A Comprehensive Experimental Study to Understand the Hofmeister

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A Comprehensive Experimental Study to Understand the Hofmeister Series of Anions of Aqueous Imidazolium-based ILs on Glycine Peptides Varadhi Govinda, and Pannuru Venkatesu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503736g • Publication Date (Web): 24 Nov 2014 Downloaded from http://pubs.acs.org on November 27, 2014

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Industrial & Engineering Chemistry Research

A Comprehensive Experimental Study to Understand the Hofmeister Series of Anions of Aqueous Imidazolium-based ILs on Glycine Peptides Varadhi Govinda and Pannuru Venkatesu∗ Department of Chemistry, University of Delhi, Delhi 110 007, India ABSTRACT The solubility and stability of biomolecules may be influenced by the addition of cosolvents. Ionic liquids (ILs) have emerged as novel co-solvents for wide spread applications in biotechnology and industrial processes. In the present study we report the solubilities and apparent transfer free energies (∆Gtr' ) for zwitterions containing glycine peptides (GPs) such as glycine (Gly), diglycine (Gly2), triglycine (Gly3), tetraglycine (Gly4) and cyclicglycylglycine (c(GG)) from water to aqueous solutions of imidazolium-based ILs at 25 0C and atmospheric pressure. ILs used here are having different anions such as chloride (Cl − ) , bromide ( Br − ) , hydrogen sulfate ( HSO4− ) , acetate (CH3COO− ) and thiocyanate ( SCN − ) and a common 1butyl-3-methylimidazolium cation [Bmim]+. The ∆Gtr' values of GPs from water to aqueous IL solutions have been obtained from the solubilities, which are determined from density (ρ) measurements as a function of ILs concentration at 25 0C. Further, we have calculated salting constant, (k) from Setschenow equation and the values have analyzed with salting-out/salting-in effects. The experimental ∆Gtr' data allowed the calculation of the transfer free energy contributions (∆gtr' ) of the peptide backbone unit (−CH2C = ONH−) from water to aqueous ILs. The effects of anions of ILs on GPs have been analyzed through solubilities, stabilities and ∆Gtr' values to obtain a usual understanding of the Hofmeister series. The results are discussed in terms of solute-solute and solute-solvent interactions in the aqueous IL solution. Keywords: glycine peptides, ionic liquids, solubilities, transfer free energies, molecular interactions.

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1. INTRODUCTION Amino acids (AAs) are the building blocks of proteins and fulfill various functions in the organisms. Fundamentally, AAs are joined together by peptide bonds to form the basic structure of protein. However, owing to the many side groups that are part of the AAs, other sorts of bonds, may form between the amino acid units. These additional bonds twist and turn the protein into convoluted shapes which are unique to the respective protein and essential for its ability to perform certain functions within the human body.1, 2 AAs and glycine peptides (GPs), as model compounds of proteins, are of fundamental importance in chemical and pharmaceutical industries. Obviously, their structural and chemical properties in co-solvents have been extensively investigated by both theoretical and experimental techniques, which provides valuable insights into the chemistry of peptides and peptide backbone.3,4 Model compounds of proteins differ from each other with the variation in their AAs sequences.5 Apparently, it is very much important to have a clear idea on the solubility, stability and thermodynamic properties of model compounds and peptide backbone unit in aqueous co-solvent solutions. The folding and unfolding of proteins in co-solvent, and the biomolecular interactions are mainly depending on the nature of the co-solvent.6-9 The study of thermodynamic properties of model compounds in a variety of media can provide valuable information about the stability or denaturation of model compounds of proteins in living organisms.10 Moreover, there are extensive investigations of model compounds in aqueous and various aqueous additive solutions.11-13 Ionic liquids (ILs) have diversified applications as novel co-solvents for a varied selection of biochemical processes that include protein wrinkle domain.8, 14-19 Due to the ionic nature of the materials, ILs have essentially negligible vapour pressure and so can be envisioned as being useful in a variety of applications,20-22 such as a catalyst, co-solvent for the chemical reactions, synthetic reactions, electrochemical, nanotechnological, biotechnological, engineering processes, separation and extractions processes. Studies on the biomolecular interactions between AAs/GPs and ILs are more useful in understanding the importance of biological processes. In this regards, in our previous works,

23-29

we studied the influence of ammonium-

based ILs on the solubility and stability of AAs and model compounds of proteins and we observed that all of these ILs are stabilized the structure of AAs and model biocompounds.


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Imidazolium-based ILs such as imidazolium cation with different anions have long been used as sources for spectroscopic studies and also as a means of obtaining low-melting salt mixtures.30-32 To the best of our awareness, only limited reports are available on the solubility and stability properties of GPs with aqueous imidazolium-based ILs solutions.33, 34 Apparently, ILs solutions are significantly important for biophysical studies due to their specific ions that have been shown to influence on the properties of biocompounds, for understanding the Hofmeister series.35-39 It has been often observed that ion-specific effects are mediated through changes in the bulk properties of liquid water by altering water’s hydrogen-bonding network.35 As organized in Hofmeister series, the species on the left

( SO4

2−

−

> H 2 PO4 > CH3COO− ) are

referred as kosmotropes, strongly hydrated ions, exhibit a salting-out effect on proteins, and favor to protein stabilization while those on the right ( NO3 − > I − > ClO4 − > SCN − ) are called chaotropes which enhances protein solubility, weakly hydrated ions, display salting-in effect on proteins and favor to protein denaturation.35-37, 39 However, Cl − and Br − are usually consider as a border line between these two different categories.37, 39 A basic level of the interactions between glycine or glycine containing peptides with ILs are quantifying by using the ∆Gtr' values of GPs from water to aqueous imidazolium ILs. In this context, the ∆Gtr' values increase with the increasing concentration of IL, which indicates unfavorable interactions (salting-out effect) are involved between the surfaces of GPs/AAs with ILs, and that leads stabilization of the structure of GPs/AAs in the aqueous IL solution.28,29 On other hand, the ∆Gtr' values decrease with the increasing concentration of IL, which indicates favorable interactions (salting-in effect) are involved between the surfaces of GPs/AAs with ILs that accounts for destabilization of the structure of GPs/AAs in the aqueous IL solution.28, 29 Numerous studies have been explored, which mainly focus on the influence of ions of ionic salts on the activity and stability of biomolecules. Further, these studies suggest that the biomolecule is stabilized by kosmotropic anions and chaotropic cations while destabilized by chaotropic anions and kosmotropic cations.38, 39 Very recently, we reviewed and summarized that Hofmeister series of ions do not necessarily stabilized/destabilize the biomolecules, in perfectly same Hofmeister order.39 The underlying basis of the Hofmeister series, in ion-specific effects on GPs are very poorly investigated and this study is still highly debated. In the current study, we 3 ACS Paragon Plus Environment


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have explored the Hofmeister series of anions effect on GPs through apparent transfer free energies (∆Gtr' ) of GPs from water to aqueous ILs solutions as a function of concentration of IL. The information regarding the biomolecular interactions occurring in aqueous imidazolium-based ILs and GPs are still scare and further studies on this direction is essentially required. In the present study, we have studied the thermodynamic contributions of the ∆Gtr' for GPs, such as glycine (Gly), diglycine (Gly2), triglycine (Gly3), tetraglycine (Gly4) and cyclicglycylglycine (c(GG)) from water to aqueous imidazolium-based ILs solutions. The ILs used are

1-butyl-3-methylimidazolium chloride [Bmim][Cl], 1-butyl-3-methylimidazolium

bromide [Bmim][Br], 1-butyl-3-methylimidazolium hydrogen sulfate [Bmim][HSO4], 1-butyl-3methylimidazolium

acetate [Bmim][CH3COO],

1-butyl-3-methylimidazolium thiocyanate

[Bmim][SCN]. The individual observed thermodynamic contributions of GPs in various ILs help us to characterize the features of peptide surface and its interactions with the co-solvents. Thus, the intent of the present work is, to study the effect of co-solvents like ILs on GPs by using the

∆Gtr' values, to understand the ion-specific effects on GPs structure. Later, the IL effect on biomolecules is characterized by the Setschenow equation and the salting constant (k) has analysed with salting-out and salting-in effects. Further adminicile, we have estimated the transfer free energy contribution, (∆gtr' ) of the glycyl residue (peptide backbone unit) from water to IL solutions by using the ∆Gtr' values of GPs from water to ILs solutions. 2. METHODS AND EXPERIMENTAL SECTION 2.1. Materials.

