ionic liquids and fhroro-solvents. In all cases, however, intriguing results have lead to further investigation. As increasing pressure is exerted upon scientists in both academia and industry alike to develop and commercialize greener reaction systems, it is expected that biocatalysis and reactions in these and other exotic solvents will continue to be of considerable interest into the future, as they have been over the previous decades. 

In a helpful approach, Abildskov et al. described a computer-aided solvent screening methodology for biocatalytic systems that used group contribution methods for calculating the constrained properties related to chemical reaction equilibrium, substrate and product solubility, water solubility, boiling points, toxicity, and others. 

Ionic liquids have been employed as the reaction media for a wide range of (bio)catalytic processes, as catalysts or as (bio)catalyst-supports. These modem solvents have been shown to possess a significant effect on the stabilization of charged intermediates in chemical catalytic processes, as well as on enzymes stability. Some ionic liquids have been shown to be by far the best non-aqueous media for biocatalytic processes due to their positive influences on enzyme stability and activity, as well as on the enantioselectivity of the reactions catalyzed by them (Park Kazlauskas, 2003). When the right ionic liquid is chosen for a given reaction, it can lead not only to enhanced selectivity, yield or reaction rate, but it also improves the result of the work-up, from product separation (extraction/distillation) to recycling of the system ionic liquid/(bio)catalysts (Dyson et al, 2003 Parvulescu Hardacre, 2007 van Rantwijk Sheldon, 2007). Separation of products from ionic liquid and ionic liquid recovery can be accomplished by distillation of product (if the product is sufficiently volatile), extraction with supercritical CO2, or simple phase separation (if the product is immiscible in the ionic liquid) (Cornils, 1999). 

Since the beginning of the 20th century, organic solvents have been used in enzymatic reaction media [30]. Biocatalytic reactions in water-organic biphasic media were first carried out by Cremonesi et al. [31] and by Buckland et al. [32] less than 30 years ago. Their work aimed at the conversion of high concentrations of poorly water soluble components, particularly steroids. Later, biphasic systems were used for enzyme-catalyzed synthesis reactions that were unfavored in water, changing the reaction equilibrium towards the higher yield of the product, such as esters or peptides. 

Due to the water requirement of biocatalytic systems, BDS is typically carried out as a two-phase aqueous-oil process. However, increased sulfur removal rates could be accomplished by using an aqueous-alkane solvent catalytic system [46,203,220,255], The BDS catalytic activity depends on both, the biocatalysts and the nature of the feedstock. It can vary from low activity for crude oil to as high as 60% removal for light gas-oil type feedstocks [27,203,256], or 70% for middle distillates, 90% for diesel, 70% for hydrotreated diesel, and 90% for cracked feedstocks [203,256], The viscosity of the crude oil poses mixing issues in the two-phase oil-water systems however, such issues are minimal for distillate feedstocks, such as diesel or gasoline [257]. 

The rate of the reaction in [BDMIM]BF4 was superior to that in [BMIM]BF4. It is significant that no drop in the reaction rate was observed in the [BDMIM]BF4 system after 10 repeated uses of the enzyme, whereas the reaction rate was significantly reduced when the reaction was conducted in [BMIM]BF4 or in [BMIM]PF6 (282). [BDMIM]BF4 was found to be the best solvent for the recycled use of the enzyme under normal pressure conditions when vinyl acetate was used as the acyl donor. No reaction took place when [BDMIM]PF6 was used as the solvent in the lipase-catalyzed reaction. (The [BDMIM]PF6 was purified with particular care to rule out the possibility of contamination.) The replacement of the C2 proton of [BMIM]PF6 by a methyl group was therefore concluded to have a large influence on its biocatalytic compatibility. 

Biphasic systems proved to be advantageous as well in the biocatalytic synthesis of (-)-l-trimethylsilylethanol which was performed by asymmetric reduction of acetyltrimethylsilane with an isolate from Rhodotorula sp. AS2.2241 [144]. Immobilized cells were employed due to the easy separation of the product as well as the improved tolerance against unfavorable factors. In an aqueous/organic solvent biphasic system higher product yield and enantiomeric excess were achieved as compared to an aqueous monophasic system. Several organic solvents were examined, and isooctane was found to be the most suitable organic phase for the reaction. 

The use of various low-water reaction systems, including anhydrous organic solvents as well as imidazolium-based ionic liquids, is investigated for the biocatalytic preparation of lipophilic derivatives of various natural phenohc compounds (Figure 9.1) including phenohc and flavonoid glycosides (escuhn, salicin, helicin. 

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