Electrolytic Effect on the Solubility and Solvation

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Electrolytic Effect on the Solubility and Solvation Thermodynamics of L‑Serine and L‑Isoleucine in Aqueous Media Saroj Chowdhury,† Prasenjit Mandal,† Aslam Hossain,‡ Partha Sarathi Guin,§ Sanjay Roy,*,§ and Kalachand Mahali*,† †

Department of Chemistry, University of Kalyani, Kalyani, 741235, Nadia, India Department of Physical and Inorganic Chemistry, Institute of Natural Sciences and Mathematics, Ural Federal University, Yekaterinburg, Russia § Shibpur Dinobundhoo Institution (College), 412/1, G.T. Road (South), Howrah, Pin-711102, India Downloaded via NOTTINGHAM TRENT UNIV on September 6, 2019 at 07:13:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: By the analytical gravimetric technique, the equilibrium saturated solubilities of L-serine and L-isoleucine in aqueous solutions of sodium nitrate (NaNO3) and potassium nitrate (KNO3) were measured at six different temperatures in the range of 288.15 to 313.15 K. The theoretical studies on the standard transfer Gibbs free energies were performed with the help of experimental solubility data. The molar volume, transfer enthalpy of cavity, transfer Gibbs free energy, and dipole−dipole interactions were also measured to explore complete thermodynamical aspects of these molecules in aqueous electrolytes. In addition, the solute−solute, solute−solvent, and solvent−solvent interactions in terms of transfer Gibbs energies due to chemical interactions and transfer entropy for the present amino acids (AA’S) were calculated by deducting the cavity interaction and dipole−dipole interaction from the total transfer Gibbs free energy (ΔG0t (i)) and transfer entropies (TΔS0t (i)). Finally, the stability owing to the chemical nature of the amino acids in aqueous electrolytic solutions is explained by introducing the abovementioned thermodynamical parameters.

1. INTRODUCTION The fundamental unit of protein is the amino acid, which is the most important species in directing several biological reactions in human physiology. Amino acids have been important not only in the biological ground but also in various fields of industry like chemical, food, pharmaceutical, foundation, and eco-friendly plastics for a long time.1−5 Information about the solubility of protein building units, that is, amino acids in various cosolvent mixtures in the presence and absence of electrolytes, is noteworthy for researchers in the pharmaceutical field as this is involved in drug purification, preformulation analysis, and design of pharmaceutical dosage.6,7 Different thermodynamic parameters like free energy are related to the solubilities of these molecules. Temperature plays a crucial role in characterizing the interaction between ions and small molecules with biomolecules in the experimental solution.7−10 A lot of studies on amino acids have established that their solubilities and solvation thermodynamics determine their biochemical role in animal metabolism.11−13 In many cases, it has been seen that thermodynamic parameters were very much helpful in characterizing the conformational changes of biomacromolecules such as proteins.14,15 With the addition of electrolytes in the solution of these molecules, the chemical reactivity and zwitterionic characters have been seen to direct other biophysical roles such as protein folding and unfolding.16−21 Otherwise, amino acids © XXXX American Chemical Society

are usually used as model compounds for more complex molecules like proteins. However, a more attentive study of the electrolytic effect on amino acid solution is still enviable to expose the molecular type of interaction between electrolyte ions and protein functional groups.20,21 That is why in this study, we started a project regarding the solubility and solvation thermodynamics of some amino acids in aqueous electrolyte solutions. In this regard, we studied solubility and solvation chemistry in aqueous sodium/potassium sulfate mixtures on the same amino acids.22 According to theory, the interactions between the electrolytic ions or small molecules with biological macromolecules have a considerable meaning in deciding the nature of macromolecules. It is true that the information on the thermodynamic solvation chemistry of biomolecules in aqueous electrolyte solutions assists to realize the conformational transforms of molecules in solution formed in the presence of denaturants or by the convey of charged solutes molecules throughout the membranes. In nature, the free state of amino acids is rarely observed. Amino acids are usually acquired through the hydrolysis of protein-containing materials. Most of the methods by which amino acids are usually obtained Received: April 25, 2019 Accepted: August 22, 2019