The Gly and Gly2 (mass fraction purity > 99%) were bought from Acros

organics (USA) chemical company. The Gly3, Gly4 and c(GG) (mass fraction purity > 99.9%), and all ILs such as [Bmim][Cl] (≤0.2% water), [Bmim][Br] (≤200 ppm water), [Bmim][HSO4] (≤1.0% water), [Bmim][CH3COO] (≤0.5% water) and [Bmim][SCN] (≤0.7% water) were supplied by Sigma-Aldrich Chemical Company (USA) and they were used as received. All the sample solutions were prepared freshly with distilled and deionized with a resistivity of 18.3 Ω. cm water, which was obtained from Nano pure-ultra pure water system. All the samples were carefully weighed by mass using a Mettler Toledo analytical balance with a precision of ±0.0001


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g. The chemical structures of glycine peptides and imidazolium-based ILs are as shown in the Figure 1. 2.2. Solubility Measurements. The solubility limits of GPs in the aqueous or aqueous ILs solutions were determined by plotting the solution density (ρ) verses composition of GPs in each IL solution. The pre saturation points and post saturation points are plotted graphically and at the point of intersection of the two fitted lines for each system is taken as the solubility limit, which was explicitly elucidated in our earlier papers.27,40,41 The solubility measurements of GPs were performed in a similar manner to that delineated elsewhere.42-44 The ρ values of all the samples are measured by using a vibrating tube densimeter (DMA-4500M, Anton-Parr, Austria) at 25 0C with a precision of ±0.00005 g.cm-3. The instrument was equipped with a built-in solidstate thermostat and a resident program with an accuracy of temperature of ±0.03 K. The densimeter was initially calibrated by measuring the ρ of atmospheric air and double distilled water, according to manufacture recommendations. The observed ρ values were nicely agreed with the manufacture recommended values. The complete procedure used in this work, includes that each of the sample vials containing a fixed amount of solvent (water or aqueous IL solutions) was added to a weighed amount of a protein model compounds such as Gly, Gly2, Gly3, Gly4 and c(GG) to provide a series of mixtures with increasing composition of the GPs to that required for saturation. In the present study, at least ten samples were prepared for each aqueous ILs system. Clearly, five to six are prepared with unsaturated solutions and the remaining are saturated solutions with excess GPs. After preparations of all the concentration sample vials were sealed with Teflon coated screw cap and the cap is sealed with parafilm to protect from air and water. Later, the completion of this process, vials were completely submerged in a low temperature shaker equipped with a water bath (Metrex, New Delhi, India) for continuous shaking up to 2436 h to reach equilibrium. The bath temperature was controlled to keep a constant temperature (25 ±0.01) 0C and always the water bath is placed in a constant room temperature, also maintained at nearly 25 0C. After 36 h stirring was discontinued and the solution is allowed to settle for at least 5 h before the measurements. The supernatant of each solution was removed by using a syringe and filtered through a 0.47µm disposal filter (Millipore, Millex-GS) before ρ measurements. The uncertainty of the solubility limit was lower than ±1.2%. 5 ACS Paragon Plus Environment


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O H 2N OH

Glycine (Gly) O OH N H NH2

O

Diglycine (Gly2) O

O H N N H

OH

NH2

O

Triglycine (Gly3) O

H N N H

NH2

O OH N H

O

O

Tetraglycine (Gly4) O

H N

N H

O

Cyclicglycylglycine (c(GG)) Figure 1. The schematic chemical structures of glycine peptides and imidazolium-based ILs.


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2.3. Apparent Transfer Free Energies, (∆Gtr' ) of GPs from Water to Aqueous Imidazoliumbased ILs. Indeed, the structure and stability of biocompounds are greatly influenced by their solubilities. The apparent transfer free energy (∆Gtr' ) provides valuable information about the stability of protein model compounds.40-44 To understand the effect of imidazolium-based ILs on the structure and stability of GPs, we obtained ∆Gtr' values for GPs in aqueous IL solutions from their respective solubility measurements at 25 0C under atmospheric pressure as a function of IL concentrations. The detailed procedure of the obtaining ∆Gtr' has been demonstrated in our earlier papers28, 40,41. The uncertainty of ∆Gtr' is lower than ±1.6%. 3. RESULTS AND DISCUSSION 3.1. Solubility of Glycine Peptides (GPs) from Water to Imidazolium-based ILs. The ρ values of each sample containing both unsaturated and saturated solutions of GPs in the aqueous and aqueous IL solutions are collected in Table 1S. The obtained solubility (SGPs) results of GPs in water and in 0.5, 1.0, 1.5, 2.0 and 2.5 M concentrations of imidazolium-based ILs and the density ( ρ GP ) at solubility limit in aqueous or in aqueous ILs with various concentrations are reported in Table 1 and graphically represented in Figure 2. In the present study the solubilities of GPs in aqueous solutions are in splendid agreement with those reported by literature.41, 45-49 From these results, the solubilities of GPs in water decrease from Gly to Gly4 as presented in Figure 2. As can be seen from Figure 2 (a and b), the solubilities of Gly or Gly2 decrease with increasing concentration of all studied ILs in aqueous solutions. This indicates that salting-out effect is dominant. The salting-out of these glycines show that the stability of Gly or Gly2 structure increases as increase the concentrations of all studied ILs. The IL-induced solubility decrease may be due to hydrophobic effects on the Gly or Gly2.27 On the other hand, we observed an increase in the solubilities of GPs of Gly3 or Gly4 at lower concentration of HSO 4− ,

SCN − , Cl − based ILs. Later, the values decrease with increasing the concentration of these ILs. This indicates as a balance between the salt-out of non-polar groups of higher GPs and salting-in of the peptide group in ILs. As shown in Figure 2 (c and d), Br − or CH3COO− anion of ILs,



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promote salting-out of Gly3 or Gly4 almost in the whole concentration region of ILs (except Gly4 in CH3COO− at the lowest concentration). The magnitude of the salting-out or salting-in effect depends on the nature of the ions of ILs and the physical interface, and variable surfaces of glycines. From these results, it can be seen that the highest solubility in ILs is obtained for Gly and the gradual lower solubility in ILs is obtained for Gly4. Analysis of the solubilities of GPs in ILs (Figure 2) reveals that there is monotonic decrease in the solubilities from Gly to Gly4, due to decrease in attraction between the solvent molecules and the long chain of GPs and also increasing hydrophobicity from Gly to Gly4. Hence, the solubility is gradually decreased in GPs from Gly to Gly4 in all ILs, which indicates that the salting-out effect is dominant. According to the Hofmeister series, the anions of ILs follow the stability order50 as F ≈

SO4

2−

−

−

> H2 PO4 > CH3COO− > Cl − > NO3 − > Br− > ClO4 > SCN − . The solubility of

glycines (Gly, Gly2, Gly3 and Gly4) in imidazolium-based ILs decrease in the following order of their anions: HSO 4− > SCN − > Cl − > CH 3COO− > Br − . Our results reveal that the highest solubility is observed for glycines in HSO4− IL, which indicates very low stabilizing effect of this IL on glycine structures. However, these observed results are not consistent with the position HSO 4− of the Hofmeister series. Further, our results show that Br − plays kosmotropic anion,

which is not consistent the Hofmeister series. These discrepancies are mainly due to the weak hydration as well as an increase in hydrophobicity of the anion.51-53 Consequently,

HSO 4−

seems to be weak salting-out anion for glycines, which is not in good agreement with its kosmotropic nature. Clearly, these anions have found to differ greatly from each other in their solubilities on GPs and inclusively indicated that the investigated anions are completely reversed the Hofmeister series.


