Can deep eutectic solvents be the best alternatives to ionic liquids and organic solvents: A perspective in enzyme catalytic reactions

As a new generation of green solvents, deep eutectic solvents (DESs) have been considered as a promising alternative to classical organic solvents and ionic liquids (ILs). DESs are normally formed by two or more components via various h-bonds interactions. Up to date, four types of DESs are found, namely, type I DESs (formed by MClx, namely FeCl₂, AlCl₃, ZnCl₂, CuCl₂ and AgCl et al., and quaternary ammonium salts); type II DESs (formed by metal chloride hydrates and quaternary ammonium salts); type III DESs (formed by choline chlorides and different kinds of HBDs) and type IV DESs (formed by salts of transition metals and urea). DESs share many advantages, such as low vapor pressure, good substrate solubility and thermal stability, with ILs, and offering a high potential to be the medium of biocatalysis reactions. In this case, this paper reviews the applications of DESs in enzymatic reactions. Lipases are the most widely used enzyme in DESs systems as their versatile applications in various reactions and robustness. Interestingly, DESs can improve the efficiency of these reactions via enhancing the substrates solubility and the activity and stability of enzymes. Therefore, the directed engineering of DESs for special reactions such as degradation of polymers in high temperature or strong acid-base conditions will be one of the future perspectives of the investigation DESs.

Green solvents for the formation of amide linkages

The formation of the amide bond is among the most commonly performed transformations in the pharmaceutical industry and the wider chemical industry. The current methods for its installation in organic compounds frequently rely on the use of large amounts of organic solvents, mainly N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), and dichloromethane (DCM), which have been associated with adverse environmental and health concerns over the last decades. This fact led academia and industry to make significant efforts toward the development of synthetic routes with the aim to avoid, reduce or replace the use of hazardous solvents. The present review fits into this framework and discusses the literature existing over the past ten years on strategies for reducing and replacing hazardous solvents, focusing on the use of biobased and neoteric solvents, such as ionic liquids and deep eutectic solvents (ILs and DESs, respectively), and on the reaction media that proved to be greener alternatives for amide bond formation.

Deep Eutectic Solvents and Multicomponent Reactions: Two Convergent Items to Green Chemistry Strategies

One of the highlights of green chemistry is the development of techniques and procedures with low environmental impact. In the last years, deep eutectic solvents (DES) have become an important alternative to conventional organic solvents. For a period ionic liquids have provoked remarkable interest, but they have been displaced by DES because they show easier preparation methods, lower prices, many of them are biodegradable and compatible with biological systems. In addition, they show adjustable physicochemical properties, high thermal stability, low volatility and are compatible with water. In this paper is reviewed the state of the art of the use of DES paying special attention to the role of reaction media in organic synthesis.

Immobilization of Pseudomonas stutzeri lipase through Cross-linking Aggregates (CLEA) for reactions in Deep Eutectic Solvents

The use of deep eutectic solvents (DES) with buffer as cosolvent (up to 10 % v/v) leads to low-viscous media in which lipases can perform synthetic reactions, instead of hydrolysis. This paper explores the immobilization of Pseudomonas stutzeri lipase (TL) in cross-linking aggregates (CLEA) to deliver robust derivatives that are active in media like choline chloride - glycerol DES with buffer as cosolvent. While the free TL enzyme was markedly inactive in these media, TL-CLEA derivatives perform esterifications and can be reused several times. Overall, results are consistent with previous experiments reported for other lipases in these DES-water media and confirm that CLEA immobilization turns out a very useful and straightforward alternative for generating active (bio)catalysts for DES-aqueous media systems. Immobilized systems open the possibility of performing continuous processes in low-viscous DES-buffer media.

Greener, Faster, Stronger: The Benefits of Deep Eutectic Solvents in Polymer and Materials Science

Deep eutectic solvents (DESs) represent an emergent class of green designer solvents that find numerous applications in different aspects of chemical synthesis. A particularly appealing aspect of DES systems is their simplicity of preparation, combined with inexpensive, readily available starting materials to yield solvents with appealing properties (negligible volatility, non-flammability and high solvation capacity). In the context of polymer science, DES systems not only offer an appealing route towards replacing hazardous volatile organic solvents (VOCs), but can serve multiple roles including those of solvent, monomer and templating agent-so called "polymerizable eutectics." In this review, we look at DES systems and polymerizable eutectics and their application in polymer materials synthesis, including various mechanisms of polymer formation, hydrogel design, porous monoliths, and molecularly imprinted polymers. We provide a comparative study of these systems alongside traditional synthetic approaches, highlighting not only the benefit of replacing VOCs from the perspective of environmental sustainability, but also the materials advantage with respect to mechanical and thermal properties of the polymers formed.