A

DOI: 10.1021/acs.jced.9b00363 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Specifications of Chemical Samples

estimate the solubilities of the experimental amino acids in electrolyte solutions of Na/K nitrate salts. Saturated solutions of the present amino acids in water−electrolyte mixtures were allowed to settle down the undissolved amino acids for 7 h before the sampling. After that, 5 mL of mixture was taken out from the supernatant liquid by using a dried pipette and filtered with the help of a 0.22 μm HPLC disposable filter. The filtrate was then transferred into a vessel and then weighed rapidly. The solution was evaporated to complete dryness so that amino acid crystals appeared and finally dried entirely in a drying stove at a temperature of 420.15 K. The dried mass was then cooled in a silica gel containing dehydrator for 1 day and finally weighed. The method was repeated until a constant weight appeared. Since the gravimetric method is a precise analytical method of measurement for amino acid like molecules, hence various researchers have measured the solubilities of amino acids in binary/ternary solvent systems. The solubilities of amino acids also show the accurate results even in higher concentration of electrolyte solution. In the current work, to make sure the chance of adsorption or assimilation of the electrolyte and degradation of the electrolyte on the solid phase of the amino acids, atomic absorption spectroscopy was applied to examine the ions in the mixture like earlier studies.15,16 We compared the concentrations of cations in the aqueous electrolyte and in the amino acid−water− electrolyte systems to ensure the fact that the electrolytes were not immersed or included on the solid phase of amino acids surface; that is, the precipitate was produced only by the amino acid.37 Electrolyte mixtures having various concentrations of amino acids were prepared, and electrolytic cation concentrations were investigated for comparison in each solution. The maximum variation in the experimental outcomes was found to be ±0.005 mol kg−1. This indicates that, regardless of the existence of dissimilar amounts of amino acid in the mixture, no countable amount of electrolyte was precipitated or adsorbed on the solid-phase amino acid.37−39 This means that the solid material recovered only the amino acid.37 To check the reliability of the current experimental method to measure the saturated solubility of the amino acid, a comparative study was done in pure water at different temperatures (Table 2).

necessitate different salts at specific pH or pOH in experimental solution. So, the investigation on the effect of salts on the solubilities of amino acids in different concentrations is very much imperative. Though there are a lot of studies that were done by several groups on amino acid solubility in water,23−25 aqueous-organic solution,5,26−28 nonaqueous solution,29−31 and aqueous-electrolyte mixtures,15,22,32,33 an attempt on the solubility-related studies of L-isomers of amino acids in aqueous and aqueous NaNO3 and KNO3 at various concentrations and temperatures is still deficient. Since amino acids mainly occur in nature (except glycine) in two isomeric forms, namely, L- and Dforms, only the L-amino acids integrate into proteins but insincere in cells. Some of the D-amino acids are not found in bacterial proteins, but these are vital in the bacterial cell walls. So, L-isomers of amino acids are very important as solutes to get the complete idea about the solvation chemistry of amino acids as well as protein builders’ molecules. In the current study, we have chosen L-serine and L-isoleucine as solutes and tried to give an effort to make a complete study in this area for the enrichment of amino acid research.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Their Purifications. L-Serine and Lisoleucine (99.8%) were procured from Sigma Aldrich. These were used after drying in a dehydrator with silica gel. Sodium nitrate and potassium nitrate (98%) were purchased from E. Merck, India, and were dried up in a temperature-restricted oven for 1 week at 400.15 K and then cooled and store up in a vacuum desiccator for 5 days before their use. Triple distilled water with conductivity less than 0.9 mS/cm was employed in preparing all solutions in the entire study. The specifications of the chemical samples are mentioned in Table 1. 2.2. Preparations of Saturated Solutions. The aqueous electrolyte mixtures with concentrations of 0.0, 0.5, 1.0, 1.5, 2.0, and 2.5 in mol·kg−1 NaNO3 and KNO3 were prepared and mixed with a small excess of each of the experimental amino acids in fine stoppered glass tubes. The tubes were kept incompletely filled so that a promising mixing of the amino acids could be reached. A temperature-controlled thermostat (accuracy: ±0.10 K) was employed in all experimental measurements. Four sets of saturated solutions containing each of the amino acids and aqueous electrolytes were prepared at desired six temperatures, and the mixtures were stirred continuously for 24 h to reach equilibrium. 2.3. Measurement of Solubility. The analytical gravimetric method34−37 was introduced in the present study to

