Review Article Enzymes in Non-Conventional Phases

Analysis of the Behavior of Deep Eutectic Solvents upon Addition of Water: Its Effects over a Catalytic Reaction

This study presents the potential role of deep eutectic solvents (DESs) in a lipase-catalyzed hydrolysis reaction as a co-solvent in an aqueous solution given by a phosphate buffer. Ammonium salts, such as choline chloride, were paired with hydrogen bond donors, such as urea, 1,2,3-propanetriol, and 1,2 propanediol. The hydrolysis of p-nitrophenyl laureate was carried out with the lipase Candida antarctica Lipase B (CALB) as a reaction model to evaluate the solvent effect and tested in different DES/buffer phosphate mixtures at different % w/w. The results showed that two mixtures of different DES at 25 % w/w were the most promising solvents, as this percentage enhanced the activities of CALB, as evidenced by its higher catalytic efficiency (kcatKM). The solvent analysis shows that the enzymatic reaction requires a reaction media rich in water molecules to enable hydrogen-bond formation from the reaction media toward the enzymatic reaction, suggesting a better interaction between the substrate and the enzyme-active site. This interaction could be attributed to high degrees of freedom influencing the enzyme conformation given by the reaction media, suggesting that CALB acquires a more restrictive structure in the presence of DES or the stabilized network given by the hydrogen bond from water molecules in the mixture improves the enzymatic activity, conferring conformational stability by solvent effects. This study offers a promising approach for applications and further perspectives on genuinely green industrial solvents.

Biocatalyzed Flow Oxidation of Tyrosol to Hydroxytyrosol and Efficient Production of Their Acetate Esters

Tyrosol (Ty) and hydroxytyrosol (HTy) are valuable dietary phenolic compounds present in olive oil and wine, widely used for food, nutraceutical and cosmetic applications. Ty and HTy are endowed with a number of health-related biological activities, including antioxidant, antimicrobial and anti-inflammatory properties. In this work, we developed a sustainable, biocatalyzed flow protocol for the chemo- and regio-selective oxidation of Ty into HTy catalyzed by free tyrosinase from Agaricus bisporus in a gas/liquid biphasic system. The aqueous flow stream was then in-line extracted to recirculate the water medium containing the biocatalyst and the excess ascorbic acid, thus improving the cost-efficiency of the process and creating a self-sufficient closed-loop system. The organic layer was purified in-line through a catch-and-release procedure using supported boronic acid that was able to trap HTy and leave the unreacted Ty in solution. Moreover, the acetate derivatives (TyAc and HTyAc) were produced by exploiting a bioreactor packed with an immobilized acyltransferase from Mycobacterium smegmatis (MsAcT), able to selectively act on the primary alcohol. Under optimized conditions, high-value HTy was obtained in 75% yield, whereas TyAc and HTyAc were isolated in yields of up to 80% in only 10 min of residence time.

Continuous operation of lipase-catalyzed reactions in nonaqueous solvents: Influence of the production of hydrophilic compounds

In the field of biocatalysis in nonaqueous media, water has been identified as a crucial parameter which has to be carefully controlled. This article studies the continuous operation of a water-producing enzymatic reaction, here the esterification of oleic acid by ethanol in n-hexane catalyzed by Lipozyme(TM). The conversion decreased significantly over time, eventually coming to a lower steady-state level. This would be due to the accumulation of the produced water into the enzyme fixed-bed reactor, n-hexane being unable to evacuate this water out of the reaction vessel, because of the low polarity of this solvent. Therefore the conversion decreased until the produced water could be eliminated by the solvent achieving a steady state with a lower conversion. In supercritical carbon dioxide, a more hydrophilic solvent, steady state is at once obtained. This approach has been extended to reaction producing a hydrophilic compound, here glycerol during the transesterification between triolein and ethanol, and similar conclusions can be made. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 232-237, 1997.

Mannanase transfer into hexane and xylene by liquid-liquid extraction

The formation of noncovalent complexes between glycosidase, endo-1,4-beta-D-mannanase, and ionic surfactant di(2-ethylhexyl) sodium sulfosuccinate (AOT) was shown to promote protein transfer into organic solvents such as xylene and hexane. It was found that mannanase can be solubilized in hexane and in xylene with concentration at least 2.5 and 2.0 mg/ml, respectively. The catalytic activity of the enzyme in hexane spontaneously increases with the concentration of AOT and is about 10% of the activity in aqueous system. In xylene, a catalytic activity higher than that in bulk aqueous conditions was found for the samples containing 0.1-0.3 mg/ml of mannanase, while for the samples with a higher concentration of enzyme, the activity was hardly detected.