The study and application of biomolecules in deep eutectic solvents

Deep eutectic solvents offer stimulating possibilities for biomolecular stabilization and manipulation, biocatalysis, bioextraction, biomass processing, and drug delivery and therapy.

Preparation of thermoresponsive hydrogels via polymerizable deep eutectic monomer solvents

We report the preparation of thermoresponsive hydrogels via free-radical polymerization and crosslinking of NIPAM based deep eutectic monomer solvents (DEMs).

A review of sustainable lignocellulose biorefining applying (natural) deep eutectic solvents (DESs) for separations, catalysis and enzymatic biotransformation processes

Conventional biorefinery processes are complex, engineered and energy-intensive, where biomass fractionation, a key functional step for the production of biomass-derived chemical substances, demands industrial organic solvents and harsh, environmentally harmful reaction conditions. There is a timely, clear and unmet economic need for a systematic, robust and affordable conversion method technology to become greener, sustainable and cost-effective. In this perspective, deep eutectic solvents (DESs) have been envisaged as the most advanced novel polar liquids that are entirely made of natural, molecular compounds that are capable of an association via hydrogen bonding interactions. DES has quickly emerged in various application functions thanks to a formulations’ simple preparation. These molecules themselves are biobased, renewable, biodegradable and eco-friendly. The present experimental review is providing the state of the art topical overview of trends regarding the employment of DESs in investigated biorefinery-related techniques. This review covers DESs for lignocellulosic component isolation, applications as (co)catalysts and their functionality range in biocatalysis. Furthermore, a special section of the DESs recyclability is included. For DESs to unlock numerous new (reactive) possibilities in future biorefineries, the critical estimation of its complexity in the reaction, separation, or fractionation medium should be addressed more in future studies.

Deep Eutectic Solvents as Media for the Prebiotic DNA-Templated Synthesis of Peptides

Translation of genetic information into peptide products is one of the fundamental processes of biology. How this occurred prebiotically, in the absence of enzyme catalysts, is an intriguing question. Nucleic acid-templated synthesis (NATS) promotes reactions by bringing building blocks tethered to complementary DNA strands into close proximity and has been shown to enable peptide synthesis without enzymes—it could therefore serve as a model for prebiotic translation of information stored in nucleic acid sequences into functional peptides. The decomposition of highly reactive DNA adapters has so far limited the effectiveness of NATS, but these studies have been performed exclusively in aqueous solution. Deep eutectic solvents (DESs) have been proposed as a feasible solvent for prebiotic replication of nucleic acids, and here are studied as media for prebiotic translation using NATS as a model. DESs are shown to enhance the stability of DNA-conjugated activated esters, the precursors of peptides. However, this enhanced stability was coupled with decreased amine reactivity that hampered the formation of peptide bonds in DESs. These properties are exploited to demonstrate the storage of DNA-conjugated activated esters in a DES followed by transfer into aqueous buffer to activate the NATS of peptides “on demand.” These findings, together with the reported functions of DESs in prebiotic processes, shed light on how DESs could have facilitated the non-enzymatic translation of genetic information into functional peptides on the early Earth.

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.

From batch to fed-batch and to continuous packed-bed reactors: Lipase-catalyzed esterifications in low viscous deep-eutectic-solvents with buffer as cosolvent

This work explores for the first time the use of Deep Eutectic Solvents (DES) with phosphate buffer 100 mM pH 7 as cosolvent (10% v/v) in biocatalytic reactions in fed-batch and packed-bed bioreactors. The lipase-catalyzed esterification of glycerol and benzoic acid is studied, as it involves two substrates with different polarities (for which DES are needed). In the fed-batch bioreactor, the highest conversion (90%) was obtained at a substrate flow rate of 0.01 mL/min. The fed-batch operation increased the conversion by 59% compared to the batch mode. Regarding productivity, semi-continuous and continuous bioreactors showed analogous results. Upon recirculation of the reaction media in the continuous bioreactor, a conversion of 67% was achieved in 7 cycles of operation. The stability of the biocatalyst in the packed-bed bioreactor decreased only 2% in 10 days, demonstrating the attractiveness that low viscous DES-water mixtures with continuous processes may have.