3. RESULTS We have measured the solubilities of amino acids by considering the simple mathematical calculation. If we consider the concentration of the electrolyte (NaNO3 or KNO3) in water as c, then the weight of the electrolyte in 5 mL of solution is w1 = B



Article

(M × c × 5/1000) g, where M is the molecular mass of the electrolyte. Considering the weights of the blank dry glass vessel (w2 g) and the glass vessel containing dried solute amino acid (w3 g), the quantity of the actual solvated amino acid in 5 mL of mixture is w = (w3 − w2 − w1) g.40 Experimental equilibrium solubilities of the amino acids in mol kg−1 in pure water were measured at different compositions of the electrolytes, and the solubility results are presented in Table 3. The measurements were done at desired temperatures by keeping the solution above and below ±0.10 K of investigation temperature. The repeatedly measured solubilities of the amino acids were found to agree within 3.0%. From experimental solubility data, we estimated the relative solubility and salting-in or salting-out constants of the present amino acids in water−electrolyte solution in different concentrations of the electrolyte. The results are tabulated in Tables 4 and 5, respectively. The equilibrium solubilities (mol kg−1) in pure water and in aqueous NaNO3 and KNO3 solutions were applied for evaluation of the apparent standard transfer Gibbs free energies of solutions using eq 126,27,29,30 and are cited in Table S1.

Table 2. Comparison of the Present Experimental Solubility Data (Mole Fraction Scale) of L-Isoleucine and L-Serine with Earlier Results by Different Analytical Techniques in Aqueous Media at Various Temperaturesa mole fraction solubility of L-isoleucine and L-serine in pure water solute

T (K)

present study

L-isoleucine

273.15 288.15 293.15 298.15

not done 0.00530 0.00535 0.00553

303.15 308.15 313.15 323.15 348.15 373.15 298.15

0.00569 0.00584 0.00607 not done not done not done 0.06911

313.15 323.15 333.15

not done not done not done

L-serine

literature data 0.0051851 0.0053922 0.0055422 22 0.00569; 0.00563;51 0.00477;52 0.00466;53 0.00437;54 0.00459;55 0.0045256 0.0058022 0.0059122 not available 0.00658;51 0.00576;52 0.0056353 0.0082951 0.0112151 22 0.06909; 0.06742;52 0.06693;53 0.06844;55 0.067457 0.092157 0.1061;52 0.105855 0.120157

ΔGt0(s) = RT ln(SR /Ss)

(1)

SR and Ss are the saturated solubilities of amino acid in pure water and water−electrolyte solution, respectively. In this case, the activity coefficients of L-serine and Lisoleucine in these solvent mixtures are considered as unity

a

Standard uncertainties u are u(T) = 0.10 K; relative uncertainties ur are ur(p) = 0.02 and ur(S) = 0.04.

Table 3. Solubilities of L-Isoleucine and L-Serine in mol·kg−1 Water in the Presence of Different Concentrations of NaNO3 and KNO3 at Different Temperatures under Atmospheric Pressure P = 0.1 MPaa solubility (S) in mol·kg−1 salt molality (mol kg−1) 0.0

0.5

1.0

1.5

2.0

2.5

T (K)

0.0

0.89

1.77

2.63

3.48

4.31

288.15 293.15 298.15 303.15 308.15 313.15 288.15 293.15 298.15 303.15 308.15 313.15 288.15 293.15 298.15 303.15 308.15 313.15 288.15 293.15 298.15 303.15 308.15 313.15

0.2951 0.3021 0.3087 0.3176 0.3262 0.3390 0.2951 0.3021 0.3087 0.3176 0.3262 0.3390 3.8252 3.9733 4.1213 4.2524 4.3103 4.9986 3.8252 3.9733 4.1213 4.2524 4.3103 4.9986

0.2731 0.2808 0.2884 0.2956 0.3033 0.3150 0.2733 0.2822 0.2899 0.3005 0.3109 0.3184 3.9131 4.1456 4.6256 4.7612 4.9355 5.5094 3.8851 4.1016 4.5618 4.6798 4.8024 5.3138

0.2411 0.2486 0.2551 0.2632 0.2745 0.2830 0.2646 0.2698 0.2763 0.2846 0.2908 0.2968 4.1638 4.4903 5.0613 5.1587 5.5542 5.7154 4.0986 4.3231 4.9689 5.1038 5.2131 5.4660

