Use of an ionic liquid to improve asymmetric reduction of 4′-methoxyacetophenone catalyzed by immobilized Rhodotorula sp. AS2.2241 cells

Mining of a novel reductase and its application for asymmetric reduction of p-methoxyacetophenone

(R)-1-(4-methoxyphenyl) ethanol [(R)-1b] is an essential precursor for the synthesis of aryl propanoic acids' anti-inflammatatory drugs. Biocatalysts for (R)-1b preparation are limited and reductase has problems of low substrate concentration and low conversion rate. As a result, there is a constant need for discovering novel biocatalysts with excellent catalytic performances. In this study, a novel reductase LpSDR from Lacisediminihabitans profunda for the biocatalytic reduction of p-methoxyacetophenone (1a) to (R)-1b was obtained based on gene-mining technology, and some key reaction parameters were also investigated to improve the conversion rate of 1a using whole cells of recombinant Escherichia coli expressing reductase LpSDR as biocatalysts. It was found that the optimal concentration of isopropanol, ZnSO4·7H2O solution, 1a, and recombinant E. coli resting cells, the optimal reaction temperature, buffer pH, and reaction time were 1.95 mol l-1, 0.75 mmol l-1, 75 mmol l-1, 250 g (wet weight) l-1, 28°C, 7.0, and 21 h, respectively. Under the above conditions, a conversion rate of 99.5% and an enantiomeric excess of 99.6% were obtained, which were superior to the corresponding values previously reported. This study provides a novel reductase LpSDR, which is helpful in reducing 1a to (R)-1b.

High production of d-psicose from d-fructose by immobilized whole recombinant Bacillus subtilis cells expressing d-psicose 3-epimerase from Agrobacterium tumefaciens

d-Psicose 3-epimerase (DPEase) can catalyze the isomerization of d-fructose to be rare sugar d-psicose, which has wide application prospects in the food and medical fields. In this study, the DPEase gene from Agrobacterium tumefaciens was constructed into plasmid pMA5, and was successfully expressed in the host Bacillus subtilis WB600 (B. subtilis). After optimization of the fermentation conditions, whole recombinant B. subtilis WB600/pMA5-At-DEPase(O) cells produced d-psicose from d-fructose with a conversion rate of 29.01 ± 0.19%, which could be used for the efficient synthesis of d-psicose. To further improve the whole recombinant B. subtilis application, B. subtilis cells were immobilized onto a gel bead biocatalyst by Ca-alginate. After optimization of the biotransformation conditions, the conversion rate of the immobilized biocatalyst reached 20.74 ± 0.39%, which was lower than the free cells. However, the results showed that the immobilized biocatalyst had higher thermal/pH stability and storability, and the gel beads could be recycled for at least six batches. The results showed that the amount of d-psicose generated reached 32.83 ± 2.56 g/L with the immobilized biocatalyst after six times biotransformation, whereas the free cells produced only approximately 10.44 ± 0.07 g/L. The results showed that immobilized recombinant B. subtilis cells are promising to use for the efficient synthesis of d-psicose.

Immobilized and Free Cells of Geotrichum candidum for Asymmetric Reduction of Ketones: Stability and Recyclability

Marine-derived fungus Geotrichum candidum AS 2.361 was previously reported by our group as an active strain for the enantioselective reduction of ketones. Although some other Geotrichum strains were also found from the terrestrial sources, information on their stability and reusability is scarce. Herein, the stabilities—in terms of pH tolerance, thermostability, and storage stability, and reusability—of G. candidum AS 2.361 were described for the asymmetric reduction of a series of aromatic ketones. Two differently immobilized cells (agar immobilization and calcium alginate immobilization) as well as free cells were prepared. For three substrates (1-(3-bromophenyl) ethan-1-one (1b), 1-(2-chlorophenyl) ethan-1-one (1d), and acetophenone (1g)) immobilized cells on agar showed a great improvement in the bioreduction activities compared to the free cells, increasing yields up to 97% with ee values of 99%. Cells immobilized on agar/calcium alginate could maintain more than 90% of the original activities within the assayed pH ranges of 3.5–11, while free cells were highly sensitive to alkaline and acidic conditions. Concerning thermostability, immobilized cells on agar kept 99% of their original activities after incubation at 60 °C for 1 h, while almost no activity was detected for the free cells under the same condition. Immobilized cells were stable at 4 °C for 80 days without any activity loss, while free cells started to decrease the activity after storage at 4 °C for six days. The immobilized cells retained almost 99% activity after four reuse cycles, while free cells lost almost all the activities at on the third cycle.