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25 25

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0 0.0

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Figure 2. Plot of solubility against concentration to illustrate the solubility limits (SAA) for (a) Gly; (b) Gly2; (c) Gly3; (d) Gly4 and (e) c(GG) in an aqueous solution of ILs: [Bmim] [HSO4] (○), [Bmim] [CH3COO] (●), [Bmim] [Br] (∆), [Bmim] [SCN] (▲) and [Bmim] [Cl] (□) at 25 0C. Solid lines show the smoothness of the solubility points. 9 ACS Paragon Plus Environment


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In contrast, c(GG) behaves quite differently from other GPs on solubilities of these model compounds in ILs. As shown in Figure 2e, c(GG) solubility decrease with increasing concentrations of [Bmim][CH3COO], [Bmim][Cl] and [Bmim][SCN] ILs, while the values increase with increasing concentration of [Bmim][Br], due to the variation in intermolecular interactions of ions of IL with c(GG). In other words, we found that the solubilities of c(GG) in [Bmim][Br] cause salting-in effect, where as [Bmim][CH3COO], [Bmim][Cl] and [Bmim][SCN] cause salt-out effect. It has been illustrated from Figure 2e that both salting-in and salting-out effects increase with increasing the concentrations of the ILs. For c(GG) in [Bmim][HSO4], we observed an initial increase in the solubility, later it decreases with increasing the concentration of this IL. The salting-out effect significantly contributes to increase in the stability of the native structure of model compounds in aqueous IL solutions. The salting-in effect mainly contributes to decrease in the stability of the native structure of peptide compounds in aqueous IL solution. Therefore, the magnitude of salting-out/salt-in effect depends on the nature of ILs and the physical interface of biocompounds. In contrast, among all the ILs, [Bmim][Br] showed strong favourable interactions with c(GG), which indicates that the salting-in effect is dominant due to strong polar interactions involved between the ions of [Bmim][Br] and c(GG) as compared to remaining ILs. Specifically, weakly hydrated ions, such as chaotropic ions display a salt-in effect on the model compound of proteins.35, 36 Interestingly, our results (Figure 2e) show that the high solubility of c(GG) in [Bmim][Br] is consistent with denaturing effect of Br − on c(GG) structure, which is behaving as a chaotropic anion. In other words, the solubility ability of other ILs for c(GG) follows in the order: HSO 4− > SCN − > Cl − > CH 3COO− . This order does not agree well with Hofmeister series of the anion. Besides, Tome et al. proposed through experimental33 as well as computational techniques34 that salting-in and salting-out phenomena possess a common basis which are in the competition between water-AA side chain, IL-AA side chain and water-IL interactions. Eventually, they concluded that the exquisite balances between these interactions are dependent on the relative affinities of the peptides to water or to ions of IL. The solubility data from Table 1, show that [Bmim][Br] IL is acted as the strongest salting-out effect for glycines while the same IL behaves strong salting-in effect for c(GG). In addition, [Bmim][HSO4] has lowest salting-out effect for all GPs. 10 ACS Paragon Plus Environment

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3.2. The Influence of Salting Constant (k) on the Solubilities of GPs in aqueous Imidazolium-based ILs Solutions. Generally, the salting constant (k) values are used to find the variation in the solubility of compounds in the aqueous salt solution. The k values elucidate the nature of the interactions between the model compounds and the ions of the salt, with respect to salting effects. The solubilities of compounds in salts can either decrease or increase with increasing concentrations of salts in the aqueous solutions, which are known as salting-out or salting-in effect, respectively.38, 54 Thus, the solubility of GPs can be known by the effects of IL in the aqueous solutions. Therefore, the salting effect has been derived by the Setschenow equation.38, 54

s  log  0  = kc s  s 

(1)

where s0 and s are the solubilities of the GPs in water and in the ILs solution, respectively; cs is the concentration of the IL, and k is the salting constant. In the present study, the k-values obtained from the logarithm of the ratio of solubilities of GPs in absence and presence of aqueous ILs (S0/S) and the values are displayed in Figure 3 and presented in Table 2 as a function of IL concentration. Generally, the positive k values indicate salting-out and negative k values indicate salting-in effect.38,

54

As can be seen from

Table 2, we have obtained positive k values for GPs of Gly or Gly2 in presence of all ILs in aqueous solutions, which support salting-out effect of these GPs. Further, we observed the magnitude of k values are negative for GPs of Gly3 and Gly4 at lower concentration and positive k values observed with increase concentration of Cl − , SCN − , HSO 4− of imidazolium-based ILs,

which indicates salting-in at the lower concentration and salting-out at the higher concentrations for these GPs in the IL solutions. On other hand, the k values are positive for GPs of Gly3 or Gly4 in the remaining of anions such as Br − or CH3COO− of imidazolium-based ILs, except at the lowest concentration of CH 3COO− ion of IL in Gly4. Moreover, the cyclic structure of c(GG) shown the positive k-values in Cl − , SCN − , CH3COO− while negative k-values in Br − of



(a)

(b) 1.1

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0.1 0.0 -0.1 -0.2 0.5

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Figure 3. The solubility constant (k) for (a) Gly; (b) Gly2; (c) Gly3; (d) Gly4 and (e) c(GG) in an aqueous solution of ILs: [Bmim] [HSO4] (○), [Bmim] [CH3COO] (●), [Bmim] [Br] (∆), [Bmim] [SCN] (▲) and [Bmim] [Cl] (□) at 25 0C. Solid lines show the smoothness of the solubility points. 12 ACS Paragon Plus Environment

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aqueous imidazolium-based ILs. The negative values are observed for c(GG) in HSO4− at lower concentrations of this IL, later the values are positive at higher concentration of this IL. The magnitude of k values and the salting-out and salting-in effects are mainly depending on different types of intermolecular interactions such as of hydrogen bonding, van der Waal interactions and hydrophilic as well as hydrophobic interaction are involved between the ions of ILs and water molecules on surface of GPs in the aqueous IL solution. The obtained magnitudes of the k values are quite consistent with the experimental solubilities of GPs in all investigated ILs. 3.3. Apparent Transfer Free Energy (∆Gtr' ) of GPs from Water to Imidazolium-based ILs. From last few decades, many reports are showing interest in the interactions between model protein compounds and co-solvents due to their many applications in biosciences, foods, cosmetics, drug delivery and biotechnological processes.10-13,

40-45

Clearly, ∆Gtr' measures the

interactions between model compounds of protein and co-solvents. Nozaki and Tanford42-44, Auton and Bolen,46 and Tomar et al.55 developed the ∆Gtr' for model compounds from water to co-solvents, which is identifying the favorable or unfavorable interactions of model compounds and co-solvents. By definition, if obtained ∆Gtr' > 0, indicates unfavorable interactions between model compound and co-solvent, means that the peptide becomes hydrophobic on transfer to a co-solvent, which accounts for an increase in peptide stability, whereas a favorable interaction, then ∆Gtr' < 0, represents that the peptide becomes hydrophilic on transfer to a kind of cosolvents, which indicates the destabilization of proteins.40-44 We analysed the imidazolium ILs effect on GPs stability with the aid of the ∆Gtr' from water to IL solutions. The ∆Gtr' values of GPs from water to aqueous ILs are collected in Table 3 and graphically illustrated in Figure 4 as a function of IL concentrations at 25 0C. Figure 4 (a and b) illustrates that the ∆Gtr' values of GPs of small glycines Gly or Gly2, increase with increasing the concentration of all studied ILs. The increase in ∆Gtr' values of GPs with increasing concentration of the IL, indicates the stabilization of the structure of Gly or Gly2 through unfavorable interactions between the Gly or Gly2 and the ions of ILs. The ∆Gtr' of the small GPs, show that [Bmim][Br] is a powerful stabilizer, while [Bmim][HSO4] is a weak



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stabilizer and [Bmim][CH3COO], [Bmim][Cl] and [Bmim][SCN] are moderate stabilizers. The results in Figure 4c reveal that ∆Gtr' values of Gly3 from water to [Bmim][Br] or [Bmim][CH3COO] ILs are monotonically positive at all studied concentrations and the values increase with increasing the concentration of Br− or CH 3COO− based ILs. These results indicate that unfavorable interactions are occurred between ions of these ILs and higher GPs. On the other hand, the ∆Gtr' values are negative for Gly3 in Cl − , SCN − , or HSO 4− of ILs at lower concentrations (≈ 0.1, ≈ 1.5, or ≈ 1.8 M of Cl− , SCN − or HSO4− ILs, respectively), later an increase in ∆Gtr' values are observed when Cl − , SCN − or HSO 4− of ILs concentration enhances up to 2.5 M of these ILs. Analogously, [Bmim][Br] IL increases the positive ∆Gtr' values of Gly4 at all studied concentrations (Figure 4d). The observed ∆Gtr' values are negative for Gly4 in

CH 3COO− , Cl − , SCN − or HSO4− ILs at lower concentrations (≈ 0.8, ≈ 1.4, ≈ 1.5 or ≈ 1.75 M for CH 3COO− , Cl − , SCN − or HSO4− ILs, respectively), further these absolute values are positive and these values increase with respect IL concentrations as shown in Figure 4d. These results indicate that a balance between the steric repulsion of IL from the water molecules at the surface of higher glycines and van der Waals attraction between ions of ILs and the peptide bond of the higher glycines. Interestingly, the higher positive ∆Gtr' values are observed for Gly in all studied ILs whereas lowest values are obtained for Gly4 in all ILs. Gly is the simplest model compound with no side chain, it can easily adopt the conformations. Moreover, due to its small size it allows adjacent polypeptide chains to pack together closely and has clearly preferential hydration,56, 57 which inducing strongest salting-out effect. As results, a large increase in the ∆Gtr' for Gly in ILs is observed due to the strongest salting-out effect. Obviously, Gly2 is the simplest peptide bond with all typical characteristics of complexing sites of higher oligomers.58 Therefore, we observed lower ∆Gtr' values for Gly2 in the ILs as compared to the values of Gly in the ILs. The salting-out effect decrease with increase in the size of the polypeptide increases (high hydrophobic character) consequently leading to lower/negative ∆Gtr' values for higher glycines in ILs.


