Reversible folding-unfolding, aggregation protection, and multi-year stabilization, in high concentration protein solutions, using ionic liquids

Protein destabilisation in ionic liquids: the role of preferential interactions in denaturation

The preferential binding of anions and cations in aqueous solutions of the ionic liquids (ILs) 1-butyl-3-methylimidazolium ([C4mim](+)) and 1-ethyl-3-methylimidazolium ([C2mim](+)) chloride and dicyanamide (dca(-)) with the small alpha-helical protein Im7 was investigated using a combination of differential scanning calorimetry, NMR spectroscopy and molecular dynamics (MD) simulations. Our results show that direct ion interactions are crucial to understand the effects of ILs on the stability of proteins and that an anion effect is dominant. We show that the binding of weakly hydrated anions to positively charged or polar residues leads to the partial dehydration of the backbone groups, and is critical to control stability, explaining why dca(-) is more denaturing than Cl(-). Direct cation-protein interactions also mediate stability; cation size and hydrophobicity are relevant to account for destabilisation as shown by the effect of [C4mim](+) compared to [C2mim](+). The specificity in the interaction of IL ions with protein residues established by weak favourable interactions is confirmed by NMR chemical shift perturbation, amide hydrogen exchange data and MD simulations. Differences in specificity are due to the balance of interaction established between ion pairs and ion-solvent that determine the type of residues affected. When the interaction of both cation and anion with the protein is strong the net result is similar to a non-specific interaction, leading ultimately to unfolding. Since the nature of the ions is a determinant of the level of interaction with the protein towards denaturation or stabilisation, ILs offer a unique possibility to modulate protein stabilisation or even folding events.

Ionic liquid-induced formation of the α-helical structure of β-lactoglobulin

Structural modification of bovine milk β-lactoglobulin (β-LG) in aqueous 1-butyl-3-methylimidazolium nitrate ([bmim][NO3]) and ethylammonium nitrate ([EAN][NO3]) solutions has been investigated by Fourier transform infrared and circular dichroism spectroscopy. Remarkably, high ionic liquid (IL) concentrations (>15 mol %IL) caused formation of a non-native α-helical structure of β-LG and disruption of its tertiary structure. Furthermore, while [bmim][NO3] promoted protein aggregation, [EAN][NO3] inhibited it probably owing to differences in the unique solution structure (nanoheterogeneity) of the ILs by the different cationic species. The IL-induced α-helical formation of β-LG shows a behavior similar to the alcohol denaturation, but a disordered structure-rich state was observed in the β-α transition process by adding IL, in contrast to the case of an aqueous alcohol solution of protein. We propose that the molten salt-like property of aqueous IL solutions strongly support α-helical formation of proteins.

Phase transition of poly(N-isopropylacrylamide) in aqueous protic ionic liquids: kosmotropic versus chaotropic anions and their interaction with water

We have investigated the influence of a series of triethylammonium-based protic ionic liquid-water solutions on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM). We find that kosmotropic anions lower the LCST of PNIPAM more dramatically when compared with chaotropic anions. In addition, we have probed the solvent properties of the hydrated protic ionic liquid solutions using (1)H NMR, polarity measurements, and solvatochromic analysis of the Kamlet-Taft parameters, β and π*. We find that the hydrogen bond character--more specifically, the interactions between water and pIL--is the dominant parameter responsible for lowering the LCST of PNIPAM. We have added choline dihydrogen phosphate (choline dhp) into this study on the basis of positive results from previously reported protein folding studies using this ionic liquid.

Contracting cardiomyocytes in hydrophobic room-temperature ionic liquid

Room-temperature ionic liquids (RTILs) are drawing attention as a new class of nonaqueous solvents to replace organic and aqueous solvents for chemical processes in the liquid phase at room temperature. The RTILs are notable for their characteristics of nonvolatility, extremely low vapor pressure, electric conductivity, and incombustibility. These distinguished properties of RTILs have brought attention to them in applications with biological cells and tissue in vacuum environment for scanning electron microscopy, and in microfluidic devices for micro-total analysis system (micro-TAS). Habitable RTILs could increase capability of nonaqueous micro-TAS for living cells. Some RTILs seemed to have the capability to replace water in biological applications. However, these RTILs had been applied to just supplemental additives for biocompatible test, to fixed cells as a substitute for an aqueous solution, and to simple molecules. None of RTILs in which directly soaks a living cell culture. Therefore, we demonstrated the design of RTILs for a living cell culture and a liquid electrolyte to stimulate contracting cardiomyocytes using the RTILs. We assessed the effect of RTILs on the cardiomyocytes using the beating lifetime to compare the applicability of RTILs for biological applications. Frequent spontaneous contractions of cardiomyocytes were confirmed in amino acid anion RTILs [P(8,8,8,8)][Leu] and [P(8,8,8,8)][Ala], phosphoric acid derivatives [P(8,8,8,8)][MeO(H)PO(2)], and [P(8,8,8,8)][C(7)CO(2)]. The anion type of RTILs had influence on applicable characteristics for the contracting cardiomyocyte. This result suggested the possibility for biocompatible design of hydrophobic group RTILs to achieve biological applications with living cells.