Ionic liquids in whole-cell biocatalysis: a compromise between toxicity and efficiency

Comparison of chemical catalysis by metal complexes, enzymatic catalysis and whole-cell biocatalysis shows well-addressed advantages of the latter approach. However, a critical limitation in the practical applications originates from the high sensitivity of microorganisms to the toxic effects of organic solvents. In the present review, we consider toxic solvent properties of ionic liquid/water systems towards the development of efficient applications in practical organic transformations.

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.

Efficient asymmetric biosynthesis of (R)-(−)-epinephrine in hydrophilic ionic liquid-containing systems

Acinetobacter sp. UN-16 cell biocatalytic process with [HOOCEMIM]NO₃ is very promising for efficient preparation of (R)-(−)-epinephrine.

Efficient biosynthesis of γ-decalactone in ionic liquids by immobilized whole cells of Yarrowia lipolytica G3-3.21 on attapulgite

In this study, the biosynthesis of γ-decalactone (GDL) was successfully conducted in an ionic liquid (IL)-containing cosolvent system using immobilized cells of Yarrowia lipolytica G3-3.21 on attapulgite (ATG). We found the immobilized Y. lipolytica G3-3.21 cells in N-butyl-pyridinium tetrafluoroborate ([BPy]BF4) solution gave the highest activity of C16-Acyl-CoA oxidase and the maximum yield of GDL. The optimum immobilization conditions for the highest yield of GDL were 20 g/L of ATG, 1.5 % of CaCl2 and 2 % of sodium alginate (NaAlg). The optimal [BPy]BF4 content, buffer pH, reaction temperature, shaking speed, castor oil and glucose contents were 7.5 %, 26 °C, 150 rpm, 100 g/L and 10 %, respectively. Under the optimized conditions, the GDL yield was up to 8.05 g/L. After ten times of reuse, the GDL yield was 7.51 g/L, corresponding to 93.3 % of that obtained in the first batch, suggesting a good reusability and potential for industrial applications.

(R)-PAC biosynthesis in [BMIM][PF₆]/aqueous biphasic system using Saccharomyces cerevisiae BY4741 cells

(R)-phenylacetylcarbinol or (R)-PAC is a pharmaceutical precursor of (1R, 2S) ephedrine and (1S, 2S) pseudoephedrine. Biotransformation of benzaldehyde and glucose by pyruvate decarboxylase produces (R)-PAC. This biotransformation suffers from toxicity of the substrate, product [(R)-PAC] and by-product (benzyl alcohol). In the present study, ionic liquid/aqueous biphasic system was employed to enhance (R)-PAC production. Fermented broth was the reaction medium in which Saccharomyces cerevisiae BY4741 was the source of pyruvate decarboxylase. Hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) was the non-aqueous phase in which toxic compounds reside. Biocompatibility of [BMIM][PF6] and adequate distribution coefficients of benzaldehyde, (R)-PAC and benzyl alcohol were determined. A Box-Behnken design and response surface methodology were used for the optimization of biotransformation variables in order to maximize (R)-PAC yield and productivity. The results showed higher (R)-PAC yield and productivity of ∼1.5-fold each in the biphasic biotransformation of phase volume ratio 0.05 as compared to the monophasic (conventional) biotransformation. Moreover, the level of major by-product benzyl alcohol was also 3.5-fold lower in biphasic biotransformation. [BMIM][PF6]/aqueous biphasic system is a new approach which could intensify the (R)-PAC production.

