Biocatalysis in ionic liquids: the stereoconvergent hydrolysis of trans-β-methylstyrene oxide catalyzed by soluble epoxide hydrolase

Biocatalytic, Stereoconvergent Alkylation of (Z/E)-Trisubstituted Silyl Enol Ethers

Alkene functionalization has garnered significant attention due to the versatile reactivity of C=C bonds. A major challenge is the selective conversion of isomeric alkenes into chiral products. Researchers have devised various biocatalytic strategies to transform isomeric alkenes into stereopure compounds; while selective, the enzymes often specifically convert one alkene isomer, thereby diminishing overall yield. To increase the overall yield, scientists have introduced additional driving forces to interconvert alkene isomers. This improves the yield of biocatalytic alkene functionalization at the cost of increased energy consumption and chemical waste. Developing a stereoconvergent enzyme for alkene functionalization offers an ideal solution, although such catalysts are rarely reported. Here we present engineered hemoproteins derived from a bacterial cytochrome P450 that efficiently catalyze the stereoconvergent α-carbonyl alkylation of isomeric silyl enol ethers, producing stereopure products. Through screening and directed evolution, we generated P450BM3 variant SCA-G2, which catalyzes stereoconvergent carbene transfer in E. coli, with high efficiency and stereoselectivity toward various Z/E mixtures of silyl enol ethers. In contrast to established stereospecific transformations that leave one isomer unreacted, SCA-G2 converts both isomers to a stereopure product. This biocatalytic approach simplifies the synthesis of chiral α-branched ketones by eliminating the need for stoichiometric chiral auxiliaries, strongly basic alkali-metal enolates, and harsh conditions, delivering products with high efficiency and excellent chemo- and stereoselectivities.

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.

Microwave-assisted method for simultaneous extraction and hydrolysis for determination of flavonol glycosides in Ginkgo foliage using Brönsted acidic ionic-liquid [HO(3)S(CH(2))(4)mim]HSO(4) aqueous solutions

The Brönsted acidic ionic-liquid [HO₃S(CH₂)₄mim] HSO₄, a novel dual catalyst–solvent, has been successfully applied in simultaneous microwave-assisted extraction and hydrolysis for the determination of flavonol glycosides in Ginkgo foliage. The parameters, namely the [HO₃S(CH₂)₄mim]HSO₄ concentration, microwave-irradiation power, microwave-irradiation time, and solid–liquid ratio, were optimized. The optimum conditions were: an amount of 1.5 M [HO₃S(CH₂)₄mim]HSO₄, a microwave-irradiation power of 120 W, an irradiation time of 15 min, and a solid–liquid ratio of 1:30 g/mL. Compared with traditional methods the proposed approach demonstrates higher efficiency in a shorter operating time, and is an efficient, rapid, and simple sample preparation method.

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.

Temperature and pH dependence of enzyme-catalyzed hydrolysis of trans-methylstyrene oxide. A unifying kinetic model for observed hysteresis, cooperativity, and regioselectivity

The underlying enzyme kinetics behind the regioselective promiscuity shown by epoxide hydrolases toward certain epoxides has been studied. The effects of temperature and pH on regioselectivity were investigated by analyzing the stereochemistry of hydrolysis products of (1R,2R)-trans-2-methylstyrene oxide between 14-46 degrees C and pH 6.0-9.0, either catalyzed by the potato epoxide hydrolase StEH1 or in the absence of enzyme. In the enzyme-catalyzed reaction, a switch of preferred epoxide carbon that is subjected to nucleophilic attack is observed at pH values above 8. The enzyme also displays cooperativity in substrate saturation plots when assayed at temperatures < or = 30 degrees C and at intermediate pH. The cooperativity is lost at higher assay temperatures. Cooperativity can originate from a kinetic mechanism involving hysteresis and will be dependent on the relationship between k(cat) and the rate of interconversion between two different Michaelis complexes. In the case of the studied reactions, the proposed different Michaelis complexes are enzyme-substrate complexes in which the epoxide substrate is bound in different binding modes, allowing for separate pathways toward product formation. The assumption of separated, but interacting, reaction pathways is supported by that formation of the two product enantiomers also displays distinct pH dependencies of k(cat)/K(M). The thermodynamic parameters describing the differences in activation enthalpy and entropy suggest that (1) regioselectivity is primarily dictated by differences in activation entropy with positive values of both DeltaDeltaH(++) and DeltaDeltaS(++) and (2) the hysteretic behavior is linked to an interconversion between Michaelis complexes with rates increasing with temperature. From the collected data, we propose that hysteresis, regioselectivity, and, when applicable, hysteretic cooperativity are closely linked properties, explained by the kinetic mechanism earlier introduced by our group.

