Using Ionic Liquid in a Biphasic System to Improve Asymmetric Hydrolysis of Styrene Oxide Catalyzed by Cross-Linked Enzyme Aggregates (CLEAs) of Mung Bean Epoxide Hydrolases

Epoxide Hydrolases: Multipotential Biocatalysts

Epoxide hydrolases are attractive and industrially important biocatalysts. They can catalyze the enantioselective hydrolysis of epoxides to the corresponding diols as chiral building blocks for bioactive compounds and drugs. In this review article, we discuss the state of the art and development potential of epoxide hydrolases as biocatalysts based on the most recent approaches and techniques. The review covers new approaches to discover epoxide hydrolases using genome mining and enzyme metagenomics, as well as improving enzyme activity, enantioselectivity, enantioconvergence, and thermostability by directed evolution and a rational design. Further improvements in operational and storage stabilization, reusability, pH stabilization, and thermal stabilization by immobilization techniques are discussed in this study. New possibilities for expanding the synthetic capabilities of epoxide hydrolases by their involvement in non-natural enzyme cascade reactions are described.

Highly enantioselective resolution of racemic 1-phenyl-1,2-ethanediol to (S)-1-phenyl-1,2-ethanediol by Kurthia gibsonii SC0312 in a biphasic system

The asymmetric resolution of racemic 1-phenyl-1,2-ethanediol (PED) to (S)-PED by Kurthia gibsonii SC0312 (K. gibsonii SC0312) was conducted in a biphasic system comprised of an organic solvent and aqueous phosphate buffer. The impacts of organic solvents on the whole cell catalytic activity, metabolic activity, membrane integrity, and material distribution were first evaluated. The results showed that all organic solvents, except for dibutyl phthalate, showed a detrimental effect on the metabolic activity of the cells, especially for those with low log P values. All organic solvents were capable of changing the membrane permeability and membrane integrity of the cells. Moreover, some organic solvents showed a good extraction of the oxidation product. Finally, a high yield of 47.7 % of (S)-PED was obtained by the asymmetric resolution of racemic PED using K. gibsonii SC0312 in a biphasic system under the optimal conditions: racemic PED 120 mM, temperature 35 °C, reaction time 6 h, 180 rpm, and a volume ratio of dibutyl phthalate to aqueous phosphate buffer of 1:1. The optical purity of (S)-PED increased from 51.3 % to >99 %. This work described an efficient approach to improve reaction efficiency, and constructed a highly effective biphasic reaction system for the fabrication of (S)-PED via K. gibsonii SC0312.

Recent developments in biocatalysis in multiphasic ionic liquid reaction systems

Ionic liquids are well known and frequently used 'designer solvents' for biocatalytic reactions. This review highlights recent achievements in the field of multiphasic ionic liquid-based reaction concepts. It covers classical biphasic systems including supported ionic liquid phases, thermo-regulated multi-component solvent systems (TMS) and polymerized ionic liquids. These powerful concepts combine unique reaction conditions with a high potential for future applications on a laboratory and industrial scale. The presence of a multiphasic system simplifies downstream processing due to the distribution of the catalyst and reactants in different phases.

An Overview on the Enhancement of Enantioselectivity and Stability of Microbial Epoxide Hydrolases

Epoxide hydrolases (EHs; 3.3.2.x) catalyze the enantioselective ring opening of racemic epoxides to the corresponding enantiopure vicinal diols and remaining equivalent unreacted epoxides. These epoxides and diols are used for the synthesis of chiral drug intermediates. With an upsurge in the methods for identification of novel microbial EHs, a lot of EHs have been discovered and utilized for kinetic resolution of racemic epoxides. However, there is still a constraint on the account of limited EHs being successfully applied on the preparative scale for industrial biotransformations. This limitation has to be overcome before application of identified functional EHs on large scale. Many strategies such as optimizing reaction media, immobilizing EHs and laboratory-scale directed evolution of EHs have been adopted for enhancing the industrial potential of EHs. In this review, these approaches have been highlighted which can serve as a pathway for the enrichment of already identified EHs for their application on an industrial scale in future studies.