0.2119 0.2242 0.2362 0.2521 0.2655 0.2715 0.2386 0.2463 0.2547 0.2640 0.2743 0.2823 4.4210 4.7119 5.2144 5.2244 5.7666 5.9408 4.2741 4.5770 5.0362 5.1110 5.4932 5.8216

0.1921 0.2005 0.2099 0.2200 0.2458 0.2520 0.2059 0.2121 0.2312 0.2374 0.2461 0.2524 4.4672 4.8188 5.4037 5.5682 5.9796 6.0778 4.3189 4.7111 5.1477 5.4882 5.4021 5.8469

0.1783 0.1844 0.1957 0.2014 0.2234 0.2308 0.1909 0.1987 0.2056 0.2122 0.2304 0.2386 4.5619 4.9325 5.5388 5.6116 6.0872 6.2778 4.4882 4.8065 5.2260 5.5507 5.7047 6.0464

mol % NaNO3/KNO3 solute/solvent system L-isoleucine

L-isoleucine

L-serine

L-serine

in NaNO3 + H2O system

in KNO3 + H2O system

in NaNO3 + H2O system

in KNO3 + H2O system

Standard uncertainties u are u(T) = 0.10 K and u(m) = 0.01 mol kg−1; relative uncertainties ur are ur(p) = 0.02 and ur(S) = 0.04.

a

C


a

D

0.9254 0.8170 0.7181 0.6509 0.6042

0.9231 0.8966 0.8085 0.6977 0.6469

1.0230 1.0885 1.1557 1.1678 1.1926

1.0159 1.0715 1.1173 1.1291 1.1733

0.5 1.0 1.5 2.0 2.5

0.5 1.0 1.5 2.0 2.5

0.5 1.0 1.5 2.0 2.5

0.5 1.0 1.5 2.0 2.5

u(T) = 0.10 K..

relative solubility (Ss/SR) 288.15 K

salt molality (mol kg−1)

0.9341 0.9013 0.8153 0.7021 0.6577

−0.0333 −0.0474 −0.0923 −0.1563 −0.1892

0.0067 0.0300 0.0482 0.0527 0.0694

1.0323 1.0880 1.1519 1.1857 1.2097

1.0434 1.1301 1.1859 1.2128 1.2414

0.9295 0.8444 0.7084 0.6948 0.6104

−0.0336 −0.0878 −0.1438 −0.1864 −0.2188

0.0099 0.0368 0.0628 0.0674 0.0765


log(Ss/SR) at 288.15 K


relative solubility relative solubility (Ss/SR) 298.15 K

L-isoleucine in NaNO3 + H2O system −0.0317 0.9342 −0.0295 −0.0734 0.8911 −0.0500 −0.1497 0.7651 −0.1162 −0.1581 0.6485 −0.1881 −0.2144 0.6339 −0.1979 L-isoleucine in KNO3 + H2O system −0.0296 0.9390 −0.0273 −0.0451 0.9038 −0.0439 −0.0887 0.8251 −0.0835 −0.1536 0.7489 −0.1255 −0.1819 0.6660 −0.1765 L-serine in NaNO3 + H2O system 0.0184 1.1224 0.0501 0.0531 1.2281 0.0892 0.0740 1.2652 0.1022 0.0838 1.3112 0.1176 0.0939 1.3439 0.1284 L-serine in KNO3 + H2O system 0.0138 1.1069 0.0441 0.0366 1.2057 0.0812 0.0614 1.2220 0.0871 0.0740 1.2490 0.0966 0.0827 1.2680 0.1031


