Lysozyme Solubility and Conformation in Neat Ionic Liquids and Their Mixtures with Water

Do Ionic Liquids Exhibit the Required Characteristics to Dissolve, Extract, Stabilize, and Purify Proteins? Past-Present-Future Assessment

Proteins are highly labile molecules, thus requiring the presence of appropriate solvents and excipients in their liquid milieu to keep their stability and biological activity. In this field, ionic liquids (ILs) have gained momentum in the past years, with a relevant number of works reporting their successful use to dissolve, stabilize, extract, and purify proteins. Different approaches in protein-IL systems have been reported, namely, proteins dissolved in (i) neat ILs, (ii) ILs as co-solvents, (iii) ILs as adjuvants, (iv) ILs as surfactants, (v) ILs as phase-forming components of aqueous biphasic systems, and (vi) IL-polymer-protein/peptide conjugates. Herein, we critically analyze the works published to date and provide a comprehensive understanding of the IL-protein interactions affecting the stability, conformational alteration, unfolding, misfolding, and refolding of proteins while providing directions for future studies in view of imminent applications. Overall, it has been found that the stability or purification of proteins by ILs is bispecific and depends on the structure of both the IL and the protein. The most promising IL-protein systems are identified, which is valuable when foreseeing market applications of ILs, e.g., in "protein packaging" and "detergent applications". Future directions and other possibilities of IL-protein systems in light-harvesting and biotechnology/biomedical applications are discussed.

Concentrated ionic liquids for proteomics: Caveat emptor!

The use of concentrated ionic liquids (ILs) in the bioanalytical chemistry of proteins is sparse; typically, dilute aqueous IL solutions are used. Concentrated ILs have unique properties that may allow researchers to dissolve previously insoluble protein analytes, to increase the depth and robustness of sample preparation and the analysis of proteins. Previous research using concentrated ILs for this purpose is sparse and there is a need to systematically investigate the structure-activity relationship between the IL structure and its capacity to solubilise proteins. Here, bovine serum albumin was dissolved in various ionic liquids and monitored over time by light microscopy and SDS-PAGE. While qualitative, these measures provide a good estimate of, respectively, the dissolving power of an IL towards the given protein and the retained integrity of the protein. Hydrophilic ILs show the best solubilisation capacity and higher temperatures (in a restricted sense) improve the solubility of the protein. Higher temperatures and longer reaction times reduce the molecular weight of the protein, which could inhibit their applicability in proteomics, unless the conditions are judiciously controlled. Researchers should exercise caution when using concentrated ILs for protein analysis until the full scope and limitations are known, an aspect we are presently investigating.

Recovery of enzyme structure and activity following rehydration from ionic liquid

Long-term preservation of proteins at room temperature continues to be a major challenge. Towards using ionic liquids (ILs) to address this challenge, here we present a combination of experiments and simulations to investigate changes in lysozyme upon rehydration from IL mixtures using two imidazolium-based ILs (1-ethyl-3-methylimidazolium ethylsulfate, [EMIM][EtSO₄] and 1-ethyl-3-methylimidazolium diethylphosphate, [EMIM][Et₂PO₄]). Various spectroscopic experiments and molecular dynamics simulations are performed to ascertain the structure and activity of lysozyme. Circular dichroism spectroscopy confirms that lysozyme maintains its secondary structure upon rehydration, even after 295 days. Increasing the IL concentration decreases the activity of lysozyme and is ultimately quenched at sufficiently high IL concentrations, but the rehydration of lysozyme from high IL concentrations completely restores its activity. Such rehydration occurs in the most common lysozyme activity assay, but without careful attention, this effect on the IL concentration can be overlooked. From simulations we observe occupation of [EMIM⁺] ions near the vicinity of the active site and the ligand-lysozyme complex is less stable in the presence of ILs, which results in the reduction of lysozyme activity. Upon rehydration, fast leaving of [EMIM⁺] is observed and the availability of active site is restored. In addition, suppression of structural fluctuations is also observed when in high IL concentrations, which also explains the decrease of activity. This structure suppression is recovered after undergoing rehydration. The return of native protein structure and activity indicates that after rehydration lysozyme returns to its original state. Our results also suggest a simple route to protein recovery following extended storage.