In vitro assessment of choline dihydrogen phosphate (CDHP) as a vehicle for recombinant human interleukin-2 (rhIL-2)

Choline dihydrogen phosphate (CDHP) is an ionic liquid reported to increase thermal stability of model proteins. The current work investigated CDHP effect on structural integrity and biological activity of recombinant human interleukin-2 (rhIL-2), a therapeutic protein used for treating advanced melanoma. In vitro CDHP biocompatibility was also evaluated using primary cell cultures, or B16-F10 cell line, chronically exposed to the ionic liquid. Formulation of rhIL-2 in an aqueous 680mM CDHP pH 7.4 solution resulted in a 12.5°C increase in the T_m of rhIL-2 compared to a basic buffer formulation, and provided conformational rhIL-2 stabilization when the solution was heated to 23.3°C above the T_m. CDHP solutions (≤80mM), exhibited no cytotoxic activity toward primary splenocytes or B16-F10 cells in culture. However, a 10-fold loss in biological activity was observed when rhIL-2 was used in a 30mM CDHP aqueous solution with NaHCO₃ (pH≥7.2) compared to controls without CDHP. While increased T_m is associated with a diminished rhIL-2 biological activity, the therapeutic protein remains structurally intact and functional.

Enhanced stabilization of the Tobacco mosaic virus using protic ionic liquids

We report on the use of protic ionic liquids, pILs, as solvents for the solubilisation and stabilization of viruses. We show that the shelf life of the pIL stabilized tobacco mosaic virus is significantly enhanced when compared to traditional phosphate buffer. This has new opportunities for the preparation, characterization and storage of viruses and virus based technologies.

Ionic liquid/water mixtures: from hostility to conciliation

Water was originally inimical to ionic liquids (ILs) especially in the analysis of their detailed properties. Various data on the properties of ILs indicate that there are two ways to design functions of ionic liquids. The first is to change the structure of component ions, to provide "task-specific ILs". The second is to mix ILs with other components, such as other ILs, organic solvents or water. Mixing makes it easy to control the properties of the solution. In this strategy, water is now a very important partner. Below, we summarise our recent results on the properties of IL/water mixtures. Stable phase separation is an effective method in some separation processes. Conversely, a dynamic phase change between a homogeneous mixture and separation of phases is important in many fields. Analysis of the relation between phase behaviour and the hydration state of the component ions indicates that the pattern of phase separation is governed by the hydrophilicity of the ions. Sufficiently hydrophilic ions yielded ILs that are miscible with water, and hydrophobic ions gave stable phase separation with water. ILs composed of hydrophobic but hydrated ions undergo a dynamic phase change between a homogeneous mixture and separate phases according to temperature. ILs having more than seven water molecules per ion pair undergo this phase transition. These dynamic phase changes are considered, with some examples, and application is made to the separation of water-soluble proteins.

Stable G-quadruplex structure in a hydrated ion pair: cholinium cation and dihydrogen phosphate anion

A combination of cholinium cations and dihydrogen phosphate anions is indispensable for the G-quadruplex structure regardless of water content. No G-quadruplex structure was observed in most salts even containing either cholinium cations or dihydrogen phosphate anions.

Structural heterogeneity and unique distorted hydrogen bonding in primary ammonium nitrate ionic liquids studied by high-energy X-ray diffraction experiments and MD simulations