1.1005 1.2002 1.2019 1.2906 1.3053

1.1196 1.2131 1.2286 1.3094 1.3196

0.9462 0.8992 0.8312 0.7475 0.6681

0.9307 0.8287 0.7938 0.6927 0.6341


0.0416 0.0792 0.0799 0.1108 0.1157

0.0491 0.0839 0.0894 0.1171 0.1204

−0.0240 −0.0461 −0.0803 −0.1264 −0.1751

−0.0312 −0.0816 −0.1003 −0.1594 −0.1978


1.1142 1.2094 1.2744 1.2858 1.3235

1.1450 1.2886 1.3379 1.3873 1.4122

0.9531 0.9129 0.8409 0.7544 0.7063

0.9298 0.8415 0.8139 0.7535 0.6848


0.0469 0.0826 0.1053 0.1092 0.1217

0.0588 0.1101 0.1264 0.1422 0.1499

−0.0209 −0.0395 −0.0752 −0.1224 −0.1510

−0.0316 −0.0749 −0.0894 −−0.1229 −0.1644


Table 4. Relative Solubility (Ss/SR) and log(Ss/SR) of L-Isoleucine and L-Serine in Water and Water + NaNO3 and Water + KNO3 in Different Compositions of NaNO3 and KNO3 at Different Temperatures under Atmospheric Pressure P = 0.1 MPaa

Journal of Chemical & Engineering Data Article



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contribution of the dipole-induced dipole term for avoiding difficulty like in the previous works:25,37

Table 5. Salting-In Constants of L-Isoleucine and L-Serine in Aqueous-NaNO3 and KNO3 Solutions at 298.15 Ka amino acids

Ksi at 298.15 K in NaNO3

Ksi at 298.15 K in KNO3

L-isoleucine

−0.09498 +0.037

−0.076 +0.02668

L-serine

0 0 0 ΔGt0(i) = ΔGt,cav (i) + ΔGt,d − d (i) + ΔGt,ch (i)

Here, the well-established scaled particle theory was used to estimate the values of ΔG0t,cav(i). In this case, we assumed the solute and solvent molecules as hard sphere models as stated by their individual diameters (Table S2). Equation 4 was used in estimating cavity interaction15,37,44 as follows:

a

u(T) = 0.10 K..

because, in these cases, mole fraction concentrations of the amino acids are very small. Standard transfer free energies ΔG0t (i) were calculated by eq 2.26,29,37 ΔGt0(i)

=

ΔGt0(s)

− RT ln(Ms /MR )

(3) 41−43

0 ΔGcav (i) = GC + RT ln(RT /VS)

(4)

where GC = RT[−ln(1 − Z) + {3X/(1 − Z)}σx + {3Y/(1 − Z)}σx2 + {9X2/2(1 − Z)2}σx2], Z = πNA/6Vs(zRσR3 + zsσs3), X = πNA/6Vs(zRσR2 + zsσs2), Y = πNA/6Vs(zRσR + zsσs), Vs = Ms/ds, NA is Avogadro’s number, zR is the mole fraction of water, zs is the mole fraction of electrolytes, σx is the hard sphere diameter of L-serine, σR is the hard sphere diameter of L-isoleucine, σs is the hard sphere diameter of water and aqueous electrolytes, VS is the molar volume, Ms is the molar mass, and ds means the molar density of the aqueous electrolyte mixtures. The values of these parameters are given in Table S2. Hence, ΔG0t, cav(i) signifies the difference as eq 5.15,37,44

(2)

where Ms and MR mean the molar mass of aqueous electrolyte (NaNO3 or KNO3) mixtures and reference solvent (water), respectively. The theoretically determined ΔG0t (i) values in the mole fraction scale are presented in Table 6. Actually, ΔG0t (i) is a complicated result of different transfer free energies terms; that is, it is the sum of transfer free energies due to cavity-forming interaction (ΔG0t,cav(i)) regarding the creation of cavities for the species in water and such aquo-ionic solvents. Dipole−dipole 0 interactions (ΔGt,d−d (i)) are experienced between dipolar zwitterionic amino acid and solvent molecules. Chemical transfer energetics (ΔG0t,ch(i)) comprises all other issues arising from various types of interactions like acid−base, dispersion, dipole−dipole, dipole-induced dipole, hydrophilic or hydrophobic hydration, and structural properties of the solute and solvent molecules, though in this case we have neglected the

0 ΔGt,cav (i) = sΔGt(cav) − R ΔGt(cav)

= (Gcs − GcR) + RT ln(VR /Vs)

(5)

On the other hand, the ΔGt,0 d − d(i) values were determined by 45

using the Keesom-orientation expression below:

(eq 6) as stated

Table 6. Coefficients a, b, and c, Gibbs Energies ΔG0t (i), and Entropies TΔS0t (i) of Transfer of L-Leucine and L-Serine on Mole Fraction Scale from H2O to H2O−NaNO3 and H2O−KNO3 Mixtures at 298.15 K salt molality (mol kg−1)

a (kJ·mol−1)