Protein aggregation and crystallization with ionic liquids: Insights into the influence of solvent properties

Ionic liquids (ILs) have been used in solvents for proteins in many applications, including biotechnology, pharmaceutics, and medicine due to their tunable physicochemical and biological properties. Protein aggregation is often undesirable, and predominantly occurs during bioprocesses, while the aggregation process can be reversible or irreversible and the aggregates formed can be native/non-native and soluble/insoluble. Recent studies have clearly identified key properties of ILs and IL-water mixtures related to protein performance, suggesting the use of the tailorable properties of ILs to inhibit protein aggregation, to promote protein crystallization, and to control protein aggregation pathways. This review discusses the critical properties of IL and IL-water mixtures and presents the latest understanding of the protein aggregation pathways and the development of IL systems that affect or control the protein aggregation process. Through this feature article, we hope to inspire further advances in understanding and new approaches to controlling protein behavior to optimize bioprocesses.

Applications of Ionic Liquids in Whole-Cell and Isolated Enzyme Biocatalysis

Ionic liquids have unique chemical properties that have fascinated scientists in many fields. The effects of adding ionic liquids to biocatalysts are many and varied. The uses of ionic liquids in biocatalysis include improved separations and phase behaviour, reduction in toxicity, and stabilization of protein structures. As the ionic liquid state of the art has progressed, concepts of what can be achieved in biocatalysis using ionic liquids have evolved and more beneficial effects have been discovered. In this review ionic liquids for whole-cell and isolated enzyme biocatalysis will be discussed with an emphasis on the latest developments, and a look to the future.

Unfolding and Aggregation of Lysozyme under the Combined Action of Dithiothreitol and Guanidine Hydrochloride: Optical Studies

Using a number of optical techniques (interferometry, dynamic light scattering, and spectroscopy), denaturation of hen egg white lysozyme (HEWL) by treatment with a combination of dithiothreitol (DTT) and guanidine hydrochloride (GdnHCl) has been investigated. The denaturing solutions were selected so that protein denaturation occurred with aggregation (Tris-HCl pH = 8.0, 50 mM, DTT 30 mM) or without aggregation (Tris-HCl pH = 8.0, 50 mM, DTT 30 mM, GdnHCl 6 M) and can be evaluated after 60 min of treatment. It has been found that denatured by solution with 6 M GdnHCl lysozyme completely loses its enzymatic activity after 30 min and the size of the protein molecule increases by 1.5 times, from 3.8 nm to 5.7 nm. Denaturation without of GdnHCl led to aggregation with preserving about 50% of its enzymatic activity. Denaturation of HEWL was examined using interferometry. Previously, it has been shown that protein denaturation that occurs without subsequent aggregation leads to an increase in the refractive index (Δn ~ 4.5 × 10-5). This is most likely due to variations in the HEWL-solvent interface area. By applying modern optical techniques conjointly, it has been possible to obtain information on the nature of time-dependent changes that occur inside a protein and its hydration shell as it undergoes denaturation.

NMR investigations on binding and dynamics of imidazolium-based ionic liquids with HEWL

Molecular level insights on protein-ionic liquid (P-IL) interactions are beneficial for assessing protein stability, binding and dynamics. In the present work, interactions of ILs, namely, 1-butyl 3-methylimidazolium methyl sulfate (IL1), 1-butyl 3-methylimidazolium octyl sulfate (IL2) and 1-butyl 3-methylimidazolium chloride (IL3) with hen egg white lysozyme (HEWL) protein were investigated using solution-state nuclear magnetic resonance (NMR) spectroscopy. To ascertain the binding and dynamics from the perspective of both protein and IL, various ligand based NMR approaches such as selective and non-selective nuclear spin-relaxation (R_1SEL and R_1NS), saturation transfer difference (STD), difference of inversion recovery rate with and without target irradiation (DIRECTION), ³⁵Cl line-shape and spin-relaxation, and protein back bone amide chemical shift perturbations (CSPs) from ¹H-¹⁵N HSQC were utilized. Among the ILs investigated, IL2 experiences significant interaction relative to those of IL1 and IL3, as revealed by the combined R_1SEL and R_1NS analysis, which is further supported by STD NMR. CSP analyses of ¹H-¹⁵N HSQC spectra of aqueous P-IL mixtures enabled to identify the potential binding sites of ILs with HEWL. Whereas, ¹⁵N longitudinal (R₁) and transverse (R₂) spin-relaxation rates and ¹⁵N{¹H} heteronuclear nuclear Overhauser effect (hetNOE) data subjected to the model free analysis for IL2 yielded the rotational correlation times and order parameters of various residues of HEWL. Furthermore, the results could discern the nature of interactions between studied ILs and HEWL in terms of specific and non-specific interactions.

