Monophasic aqueous-organic systems

In this chapter, latest advancements in solvent engineering in bioreductions and greener needs for bioreaction media have been discussed in depth with recent examples. Solvents for bioreductions may be categorized as (i) aqueous (ii) water/water-miscible (monophasic aqueous-organic system) (iii) water/ water-immiscible (biphasic aqueous-organic system) (iv) nonaqueous (mono-phasic organic system, including solvent-free system) and (v) nonconventional media (e.g., ionic liquids, supercritical fluids, gas-phase media, and reverse micelles). 

Administration of water-miscible organic solvents is a straightforward method to increase the solubility of hydrophobic reactants. A monophasic aqueous-organic system can considerably reduce the mass-transfer limitations, leading to more rapid reaction rates for hydrophobic compounds. 

Lowering the water-activity of the medium [1565] by using water-miscible organic cosolvents such as ethanol or methanol. Alternatively, the reaction can be carried out in a biphasic aqueous-organic system or in a monophasic organic solvent (e.g., ethyl acetate, di-/-propyl, or methyl t-butyl ether) which contains only traces of water to preserve the enz3mie s activity. 

The addition of nucleophiles such as carbon nucleophiles to electrophiles such as unsaturated carbon-heteroatom bonds is an important reaction in organic chemistry, allowing carbon-carbon bond formation. Although these reactions are usually performed in organic or eventually in organic/aqueous two-phase system, it was recently shown that they could be performed in water alone or in a monophasic water/organic system. This development has also allowed reactions to proceed under less drastic conditions. 

Tsuchiyama, S., Doukyu, N., Yasuda, M., Ishimi, K., Ogino, H. (2007). Peptide synthesis of aspartame precursor using organic-solvent-stable PST-01 protease in monophasic aqueous-organic solvent systems. Biotechnol. Progr., 23,820-823. 

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. 

In peptidase-catalyzed peptide synthesis the solubility of the starting components dramatically influences the course of the synthesis. From the ideal medium, water, the spectrum of solvents ranges from water-miscible organic solvents and aqueous-organic biphasic systems to monophasic organic solvents with trace amounts of... 

NADP+ in a reaction with 2-propanol accompanied by formation of acetone as coproduct. Both ketone/alcohol reactions are equilibrium processes and therefore high 3delds of (f )-2-octanol are not available in a monophasic aqueous system, or in an organic-aqueous biphasic system where the partition coefficients of 2-propanol and acetone are approximately the same. It was found, however, that in a biphasic water/[bmim][(CF3S02)2N] system acetone was preferentially dissolved by the IL phase and this pulled the catalytic transfer hydrogenation of NADP+ by 2-propanol in the aqueous phase to near completion. Consequently, almost quantitative yields of (i )-2-octanol were obtained (275). 

Lin and coworkers disclosed that, at room temperature, nonenzymatic chemical addition was still observed in a water-organic solvent biphasic reaction system, though the volume of aqueous phases was relative small. Lin developed a method of preparing an active enzyme meal that contained essential water to retain its power for catalysis and found a new catalytic reaction system by application of the prepared meal in a nonaqueous monophasic organic medium (Figure 5.7). There was no problem over a wide range of temperature (from 0-30 °C) when the reactions were carried out under micro-aqueous conditions [50]. 

Enzyme catalysis in nonconventional media can be divided into a number of different categories depending on whether the aqueous and organic phases are miscible or immiscible and whether the biocatalyst is dissolved or not. In this section, only free enzymes will be considered. Thus, the field can be simplified to just two categories, depending on whether the solvent is water miscible or immiscible (systems employing water-immiscible solvents, where water is present in quantities that are below its solubility limit, have been considered as monophasic) ... 

It must be remembered that irradiation experiments can be performed with monophasic systems (in the absence of an aqueous phase) or in extraction conditions in the presence of an aqueous phase, in static or dynamic conditions (organic and aqueous phases continuously stirred with a dedicated stirrer unit (20), magnetic stirrer, or sparging with air). 

This was the starting point of Kimtz s work at Rhone-Poulenc, trying to transfer rhodium catalysts from the organic monophase to the aqueous biphase not by triphenylphosphine monosulfonate (which had been tried by Joo or WiUdnson (4, 8]) but by TPPTS. From 1974 onward, the scope of different reactions using biphasic catalyst systems was tested in laboratory-scale experiments. Among these were... 

When the phenylacetyl carbinol-generating biottansformations catalyzed by yeast whole cells were carried out in monophasic systems consisting of an aqueous phase and water-miscible organic solvents, a decrease in benzyl alcohol formation was observed [44,45]. 

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