Insights into the impact of deep eutectic solvents on horseradish peroxidase: Activity, stability and structure

Deep Eutectic Solvents as Efficient Solvents in Biocatalysis

'Ideal' solvents in biocatalysis have to fulfill a large number of requirements, such as high substrate solubility, high enzyme activity and stability, and positive effects on reaction equilibrium. In the past decades, many enzymatic synthesis routes in water-based and nonaqueous (organic solvents, ionic or supercritical fluids) reaction media have been developed. However, no solvent meets every demand for different reaction types at the same time, and there is still a need for novel solvents suited for different reaction types and applications. Deep eutectic solvents (DESs) have recently been evaluated as solvents in different biocatalytic reactions. They can improve substrate supply, conversion, and stability. The best results were obtained when the DES is formed by the substrates of an enzymatic reaction.

Ionic deep eutectic solvents for the extraction and separation of natural products

Room ionic liquids (ILs) used as green solvents have received considerable attention and wide application in different research and industrial fields, such as chemistry, biology, catalysis, energy, and even environmental sciences. Recently, a new class of sustainable solvents named deep eutectic solvents (DESs) have been developed, which share the promising solvent characteristics of ILs, such as thermal and chemical stability, low vapor pressure and design ability. In addition, the major advantages of DESs over ILs are their lower prices and easier preparation. Therefore, DESs have been considered to be a potential alternative to replace conventional organic solvents and ILs. Currently, the developed DESs may be classified into ionic and nonionic liquids. Typically, choline chloride (ChCl)/urea (1:2) is an ionic DES, while glucose/sucrose (1:1) is a nonionic DES. Although several reviews have covered advancements in DESs, in this review, we aim to provide a general insight into DESs, particularly ionic DESs, like choline-based DES, in terms of their preparation and application in the extraction of natural products (NPs) mainly from traditional Chinese medicines and the recovery of extracted compounds from their extracts. Additionally, various factors affecting the extraction efficiency of DESs are discussed.

Increased Selectivity of Novozym 435 in the Asymmetric Hydrolysis of a Substrate with High Hydrophobicity Through the Use of Deep Eutectic Solvents and High Substrate Concentrations

The effects of the reaction medium and substrate concentration were studied on the selectivity of Novozym 435 using the asymmetric hydrolysis of dimethyl-3-phenylglutarate as a model reaction. Results show that the use of choline chloride ChCl:urea/phosphate buffer 50% (v/v) as a reaction medium increased the selectivity of Novozym 435 by 16% (e.e = 88%) with respect to the one in 100% phosphate buffer (e.e = 76%). Best results were obtained when high substrate concentrations (well above the solubility limit, 27-fold) and ChCl:urea/phosphate buffer 50% (v/v) as reaction medium at pH 7 and 30 °C were used. Under such conditions, the R-monoester was produced with an enantiomeric purity of 99%. Novozym 435 was more stable in ChCl:urea/phosphate buffer 50% (v/v) than in phosphate buffer, retaining a 50% of its initial activity after 27 h of incubation at pH 7 and 40 °C. Results suggest that the use of deep eutectic solvents (ChCl:urea/phosphate buffer) in an heterogeneous reaction system (high substrate concentration) is a viable and promising strategy for the synthesis of chiral drugs from highly hydrophobic substrates.

Increased Selectivity of Novozym 435 in the Asymmetric Hydrolysis of a Substrate with High Hydrophobicity Through the Use of Deep Eutectic Solvents and High Substrate Concentrations

The effects of the reaction medium and substrate concentration were studied on the selectivity of Novozym 435 using the asymmetric hydrolysis of dimethyl-3-phenylglutarate as a model reaction. Results show that the use of choline chloride ChCl:urea/phosphate buffer 50% (v/v) as a reaction medium increased the selectivity of Novozym 435 by 16% (e.e = 88%) with respect to the one in 100% phosphate buffer (e.e = 76%). Best results were obtained when high substrate concentrations (well above the solubility limit, 27-fold) and ChCl:urea/phosphate buffer 50% (v/v) as reaction medium at pH 7 and 30 °C were used. Under such conditions, the R-monoester was produced with an enantiomeric purity of 99%. Novozym 435 was more stable in ChCl:urea/phosphate buffer 50% (v/v) than in phosphate buffer, retaining a 50% of its initial activity after 27 h of incubation at pH 7 and 40 °C. Results suggest that the use of deep eutectic solvents (ChCl:urea/phosphate buffer) in an heterogeneous reaction system (high substrate concentration) is a viable and promising strategy for the synthesis of chiral drugs from highly hydrophobic substrates.