Liquid structure and the closest ion-ion interactions in a series of primary alkylammonium nitrate ionic liquids [C(n)Am(+)][NO(3)(-)] (n = 2, 3, and 4) were studied by means of high-energy X-ray diffraction (HEXRD) experiments with the aid of molecular dynamics (MD) simulations. Experimental density and X-ray structure factors are in good accordance with those evaluated with MD simulations. With regard to liquid structure, characteristic peaks appeared in the low Q (Q: a scattering vector) region of X-ray structure factors S(Q)'s for all ionic liquids studied here, and they increased in intensity with a peak position shift toward the lower Q side by increasing the alkyl chain length. Experimentally evaluated S(Q(peak))(r(max)) functions, which represent the S(Q) intensity at a peak position of maximum intensity Q(peak) as a function of distance (actually a integration range r(max)), revealed that characteristic peaks in the low Q region are related to the intermolecular anion-anion correlation decrease in the r range of 10-12 Å. Appearance of the peak in the low Q region is probably related to the exclusion of the correlations among ions of the same sign in this r range by the alkyl chain aggregation. From MD simulations, we found unique and rather distorted NH···O hydrogen bonding between C(n)Am(+) (n = 2, 3, and 4) and NO(3)(-) in these ionic liquids regardless of the alkyl chain length. Subsequent ab initio calculations for both a molecular complex C(2)H(5)NH(2)···HONO(2) and an ion pair C(2)H(5)NH(3)(+)···ONO(2)(-) revealed that such distorted hydrogen bonding is specific in a liquid state of this family of ionic liquids, though the linear orientation is preferred for both the N···HO hydrogen bonding in a molecular complex and the NH···O one in an ion pair. Finally, we propose our interpretation of structural heterogeneity in PILs and also in APILs.

Role of Cation in Enhancing the Conversion of the Alzheimer’s Peptide into Amyloid Fibrils Using Protic Ionic Liquids

We report on the impact of changes in the protic ionic liquid (pIL) cation on the fibrilisation kinetics and the conversion of the Aβ 16–22 from monomers to amyloid fibrils. When we compare the use of primary, secondary, and tertiary amines we find that the primary amine results in the greatest conversion into amyloid fibrils. We show that the pIL is directly interacting with the peptide and this likely drives the difference in conversion and kinetics observed.

How ionic liquids can help to stabilize native proteins

The native state of a globular protein is essential for its biocatalytic function, but is marginally stable against unfolding. While unfolding equilibria are often reversible, folding intermediates and misfolds can promote irreversible protein aggregation into amorphous precipitates or highly ordered amyloid states. Addition of ionic liquids-low-melting organic salts-offers intriguing prospects for stabilizing native proteins and their enzymatic function against these deactivating reaction channels. The huge number of cations and anions that form ionic liquids allows fine-tuning of their solvent properties, which offers robust and efficient strategies for solvent optimization. Going beyond case-by-case studies, this article aims at discussing principles for a rational design of ionic liquid-based formulations in protein chemistry and biocatalysis.

Structure and function of proteins in hydrated choline dihydrogen phosphate ionic liquid

Ionic liquids are being intensely studied as promising media for the stabilization of proteins and other biomolecules. Choline dihydrogen phosphate (CDHP) has been identified as one of the most promising candidates for this application. In this work we have probed in more detail the effects that CDHP may have on the thermodynamics, structure, and stability of proteins, including one of therapeutic interest. Microcalorimetry and circular dichroism spectropolarimetry (CD) were used to assess the thermal stability of protein solutions in CDHP/water mixtures at various concentrations. Increasing thermal stability of lysozyme and interleukin-2 in proportion to CDHP concentration was observed. Isothermal titration calorimetry (ITC) was used to quantify binding interactions, and indicate that the mechanism for stability does not appear to be dependent upon CDHP binding to protein. CD and small angle X-ray scattering (SAXS) analyses were used to probe for structural changes due to the presence of CDHP. SAXS indicates charge effects on the surface of the protein play a role in protein stability in ionic liquids, and no significant alteration of the overall tertiary conformation of lysozyme was observed at 25 °C. However, after incubation at 37 °C or at higher concentrations of CDHP, small changes in protein structure were seen. Effects on protein activity were monitored using turbidity assays, and CDHP decreases protein activity but does not eliminate it. Protein solubility was also monitored using a turbidity assay and was found to be inversely proportional to the concentration of CDHP in solution.

An FCS study of unfolding and refolding of CPM-labeled human serum albumin: role of ionic liquid