Highly active nanobiocatalysis in deep eutectic solvents via metal-driven enzyme-surfactant nanocomposite

Metal-driven papain-surfactant nanocomposite (PA@MSNC), a novel soft nanobiocatalyst, was successfully prepared via one-pot self-assembly technique in aqueous solution for the biosynthesis of N-(benzyloxycarbonyl)-L-alanyl-L-glutamine (Z-Ala-Gln) dipeptide in deep eutectic solvents (DESs). The metal-driven self-assembly process generated PA@MSNC as nanospheres of ˜130 nm in diameter, with high protein loading and relative enzyme activity of 420 mg/g and 80% (4270 U/g protein), respectively. PA@MSNC showed high apparent substrate affinity and catalytic efficiency. The stability of PA@MSNC at high temperature and extreme pH was significantly higher than that of free PA. Catalysis efficiency for the biosynthesis of Z-Ala-Gln by PA@MSNC in choline chloride: glycerol reaction medium was 1.69-fold higher than that of free PA, achieving a high product yield of 75.7% within 4 h. PA@MSNC also showed better techno-economic performance. We propose that enzyme-surfactant nanocomposite via metal-driven dynamically reversible coordination interactions contribute simultaneously promotes catalytic flexibility and configurational stability. The generated PA@MSNC has potential practical implications for green synthesis of dipeptide in DESs.

Lipases in Green Chemistry: Deep Eutectic Solvents (DES) as New Green Solvents

Deep eutectic solvents (DES) may become important alternatives as versatile, biodegradable, and cost-effective solvents for biocatalysis. Especially for reactions where substrates and products of different polarities are combined, the design of a tailored solvent that may dissolve all compounds-while being enzyme-compatible at the same time-appears to be a strong ally in sustainable chemistry. Herein it is shown that the combination of DES with "water as cosolvent" (in a range from 5% to 20% water, v/v) leads to non-conventional solvents with significantly reduced viscosity. In these media, lipases and proteases can perform synthetic reactions efficiently, and hydrolytic side reactions remain suppressed (even at 20% water, v/v). The use of these less viscous non-conventional media could also provide options for hydrolase-catalyzed synthetic reactions even in continuous fashion.

Deep eutectic-water binary solvent associations investigated by vibrational spectroscopy and chemometrics

Investigation of the behaviour of deep eutectic solvents (DESs) as novel green solvents in the presence of other solvents is of great interest. In this study the behaviour of a common natural DES, namely choline chloride-glycerol deep eutectic solvent (GDES), was studied in the presence of water. A detailed study of the association of the two solvents was performed by integration of two vibrational spectroscopic methods (FTIR and Raman spectroscopy) followed by multivariate analysis. Moreover, a binary mixture of glycerol (Gly) as one of the liquid constituents of GDES and water was explored under the same conditions. A quintuplet and ternary systems were resolved for GDES-water and Gly-water probes, respectively, using multivariate analysis of global data (multi-technique and multi-experiment data arrangements). The results confirmed that in the presence of water the GDES showed different behaviour from its components. Therefore, a DES can be introduced as an independent solvent with its unique properties. Also, different H-bond interaction energies of GDES and its pure components in the presence of water were shown by theoretical calculations based on a density functional theory framework. To investigate the effects of water on the structure of GDES, molecular dynamics (MD) simulations of GDES-water liquid mixtures were performed at 0.9 mole fraction of water.

Thiol-ene photoimmobilization of chymotrypsin on polysiloxane gels for enzymatic peptide synthesis

Chemical incorporation of enzymes onto polymeric materials has recently attracted intense scientific attention. Cross-linked polysiloxane gels as a typical super-hydrophobic support, are a good candidate for supporting enzymes in low-water organic medium to efficiently catalyze peptide synthesis because the hydrophobic polysiloxane matrix can prevent water from attacking the acyl-enzyme intermediate, which is beneficial for the shift in equilibrium to peptide formation. In this work, we develop a facile strategy to photoimmobilize olefin-functionalized chymotrypsin onto cross-linked polysiloxane gels via UV-initiated thiol-ene click chemistry. The impacts of water addition amount, heat-treatment and recyclability of the immobilized chymotrypsin influencing the peptide synthesis efficiency are investigated. Compared with the native chymotrypsin, polysiloxane-immobilized chymotrypsin showed advantageous catalytic activity, higher thermal stability and superior recyclability.