Purification and characterization of a glucose-tolerant β-glucosidase from black plum seed and its structural changes in ionic liquids

The objective of this study was to characterize a plant origin β-glucosidase from black plum seeds and identify its conformational changes in twenty-six imidazolium- and amino acid-based ionic liquids (ILs). The results revealed that the purified 60 kDa enzyme was monomeric in nature, maximally active at 55 °C and pH 5.0, and nearly completely inhibited by Hg²⁺ and Ag⁺. Attractive peculiarities of the relative low kinetic and higher glucose inhibition constants (K_m = 0.58 mM [pNPG]; K_i = 193.5 mM [glucose]) demonstrated its potential applications in food industry. Circular dichroism studies showed that the secondary structural changes of the enzyme depended not only on the anions, but also on the cations of the assayed ILs. Interestingly, no corresponding relations were observed between the changes in enzyme structure induced by ILs and its catalytic activities, suggesting that the influences of ILs on enzymatic processes don't rely simply on enzyme conformational changes.

Green biotransformations catalysed by enzyme-inorganic hybrid nanoflowers in environmentally friendly ionic solvents

Environmentally friendly ionic solvents such as (a) ionic liquids (ILs) formulated with hydroxyl ammonium cations and various carboxylic acid anions and (b) choline chloride or ethyl ammonium chloride-based deep eutectic solvents (DES) were tested as media for hydrolytic and synthetic reactions catalysed by lipase-inorganic hybrid nanoflowers. The nature of ionic solvents used has a significant effect on the hydrolytic and synthetic activity of the immobilized lipase, as well as on its stability and reusability. In choline chloride-based DES, the activity and especially the operational stability of the biocatalyst are significantly increased compared to those observed in buffer, indicating the potential application of these solvents as green media for various biocatalytic processes of industrial interest.

Self-assembly and surface behaviour of pure and mixed zwitterionic amphiphiles in a deep eutectic solvent

Recent investigations have shown that deep eutectic solvents provide a suitable environment for self-organisation of biomolecules, in particular phospholipids and proteins. However, the solvation of complex lyophilic moieties by deep eutectic solvents still remains unclear. Here we explore the behaviour of zwitterionic surfactants in choline chloride:glycerol eutectic mixture. Dodecyl-2-(trimethylammonio)ethylphosphate and N-alkyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (alkyl = dodecyl, tetradecyl) surfactants were investigated by means of surface tension, X-ray reflectivity and small-angle neutron scattering. These surfactants were found to remain surface active and form globular micelles in deep eutectic solvents. Still, the surface behaviour of these species was found to differ depending on the headgroup and tail structure. The morphology of the micelles also slightly varies between surfactants, demonstrating differences in the packing of individual monomers. The characteristics of mixtures of the dodecyl surfactants is also reported, showing a deviation from ideal mixing associated with attractive interactions between sulfobetaine and phosphocholine headgroups. Such non-ideality results in variation of the surface behaviour and self-assembly of these surfactant mixtures. The results presented here will potentially lead to the development of new alternatives for drug-delivery, protein solubilisation and biosensing through a better fundamental understanding of the behaviour of zwitterionic surfactants in deep eutectic solvents.

Interaction of a digestive protease, Candida rugosa lipase, with three surfactants investigated by spectroscopy, molecular docking and enzyme activity assay

The extensive use of surfactants in food, laundry products and agriculture has caused concern about their biosafety. However, few studies have been done on their potential effect on the lipase which has always been used with surfactants in food and laundry industry. Herein, we investigated the interaction of three surfactants (sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium lauryl sulfonate (SLS)) with Candida rugosa lipase (CRL), which is a popular biocatalyst used regularly with surfactants. The effect of the three surfactants on the conformation and activity of CRL was evaluated by using multiple spectral methods, enzyme activity assay and molecular docking modeling. The results demonstrated that CRL interacted with SDS, SDBS and SLS primarily through hydrophobic forces, H-bonding and electrostatic forces, respectively. The binding constants (K_A) of SDBS with CRL varied with temperature: 1.99×10³mol/L at 298K and 4.13×10³mol/L at 318K. SDS and SDBS affected the secondary structure and skeleton of CRL, which changed the polarity of CRL and enhanced its activity. SLS also changed the secondary structure and activity of CRL moderately, but had little effect on its polarity and chromophore microenvironment. Accordingly, all three surfactants exhibited effect to CRL on the molecular level calling for more attention to pay on their biosafety. The work demonstrates that SDS, SDBS and SLS could cause negative effects to CRL from different angles and therefore are not bio-friendly detergents.