The effect of a room temperature ionic liquid (RTIL) on the conformational dynamics of a protein, human serum albumin (HSA), is studied by fluorescence correlation spectroscopy (FCS). For this, the protein was covalently labeled by a fluorophore, 7-dimethylamino-3-(4-maleimidophenyl)-4-methylcoumarin (CPM). On addition of a RTIL ([pmim][Br]) to the native protein, the diffusion coefficient (D(t)) decreases and the hydrodynamic radius (R(h)) increases. This suggests that the RTIL ([pmim][Br]) acts as a denaturant when the protein is in the native state. However, addition of [pmim][Br] to a protein denatured by GdnHCl causes an increases in D(t) and decrease in R(h). This suggests that in the presence of GdnHCl addition of RTIL helps the protein to refold. In the native state, the conformational dynamics of protein is described by three distinct time constants: ~3.6 ± 0.7, ~29 ± 4.5, and 133 ± 23 μs. The faster components (~3.6 ± 0.7 and ~29 ± 4.5 μs) are ascribed to chain dynamics of the protein, while the slowest component (133 μs) is responsible for interchain interaction or concerted motion. On addition of [pmim][Br], the conformational dynamics of HSA becomes slower (~5.1 ± 1, ~43.5 ± 2.8, and ~311 ± 2.3 μs in the presence of 1.5 M [pmim][Br]). The time constants for the protein denatured by 6 M GdnHCl are 3.2 ± 0.4, 34 ± 6, and 207 ± 38 μs. When 1.5 M [pmim][Br] is added to the denatured protein (in 6 M GdnHCl), the time constants become ~5 ± 1, ~41 ± 10, and ~230 ± 45 μs. The lifetime histogram shows that, on addition of GdnHCl to HSA, the contribution of the shorter lifetime component decreases and vanishes at 6 M GdnHCl. The shorter lifetime component immediately reappears after addition of RTIL to unfolded HSA. This suggests recoiling of the unfolded protein by RTIL.

On the decoupling of relaxation modes in a molecular liquid caused by isothermal introduction of 2 nm structural inhomogeneities

To support a new interpretation of the origin of the dynamic heterogeneity observed pervasively in fragile liquids as they approach their glass transition temperatures T(g), we demonstrate that the introduction of ~2 nm structural inhomogeneities into a homogeneous glass former leads to a decoupling of diffusion from viscosity similar to that observed during the cooling of orthoterphenyl (OTP) below T(A,) where Arrhenius behavior is lost. Further, the decoupling effect grows stronger as temperature decreases (and viscosity increases). The liquid is cresol, and the ~2 nm inhomogeneities are cresol-soluble asymmetric derivatized tetrasiloxy-based (polyhedral oligomeric silsesquioxane (POSS)) molecules. The decoupling is the phenomenon predicted by Onsager in discussing the approach to a liquid-liquid phase separation with decreasing temperature. In the present case the observations support the notion of a polyamorphic transition in fragile liquids that is hidden below the glass transition. A similar decoupling can be expected as a globular protein is dissolved in dilute aqueous solutions or in protic ionic liquids.

Evaluation of a non-Arrhenius model for therapeutic monoclonal antibody aggregation

Understanding antibody aggregation is of great significance for the pharmaceutical industry. We studied the aggregation of five different therapeutic monoclonal antibodies (mAbs) with size-exclusion chromatography-high-performance liquid chromatography (SEC-HPLC), fluorescence spectroscopy, electron microscopy, and light scattering methods at various temperatures with the aim of gaining insight into the aggregation process and developing models of it. In particular, we find that the kinetics can be described by a second-order model and are non-Arrhenius. Thus, we develop a non-Arrhenius model to connect accelerated aggregation experiments at high temperature to long-term storage experiments at low temperature. We evaluate our model by predicting mAb aggregation and comparing it with long-term behavior. Our results suggest that the number of monomers and mAb conformations within aggregates vary with the size and age of the aggregates, and that only certain sizes of aggregates are populated in the solution. We also propose a kinetic model based on conformational changes of proteins and monomer peak loss kinetics from SEC-HPLC. This model could be employed for a detail analysis of mAb aggregation kinetics.

Fluorescence and circular dichroism spectroscopy of cytochrome c in alkylammonium formate ionic liquids

The structural stability of cytochrome c has been studied in alkylammonium formate (AAF) ionic liquids such as methylammonium formate (MAF) and ethylammonium formate (EAF) by fluorescence and circular dichroism (CD) spectroscopy. At room temperature, the native structure of cytochrome c is maintained in relatively high ionic liquid concentrations (50-70% AAF/water or AAF/phosphate buffer pH 7.0) in contrast with denaturation of cytochrome c in similar solutions of methanol or acetonitrile with water or buffer cosolvents. Fluorescence and CD spectra indicate that the conformation of cytochrome c is maintained in 20% AAF-80% water from 30 to 50 °C. No such temperature stability is found in 80% AAF-20% water. About one-third of the enzyme activity of cytochrome c in 80% AAF-20% water can be maintained as compared with phosphate buffer, and this is greater than the activities measured in corresponding methanol and acetonitrile aqueous solutions. This biophysical study shows that AAFs have potential application as organic solvent replacements at moderate temperature in the mobile phase for the separation of proteins in their native form by reversed phase liquid chromatography.