Optimization of lipase-catalyzed synthesis of acetylated tyrosol by response surface methodology

The ability of a noncommercial immobilized lipase from Staphylococcus xylosus (SXLi) to catalyze the transesterification of tyrosol and ethyl acetate was investigated. Response surface methodology was used to evaluate the effects of the temperature (40-60 degrees C), the enzyme amount (50-500 UI), and the ethyl acetate/hexane volume ratio (0.2-1) on the tyrosol acetylation conversion yield. Two independent replicates were carried out under the optimal conditions predicted by the model (reaction temperature 54 degrees C, enzyme amount 500 UI, and volume ratio ethyl acetate/hexane 0.2). The maximum conversion yield reached 95.36 +/- 3.6%, which agreed with the expected value (96.8 +/- 3.7%). The ester obtained was characterized by spectroscopic methods. Chemical acetylation of tyrosol was performed, and the products were separated using HPLC. Among the eluted products from HPLC, mono- and diacetylated derivatives were identified by positive mass spectrometry. Tyrosol and its monoacetylated derivative exert similar antiradicalar activity on 2,2-diphenyl-1-picrylhydrazyle.

On the Possibility That Cyclodextrins' Chiral Cavities Can Be Available on AOT n-Heptane Reverse Micelles. A UV−Visible and Induced Circular Dichroism Study

The formation of reverse micelles (RMs) of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) in n-heptane including two different beta-cyclodextrin (beta-CD) derivatives (hydroxypropyl-beta-CD, hp-beta-CD, and decenyl succinyl-beta-CD, Mod-beta-CD) is reported. Both cyclodextrins can be incorporated into AOT RMs in different zones within the aggregate, while beta-CD cannot. Using UV-vis and induced circular dichroism (ICD) spectroscopy and different achiral molecular probes (some azo dyes, p-nitroaniline and ferrocene), it was possible to determine that Mod-beta-CD is located with its cavity at the oil side of the AOT RM interface, while for hp-beta-CD the cavity is inside the RM water pool. Among the molecular probes used, methyl orange (MO) was the only one which gave the ICD signal when dissolved in the AOT RMs with hp-beta-CD, so a detailed study of MO behavior in homogeneous media was also performed to compare with the microheterogeneous media. The solvatochromic behavior of the dye depends not only on the polarity of the media but also on other specific solvent properties. A Kamlet-Taft analysis shows that the MO absorption spectrum shifts to longer wavelength with an increase in the solvent polarity-polarizability (pi*) and the hydrogen donor ability (alpha) of the medium. MO appears to be almost 3 times more sensitive to the pi* parameter than to the alpha parameter. In addition, from the MO absorption spectral changes with the hp-beta-CD concentration, the association equilibrium constants in pure water (K11W) and inside the RMs (K11RM) were computed. The results show that K11W is almost 10 times larger than the value inside the RMs. The latter can be explained considering that MO resides anchored to the RM interface through hydrogen bond interaction with the hydration bound water. This study shows for the first time that the cyclodextrin chiral cavity is available for a guest in an organic medium such as the RMs; therefore, we have created a potentially powerful nanoreactor with two different confined regions in the same aggregate: the polar core of the RMs and the chiral hydrophobic cavity of cyclodextrin.

Effects of pressure and temperature on enzymatic reactions in supercritical fluids

Supercritical fluids (SCFs) are receiving increasing attention as reaction media because they permit higher reaction rates compared with the conventional solvents. The ease of manipulating the physical properties of the SCFs enables easier control of the reaction conditions and easier solvent removal after the reaction. This review focuses on effects of pressure, temperature and the properties of the SCFs, on enzymatic reactions. Phase behavior, reaction rate and activation volume in SCFs are discussed. Within the ranges of pressure (10-40 MPa) and temperature (35-60 degrees C) that typically characterize the supercritical region, an increase in pressure and/or a decrease in temperature lead to a decrease in the enzyme turnover because the diffusion coefficients of the substrates migrating to the active sites of enzymes are affected.

New Insights on the Photophysical Behavior of PRODAN in Anionic and Cationic Reverse Micelles: From Which State or States Does It Emit?