Applications and mechanisms of ionic liquids in whole-cell biotransformation

Ionic liquids (ILs), entirely composed of cations and anions, are liquid solvents at room temperature. They are interesting due to their low vapor pressure, high polarity and thermostability, and also for the possibility to fine-tune their physicochemical properties through modification of the chemical structures of their cations or anions. In recent years, ILs have been widely used in biotechnological fields involving whole-cell biotransformations of biodiesel or biomass, and organic compound synthesis with cells. Research studies in these fields have increased from the past decades and compared to the typical solvents, ILs are the most promising alternative solvents for cell biotransformations. However, there are increasing limitations and new challenges in whole-cell biotransformations with ILs. There is little understanding of the mechanisms of ILs' interactions with cells, and much remains to be clarified. Further investigations are required to overcome the drawbacks of their applications and to broaden their application spectrum. This work mainly reviews the applications of ILs in whole-cell biotransformations, and the possible mechanisms of ILs in microbial cell biotransformation are proposed and discussed.

Immobilization of Acetobacter sp. CCTCC M209061 for efficient asymmetric reduction of ketones and biocatalyst recycling

Background

The bacterium Acetobacter sp. CCTCC M209061 is a promising whole-cell biocatalyst with exclusive anti-Prelog stereoselectivity for the reduction of prochiral ketones that can be used to make valuable chiral alcohols such as (R)-4-(trimethylsilyl)-3-butyn-2-ol. Although it has promising catalytic properties, its stability and reusability are relatively poor compared to other biocatalysts. Hence, we explored various materials for immobilizing the active cells, in order to improve the operational stability of biocatalyst.

Results

It was found that Ca-alginate give the best immobilized biocatalyst, which was then coated with chitosan to further improve its mechanical strength and swelling-resistance properties. Conditions were optimized for formation of reusable immobilized beads which can be used for repeated batch asymmetric reduction of 4′-chloroacetophenone. The optimized immobilized biocatalyst was very promising, with a specific activity of 85% that of the free-cell biocatalyst (34.66 μmol/min/g dw of cells for immobilized catalyst vs 40.54 μmol/min/g for free cells in the asymmetric reduction of 4′-chloroacetophenone). The immobilized cells showed better thermal stability, pH stability, solvent tolerance and storability compared with free cells. After 25 cycles reaction, the immobilized beads still retained >50% catalytic activity, which was 3.5 times higher than degree of retention of activity by free cells reused in a similar way. The cells could be recultured in the beads to regain full activity and perform a further 25 cycles of the reduction reaction. The external mass transfer resistances were negligible as deduced from Damkohler modulus Da < <1, and internal mass transfer restriction affected the reduction action but was not the principal rate-controlling step according to effectiveness factors η < 1 and Thiele modulus 0.3<∅ <1.

Conclusions

Ca-alginate coated with chitosan is a highly effective material for immobilization of Acetobacter sp. CCTCC M209061 cells for repeated use in the asymmetric reduction of ketones. Only a small cost in terms of the slightly lower catalytic activity compared to free cells could give highly practicable immobilized biocatalyst.

15α-Hydroxylation of a steroid (13-ethyl-gon-4-en-3,17-dione) by Penicillium raistrickii in an ionic liquid/aqueous biphasic system

Biphasic processes are used in whole-cell biotransformation to overcome the low water solubility of substrates and products as well as their inhibitory effects on the biocatalyst. Commercially available [NTf(2)]- and [PF(6)]-based ionic liquids (ILs) were used in a biphasic system for the 15α-hydroxylation of 13-ethyl-gon-4-en-3,17-dione by Penicillium raistrickii. With the substrate at 5 g l(-1) and a volume ratio of IL to buffer, buffer pH and cell density at, 1:9, 6.5, 16.8 g(DW) l(-1), respectively, the 15α-hydroxylation of 13-ethyl-gon-4-en-3,17-dione was achieved with a yield of 70 % after 72 h using [BMIm][NTf(2)] in a 50 ml biphasic system. This is compared to a 30 % yield in a monophasic aqueous system. This suggests the potential industrial application of ILs-based biphasic systems for steroid biotransformation.