Toward advanced ionic liquids. Polar, enzyme-friendly solvents for biocatalysis

Ionic liquids, also called molten salts, are mixtures of cations and anions that melt below 100 °C. Typical ionic liquids are dialkylimidazolium cations with weakly coordinating anions such as [MeOSO₃] or [PF₆]. Advanced ionic liquids such as choline citrate have biodegradable, less expensive and less toxic anions and cations. Deep eutectic solvents are also included in the advanced ionic liquids. Deep eutectic solvents are mixtures of salts such as choline chloride and uncharged hydrogen bond donors such as urea, oxalic acid, or glycerol. For example, a mixture of choline chloride and urea in 1:2 molar ratio liquifies to form a deep eutectic solvent. Their properties are similar to those of ionic liquids. Water-miscible ionic liquids as cosolvents with water enhance the solubility of substrates or products. Although traditional water-miscible organic solvents also enhance solubility, they often inactivate enzymes, while ionic liquids do not. The enhanced solubility of substrates can increase the rate of reaction and often increases the regio- or enantioselectivity. Ionic liquids can also be solvents for non-aqueous reactions. In these cases, they are especially suited to dissolve polar substrates. Polar organic solvent alternatives inactivate enzymes, but ionic liquids do not even when they have similar polarities. Besides their solubility properties, ionic liquids and deep eutectic solvents may be greener than organic solvents because ionic liquids are non-volatile and can be made from non-toxic components. This review covers selected examples of enzyme catalyzed reaction ionic liquids that demonstrate their advantages and unique properties and point out opportunities for new applications. Most examples involve hydrolases, but oxidoreductases and even whole cell reactions have been reported in ionic liquids.

"Nonsolvent" applications of ionic liquids in biotransformations and organocatalysis

The application of room-temperature ionic liquids (RTILs) as (co)solvents and/or reagents is well documented. However, RTILS also have "nonsolvent" applications in biotransformations and organocatalysis. Examples are the anchoring of substrates to RTILs; ionic-liquid-coated enzymes (ILCE) and enzyme-IL colyophilization; the construction of biocatalytic ternary reaction systems; the combination of enzymes, RTILs, membranes, and (bio)electrochemistry; and ionic-liquid-supported organocatalysts. These strategies provide more robust, more efficient, and more enantioselective bio- and organocatalysts with many practical applications. As shown herein, RTILs offer a wide range of promising alternatives to conventional chemistry.

Kinetics of reactions catalyzed by enzymes in solutions of surfactants

The effect of surfactants, both in water-in-oil microemulsions (hydrated reverse micelles) and aqueous solutions upon enzymatic processes is reviewed, with special emphasis on the effect of the surfactant upon the kinetic parameters of the process. Differences and similarities between processes taking place in aqueous and organic solvents are highlighted, and the main models currently employed to interpret the results are briefly discussed.

Effect of ionic liquids on epoxide hydrolase-catalyzed synthesis of chiral 1,2-diols

Ionic liquids (ILs) offer new possibilities for epoxide hydrolase (EH) catalyzed resolution of epoxides and for synthesis of chiral 1,2-diols. Soluble EHs from cress and mouse (csEH and msEH) and microsomal EH from rat (rmEH) were tested in several ILs. For all the enzymes tested, higher enantioselectivities were obtained in [bmim][N(Tf)(2)] and [bmim][PF(6)]. The optimized amount of water for EH activity in these ILs was established. Classical problems arising from low solubility of epoxides in water or from the high tendency of the oxirane ring to undergo chemical hydrolysis were avoided using these new media.

Penicillin acylase catalysis in the presence of ionic liquids

Several ionic liquids were used as reaction media for penicillin G acylase catalysis. In all the assayed ionic liquids, [bmim]PF6 proved good media for PGA-catalyzed hydrolysis. A novel [bmim]PF6/water two-phase system is provided for 6-aminopenicillanic acid (APA) production, which will be more benefical than aquous batch systems used widely in industrial production of APA.

Study of Aromatic Nucleophilic Substitution with Amines on Nitrothiophenes in Room-Temperature Ionic Liquids: Are the Different Effects on the Behavior of para-Like and ortho-Like Isomers on Going from Conventional Solvents to Room-Temperature Ionic Liquids Related to Solvation Effects?

The kinetics of the nucleophilic aromatic substitution of some 2-L-5-nitrothiophenes (para-like isomers) with three different amines (pyrrolidine, piperidine, and morpholine) were studied in three room-temperature ionic liquids ([bmim][BF4], [bmim][PF6], and [bm(2)im][BF4], where bmim = 1-butyl-3-methylimidazolium and bm(2)im = 1-butyl-2,3-dimethylimidazolium). To calculate thermodynamic parameters, a useful instrument to gain information concerning reagent-solvent interactions, the reaction was carried out over the temperature range 293-313 K. The reaction occurs faster in ionic liquids than in conventional solvents (methanol, benzene), a dependence of rate constants on amine concentration similar to that observed in methanol, suggesting a parallel behavior. The above reaction also was studied with 2-bromo-3-nitrothiophene, an ortho-like derivative able to give peculiar intramolecular interactions in the transition state, which are strongly affected by the reaction medium.

Thermal stability and activity regain of horseradish peroxidase in aqueous mixtures of imidazolium-based ionic liquids

Thermal deactivation kinetics of horseradish peroxidase (HRP) were studied from 45 to 90 degrees C in phosphate buffer and 5-25% (v,w/v) 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] and 1-butyl-3-methylimidazolium chloride [BMIM][Cl]. HRP activity at 25 degrees C was not affected by the presence of ionic liquids up to 20% (v,w/v). Increasing the ionic liquids concentration up to 25% (v,w/v) changed the biphasic character of deactivation kinetics to an apparent single first-order step. The presence of 5-10% (v/v) [BMIM][BF4] significantly improved HRP thermal stability with lower activation energies for the deactivation second phase (83-87 kJ mol(-1)). After deactivation, enhanced activity regain of the enzyme, up to 70-80% of the initial activity, was found in 25% (v/v) [BMIM][BF4] and 10% (w/v) [BMIM][Cl] and correlated to prevalence of the deactivation first phase.