6-propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe because of its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer states (ICT) that can be particularly useful as a sensor. The nature of the emissive states has not yet been established despite the detailed experimental and theoretical investigations done on this fluorophore. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES) and time-resolved area normalized emission (TRANES) spectroscopies on the molecular probe PRODAN in the anionic water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane and the cationic water/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/benzene reverse micelles (RMs). The experiments were done by varying the surfactant concentrations at a fixed molar ratio (W = [H2O]/[Surfactant]) and changing the water content at a constant surfactant concentration. The results obtained varying the surfactant concentration at W = 0 show a bathochromic shift and an increase in the intensity of the PRODAN emission band due to the PRODAN partition process between the external solvent and the RMs interface. The partition constants, Kp, are quantified from the changes in the PRODAN emission spectra and the steady-state anisotropy (<r>) with the surfactant concentration in both RMs. The Kp value is larger in the BHDC than the AOT RMs, probably due to the interaction between the cationic polar head of the surfactant and the aromatic ring of PRODAN. The partition process is confirmed with the TRES experiments, where the data fit to a continuous model, and with the time-resolved area normalized emission spectroscopy (TRANES) spectra, where only one isoemissive point is detected. On the other hand, the emission spectra at W = 10 and 20 show a dual fluorescence with a new band that emerges in the low-energy region of the spectra, a band that was previously assigned to the PRODAN emission from the water pool of RMs. Our studies demonstrate that this band is due to the emission from an ICT state of the molecular probe PRODAN located at the interface of the RMs. These results are also confirmed by the lifetime measurements, the TRES experiments where the results fit to a two-state model, and the time-resolved area normalized emission spectroscopy (TRANES) spectra where three or two isoemissive points are detected in the AOT and BHDC RMs, respectively. In the AOT RMs, Kp values obtained at W = 10 and 20 are almost independent of the water content; the values are higher for the BHDC RMs due to the higher micropolarity of this interface.

Combining Enabling Techniques in Organic Synthesis: Continuous Flow Processes with Heterogenized Catalysts

The concepts article describes enabling techniques (solid-phase assisted synthesis, new reactor design, microwave irradiation and new solvents) in organic chemistry and emphasizes the combination of several of them for creating new synthetic technology platforms. Particular focus is put on the combination of immobilized catalysts as well as biocatalysts with continuous flow processes. In this context, the PASSflow continuous flow technique fulfils both chemical as well as chemical engineering requirements. It combines reactor design with optimized, monolithic solid phases as well as reversible immobilization techniques for performing small as well as large scale synthesis with heterogenized catalysts under continuous flow conditions.

A novel lipolytic activity of Rhodotorula glutinis cells: production, partial characterization and application in the synthesis of esters

Cell-bound lipase activity (10 pNPL units/g dry cell weight) was released when the yeast Rhodotorula glutinis was cultured in a 7-l stirred tank fermentor using palm-oil as the sole carbon source. The enzyme showed relative specificity towards medium chain organic acids since the apparent K(m) values for pNPB (p-NitroPhenyl-Butyrate) and pNPL (p-NitroPhenyl-Laurate) were equal to 2.7 and 0.7 mM, respectively. In addition, 80% of this activity could be detected on the surface of the cells. The cell-bound nature of the enzyme increased its thermal stability showing half-life times of 200 and 60 min at 50 and 60 degrees C, respectively, as well as good stability in organic solvents. Freeze-dried cell preparations were successfully used to catalyze the synthesis of fatty acid esters of butanol and heptanol in nearly anhydrous organic solvents. A conversion of 60-62% was obtained upon esterification of palmitic or oleic acid with butanol, within 96 h. The enzyme preparation was used in four consecutive batch reactions with only 10% loss of activity.

Optimization of lipase-catalyzed synthesis of octyl hydroxyphenylpropionate by response surface methodology

The ability of immobilized lipase Candida antarctica (Novozyme 435) to catalyze the direct esterification of hydroxyphenylpropionic acid and octanol in a solvent-free system was investigated in this study. Response surface methodology (RSM) and five-level-four-factor central composite rotatable design (CCRD) were employed to evaluate the effects of synthesis parameters, such as reaction time, temperature, enzyme amount, and pH memory, on percentage molar conversion of phenolic acid esters. Reaction time, temperature, and enzyme amount were the most important variables. On the basis of canonical analysis and ridge max analysis, the optimum synthesis conditions with 95.9% molar conversion were reaction time of 58.2 h, temperature of 52.9 degrees C, enzyme amount of 37.8% (w/w), and pH memory of pH 7.