0.0 0.5 1.0 1.5 2.0 2.5

−20.57 03.91 −40.56 −12.88 −183.12 −146.84

0.0 0.5 1.0 1.5 2.0 2.5

−20.57 −21.16 −36.73 −24.51 53.49 −111.11

0.0 0.5 1.0 1.5 2.0 2.5

64.03 148.52 119.92 26.08 106.86 110.14

0.0 0.5 1.0 1.5 2.0 2.5

64.03 166.26 215.13 82.86 216.92 125.07

b (kJ·mol−1)

c (kJ·mol−1)

in NaNO3 + H2O system 0.5428 −0.08144 −0.0037 0.00017 1.0123 −0.15180 0.4619 −0.07139 4.2936 −0.64349 3.4659 −0.51946 L-isoleucine in KNO3 + H2O system 0.5428 −0.08144 0.5763 −0.08689 0.9162 −0.13732 0.6605 −0.09950 −1.0572 0.15623 2.6350 −0.39475 L-serine in NaNO3 + H2O system −1.3642 0.19968 −3.1802 0.46852 −2.5064 0.36698 −0.4203 0.05605 −2.2098 0.32251 −2.2899 0.33461 L-serine in KNO3 + H2O system −1.3642 0.19968 −3.5919 0.53037 −4.6651 0.68983 −1.7026 0.24772 −4.7046 0.69563 −2.6484 0.38819

ΔG0t (i) (kJ·mol−1)

TΔS0t (i) (kJ·mol−1)

L-isoleucine

E

0 0.0944 0.3102 0.4107 0.6735 0.8024

0 0.0536 0.6776 4.2829 4.3517 3.5232

0 0.0402 0.0492 0.1892 0.3902 0.5683

0 0.9947 0.4514 1.2553 2.8085 2.3051

0 −0.3422 −0.6160 −0.7473 −0.9312 −1.0451

0 4.6776 6.6255 5.6205 7.1394 6.9257

0 −0.3027 −0.6212 −0.7404 −0.9299 −1.0452

0 3.9374 5.5830 5.2472 5.9524 6.9002 DOI: 10.1021/acs.jced.9b00363 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Table 7. Gibbs Energies of Transfer ΔG0t (i), ΔG0t,cav(i), ΔG0t,d−d(i), and ΔG0t,ch(i), Enthalpy ΔH0t,cav(i), and Entropies of Transfer TΔS0t (i), TΔS0t,cav(i), TΔS0t,d−d(i), and TΔS0t,ch(i) of L-Isoleucine and L-Serine in H2O + NaNO3 and H2O + KNO3 Mixtures at 298.15 K in kJ·mol−1 a salt molality (mol kg−1).

ΔG0t (i)

ΔG0t,cav(i)

0.0 0.5 1.0 1.5 2.0 2.5

0 0.0944 0.3102 0.4107 0.6735 0.8024

0 −0.1921 −0.3683 −0.5309 −0.6808 −0.8197

0.0 0.5 1.0 1.5 2.0 2.5

0 0.0402 0.0492 0.1892 0.3902 0.5683

0 −0.2718 −0.5197 −0.7441 −0.9515 −1.1400

0.0 0.5 1.0 1.5 2.0 2.5

0 −0.3422 −0.6160 −0.7473 −0.9312 −1.0451

0 −0.2064 −0.3953 −0.5692 −0.7293 −0.8773

0.0 0.5 1.0 1.5 2.0 2.5

0 −0.3027 −0.6212 −0.7404 −0.9299 −1.0452

0 −0.2919 −0.5575 −0.7973 −1.0184 −1.2189

0 ΔGt,d−d (i)

ΔG0t,ch(i)

TΔS0t (i)

L-isoleucine in H2O + NaNO3 system 0 0 0 0.0457 0.2408 0.0536 0.1600 0.5185 0.6776 0.2988 0.6428 4.2829 0.4148 0.9395 4.3517 0.5678 1.0543 3.5232 L-isoleucine in H2O + KNO3 system 0 0 0 0.0599 0.2521 0.9947 0.2056 0.3633 0.4514 0.3743 0.5590 1.2553 0.5056 0.8361 2.8085 0.6800 1.0283 2.3051 L-serine in H2O + NaNO3 system 0 0 0 0.0350 −0.1708 4.6776 0.1154 −0.3361 6.6255 0.2156 −0.3937 5.6205 0.2993 −0.5012 7.1394 0.4098 −0.5776 6.9257 L-serine in H2O + KNO3 system 0 0 0 0.0433 −0.0541 3.9374 0.1487 −0.2124 5.5830 0.2708 −0.2139 5.2472 0.3658 −0.2773 5.9524 0.4921 −0.3184 6.9002