Lysozyme conformational changes with ionic liquids: Spectroscopic, small angle x-ray scattering and crystallographic study

Solvents that support protein functionality are important for biochemical applications, and new solvents are required. Here we employ FTIR and fluorescence spectroscopies, small angle X-ray scattering (SAXS) and X-ray crystallography to understand conformational changes of lysozyme with ionic liquids (ILs) added. Spectroscopic techniques identified that the secondary structure of lysozyme was maintained at the lower IL concentrations of 1 and 5 mol%, though the Tryptophan environment was significantly altered with nitrate-based ILs present. SAXS experiments indicated that the radius of gyration of lysozyme increased with 1 mol% IL present, and then decreased with increasing IL concentrations. The tertiary structure, particularly the loop regions, changed as a function of IL concentration, and this depended on the IL type. The crystallographic structure of lysozyme with the IL of ethylammonium nitrate present confirmed the loop region was extended, and identified three specific binding sites with nitrate ions, and that the positively charged areas were IL sensitive regions. This work provides a detailed understanding of lysozyme conformational changes in the presence of ILs. This approach can be extended to other functionally-important proteins.

Neat Ionic liquid and α-Chymotrypsin-Polymer Surfactant Conjugate-Based Biocatalytic Solvent

Performing biocatalysis in nonaqueous solvents is advantageous as it imparts enhanced solubility to hydrophobic substrates and an ability to increase the temperature for shifting reaction equilibrium in the forward direction. In this work, we show the design and development of another class of nonaqueous composite solvent obtained by mixing surface modified enzyme and neat ionic liquid (IL). We systematically probe the interaction and solubility of industrially relevant α-chymotrypsin in its native or surface-bound polymer-surfactant bioconjugated form, with neat protic (N-methyl-2-pyrrolidonium trifluoromethanesulfonate; [NMP][OTf]), or aprotic (1-methyl-3-(4-sulfobutyl)-1H-imidazol-3-ium trifluoromethanesulfonate; [HO₃S(CH₂)₄MIm][OTf]), ILs. Polarized optical micrographs show that the lyophilized powder of native α-chymotrypsin, nCT, does not disperse in either of the neat ILs, however, its polymer surfactant (PS)-coated bioconjugate counterparts, PScCT, in the waterless state, can be well-dispersed and solubilized in the neat [HO₃S(CH₂)₄MIm][OTf]. The solubilization of waterless bioconjugates of PScCT in neat aprotic IL provides a composite liquid, WL-ImPScCT (WL: waterless, Im: [HO₃S(CH₂)₄MIm][OTf]), having a viscosity of 69.6 Pa·s at 25 °C with a shear-thinning behavior, ≈ 15 w/w % α-chymotrypsin, and ≈ 1.2 w/w % residual water content. Detailed secondary structural analysis using circular dichroism and Fourier self-deconvolution on the ATR-FTIR data of WL-ImPScCT liquid reveals retention of the near native secondary structure of α-chymotrypsin. Further, using a combination of fluorescence spectroscopy and electron spray ionization mass spectrometry, we show that scattering of dry and powdered bovine serum albumin (BSA) protein on the WL-ImPScCT composite liquid results in the solubilization of the former, followed by limited proteolysis of BSA by the α-chymotrypsin. Our results, therefore, show the stabilization of α-chymotrypsin in a neat aprotic IL environment to yield a composite liquid, which not only acts as a nonaqueous, nonvolatile, and environmentally benign solvent, but also provides a biocatalytic platform capable of carrying out reactions relevant for biotransformations, food processing, drug delivery, and various other applications.

Proteins in Ionic Liquids: Current Status of Experiments and Simulations

In the last two decades, while searching for interesting applications of ionic liquids as potent solvents, their solvation properties and their general impact on biomolecules, and in particular on proteins, gained interest. It turned out that ionic liquids are excellent solvents for protein refolding and crystallization. Biomolecules showed increased solubilities and stabilities, both operational and thermal, in ionic liquids, which also seem to prevent self-aggregation during solubilization. Biomolecules can be immobilized, e.g. in highly viscous ionic liquids, for particular biochemical processes and can be designed to some extent by the proper choice of the ionic liquid cations and anions, which can be characterized by the Hofmeister series.

Homogeneous capture and heterogeneous separation of proteins by PEG-functionalized ionic liquid–water systems

An efficient homogeneous capture and heterogeneous separation strategy for proteins is reported using PEG-functionalized ionic liquids with LCST phase behavior in water.