(R)-phenylacetylcarbinol production in aqueous/organic two-phase systems using partially purified pyruvate decarboxylase fromCandida utilis

Aqueous/organic two-phase systems have been evaluated for enhanced production of (R)-phenylacetylcarbinol (PAC) from pyruvate and benzaldehyde using partially purified pyruvate decarboxylase (PDC) from Candida utilis. In a solvent screen, octanol was identified as the most suitable solvent for PAC production in the two-phase system in comparison to butanol, pentanol, nonanol, hexane, heptane, octane, nonane, dodecane, methylcyclohexane, methyl tert butyl ether, and toluene. The high partitioning coefficient of the toxic substrate benzaldehyde in octanol allowed delivery of large amounts of benzaldehyde into the aqueous phase at a concentration less than 50 mM. PDC catalyzed the biotransformation of benzaldehyde and pyruvate to PAC in the aqueous phase, and continuous extraction of PAC and byproducts acetoin and acetaldehyde into the octanol phase further minimized enzyme inactivation, and inhibition due to acetaldehyde. For the rapidly stirred two-phase system with a 1:1 phase ratio and 8.5 U/mL carboligase activity, 937 mM (141 g/L) PAC was produced in the octanol phase in 49 h with an additional 127 mM (19 g/L) in the aqueous phase. Similar concentrations of PAC could be produced in the slowly stirred phase separated system at this enzyme level, although at a much slower rate. However at lower enzyme concentration very high specific PAC production (128 mg PAC/U carboligase at 0.9 U/mL) was achieved in the phase separated system, while still reaching final PAC levels of 102 g/L in octanol and 13 g/L in the aqueous phase. By comparison with previously published data by our group for a benzaldehyde emulsion system without octanol (50 g/L PAC, 6 mg PAC/U carboligase), significantly higher PAC concentrations and specific PAC production can be achieved in an octanol/aqueous two-phase system.

Reverse Micellar Aggregates: Effect on Ketone Reduction. 1. Substrate Role

The reduction of three aromatic ketones, acetophenone (AF), 4-methoxyacetophenone (MAF), and 3-chloroacetophenone (CAF), by NaBH(4) was followed by UV-vis spectroscopy in reverse micellar systems of water/AOT/isooctane at 25.0 degrees C (AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate). The first-order rate constants, k(obs), increase with the concentration of surfactant due to the substrate incorporation at the reverse micelle interface, where the reaction occurs. For all the ketones the reactivity is lower at the micellar interface than in water, probably reflecting the low affinity of the anionic interface for BH(4)(-). Kinetic profiles upon water addition show maxima in k(obs) at W(0) approximately 5 probably reflecting a strong interaction between water and the ionic headgroup of AOT; at W(0) < 5 by increasing W(0) BH(4)(-) is repelled from the anionic interface once the water pool forms. The order of reactivity was CAF >> AF > MAF. Application of a kinetic model based on the pseudophase formalism, which considers distribution of the ketones between the continuous medium and the interface, and assumes that reaction take place only at the interface, gives values of the rate constants at the interface of the reverse micellar system. At W(0) = 5, we conclude that NaBH(4) is wholly at the interface, and at W(0) = 10 and 15, where there are free water molecules, the partitioning between the interface and the water pool has to be considered. The results were used to estimate the ketone and borohydride distribution constants between the different pseudophases as well as the second-order reaction rate constant at the micellar interface.

Reverse Micellar Aggregates: Effect on Ketone Reduction. 2. Surfactant Role

Kinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH(4)) were followed by UV-vis spectroscopy at 27.0 degrees C in different reverse micellar media, toluene/BHDC/water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such as surfactant and NaBH(4) concentration and the amount of water dispersed in the reverse micelles, W(0) = [H(2)O]/[surfactant]. In all cases, the first-order rate constant, k(obs), increases with the concentration of surfactant as a consequence of incorporating the substrate into the interface of the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface than at the anionic one probably because the negative ion BH(4)(-) is part of the cationic interface. The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to BH(4)(-) being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic profile upon water addition depends largely on the type of interface. In the BHDC system, k(obs) increases with W(0) in the whole range studied while in AOT the kinetic profile has a maximum at W(0) approximately 5, probably reflecting the fact that BH(4)(-) is part of the cationic interface while, in the anionic one, there is a strong interaction between water and the polar headgroup of AOT below W(0) = 5 and, above that, BH(4)(-) is repelled from the interface once the water pool has formed. Application of a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone between the continuous medium and the interface and assumes that reaction takes place only at the interface, has enabled us to estimate rate constants at the interface of the reverse micellar systems. At W(0) < 10, it was considered that NaBH(4) is wholly at the interface and, at W(0) >/= 10, where there are free water molecules, also the partitioning between the interface and the water pool was taken into account. The results were used to evaluate CAF and NaBH(4) distribution constants between the different pseudophases as well as the second-order reaction rate constant of the reduction reaction in the micellar interface.