Innovative aspects of protein stability in ionic liquid mixtures

Mixtures of ionic liquids (ILs) have attracted our attention because of their extraordinary performances in extraction technologies and in absorbing large amount of CO₂ gas. It has been observed that when two or more ILs are mixed in different proportions, a new solvent is obtained which is much better than that of each component of ILs from which the mixture is obtained. Within a mixture of ILs, several unidentified interactions occur among several ions which give rise to unique solvent properties to the mixture. Herein, in this review, we have highlighted the utilization of the advantageous properties of the IL mixtures in protein stability studies. This approach is exceptional and opens new directions to the use of ILs in biotechnology.

Soybean peroxidase immobilized onto silica-coated superparamagnetic iron oxide nanoparticles: Effect of silica layer on the enzymatic activity

Peroxidase immobilization onto magnetic supports is considered an innovative strategy for the development of technologies that involves enzymes in wastewater treatment. In this work, magnetic biocatalysts were prepared by immobilization of soybean peroxidase (SBP) onto different silica-coated superparamagnetic iron oxide nanoparticles. The obtained magnetic biocatalysts were tested for the degradation of malachite green (MG), a pollutant often found in industrial wastewaters and with significant drawbacks for the human and environmental health. A deep physicochemical characterization of the materials was performed by means of X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), High Resolution-Transmission Electron Microscope (HR-TEM) and magnetization measurements among others techniques. Results showed high immobilization yield of SBP onto nanomaterials with excellent properties for magnetic recoverability. A partial loss of activity with respect to free SBP was observed, compatible with the modification of the conformational structure of the enzyme after immobilization. The structural modification depended on the amount (and thickness) of silica present in the hybrid materials and the activity yield of 43% was obtained for the best biocatalyst. Thermal stability and reusability capacity were also evaluated.

Recent progress on deep eutectic solvents in biocatalysis

Deep eutectic solvents (DESs) are eutectic mixtures of salts and hydrogen bond donors with melting points low enough to be used as solvents. DESs have proved to be a good alternative to traditional organic solvents and ionic liquids (ILs) in many biocatalytic processes. Apart from the benign characteristics similar to those of ILs (e.g., low volatility, low inflammability and low melting point), DESs have their unique merits of easy preparation and low cost owing to their renewable and available raw materials. To better apply such solvents in green and sustainable chemistry, this review firstly describes some basic properties, mainly the toxicity and biodegradability of DESs. Secondly, it presents several valuable applications of DES as solvent/co-solvent in biocatalytic reactions, such as lipase-catalyzed transesterification and ester hydrolysis reactions. The roles, serving as extractive reagent for an enzymatic product and pretreatment solvent of enzymatic biomass hydrolysis, are also discussed. Further understanding how DESs affect biocatalytic reaction will facilitate the design of novel solvents and contribute to the discovery of new reactions in these solvents.

Highly Efficient Enzymatic Preparation of Daidzein in Deep Eutectic Solvents

Daidzein, which is scarce in nature, has gained significant attention due to its superior biological activity and bioavailability compared with daidzin. So far, it has been widely used in the medicine and health care products industries. The enzymatic approach for the preparation of daidzein has prevailed, benefitted by its high efficiency and eco-friendly nature. Our present research aimed at providing a preparation method of daidzein by enzymatic hydrolysis of daidzin in a new "green" reaction medium-deep eutectic solvents (DESs). Herein, the DESs were screened via evaluating enzyme activity, enzyme stability and the substrate solubility, and the DES (ChCl/EG 2:1, 30 vol %) was believed to be the most appropriate co-solvent to improve the bioconversion efficiency. Based on the yield of daidzein, response surface methodology (RSM) was employed to model and optimize the reaction parameters. Under these optimum process conditions, the maximum yield of 97.53% was achieved and the purity of daidzein crude product reached more than 70%, which is more efficient than conversions in DESs-free buffer. Importantly, it has been shown that DESs medium could be reused for six batches of the process with a final conversion of above 50%. The results indicated that this procedure could be considered a mild, environmentally friendly, highly efficient approach to the economical production of daidzein, with a simple operation process and without any harmful reagents being involved.

Trifluoromethanesulfonic acid-based DESs as extractants and catalysts for removal of DBT from model oil

The ChCl·1.5CF₃O₃S DESs is synthesized by stirring at the room temperature. The high efficient desulfurizative system using ChCl·1.5CF₃O₃S DESs as extractant and catalyst, H₂O₂ as oxidant with small amount of H₂O₂ (an O/S molar ratio of 6) and the low temperature (40 °C).