Efficient asymmetric hydrolysis of styrene oxide catalyzed by Mung bean epoxide hydrolases in ionic liquid-based biphasic systems

The asymmetric hydrolysis of styrene oxide to (R)-1-phenyl-1,2-ethanediol using Mung bean epoxide hydrolases was, for the first time, successfully conducted in an ionic liquid (IL)-containing biphasic system. Compared to aqueous monophasic system, IL-based biphasic systems could not only dissolve the substrate, but also effectively inhibit the non-enzymatic hydrolysis, and therefore markedly improve the reaction efficiency. Of all the tested ILs, the best results were observed in the biphasic system containing C(4)MIM·PF(6), which exhibited good biocompatibility with the enzyme and was an excellent solvent for the substrate. In the C(4)MIM·PF(6)/buffer biphasic system, it was found that the optimal volume ratio of IL to buffer, reaction temperature, buffer pH and substrate concentration were 1/6, 35°C, 6.5 and 100 mM, respectively, under which the initial reaction rate, the yield and the product e.e. were 18.4 mM/h, 49.4% and 97.0%. The biocatalytic process was shown to be feasible on a 500-mL preparative scale.

Activation and stabilization of enzymes in ionic liquids

As environmentally benign "green" solvents, room temperature ionic liquids (ILs) have been used as solvents or (co)solvents in biocatalytic reactions and processes for a decade. The technological utility of enzymes can be enhanced greatly by their use in ionic liquids (ILs) rather than in conventional organic solvents or in their natural aqueous reaction media. In fact, the combination of green properties and unique tailor-made physicochemical properties make ILs excellent non-aqueous solvents for enzymatic catalysis with numerous advantages over other solvents, including high conversion rates, high selectivity, better enzyme stability, as well as better recoverability and recyclability. However, in many cases, particularly in hydrophilic ILs, enzymes show relative instability and/or lower activity compared with conventional solvents. To improve the enzyme activity as well as stability in ILs, various attempts have been made by modifying the form of the enzymes. Examples are enzyme immobilization onto support materials via adsorption or multipoint attachment, lyophilization in the presence of stabilizing agents, chemical modification with stabilizing agents, formation of cross-linked enzyme aggregates, pretreatment with polar organic solvents or enzymes combined with suitable surfactants to form microemulsions. The use of these enzyme preparations in ILs can dramatically increase the solvent tolerance, enhance activity as well as stability, and improve enantioselectivity. This perspective highlights a number of pronounced strategies being used successfully for activation and stabilization of enzymes in non-aqueous ILs media. This review is not intended to be comprehensive, but rather to present a general overview of the potential approaches to activate enzymes for diverse enzymatic processes and biotransformations in ILs.

Immobilization of intracellular carbonyl reductase from Geotrichum candidum for the stereoselective reduction of 1-naphthyl ketone

Different cell disintegration methods were used for the liberation of intracellular carbonyl reductase from Geotrichum candidum, in its active form. Solid shear (bead milling) was proved to be the best method for the extraction of the enzyme. Various solid supports were checked for the immobilization of the purified enzyme. Carbonyl reductase was immobilized on silica with an optimized protein loading of 4 mg/g support. Cross-linking with glutaraldehyde rendered the preparation more stable and suitable for use in consecutive batches. Carbonyl reductase of G. candidum immobilized on silica support and cross-linked by glutaraldehyde was found to be highly efficient biocatalyst formulation for the production of S(-)-1-(1'-naphthyl) ethanol.