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Figure 4. Apparent transfer free energies ( ∆Gtr' ) for (a) Gly; (b) Gly2; (c) Gly3; (d) Gly4 and (e) c(GG) in an aqueous solution of ILs: [Bmim] [HSO4] (○), [Bmim] [CH3COO] (●), [Bmim] [Br] (∆), [Bmim] [SCN] (▲) and [Bmim] [Cl] (□) at 25 0C. Solid lines show the smoothness of the solubility points. 15 ACS Paragon Plus Environment


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Our results suggest that the salting-out/salting-in as well as the magnitude of ∆Gtr' values of the glycines in ILs are mainly depending on the preferential interactions between various biomolecular surfaces and the size of the ions of ILs. It is noteworthy to compare the present

∆Gtr' values with those of glycines in ammonium-based ILs.27 In our previous study, we also observed that the ∆Gtr' values for smaller glycines (Gly or Gly2) in ammonium family ILs are obviously higher than those of higher glycines in same family of ILs.27 Apparently, the ∆Gtr' values of glycines in ILs from Gly to Gly4, decrease with increasing backbone of the glycines. The ∆Gtr' values of c(GG) in aqueous imidazolium-based ILs are distinctly different from that of zwitterionic glycine peptides. The positive ∆Gtr' values are found in c(GG) in CH 3COO− ,

Cl − , SCN − based ILs, because these ILs have salting-out effect, whereas the remaining Br − and HSO 4− ILs have salting-in effect for c(GG), thereby negative ∆Gtr' values were observed for c(GG) in Br − or HSO 4− ILs at all studied concentrations. The ∆Gtr' values of c(GG) in ILs reveal that [Bmim][CH3COO] is a strong stabilizer whereas [Bmim][SCN] is a weak stabilizer and [Bmim][Cl] is moderate stabilizer for structure of c(GG). Surprisingly, the observed negative

∆Gtr' values for c(GG) in [Bmim][Br] show that Br − IL behaved as a strong destabilizer for the structure of c(GG) and this behavior is entirely reversed for this IL behavior in case of glycines, in which [Bmim][Br] behaved as a strong stabilizer for the structures of all glycines. The GPs stability/destability and associations depend on the hydrophobic, hydrophilic and van der Waals interactions between the GPs and co-solvent molecules. 3.4. The Influence of Hofmeister Series of Anions of ILs on the Structure of GPs. The molecular interactions of ions of ILs with model compounds or biomolecules have invoked the interest in protein science and biological objects. The ions of the Hofmeister series of ILs significantly contribute on biological function, which can be very important in living organisms. Figure 4 (a and b) demonstrates that the ∆Gtr' values for Gly or Gly2 in the anions of aqueous imidazolium-based ILs following order: Br − > CH3COO− > Cl − > SCN − > HSO 4− . This order is consistent with the salting-out behavior of Gly or Gly2 in the ILs. This order is an absolutely reversed of Hofmeister series. Our results explicitly elucidates that Br − (which is border line 16 ACS Paragon Plus Environment

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between kosmotropes and chaotropes or neutral ion) shows dominant role in unfavorable interactions with the surface of the smaller glycines. On the other hand, this order shows that strongly hydrated and kosmotrope ion HSO 4− interacts very weakly unfavorable interactions with the surface of the small glycines, which indicates that HSO 4− behaves very weak stabilizer for Gly or Gly2 structures, which is also reversed of Hofmeister series. Cl − shows moderate stabilizer for both Gly and Gly2 structure, thereby it acts as a Hofmeister-neutral ion. The CH3COO− behaves as a weaker stabilizer than Br − which is not consistent with the Hofmeister

series. Consequently, weakly hydrated SCN − IL exhibits no preferential binding/favorable interactions with the structures of small glycines, which reveal that SCN − ion is also behaving as a stabilizer for these glycines. These arguments of the current studies are entirely different from the anions of Hofmeister-series. As per Hofmeister series, the more kosmotropic anions interact with the surface of the biomolecules via mediating water molecules, while chaotropic anions should interact with a biomolecule directly.38 As can be seen from Figure 4c, the chaotrope ion such as Br − , exhibits strong stabilizer for Gly3 structure, which indicates unfavorable interactions between this IL with the amide bond of the Gly3 structure and IL leads to a decrease in solubility with increasing the IL concentration (Figure 2c). Similarly, CH3COO− also enhances the structure stability of Gly3 with increasing the concentration of IL. Unexpectedly, this Br− behavior with Gly3 is not following the Hofmeister series. On the other hand, the kosmotrope anion HSO 4− interacts favorably with the surface of Gly3 via mediating water molecules at lower concentration of this IL, whereas HSO 4− slightly stabilize the structure of Gly3 only at higher concentrations. The denaturing effect of − HSO 4− is entirely different from its position in the Hofmeister series. The position of Cl is

almost in the middle of the studied ILs (Figure 4c). Further, the results in Figure 4c show that the weakly hydrated ion, SCN − is attracted to the surface of Gly3 up to 1.8 M of this IL while the same SCN − is repelled from the immediate vicinity of Gly3 structure. Apparently, the studied anion series of the imidazolium ILs follow a reversed Hofmeister series, such as Br − > CH3COO− > Cl − > SCN − > HSO 4− .



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From Figure 4d, it is observed that the ∆Gtr' values are positive for Gly4 in Br − , which indicates that repulsion of this ion from the vicinity of the surface of Gly4 and clearly shows that reversed of Hofmeister series. As illustrated in Figure 4d, the ∆Gtr' values follow the order:

Br − > CH3COO− > Cl − > SCN − > HSO 4− , which is completely reversed of the Hofmeister series. As would be expected on the basis of the interactions between Gly or Gly2 or Gly3 or Gly4 with anions of ILs, the ordering is reversed, the position in the series can alter depending on the nature and structural arrangement of biomolecules. For c(GG), ∆Gtr' values are not affected by Hofmeister series of the ions such as kosmotrope ion HSO 4− behaves as a chaotrope ion whereas chaotrope ion SCN − acts as a kosmotrope ion. Figure 4e demonstrates that CH3COO− is a strong stabilizer, Cl − is a moderate while SCN − is a weak stabilizer. The observed results show that Br − IL behaved as a strong destabilizer for the structure of c(GG). Clearly, the anions such as CH 3COO− , Cl − , SCN − and HSO4− of ILs are excluded from the surface of c(GG) due to steric repulsions from the hydration

layer whereas preferentially binding interactions are occurring on the surface of c(GG) and Br − , which shows that inclusion of Br − IL on the surface of c(GG) due to strong intermolecular interactions with the hydration layer. The schematic description of stability or destability of c(GG) structure in presence of different anions of aqueous imidazolium-based ILs has depicted Scheme 1. The analysis of the ∆Gtr' values for c(GG) in imidazolium aqueous ILs show that the traditional Hofmeister order is not observed for all investigated anions of aqueous imidazoliumbased ILs. The traditional Hofmeister order mainly depending on the ability of anion interactions with surface of the protein via water molecules such as hydrogen bonding, nonpolar interactions and electrostatic effects and also the anion structural arrangement with the nature of cation. Our results clearly show that the imidazolium-based ILs contribution on GPs fail to follow the Hofmeister series. Interestingly, our analysis is absolutely consistent with the existing result.59


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Scheme 1. The stability or destability of the structure c(GG) in presence of different anions of aqueous imidazolium-based ILs. The anions such as CH 3COO− , Cl − , SCN − and HSO4− of ILs are excluded from the surface of c(GG) due to steric repulsions from the hydration layer (blue colour line) whereas preferentially binding interactions are occurring on the surface of c(GG) and Br − , which shows that inclusion of Br − IL on the surface of c(GG) due to strong intermolecular interactions with the hydration layer (slight blue colour line). 3.5. Transfer Free Energy ( ∆gtr' ) Contribution of Peptide Backbone Unit (or) Glycyl Residue from Aqueous Solution to Imidazolium ILs. Basically, the transfer free energies of model compounds are interpreted in terms of group additivity, which are good estimate methods to find the contribution of a functional group that is assumed to be independent of neighboring functional groups.60,