Protein conformation in pure and hydrated deep eutectic solvents

Deep eutectic solvents as media for protein stabilisation: conformation in the absence and presence of water.

Applications of deep eutectic solvents in biotechnology and bioengineering-Promises and challenges

Deep eutectic solvents (DESs) have been touted recently as potential alternatives to ionic liquids (ILs). Although they possess core characteristics that are similar to those of ILs (e.g., low volatility, non-flammability, low melting points, low vapor pressure, dipolar nature, chemical and thermal stability, high solubility, and tuneability), DESs are superior in terms of the availability of raw materials, the ease of storage and synthesis, and the low cost of their starting materials. As such, they have become the subject of intensive research in various sectors, notably the chemical, electrochemical, and biological sectors. To date, the applications of DESs have shown great promise, especially in the medical and biotechnological fields. In spite of these various achievements, the safety concern for these mixtures must be sufficiently addressed. Indeed, in order to exploit the vast array of opportunities that DESs offer to the biological industry, first, they must be established as safe mixtures. Hence, the biotechnological applications of DESs only can be implemented if they are proven to have negligible or low toxicity profiles. This review is the first of its kind, and it discusses two current aspects of DES-based research. First, it describes the properties of these mixtures with ample focus on their toxicity profiles. Second, it provides an overview of the breakthroughs that have occurred and the foreseeable prospects of the use of DESs in various biotechnological and biological applications.

Natural deep eutectic solvents: cytotoxic profile

The purpose of this study was to investigate the cytotoxic profiles of different ternary natural deep eutectic solvents (NADESs) containing water. For this purpose, five different NADESs were prepared using choline chloride as a salt, alongside five hydrogen bond donors (HBD) namely glucose, fructose, sucrose, glycerol, and malonic acid. Water was added as a tertiary component during the eutectics preparation, except for the malonic acid-based mixture. Coincidentally, the latter was found to be more toxic than any of the water-based NADESs. A trend was observed between the cellular requirements of cancer cells, the viscosity of the NADESs, and their cytotoxicity. This study also highlights the first time application of the conductor-like screening model for real solvent (COSMO-RS) software for the analysis of the cytotoxic mechanism of NADESs. COSMO-RS simulation of the interactions between NADESs and cellular membranes' phospholipids suggested that NADESs strongly interacted with cell surfaces and that their accumulation and aggregation possibly defined their cytotoxicity. This reinforced the idea that careful selection of NADESs components is necessary, as it becomes evident that organic acids as HBD highly contribute to the increasing toxicity of these neoteric mixtures. Nevertheless, NADESs in general seem to possess relatively less acute toxicity profiles than their DESs parents. This opens the door for future large scale utilization of these mixtures.

Combination of deep eutectic solvent and ionic liquid to improve biocatalytic reduction of 2-octanone with Acetobacter pasteurianus GIM1.158 cell

The efficient anti-Prelog asymmetric reduction of 2-octanone with Acetobacter pasteurianus GIM1.158 cells was successfully performed in a biphasic system consisting of deep eutectic solvent (DES) and water-immiscible ionic liquid (IL). Various DESs exerted different effects on the synthesis of (R)-2-octanol. Choline chloride/ethylene glycol (ChCl/EG) exhibited good biocompatibility and could moderately increase the cell membrane permeability thus leading to the better results. Adding ChCl/EG increased the optimal substrate concentration from 40 mM to 60 mM and the product e.e. kept above 99.9%. To further improve the reaction efficiency, water-immiscible ILs were introduced to the reaction system and an enhanced substrate concentration (1.5 M) was observed with C₄MIM·PF₆. Additionally, the cells manifested good operational stability in the reaction system. Thus, the efficient biocatalytic process with ChCl/EG and C₄MIM·PF₆ was promising for efficient synthesis of (R)-2-octanol.

Enzyme-mediated free radical polymerization of acrylamide in deep eutectic solvents

The enzyme-mediated free radical polymerization of acrylamide was performed in nearly non-aqueous DES, which allowed the exploration of higher and lower temperatures that in aqueous media.