ΔH0t,cav(i)

TΔS0t,cav(i)

TΔS0t,d−d(i)

TΔS0t,ch(i)

0 −0.0107 −0.0198 −0.0278 −0.0348 −0.0408

0 0.1814 0.3485 0.5031 0.6460 0.7789

0 0.0492 0.1723 0.3217 0.4466 0.6113

0 −0.1770 0.1568 3.4581 3.2591 2.1330

0 −0.0154 −0.0284 −0.0393 −0.0488 −0.0568

0 0.2564 0.4913 0.7048 0.9027 1.0832

0 0.0645 0.2214 0.4030 0.5443 0.7321

0 0.6738 −0.2613 0.1475 1.3615 0.4898

0 −0.0120 −0.0222 −0.0312 −0.0390 −0.0458

0 0.1944 0.3731 0.5380 0.6903 0.8315

0 0.0355 0.1243 0.2321 0.3222 0.4412

0 4.4477 6.1281 4.8504 6.1269 5.6530

0 −0.0172 −0.0318 −0.0441 −0.0547 −0.0636

0 0.2747 0.5257 0.7532 0.9637 1.1553

0 0.0466 0.1601 0.2915 0.3939 0.5298

0 3.6128 4.8972 4.2025 4.5948 5.2151

a

Diameters of NaNO3 and KNO3 are 0.628 and 0.700 nm,18,35 respectively. Dipole moments of NaNO3 and KNO3 are 15.07 and 16.8D, respectively.18,35 0 d−d ( 0 ΔGt,d − d (i) = ( s ΔG 0 (i) −R ΔGd − d (i))

ΔSt0(i) = (bR − bs) + (c R − cs)(1 + ln T )

(6)

In solution, sΔdG0− d((i) is presented as 0 s ΔGd − d (i)

=

−(8Π/9)N 2μs 2 μx 2 σs − x −3(kT )−1Vx −1

+ R ln(Ms /MR )

The subscript s stands for aqueous NaNO3/ KNO3 mixtures and R refers for H2O. MR and Ms observed are the molar masses of the water and water−electrolyte mixed solvent, respectively. In Tables 5 and 7, the values of TΔS0t (i) are shown. The uncertainty in calculating the ΔS0t (i) values was as ±0.3 kJ·K−1 mol−1. On the other hand, ΔS0t,d−d(i) = (sΔS0d−d(i) − RΔS0d−d(i)) was determined by the Keesom-orientation expression.26,42 The expression of ΔS0d−d(i) is sΔS0d−d((i) = − {δsΔG0d−d((i)/δΤ}p, that is,

= A /TVs (7)

where A = −(8Π/9)N2μs2μx2σs−x−3(k)−1 and Vs = Ms/ds. In this case, N means Avogadro’s number, μs and μx are the dipole moments of cosolvent and amino acid molecules (Table S1), and σs − x is the diameter of attractive and repulsive interactions between the solvent and amino acid molecules, which is equivalent to 1 2 (σs + σx), where σsand σx are the hard sphere diameters of the hydrated electrolyte and single amino acid molecule, respectively. The values of μs and σs for such mixed solvent systems are calculated with changing the mole fraction and are presented in Table S1. The magnitude was again multiplied by the term Xs1 like in the previous studies25,28 to find the ΔG0t,d−d(i) term on the mole fraction scale. The term Xs1 is given as Xs1 = Xs(μs /σs 3)/(μR /σR 3)

(9)

ΤsΔSd0− d(i) = sΔGd0− d(i)[1 + Τα ]

(10)

α is the isobaric thermal expansibility constant of a solvent, and its value was estimated by eq 11.27 α = δ(lnVs/δ Τ)P = − (δ lnds/δ Τ)P

(11)

The enthalpy variation due to cavity creation in water to water electrolyte solution was determined by eqs 12 and 13.46