40-46

From the ∆Gtr' values, our intent is to reveal that the influence of 19 ACS Paragon Plus Environment


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aqueous imidazolium-based ILs on the peptide backbone unit of model compounds, which is also more useful in biochemical process. The ∆Gtr' values of GPs in the IL solutions are used to determine the contribution of peptide backbone unit transfer free energy ( ∆gtr' ) from water to imidazolium-based ILs. 3.5.1 Transfer Free Energy ( ∆gtr' ) Contribution of Peptide Backbone Unit of Zwitterionic Glycine Peptides from Water to Imidazolium-based ILs. The transfer free energy (∆gtr' ) contributions of peptide backbone unit (or glycyl residue, -CH2C=ONH-) of GPs are obtained from the difference between the values of ∆Gtr' related the sequences of Gly to Gly4 as a function of IL concentration, which is schematically shown in scheme 2. The detailed description of the obtained ∆gtr' values are delineated in our earlier articles.61 We have obtained the ∆gtr' of the peptide backbone unit for glycine peptides in the imidazolium-based IL solutions from simple subtracting the corresponding values of glycine peptides of the ∆Gtr' of two glycines, such as Gly2 and Gly; Gly3 and Gly2; Gly4 and Gly3, which is explicitly elucidated in the upper panel of the scheme 2. Further, the composite constructs is obtained by subtracting the ∆Gtr' of two glycines that differ in chain length by more than that of one peptide unit, such as Gly4 and Gly, then dividing the difference by 3; [(Gly4-Gly)/3]. Similarly, for other different chain length peptides, like [(Gly4-Gly2)/2] and [(Gly3-Gly)/2], which are shown in the lower panel of the scheme 2. All of these mathematical constructs for each scheme of the model compound provide a determination for the peptide backbone unit transfer free energy contribution from water to IL solutions with reference to the difference of their definitions as well as their interactions with different ILs. The ∆gtr' values of the glycyl residue, which are obtained from scheme 2, are presented in Table 4 as a function of IL concentrations. From the Table 4, we have observed both positive and negative ∆gtr' contribution of peptide backbone unit from water to aqueous imidazolium-based IL solutions. We have obtained negative ∆gtr' values for [Gly2-Gly] in Br − , Cl − of imidazolium ILs solutions, except higher concentrations of Br − and Cl − of ILs, in which we obtained positive values. The negative ∆gtr'


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Scheme 2. Schematic illustration of the contribution of the peptide backbone unit of glycine peptides. contribution of peptide backbone unit is observed in presence of CH 3 COO − , HSO 4− , SCN − ILs (except at higher concentration of SCN − ) in the whole concentrations of the ILs, which indicates that the peptide backbone unit is significantly denatured by these ILs. Similarly, we observed the

∆gtr' values are negative for [(Gly3-Gly2)], [(Gly4-Gly3)], [(Gly3-Gly1)/2], [(Gly4-Gly2)/2] and [Gly4-Gly1)/3] in the whole concentration ranges of all ILs (except for [(Gly4-Gly3)], at the lower 21 ACS Paragon Plus Environment


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concentration of Cl − , HSO 4− and SCN − ions of ILs). The negative contribution indicates that the interactions between the IL and glycyl residue are favorable and these ILs exhibited like destabilizers for glycine residue. The positive contribution indicates that the interaction between the IL and glycyl residue are unfavorable and these ILs acted like stabilizers for glycine residues. Finally, we concluded that from these results, the values of ∆gtr' for glycine residue are very sensitive to the nature of neighboring groups and dependent on the ionic strength of the solutions. 3.5.2 Transfer Free Energy ( ∆gtr' ) Contribution of Peptide Backbone Unit of c(GG) from Water to Imidazolium-based ILs. The c(GG) is selected as the model compound of typical interactions found in aqueous ILs solution because it is the cyclic dimer of glycine. The hydrogen-bonded structure makes c(GG) a good model compound for hydrogen bonding which takes place within the peptide backbone unit of a model compound.61 The close packing of c(GG)

in the solid also brings into play van der Waals interactions between groups.

Consequently, the transfer free energy from water to aqueous IL solutions of this compound is divided by 2 to obtain the contribution of one peptide backbone unit as shown in scheme 3. The calculated ∆gtr' values of peptide backbone unit of c(GG) are also included in Table 4.

Scheme 3. Schematic illustration of determining the contribution of the peptide backbone unit of c(GG). 22 ACS Paragon Plus Environment

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The ∆gtr' values are positive for peptide backbone unit of c(GG) from water to CH 3COO − , Cl − ,

SCN − ions of IL solutions (except at lower concentration of SCN − ) and this contribution increases with increasing IL concentrations. Here, we have observed larger positive ∆gtr' values for this glycine residue in presence of CH 3COO − IL at all concentrations, which indicates that it acts as a strong stabilizer and Cl − and SCN − ions of imidazolium ILs are a moderate stabilizer for peptide backbone unit of GPs of c(GG) in the IL solution. The results in Table 4 show the

∆gtr' values are negative for peptide backbone unit of c(GG) from water to HSO 4− and Br − ions of these ILs in the solution, which indicate that a favorable interactions between this residue with ions of ILs. The obtained ∆gtr' behavior is quite contrasting to the ∆Gtr' of GPs in ILs. 5. CONCLUSIONS The present study provides a comprehensive investigation of ions of imidazolium-based ILs with glycine and glycine containing peptides by using the solubility and ∆Gtr' measurements of glycine peptides from water to aqueous ILs solutions at 25 0C. The present solubility data for simple glycine peptides (Gly or Gly2) show that the solubilities decrease with increasing the ILs concentrations, which indicates salting-out effect is dominating, accordingly we obtain positive

∆Gtr' values for these glycines in all studied ILs. Later, we observed salting-in effect at lower concentration of ILs whereas salting-out effect was obtained at higher concentration of ILs for higher glycine (Gly3 or Gly4) in all ILs, except in CH 3COO− or Br − IL. In the case of

CH3COO− or Br − anion of ILs, promote salting-out of Gly3 or Gly4 almost in the whole concentration region of ILs. Explicitly, the magnitude of the salting-out or salting-in effect depends on the nature of the ions of ILs and the physical interface, and variable surfaces of glycines. The positive ∆Gtr' values are observed for c(GG) in CH 3COO− , Cl − and SCN − based ILs, since these ILs have salting-out effect, whereas the remaining Br − and HSO 4− ILs have salting-in effect for c(GG), thereby negative ∆Gtr' values are observed for c(GG) in Br − or ' HSO 4− ILs at all studied concentrations. The observed negative ∆Gtr values for c(GG) in

[Bmim][Br] show that Br − IL behaved as a strong destabilizer for the structure of c(GG) and 23 ACS Paragon Plus Environment


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this behavior is exclusively reversed of the behavior observed in case of glycines, in which [Bmim][Br] behaved as a strong stabilizer.

Further, we have obtained the transfer free energy

( ∆gtr' ) contributions of the peptide backbone unit from the ∆Gtr' values of GPs in the ILs solutions. We have obtained both positive and negative ∆gtr' contribution of peptide backbone unit from water to aqueous imidazolium-based IL solutions. Eventually, our results concluded that the values of ∆gtr' for glycine residue are very sensitive to the nature of neighboring groups and dependent on the ionic strength of the solutions. Our results demonstrate that the structural variability of anion in imidazolium-based ILs is a great asset for affecting solubilities and free energies changes of glycine and glycine containing peptides. Further, the ions of ILs produce different effects on the solubility and stability of model protein compounds by controlling the water structure in the surroundings of the surface of model compounds. Overall, the present study elucidates that the anions of the ILs do not follow the traditional Hofmeister series on solubility, structure and stability of glycine peptides. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]; Tel: +91-11-27666646-142; Fax: +91-11-2766 6605. Notes: The authors declare no competing ﬁnancial interest. ACKNOWLEDGEMENTS We are gratefully acknowledged to the Council of Scientific Industrial Research (CSIR), New Delhi, through the Grant No. 01 (2713) 13/EMR-II for financial support.

Supporting Information. Table 1S. Concentration of glycine peptides in water and imidazolium aqueous ILs (expressed in mass/mass) and experimental densities (ρ) of the solutions at 298.15 K. This information is available free of charge via the Internet at http://pubs.acs.org.