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 Acylation of Dihydromyricetin by the Immobilized Lipase with Deep Eutectic Solvents as Cosolvent

A novel deep eutectic solvent (DES)-DMSO cosolvent system has been, for the first time, successfully used as the reaction medium for the enzymatic acylation of dihydromyricetin (DMY) catalyzed by the immobilized lipase from Aspergillus niger (ANL). The cosolvent mixture, ChCl:Glycerol-DMSO (1:3, v/v) proved to be the optimal medium. With the newly developed cosolvent, the initial reaction rate of enzymatic acylation of DMY achieved 11.1 mM/h and the conversion of DMY was 91.6%. ANL@PD-MNPs is stable and recyclable in this cosolvent, offering 90% conversion rate after repeated use of 5 times. The lipid-solubility of DMY-16-acetate was 10 times higher than that of its raw materials DMY. The results showed that the DMY-16-acetate product exhibits good antioxidative activity. The present research illustrated that the use of DES-DMSO cosolvent may become a feasible alternative for the synthesis of DMY ester.

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.

The effect of deep eutectic solvents on catalytic function and structure of bovine liver catalase

Aqueous solutions of reline and glyceline, the most common deep eutectic solvents, were used as a medium for Catalase reaction. By some spectroscopic methods such as UV-vis, fluorescence and circular dichroism (CD) function and structure of Catalase were investigated in aqueous solutions of reline and glyceline. These studies showed that the binding affinity of the substrate to the enzyme increased in the presence of 100mM glyceline solution, which contrasts with reline solution that probably relates to instructive changes in secondary structure of protein. Meanwhile, enzyme remained nearly 70% and 80% active in this concentration of glyceline and reline solutions respectively. In the high concentration of DES solutions, enzyme became mainly inactive but surprisingly stayed in nearly 40% active in choline chloride solution, which is the common ion species in reline and glyceline solvents. It is proposed that the chaotropic nature of choline cation might stop the reducing trend of activity in concentrated choline chloride solutions but this instructive effect is lost in aqueous deep eutectic solvents. In this regard, the presence of various concentrations of deep eutectic solvents in the aqueous media of human cells would be an activity adjuster for this important enzyme in its different operation conditions.

On the (Un)greenness of Biocatalysis: Some Challenging Figures and Some Promising Options

Biocatalysis is generally regarded as a “green” technology. This statement is justified by the mild reaction conditions, the use of aqueous reaction media—with water as the paradigm of green solvents—, and the renewable nature of the biocatalysts. However, researchers making these statements frequently do not take into account the entire picture of their processes. Aspects like water consumption, wastewater production, titers, and metrics of the (diluted?) biocatalytic processes are important as well. With those figures at hand, many biocatalytic reactions do not appear so green anymore. This article critically discusses some common wrong assumptions given for biocatalytic approaches, with regard to their environmental impact, and actual greenness. Some promising biocatalytic approaches, such as the use of biphasic systems involving biogenic solvents, deep-eutectic-solvents (and biogenic ionic liquids), water-free media, solvent-free processes, are briefly introduced, showing that enzyme catalysis can actually be a robust sustainable alternative for chemical processes.

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.

Prebiotic phosphate ester syntheses in a deep eutectic solvent

We report a route to synthesize a wide range of organophosphates of biological significance in a deep eutectic solvent (2:1 urea and choline chloride), utilizing various orthophosphate sources. Heating an organic alcohol in the solvent along with a soluble phosphorus source yields phosphorus esters of choline as well as that of the added organic in yields between 15 to 99 %. In addition, phosphite analogs of biological phosphates and peptides were also formed by the simple mixing of reagents and heating at 60-70 °C in the deep eutectic solvent. The presented dehydration reactions are relevant to prebiotic and green chemistry in alternative solvents.