New Generations of Ionic Liquids Applied to Enzymatic Biocatalysis

Abstract

dissolve nonpolar substrates, while avoiding enzyme inactivation like water-miscible organic solvents, as DMSO or acetonitrile, often do [6]. Another mentioned characteristic of ILs is the possibility of obtaining the desired physicochemical properties by selecting combinations of cations and anions ("tunability"), which makes them "designer solvents". For example, ILs can be produced to be water-miscible, partially miscible or totally immiscible, and can also be synthesized with different viscosities. These interesting properties make them a very important reaction media for enzyme stabilization and reaction. The use of organic solvents in bioprocess presents a number of further problems. The main concerns are the toxicity of the organic solvents to both the process operators and the environment (eco-toxicity), and also the volatile and flammable nature of these solvents, which make them a potential explosion hazard [7]. Thus, ILs have emerged as a potential replacement for organic solvents in biocatalytic processes at both laboratory and industrial scale. The negligible vapor pressure means that they emit no volatile compounds, and also introduces the additional possibility of removal of products by distillation without further contamination by the solvent. It also facilitates the recycling of ILs, decreasing operation costs. All these properties make ILs very important for the stabilization and activation of enzymes; therefore, numerous enzymatic reactions have been investigated in different types of ILs as will be shown in the next sections. Several topics about biocatalysis in ILs will be reviewed: their effect on the activity and stability of enzymes, toxicity of ILs, new generation of ILs and methods to stabilize enzymes will be discussed.

Laccase Activation in Deep Eutectic Solvents

“…3 During the last decades, promising alternative solvents, such as ionic liquids (ILs), have been reported as solvents or co-solvents in enzyme-catalysed reactions with lipase, laccase, peroxidase, among others. 4 However, some ILs based on pyridinium or imidazolium cations, most of the times combined with fluorinated anions, may raise some toxicity and biodegradability concerns 5 and may lead to enzymes deactivation. 6 In addition to ILs as alternative solvents and bioreaction media, deep eutectic solvents (DES) have been attracting widespread interest from academics and industrial sector since these solvents can be produced by the simple mixing of low-cost and safe compounds, most of the times from natural sources.…”

An Overview on the Toxicological Properties of Ionic Liquids toward Microorganisms

“…This has brought into emergence new "generations" of ILs. The first-generation ILs are mainly based on imidazolium or pyridinium cations [16,32] and synthesis of these ILs was mainly focused on obtaining unique physicochemical properties. Second-generation ILs are based on ammonium or phosphonium cations and these ILs have allowed the combination of selected chemical and physical properties.…”

The Versatile Applications of DES and Their Influence on Oxidoreductase-Mediated Transformations

“…DES are mixtures of a salt such as choline chloride (ChCl) and a hydrogen bond donor (HBD) molecule such as urea (Ur). These solvents present many advantages, including the facts that they are eco-friendly, cheap, and can increase the (bio)catalytic activity of many reactions leading to high yields of products with excellent enantiomeric excess ( ee ), including important precursors for drugs that are widely used in the pharmaceutical industry [1,2]. ILs are salts, consisting of a mixture of cations and anions that do not pack well together, that consequently have a melting point near room temperature, although they are arbitrarily defined as salts with a melting point below 100 °C [3].…”

“…The anions were replaced with halides and those ILs had low melting points and were more stable in the presence of organic solvents and were used, among other applications, in biocatalysis in the 2000s. However, these ILs had again some disadvantages such as high cost and toxicity [2].…”

“…They are easy to prepare and do not require purification. In addition to that, enzymes have shown good activities and stabilities in DES [2]. This medium presents many potential advantages in the medical and biotechnological sectors.…”