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6. REFERENCES (1)

Maksic, Z. B.; Kovacevic, B. Towards the absolute proton affinities of 20 α-amino acids, Chem. Phys. Lett. 1999, 307, 497-504.

(2)

Harrison, A. G. The gas-phase basicities and proton affinities of amino acids and peptides, Mass Spectom. Rev. 1997, 16, 201-217.

(3)

Koolman, J.; Roehm, K. H. Colour Atlas of Biochemistry, Thieme: Stuttgart, 1996.

(4)

Timberlake, K. C. Chemistry-5th Edition, Haper-Collins Publishers Inc: New York, 1992.

(5)

Dickrson, R. E.; Geis, I. The Structure and Action of Proteins, Harper & Row Publishers, New York, 1969.

(6)

Ashok, P.; Satyajit, P.; Samanta, A. Structural transformation of bovine serum albumin induced by dimethyl sulfoxide and probed by fluorescence correlation spectroscopy and additional methods, Chem. Phys. Chem. 2013, 14, 2441-2449.

(7)

Damodaran, S.; Kyung, B. S. The role of solvent polarity in the free energy of transfer of amino acid side chains from water to organic solvents, J. Biol. Chem. 1986, 261, 7220-7222.

(8)

Zancan, P.; Sola-Penna, M. Trehalose and glycerol stabilize and renature yeast inorganic pyrophosphatase inactivated by very high temperatures, Archi. Biochem. and Biophys. 2005, 444, 52-60.

(9)

England, J. L.; Haran, G.; Role of solvation effects in protein denaturation: from thermodynamics to single molecules and back, Annu. Rev. Phys. Chem. 2011, 62, 257-277.

(10)

Keswani, N.; Kishore, N. Interaction of some hydrophobic amino acids, peptides, and protein with aqueous 3-chloro-1,2-propanediol and 3-chloro-1-propanol: Biophysical studies, J. Chem. Thermodyn. 2011, 43, 591-602.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(11)

Page 26 of 36

Zafarani, M. M. T.; Behnaz, A. A. S.; Mahdi G. N. Study of thermodynamic properties of l-serine in aqueous 1-carboxymethyl-3-methylimidazolium chloride solutions at 298.15 K, Fluid Phase Equilibria 2014, 363, 32-40.

(12)

Qiu, X. M.; Lei, Q. F.; Fang, W. J.; Lin, R. S. Transfer enthalpies of amino acids and glycine peptides from water to aqueous solutions of sugar alcohol at 298.15 K, J. Chem. Eng. Data 2009, 54, 1426-1429.

(13)

Ziemer, S. P.; Woolley, E. M. Thermodynamics of the first and second proton dissociations from aqueous L-aspartic acid and L-glutamic acid at temperatures from (278.15 to 393.15) K and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volumes of zwitterionic, protonated cationic, and deprotonated anionic forms at molalities from (0.002 to 1.0) mol. kg-1, J. Chem. Thermodyn. 2007, 39, 645-666.

(14)

Shaokun, Tang.; Gary, A. B.; Hua, Z. Ether-and alcohol-functionalized task-specific ionic liquids: attractive properties and applications, Chem. Soc. Rev. 2012, 41, 4030-4066.

(15)

Alesia, A. T.; Pascal, H.; Annegret, S.; Diana, I. Ionic liquid applications in peptide chemistry: synthesis, purification and analytical characterization processes, Molecules 2012, 17, 4158-4185.

(16)

Welton, T. Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 1999, 99, 2071-2083.

(17)

Li, W.; Wu, P. Insights into the denaturation of bovine serum albumin with a thermo-responsive ionic liquid, Soft Matter 2014, 10, 6161-6171.

(18)

Li, W.; Wu, P. On the thermodynamic phase behavior of poly(N- vinylcaprolactam) solution in the presence of different ionic liquids, Polym. Chem. 2014, 5, 761-770.

(19)

Greaves, T. L.; Drummond, C. J. Protic ionic liquids: Properties and applications, Chem. Rev. 2008, 108, 206-237.


Page 27 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20)

Plechkova, N. V.; Seddon, K. R. Application of ionic liquids in the chemical industry, Chem. Soc. Rev. 2008, 37, 123-150.

(21)

Egorova, K.S.; Ananikov, V.P. Toxicity of ionic liquids: Eco(cyto) activity as complicated, but unavoidable parameter for task-specific optimization. ChemsusChem. 2007, 46, 8887-8889.

(22)

Rebelo, L. P. N.; Visak, N.; Visak, Z. P.; Nunes da Ponte, M.; Szydlowski, J.; Cerderina, C. A.; Troncoso, J.; Romani, L.; Esperanca, J. M. S. S.; Guedes, H. J. R.; de Sousa, H. C. A detailed thermodynamic analysis of [C4mim][BF4] + water as a case study to model ionic liquid aqueous solutions, Green Chem. 2004, 6, 369-381.

(23)

Vasantha, T.; Attri, P.; Venkatesu, P.; Rama Devi, R. S. Thermodynamic contributions of peptide backbone unit from water to biocompatible ionic liquids at T = 298.15 K, J. Chem. Thermodyn. 2012, 45, 122-136.

(24)

Vasantha, T.; Kumar, A.; Attri, P.; Venkatesu, P.; Rama Devi, R. S. Influence of biocompatible ammonium ionic liquids on the solubility of l-alanine and l-valine in water, Fluid Phase Equilibria 2012, 335, 39-45.

(25)

Vasantha, T.; Attri, P.; Venkatesu, P.; Rama Devi, R. S. Ammonium based ionic liquids act as compatible solvents for glycine peptides, J. Chem. Thermodyn. 2013, 56, 21-31.

(26)

Vasantha, T.; Kumar, A.; Attri, P.; Venkatesu, P.; Rama Devi, R. S. The solubility and stability of amino acids in biocompatible ionic liquids, Protein & Peptide Letters 2014, 21, 15-24.

(27)

Vasantha, T.; Attri, P.; Venkatesu, P.; Rama Devi, R. S. Structural basis for the enhanced stability of protein model compounds and peptide backbone unit in ammonium ionic liquids, J. Phys. Chem. B 2012, 116, 11968-11978.

(28)

Attri, P.; Venkatesu, P. Thermodynamic characterization of the biocompatible ionic liquid effects on protein model compounds and their functional groups. Phys. Chem. Chem. Phys. 2011, 13, 6566-6575.



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Page 28 of 36

Kumar, A.; Venkatesu, P.; Mohamed, T.; Lee, M. J. Thermodynamic contribution of amino acids in ionic liquids towards protein stability, Curr. Biochem. Eng. 2014, 1, 125-140.

(30)

Sushma P. I.; Naved I. M. Experimental and theoretical excess molar properties of imidazolium based ionic liquids with molecular organic solvents-I. 1-Hexyl-3methylimidazlouim tetrafluoroborate and 1-octyl-3-methylimidazlouim tetrafluoroborate with cyclic ethers, J. Chem. Thermodyn. 2014, 71, 236-248.

(31)

Jean-Philippe, B.; Austen, A. C. Protic ionic liquids: preparation, characterization, and proton free energy level representation, J. Phys. Chem. B 2007, 111, 4926-4937.

(32)

Huacong, Z.; Liangrong, Y.; Wei, L.; Qinghui, S.; Peng, X.; Wensong, L.; Fuchun, W.; Pinhua, Y.; Huizhou, L. Improving the stability of immobilized penicillin G acylase via the modification of supports with ionic liquids, Ind. Eng. Chem. Res. 2012, 51, 4582-4590.

(33)

Tomé, L. I. N.; Domínguez-Pèrez, M.; Claudio, A. F. M.; Freire, M. G.; Marrucho, I. M.; Cabeza, O.; Coutinho, J. A. P. On the interactions between amino acids and ionic liquids in aqueous media, J. Phys. Chem. B 2009, 113, 13971-13979.

(34)

Tome, L. I. N.; Jorge, M.; Gomes, J. R. B.; Coutinho, J. A. P. Molecular dynamics simulation studies of the interactions between ionic liquids and amino acids in aqueous solution, J. Phys. Chem. B 2012, 116, 1831-1842.

(35)

Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Negligible effect of ions on the hydrogen-bond structure in liquid water, Science 2003, 301, 347-349.

(36)

Zhang, Y.; Cremer, P. S. Chemistry of Hofmeister anions and osmolytes, Ann. Rev. Phys. Chem. 2010, 61, 63-83.