Engineering protein stability

This article defines protein stability, emphasizes its importance and surveys some notable recent publications (2004-2008) in the field of protein stability/stabilization. Knowledge of the factors stabilizing proteins has emerged from denaturation studies and from study of thermophilic (and other extremophilic) proteins. One can enhance stability by protein engineering strategies, the judicious use of solutes and additives, immobilization, and chemical modification in solution. General protocols are set out on how to measure the kinetic thermal stability of a given protein and how to undertake chemical modification of a protein in solution.

Activity and stability of α-chymotrypsin in biocompatible ionic liquids: enzyme refolding by triethyl ammonium acetate

In view the of wide scope of structural information of biomolecules in biocompatible ionic liquids (ILs) in various applications including chemical and biochemical, it is essential to study the productive preferential interactions between biological macromolecules and biocompatible ILs. We have therefore explored the stability and activity of α-chymotrypsin (CT) in the presence of five ILs from different families, such as triethyl ammonium acetate (TEAA), triethyl ammonium phosphate (TEAP) from ammonium salts, 1-benzyl-3-methylimidazolium chloride ([Bzmim][Cl]), 1-benzyl-3-methylimidazolium tetrafluoroborate ([Bzmim][BF(4)]) from imidazolium salts and tetra-butyl phosphonium bromide (TBPBr) from phosphonium families. Circular dichroism (CD) and UV-vis spectrophotometer experiments were used to study CT stabilization by ILs, related to the associated structural changes and enzyme activity studies, respectively. We observed that all ILs have a dominant contribution to the stabilization of CT. The stability and activity of CT depends on the structural arrangement of the ions of ILs. Our experimental results explicitly elucidate that more hydrophobic imidazolium and phosphonium cations carrying longer alkyl chains of ILs ([Bzmim][Cl], [Bzmim][BF(4)] and TBPBr) were weak stabilizers for CT, while small alkyl chain molecules of triethyl ammonium salts (TEAA and TEAP) are strong stabilizers and therefore more biocompatible for CT stability. Our CD and NMR measurements reveal that TEAA is a refolding additive for CT from a quenched thermal unfolded enzyme structure.

Enzymatic activity and thermal stability of metallo proteins in hydrated ionic liquids

Hydrated choline dihydrogen phosphate (Hy[ch][dhp]) containing 30 wt% water was investigated as a novel protein solvent. The Hy[ch][dhp] dissolved some metallo proteins (cytochrome c, peroxidase, ascorbate oxidase, azurin, pseudoazurin and fructose dehydrogenase) without any modification. These proteins retained the surroundings of the active site after dissolution in Hy[ch][dhp]. Some metallo proteins were found to retain their activity in the Hy[ch][dhp].

Ionic liquids: a pathway to environmental acceptability

Ionic liquids were initially proposed as replacements for conventional organic solvents; however, their chemistry has developed remarkably and offers unexpected opportunities in numerous fields, ranging from electrochemistry to biology. As a consequence of ionic liquids advancing towards potential and actual applications, a comprehensive determination of their environmental, health and safety impact is now required. This critical review aims to present an overview of the current understanding of the toxicity and environmental impact of the principal ionic liquid groups, and highlights some emerging concerns. Each cation type is considered separately, examining the significance of the biological data, and identifying the most critical questions, some yet unresolved. The need for more, and more detailed, studies is highlighted (176 references).

Dissolution and dissolved state of cytochrome C in a neat, hydrophilic ionic liquid

The dissolution and dissolved molecular state of cytochrome c were investigated in the room temperature ionic liquid ethylmethylimidazolium ethylsulfate, [EMIM][EtSO4], by viscometry, optical and vibrational spectroscopies, and peroxidase activity. In dilute mixtures, viscometry demonstrated true molecular dissolution of cytochrome c in the ionic liquid and uncovered a molecular size larger than that in aqueous buffer, suggesting altered solvation or slight denaturation. The protein's heme unit absorbs light outside the spectral range masked by [EMIM], enabling conformational assessments by UV-visible and circular dichroism spectroscopies. Adding trends from fluorescence and Fourier transform infrared spectroscopy, unchanged secondary but perturbed tertiary structures were determined, consistent with the appreciable peroxidase activity measured. Different than in aqueous buffers, denaturation is not accompanied by aggregation. Results are relevant to the proposed application of ionic liquids as media for room temperature preservation of biomacromolecules.