Esterification by immobilized lipase in solvent-free media: kinetic and thermodynamic arguments

The aim of this study is to characterize, in solvent-free systems (SFS), the kinetic and thermodynamic performance of batch lipase-catalyzed esterification. SFS are compared to a conventional organic solvent, n-hexane. The esterification of oleic acid with ethanol was chosen as a model reaction. The TABEK (thermodynamic activity-based enzyme kinetics) approach was used to rationally analyze kinetics. Influence of the reaction medium on final conversions was also studied. Several factors, such as initial molar ratio of substrates, reactant availability, initial water content, and quantity of immobilized enzyme, were examined. Special attention was also turned to enzyme stability and reuse after reaction, this last item being a prerequisite in the development of industrial processes. SFS proved to be almost as efficient as n-hexane from a kinetic and thermodynamic point of view and offered a better volumetric production.

Alpha-chymotrypsin catalysis in imidazolium-based ionic liquids

The transesterification reaction of N-acetyl-L-phenylalanine ethyl ester with 1-propanol catalyzed by alpha-chymotrypsin was examined in the ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim][PF(6)]), and in combination with supercritical carbon dioxide (SC-CO(2)). The activity of alpha-chymotrypsin was studied to determine whether trends in solvent polarity, water activity, and enzyme support properties, observed with this enzyme in conventional organic solvents, hold for the novel environment provided by ionic liquids. alpha-Chymotrypsin freeze-dried with K(2)HPO(4), KCl, or poly(ethylene glycol) demonstrated no activity in [bmim][PF(6)] or [omim][PF(6)] at very low water concentrations, but moderate transesterification rates were observed with the ionic liquids containing 0.25% water (v/v) and higher. However, the physical complexation of the enzyme with poly(ethylene glycol) or KCl did not substantially stimulate activity in the ionic liquids, unlike that observed in hexane or isooctane. Activities were considerably higher in [omim][PF(6)] than [bmim][PF(6)]. Added water was not necessary for enzyme activity when ionic liquids were combined with SC-CO(2). These results indicate that [bmim][PF(6)] and [omim][PF(6)] provide a relatively polar environment, which can be modified with nonpolar SC-CO(2) to optimize enzyme activity.

Influence of anionic and cationic reverse micelles on nucleophilic aromatic substitution reaction between 1-fluoro-2,4-dinitrobenzene and piperidine

The nucleophilic aromatic substitution (S(N)Ar) reaction between 1-fluoro-2,4-dinitrobenzene and piperidine (PIP) were studied in two different reverse micellar interfaces: benzene/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT)/water and benzene/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/water reverse micellar media. The kinetic profiles of the reactions were investigated as a function of variables such as surfactant and amine concentration and the amount of water dispersed in the reverse micelles, W0 = [H2O]/[surfactant]. In the AOT system at W0 = 0, no micellar effect was observed and the reaction takes place almost entirely in the benzene pseudophase, at every AOT and PIP concentration. At W0 = 10, a slight increment of the reaction rate was observed at low [PIP] with AOT concentration, probably due to the increase of micropolarity of the medium. However, at [PIP] > or = 0.07 M the reaction rates are always higher in pure benzene than in the micellar medium because the catalytic effect of the amine predominates in the organic solvent. In the BHDC system the reaction is faster in the micellar medium than in the pure solvent. Increasing the BHDC concentration accelerates the overall reaction, and the saturation of the micellar interface is never reached. In addition, the reaction is not base-catalyzed in this micellar medium. Thus, despite the partition of the reactants in both pseudophases the reactions effectively take place at the interface of the aggregates. The kinetic behavior can be quantitatively explained taking into account the distribution of the substrate and the nucleophile between the bulk solvent and the micelle interface. The results were used to evaluate the amine distribution constant between the micellar pseudophase and organic solvent and the second-order rate coefficient of S(N)Ar reaction in the interface. A mechanism to rationalize the kinetic results in both interfaces is proposed.