Markedly improving asymmetric oxidation of 1-(4-methoxyphenyl) ethanol with Acetobacter sp. CCTCC M209061 cells by adding deep eutectic solvent in a two-phase system

BACKGROUND

Enantiopure (S)-1-(4-methoxyphenyl) ethanol {(S)-MOPE} can be employed as an important synthon for the synthesis of cycloalkyl [b] indoles with the treatment function for general allergic response. To date, the biocatalytic resolution of racemic MOPE through asymmetric oxidation in the biphasic system has remained largely unexplored. Additionally, deep eutectic solvents (DESs), as a new class of promising green solvents, have recently gained increasing attention in biocatalysis for their excellent properties and many successful examples in biocatalytic processes. In this study, the biocatalytic asymmetric oxidation of MOPE to get (S)-MOPE using Acetobacter sp. CCTCC M209061 cells was investigated in different two-phase systems, and adding DES in a biphasic system was also explored to further improve the reaction efficiency of the biocatalytic oxidation.

RESULTS

Of all the examined water-immiscible organic solvents and ionic liquids (ILs), 1-butyl-3-methylimidazolium hexafluorophoshpate ([C4MIM][PF6]) afforded the best results, and consequently was selected as the second phase of a two-phase system for the asymmetric oxidation of MOPE with immobilized Acetobacter sp. CCTCC M209061 cells. For the reaction performed in the [C4MIM][PF6]/buffer biphasic system, under the optimized conditions, the initial reaction rate, the maximum conversion and the residual substrate e.e. recorded 97.8 μmol/min, 50.5 and >99.9 % after 10 h reaction. Furthermore, adding the DES [ChCl][Gly] (10 %, v/v) to the aqueous phase, the efficiency of the biocatalytic oxidation was rose markedly. The optimal substrate concentration and the initial reaction rate were significantly increased to 80 mmol/L and 124.0 μmol/min, respectively, and the reaction time was shortened to 7 h with 51.3 % conversion. The immobilized cell still retained over 72 % of its initial activity after 9 batches of successive reuse in the [C4MIM][PF6]/[ChCl][Gly]-containing buffer system. Additionally, the efficient biocatalytic process was feasible up to a 500-mL preparative scale.

CONCLUSION

The biocatalytic asymmetric oxidation of MOPE with Acetobacter sp. CCTCC M209061 cells was successfully conducted in the [C4MIM][PF6]-containing biphasic system with high conversion and enantioselectivity, and the reaction efficiency was further enhanced by adding [ChCl][Gly] to the reaction system. The efficient biocatalytic process was promising for the preparation of enantiopure (S)-MOPE.

Recent Developments in Chemical Synthesis with Biocatalysts in Ionic Liquids

Over the past decade, a variety of ionic liquids have emerged as greener solvents for use in the chemical manufacturing industries. Their unique properties have attracted the interest of chemists worldwide to employ them as replacement for conventional solvents in a diverse range of chemical transformations including biotransformations. Biocatalysts are often regarded as green catalysts compared to conventional chemical catalysts in organic synthesis owing to their properties of low toxicity, biodegradability, excellent selectivity and good catalytic performance under mild reaction conditions. Similarly, a selected number of specific ionic liquids can be considered as greener solvents superior to organic solvents owing to their negligible vapor pressure, low flammability, low toxicity and ability to dissolve a wide range of organic and biological substances, including proteins. A combination of biocatalysts and ionic liquids thus appears to be a logical and promising opportunity for industrial use as an alternative to conventional organic chemistry processes employing organic solvents. This article provides an overview of recent developments in this field with special emphasis on the application of more sustainable enzyme-catalyzed reactions and separation processes employing ionic liquids, driven by advances in fundamental knowledge, process optimization and industrial deployment.