(37)

Nadine, S.; Dominik H.; Roland R. N. Reversed anionic Hofmeister series: The interplay of surface charge and surface polarity, Langmuir 2010, 26, 7370-7379.

(38)

Pierandrea, L. N.; Barry, W. N. Hofmeister phenomena: An update on ion specificity in biology, Chem. Rev. 2012, 112, 2286-2322. 28 ACS Paragon Plus Environment

Page 29 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39)

Awanish, K.; Venkatesu, P. Does the stability of proteins in ionic liquids obey the Hofmeister series?, Inter. J. Biol. Macromol. 2014, 63, 244-253.

(40)

Venkatesu, P.; Lee, M.J.; Lin, H.M. Thermodynamic characterization of the osmolyte effect on protein stability and the effect of GdnHCl on the protein denatured state, J. Phys. Chem. B 2007, 111, 9045-9056.

(41)

Venkatesu, P.; Lee, M.J.; Lin, H.M. Trimethylamine N-oxide counteracts the denaturing effects of urea or GdnHCl on protein denatured state, Arch. Biochem. Biophys. 2007, 466, 106-115.

(42)

Nozaki, Y.; Tanford, C. The solubility of amino acids, diglycine, and triglycine in aqueous guanidine hydrochloride solutions, J. Biol. Chem. 1970, 245, 1648-1652.

(43)

Nozaki, Y.; Tanford, C. The solubility of amino acids and related compounds in aqueous ethylene glycol solutions, J. Biol. Chem. 1965, 240, 3568-3573.

(44)

Nozaki, Y.; Tanford, C. The solubility of amino acids and two glycine peptides in aqueous ethanol and dioxane solutions, J. Biol. Chem. 1971, 246, 2211-2217.

(45)

Talukdar, H.; Rudra, S.; Kundu, K. K. Thermodynamics of transfer of glycine, diglycine, and triglycine from water to aqueous solutions of urea, glycerol, and sodium nitrate, Can. J. Chem. 1988, 66, 461-468.

(46)

Auton, M.; Bolen, D. W. Additive transfer free energies of the peptide backbone unit that are independent of the model compound and the choice of concentration scale, Biochemistry 2004, 43, 1329-1342.

(47)

Sijpkes, A. H.; van de Kleut, G. J.; Gill, S. C. The solubilities of five cyclic dipeptides in water and in aqueous urea at 298.15 K: a quantitative model for the denaturation of proteins in aqueous urea solutions, Biophys. Chem. 1994, 52, 75-82.

(48)

Gill, S. J.; Hutson, J.; Clopton, J. R.; Downing, M. Solubility of diketopiperazine in aqueous solutions of urea, J. Phys. Chem. 1961, 65, 1432-1435.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(49)

Page 30 of 36

Liu, Y.; Bolen, D. W. The peptide backbone plays a dominant role in protein stabilization by naturally occurring osmolytes, Biochemistry 1995, 34, 12884-12891.

(50)

Tietze, A. A.; Bordusa, F.; Giernoth, R.; Imhof, D.; Lenzer, T.; Maab, A.; Mrestani-Klaus, C.; Neundorf, I.; Oum, K.; Reith, D.; Stark, A. On the nature of interactions

between ionic liquids and small amino-acid-based biomolecules,

Chem. Phys. Phys. Chem. 2013, 14, 4044-4064.

(51)

Christian, L.; Ganesh, P.; Rainer, R. Ionic liquids as refolding additives: N’-alkyl and N’-(ω- hydroxyalkyl) N-methylimidazolium chlorides, Protein Science 2005, 14, 26932701.

(52)

Catherine, A.; Summers.; Robert, A.; flowers, I. I. Protein renaturation by the liquid organic salt ethylammonium nitrate, Protein Science 2000, 9, 2001-2008.

(53)

Mann, J. P.; McCluskey, A.; Atkin, R. Activity and thermal stability of lysozyme in alkylammonium formate ionic liquids-influence of cation modification, Green Chem. 2009, 11, 785-792.

(54)

Grover, P. K.; Ryall, R. L. Critical appraisal of salting-out and its implications for chemical and biological sciences, Chem. Rev. 2005, 105, 1-10.

(55)

Tomar, D. S.; Asthagiri, D.; Weber, V. Solvation free energy of the peptide group: Its model dependence and implications for the additive-transfer free-energy model of protein stability, Biophys. J. 2013, 105, 1482-1490.

(56)

Yibing, Y.; Bruce, W. E.; Alexander, T. Free energies for folding and refolding of four types of β Turns: Simulation of the Role of D/L Chirality, J. Am. Chem. Soc. 1995, 117, 7592-7599.

(57)

Thomas, F. M.; III.; David, C. C. Quantum free energies of the conformers of glycine on an ab initio potential energy surface, Phys. Chem. Chem. Phys. 2004, 6 , 2563-2571.

(58)

Chaudhuri, P.; Canuto, S. Rayleigh scattering properties of small polyglycine molecules, J. Mol. Structure: THEOCHEM, 2006, 760, 15-20.


Page 31 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(59)

Zhao, H.; Campbell, S. M.; Jacksan, L.; Song, Z.; Olubaja, O. Hofmeister series of ionic liquids: kosmotropic effect of ionic liquids on the enzymatic hydrolysis of enantiomeric phenylalanine methyl ester, Tetrahedron: Asymmetry 2006, 17, 377-383.

(60)

Spolar, R. S.; Livingstone, J. R.; Record, M. T., Jr. Use of liquid hydrocarbon and amide transfer data to estimate contributions to thermodynamic functions of protein folding from the removal of non polar and polar surface from water, Biochemistry 1992, 31, 3947-3955.

(61)

Venkatesu, P.; Lee, M. J.; Lin, H. M. Transfer free energies of peptide backbone unit from water to aqueous electrolyte solutions at 298.15 K, Biochem. Eng. J. 2006, 32, 157-170.



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Page 32 of 36

Table 1. Glycine peptide solubilities (SGP) in an aqueous or aqueous imidazolium-based ILs solutions and densities (ρGP) at solubility limits at 25 0C. System 0M Gly Water [Bmim][Cl] [Bmim][Br] [Bmim][HSO4] [Bmim][CH3COO] [Bmim][SCN] Gly2 Water [Bmim][Cl] [Bmim][Br] [Bmim][HSO4] [Bmim][CH3COO] [Bmim][SCN] Gly3 Water [Bmim][Cl] [Bmim][Br] [Bmim][HSO4] [Bmim][CH3COO] [Bmim][SCN] Gly4 Water [Bmim][Cl] [Bmim][Br] [Bmim][HSO4] [Bmim][CH3COO] [Bmim][SCN] c(GG) Water [Bmim][Cl] [Bmim][Br] [Bmim][HSO4] [Bmim][CH3COO] [Bmim][SCN]

ρGP (g.cm-3)