Using a water-immiscible ionic liquid to improve asymmetric reduction of 4-(trimethylsilyl)-3-butyn-2-one catalyzed by immobilized Candida parapsilosis CCTCC M203011 cells

Background

Whole cells are usually employed for biocatalytic reduction reactions to ensure efficient coenzyme regeneration and to avoid problems with enzyme purification and stability. The efficiency of whole cell-catalyzed bioreduction is frequently restricted by pronounced toxicity of substrate and/or product to the microbial cells and in many instances the use of two-phase reaction systems can solve such problems. Therefore, we developed new, biphasic reaction systems with biocompatible water-immiscible ionic liquids (ILs) as alternatives to conventional organic solvents, in order to improve the asymmetric reduction of 4-(trimethylsilyl)-3-butyn-2-one (TMSB) to (S)-4-(trimethylsilyl)-3-butyn-2-ol {(S)-TMSBOL}, a key intermediate for synthesis of 5-lipoxygenase inhibitors, using immobilized Candida parapsilosis CCTCC M203011 cells as the biocatalyst.

Results

Various ILs exerted significant but different effects on the bioreduction. Of all the tested water-immiscible ILs, the best results were observed with 1-butyl-3-methylimidazolium hexafluorophosphate (C₄MIM·PF₆), which exhibited not only good biocompatibility with the cells but also excellent solvent properties for the toxic substrate and product, thus markedly improving the efficiency of the bioreduction and the operational stability of the cells as compared to the IL-free aqueous system. 2-Propanol was shown to be the most suitable co-substrate for coenzyme regeneration, and it was found that the optimum volume ratio of buffer to C₄MIM·PF₆, substrate concentration, buffer pH, 2-propanol concentration and reaction temperature were 4/1 (v/v), 24 mM, 5.5, 130 mM and 30°C, respectively. Under these optimized conditions, the maximum yield and the product e.e. wer 97.7% and >99%, respectively, which are much higher than the corresponding values previously reported. The efficient whole-cell biocatalytic process was shown to be feasible on a 250-mL scale.

Conclusion

The whole cell-catalyzed asymmetric reduction of TMSB to (S)-TMSBOL can be substantially improved by using a C₄MIM·PF₆/buffer biphasic system instead of a single-phase aqueous system and the resulting biocatalytic process appears to be effective and competitive on a preparative scale.

Efficient enantioselective reduction of 4'-methoxyacetophenone with immobilized Rhodotorula sp. AS2.2241 cells in a hydrophilic ionic liquid-containing co-solvent system

The biocatalytic enantioselective reduction of 4'-methoxyacetophenone to (S)-1-(4-methoxyphenyl)ethanol was successfully conducted in a hydrophilic IL-containing co-solvent system using immobilized Rhodotorula sp. AS2.2241 cells. Of all the tested ILs, the best results were observed with the novel IL 1-(2'-hydroxy)ethyl-3-methylimidazolium nitrate (C(2)OHMIM.NO(3)), which showed a good biocompatibility with the cells and increased the cell membrane permeability moderately, thus improving the efficiency of the bioreduction. To better understand the bioreduction, several crucial influential variables were also examined. The optimal C(2)OHMIM.NO(3) content, buffer pH, reaction temperature and substrate concentration were 5.0% (v/v), 8.5, 25 degrees C and 12 mM, respectively. Under the optimized conditions, the initial reaction rate, the maximum yield and the product e.e. were 9.8 micromol/hg(cell), 98.3% and >99%, respectively, which are much better than the results previously reported. The established biocatalytic system has proven to be highly effective for the reduction of other aryl ketones. Also, the cells exhibited excellent operational stability in the presence of C(2)OHMIM.NO(3). Moreover, the ILs can accumulate within the cells, suggesting that ILs are likely to interact with the related enzymes within the cells.