Investigating the effect of gallium curcumin and gallium diacetylcurcumin complexes on the structure, function and oxidative stability of the peroxidase enzyme and their anticancer and antibacterial activities

Curcumin has a wide spectrum of biological and pharmacological activities including anti-inflammatory, antioxidant, antiproliferative, antimicrobial and anticancer activities. Complexation of curcumin with metals has gained attention in recent years for improvement of its stability. In this study, the effect of gallium curcumin and gallium diacetylcurcumin on the structure, function and oxidative stability of horseradish peroxidase (HRP) enzyme were evaluated by spectroscopic techniques. In addition to the enzymatic investigation, the cytotoxic effect of the complexes was assessed on bladder, MCF-7 breast cancer and LNCaP prostate carcinoma cell lines by MTT assay. Furthermore, antibacterial activity of the complexes against S. aureus and E. coli was explored by dilution test method. The results showed that the complexes improve activity of HRP and also increase its tolerance against the oxidative condition. After addition of the complexes, affinity of HRP for hydrogen peroxide substrate decreases, while the affinity increases for phenol substrate. Circular dichroism, intrinsic and synchronous fluorescence spectra showed that the enzyme structure around the catalytic heme group becomes less compact and also the distance between the heme group and tryptophan residues increases due to binding of the complexes to HRP. On the whole, it can be concluded that the change in the enzyme structure upon binding to the gallium curcumin and gallium diacetylcurcumin complexes results in an increase in the antioxidant efficiency and activity of the peroxidise enzyme. The result of anticancer and antibacterial activities suggested that the complexes exhibit the potential for cancer treatment, but they have no significant antibacterial activity.

From green chemistry to nature: The versatile role of low transition temperature mixtures

In 1998, the concept of "green chemistry" was established through twelve principles with the aim of improving the eco-efficiency of chemical processes and to judge, whether or not, a chemical process is sustainable. Currently, numerous processes do not obey to most of these principles (large energy usage, formation of waste, usage of hazardous solvents and reagents, etc …), which have forced the scientists to develop and implement new strategies for upcoming researches. One of the most attractive challenges is finding, creating and developing new and green media. Over the last decades, the scientific community has mainly focused on two different classes of solvents (namely, Ionic liquids and Eutectic Solvents). These solvents share advantageous characteristics (low vapor pressure, thermally stable, non-flammable, etc …) making them an attractive option to implement sustainable chemistry and engineering. Mainly due to its environmental and economic features, DES are now growing much more interest. Indeed, although their ecotoxicological profile is still poorly known, DES are classified as "green" solvents because they are composed of molecules which are considered to be eco-friendly. The fast, numerous and broad scope of studies on these new liquids make the literature rather complex to understand. Here, we attempted to establish a succinct history and a presentation of these liquids with emphasis on their role, classification, importance and application in biological systems.

Selected issues related to the toxicity of ionic liquids and deep eutectic solvents--a review

Green Chemistry plays a more and more important role in implementing rules of sustainable development to prevent environmental pollution caused by technological processes, while simultaneously increasing the production yield. Ionic liquids (ILs) and deep eutectic solvents (DESs) constitute a very broad group of substances. Apart from many imperfections, ILs and DESs have been the most promising discoveries in the world of Green Chemistry in recent years. The main advantage of ILs is their unique physicochemical properties-they are very desirable from the technological point of view, but apart from these benefits, ILs appear to be highly toxic towards organisms from different trophic levels. DES areas of usage are very spread, because they cover organic synthesis, extraction processes, electrochemistry, enzymatic reactions and many others. Moreover, DESs seem to be a less toxic alternative to ionic liquids. New possibilities of applications and future development trends are sought and presented, including such important solutions of life branches as pharmaceuticals' production and medicine.

Assessing the toxicity and biodegradability of deep eutectic solvents

Deep eutectic solvents (DESs) have emerged as a new type of promising ionic solvents with a broad range of potential applications. Although their ecotoxicological profile is still poorly known, DESs are generally regarded as "green" because they are composed of ammonium salts and H-bond donors (HBDs) which are considered to be eco-friendly. In this work, cholinium-based DESs comprised of choline chloride (ChCl) and choline acetate (ChAc) as the salt and urea (U), acetamide (A), glycerol (G) and ethylene glycol (EG) as the HBD were evaluated for their toxic effects on different living organisms such as Escherichia coli (a bacterium), Allium sativum (garlic, a plant) and hydra (an invertebrate), and their biodegradabilities were assessed by means of closed bottle tests. These DESs possessed an anti-bacterial property and exhibited inhibitory effects on the test organisms adopted, depending on the composition and concentration of the DES. The mechanism for the impact of DESs and their components on different living organisms can be associated to their interactions with the cellular membranes. Not all DESs can be considered readily biodegradable. By extending the limited knowledge about the toxicity and biodegradation of this particular solvent family, this investigation on DESs provides insight into our structure-based understanding of their ecotoxicological behavior.

Evaluation of toxicity and biodegradability for cholinium-based deep eutectic solvents

The first study investigating the toxicity of cholinium-based DESs on freshwater fish or fungi and the biodegradability of EAC-based DESs.