SGP (g/100g solvent) 0.5 M

1.0 M

1.5 M

2.0 M

2.5 M

17.95 15.55 20.05 16.46 18.95

12.80 9.98 14.97 11.02 13.87

9.23 6.44 10.93 7.61 10.12

6.51 3.93 8.19 5.10 7.40

4.44 1.83 6.18 2.91 5.28

16.78 14.85 18.34 15.91 17.60

12.03 9.23 14.34 10.92 13.22

8.24 5.29 10.71 7.04 9.36

5.49 2.78 8.08 4.38 6.89

3.74 1.55 5.98 2.78 4.70

6.988 5.938 8.066 6.338 7.413

6.515 5.019 8.077 5.743 7.290

5.128 3.621 7.057 4.325 6.328

3.457 2.073 5.489 2.657 4.652

1.871 0.527 3.724 1.169 2.704

0.415 0.374 0.427 0.398 0.421

0.409 0.345 0.426 0.386 0.419

0.376 0.300 0.401 0.348 0.389

0.323 0.248 0.356 0.296 0.342

0.261 0.186 0.300 0.228 0.283

1.618 1.777 1.709 1.539 1.670

1.538 1.880 1.705 1.350 1.625

1.406 2.030 1.684 1.130 1.553

1.271 2.191 1.634 0.905 1.459

1.084 2.357 1.558 0.657 1.326

0M

0.5 M

1.0 M

1.5 M

2.0 M

2.5 M

1.06964 1.08464 1.09144 1.06856 1.07085

1.05978 1.09069 1.09330 1.06181 1.06231

1.05477 1.10039 1.09524 1.05491 1.05936

1.05577 1.10281 1.10816 1.05387 1.05346

1.05024 1.11184 1.11346 1.04714 1.05022

1.07177 1.08506 1.08974 1.06677 1.07127

1.06199 1.09229 1.09388 1.05708 1.07067

1.05622 1.10212 1.09461 1.05181 1.06026

1.05396 1.10496 1.10393 1.04992 1.05284

1.05072 1.11362 1.10588 1.04949 1.04813

1.03163 1.04511 1.07044 1.02331 1.02816

1.03614 1.04746 1.07385 1.02674 1.03162

1.03847 1.05163 1.07583 1.02847 1.03314

1.04172 1.05282 1.07524 1.03011 1.03522

1.04270 1.05390 1.07469 1.03182 1.03364

1.00479 1.02392 1.03236 1.01374 1.00457

1.01063 1.04111 1.03411 1.01709 1.00890

1.01650 1.05823 1.03864 1.02006 1.01256

10.2064 1.06773 1.03785 1.02358 1.01611

1.02571 1.06872 1.03621 1.02932 1.02186

1.00895 1.02872 1.03033 1.00902 1.00932

1.01405 1.04246 1.05245 1.01503 1.01421

1.01892 1.06291 1.06977 1.02093 1.01844

1.02406 1.07579 1.08697 1.02814 1.02147

1.02739 1.08831 1.09986 1.03245 1.02426

1.08299

25.09

1.07731

22.75

1.02205

6.41

0.99859

0.395

1.00250

1.68


Page 33 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Salting constant (k) values of GPs from water to aqueous imidazolium-based ILs olutions at 25 0C. GPs

ILs 0.5 M

1.0 M

k-values 1.5 M

[Bmim][Cl] [Bmim][Br] [Bmim][HSO4] [Bmim]CH3COO] [Bmim][SCN]

0.6697 0.9568 0.4485 0.8431 0.5613

0.6730 0.9219 0.5164 0.8227 0.5927

0.6667 0.9066 0.5540 0.7953 0.6053

0.6745 0.9269 0.5597 0.7966 0.6105

0.6927 1.0473 0.5572 0.8617 0.6234

[Bmim][Cl] [Bmim][Br] [Bmim][HSO4] [Bmim][CH3COO] [Bmim][SCN]

0.6088 0.8531 0.4310 0.7099 0.5133

0.6372 0.9021 0.4615 0.7340 0.5428

0.6770 0.9725 0.5023 0.7820 0.5921

0.7108 1.0511 0.5176 0.8238 0.5972

0.7221 1.0745 0.5345 0.8408 0.6308


-0.1727 0.1530 -0.4596 0.0226 -0.2907

-0.0163 0.2446 -0.2312 0.1099 -0.1286

0.1488 0.3807 -0.0641 0.2623 0.0086

0.3087 0.5644 0.0775 0.4403 0.1603

0.4925 0.9994 0.2172 0.6807 0.3452


-0.0988 0.1092 -0.1558 0.0131 -0.1275

-0.0348 0.1353 -0.0755 0.0230 -0.0590

0.0329 0.1615 -0.0101 0.0844 0.0102

0.1006 0.2327 0.0520 0.1443 0.0720

0.1657 0.3012 0.1100 0.2198 0.1334


0.0752 -0.1123 -0.0342 0.1753 0.0119

0.0883 -0.1125 -0.0145 0.2187 0.0333

0.1187 -0.1262 -0.0016 0.2644 0.0524

0.1395 -0.1328 0.0139 0.3093 0.0705

0.1752 -0.1354 0.0302 0.3755 0.0946

2.0 M

2.5 M

Gly

Gly2

Gly3

Gly4

c(GG)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

Table 3. Apparent transfer free energies ( ∆Gtr' ) of glycine peptides from water to imidazoliumbased ILs solution at 25 0C. GPs

ILs

∆Gtr' /(J .mol−1 ) 1.5 M 2.0 M

0.5 M

1.0 M

2.5 M


715.20 985.52 434.66 901.01 598.93

1465.73 1948.62 1047.56 1792.74 1284.21

2208.34 2931.48 1734.22 2649.40 1989.49

3008.93 4091.21 2358.56 3585.48 2717.33

3923.02 5915.11 2998.31 4940.06 3512.33


643.72 874.75 415.02 768.75 543.95

1388.46 1912.76 930.28 1615.25 1160.64

2254.69 3179.37 1572.20 2627.60 1954.82

3202.84 4708.05 2190.12 3746.09 2675.08

4120.53 6107.05 2883.19 4835.73 3583.11


-223.72 123.27 -646.05 23.27 -351.89

-71.77 512.92 -657.06 245.44 -321.59

483.60 1279.18 -350.47 910.76 3.25

1413.62 2621.71 237.20 2074.63 721.62

2894.89 5976.39 1158.34 4069.67 2023.82

[Bmim][Cl] [Bmim][Br] [Bmim][HSO4] [Bmim[CH3COO] [Bmim][SCN]

-137.29 72.81 -274.76 -53.01 -172.19

-115.71 230.90 -273.17 11.41 -171.09

77.69 535.82 -134.70 260.15 3.37

442.93 984.26 161.14 651.52 312.73

957.45 1693.58 587.95 1283.02 766.71


75.80 -200.79 -109.60 197.79 -2.25

187.06 -370.85 -156.54 503.27 52.38

394.40 -605.66 -166.79 924.49 152.65

628.86 -820.80 -132.84 1451.94 297.77

1010.75 -1026.51 -45.85 2229.36 524.71

Gly

Gly2

Gly3

Gly4

c(GG)


Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Apparent transfer free energy ( ∆gtr' ) contributions of GPs from water to imidazoliumbased ILs solutions at 25 0C. GPs ILs

∆gtr' / J .mol−1 0.5 M

1.0 M

1.5 M

2.0 M

2.5 M

Gly2-Gly1


-71.48 -110.76 -19.63 -132.25 -54.98

-77.26 -35.85 -117.27 -177.49 -123.56

46.36 247.89 -162.02 -21.79 -34.68

193.91 616.85 -168.44 -63.61 -42.24

197.52 991.94 -115.11 -104.32 70.77

Gly3-Gly2


-867.44 -751.48 -1061.08 -745.49 -895.85

-1460.24 -1399.84 -1587.35 -1369.80 -1482.24

-1771.09 -1900.19 -1922.68 -1716.84 -1951.57

-1789.22 -2086.35 -1952.92 -1671.45 -1953.47

-1225.64 -130.66 -1724.85 -766.07 -1559.28

Gly4-Gly3


88.15 -49.68 371.87 -76.93 181.47

-41.74 -283.72 379.26 -233.59 152.86

-409.38 -741.27 213.31 -651.31 -0.53

-972.73 -1633.34 -77.45 -1422.06 -408.17

-1938.65 -4276.19 -569.58 -2787.71 -1258.86

Gly3-Gly1/2

[Bmim][Cl] [Bmim] [Br] [Bmim][HSO4] [Bmim][CH3COO] [Bmim][SCN]

-469.46 -431.12 -540.37 -438.87 -475.41

-768.75 -717.85 -852.31 -773.65 -802.90

-862.36 -826.15 -1042.35 -869.32 -993.12

-797.65 -734.75 -1060.68 -755.42 -997.85

-514.06 30.64 -919.98 -435.19 -744.25

Gly4-Gly2/2


-389.64 -400.58 -344.61 -411.21 -357.19

-750.99 -841.78 -604.05 -801.70 -664.69

-1090.24 -1320.73 -854.68 -1184.08 -976.05

-1380.98 -1859.84 -1015.18 -1546.76 -1180.82

-1582.15 -2203.43 -1147.21 -1776.89 -1409.07

Gly4-Gly1/3


-283.59 -303.97 -236.28 -318.22 -256.45

-526.42 -573.14 -441.79 -593.63 -484.31

-711.37 -797.85 -623.79 -796.65 -662.26

-856.01 -1034.28 -732.93 -977.63 -801.24

-988.93 -1404.97 -803.18 -1219.37 -915.79

c(GG)/2


37.90 -100.39 -54.80 98.89 -1.12

93.53 -185.42 -78.27 251.63 26.19

197.20 -302.83 -83.40 462.24 76.33

314.43 -410.40 -66.42 725.97 148.89

505.37 -513.25 -22.92 1114.68 262.35



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Page 36 of 36

Graphical Abstract A Comprehensive Experimental Study to Understand the Hofmeister Series of Anions of Aqueous Imidazolium-based ILs on Glycine Peptides Varadhi Govinda and Punnuru Venkatesu∗ Department of Chemistry, University of Delhi, Delhi 110 007, India

ACS Paragon Plus Environment

A Comprehensive Experimental Study to Understand the Hofmeister

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