(8)

0 0 0 ΔHt,cav (i) = sΔHcav (i) − R ΔHcav (i )

Here, Xs1 is the actual mole fraction participation due to the dipole−dipole interaction effect. Now, to get ΔG0t,ch(i), the values of ΔG0t,cav(i) and ΔG0t,d−d(i) are subtracted from ΔG0t (i), and the values are presented in Table 7. Similarly,

(12)

0 ΔHcav (i) = (Α + Η + Κ + Ε) × Β

(13)

where Α = (ΠNA /6Vs) × (Ζ R σR + Zsσs ), B = σsRT /1 − A, H = σx × 3Y/1 − A, K = σx × 3Χ/1 − A, 3

F

3

2



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E = 9σx 2 × X2 /(1 − A)2 , X = (ΠNA /6Vs) × (Z R σR 2 + Zsσs 2), Y = (ΠNA/6Vs) × (ZRσR + Zsσs), and Π = 22/7. The values of ΔH0t, cav(i) in kJ·mol−1 in pure and mixed solvents are shown in Table 7.

be a probable reason why the results are quite different in comparison to one another. By performing the experiment several times in the present study, we observed that the results agreed well with a variation of a maximum of 3.0%. This justifies the analytical technique of measurement in the present study. From the solubility results (Table 3 and Figure 2), it is seen that, with increasing concentration of both the experimental

4. DISCUSSION 4.1. Solubility. The temperature and physicochemical characteristics such as the hard sphere diameters, dipole moments, and thermal expansibility constants of the solute, solvent, and electrolyte molecules affect the solubilities of amino acids significantly.8,20,35 In the current study, we estimated the solubilities of L-isoleucine and L-serine in water−NaNO3 and water−KNO3 solvent systems at five equidistant temperatures. To ensure the consistency of the present experimental method in finding the solubilities of L-isoleucine and L-serine in pure water at different temperatures, the results evaluated were compared with the literature values22,51−57 (Table 2), which is shown in Figure 1a,b. It clearly shows that most of the solubility data of the experimental amino acids in pure water corroborate excellently with the literature data22,51,55 with a few exceptions in the case of the studies by other groups.52−57 In such exceptional cases, the experimental pressure in the earlier studies52−55 has been found to be high compared to us (P ∼0.1 MPa). This might

Figure 2. Variation of solubilities (mol kg−1) of L-isoleucine and Lserine in pure water and aqueous electrolyte (NaNO3/KNO3) mixtures in different compositions at 298.15 K.

electrolytes in the reaction media, the solubility of L-isoleucine decreases, whereas the solubility increases for L-serine under a similar composition of electrolytes. The results indicate the salting-in effect for L-serine and salting-out for L-isoleucine in existence of sodium/potassium nitrate. Thus, the amino acid solubilities are affected noticeably by their structural parameters as well as the nature of the electrolytes. In addition, the salting-in effect was found to be acting more efficiently for L-serine in aqueous sodium nitrate in comparison to that in aqueous potassium nitrate solution. This is in accordance to our previous study37 for glycine in aqueous NaBr/KBr solutions. On the other hand, in the case of L-isoleucine, sodium nitrate leads to a greater salting-out effect than potassium nitrate. This is evident from the relative solubility data (Table 4, Figure 3). The ratio (Ss/SR) is the relative solubility where Ss is the solubility of

Figure 1. (a) Mole fraction solubility of L-isoleucine in pure water at different temperatures. (b) Mole fraction solubility of L-serine in pure water at different temperatures.

Figure 3. Variation of relative solubilities of L-isoleucine and L-serine in aqueous electrolyte (NaNO3/KNO3) mixtures in different compositions at 298.15 K. G



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amino acid in aqueous-salt mixtures and SR is the solubility in pure water. The relative solubilities in different electrolyte concentrations were considered and are presented in Table 4. The salting-in constant (Ksi) is a quantitative estimation of the salting-in effect, and it can be measured by eq 14.24,48 log(Ss/SR ) = K siC

group, which is why the former is more polar than the latter. In our earlier studies,4,22 we found a similar type of solubility trend in the presence of K+ and Na+ cations and SO42− anion as common ions for the same amino acids. Though the trend is the same, slightly higher solubility was found in the case of nitrate (NO3−) as the common anion in the present study than the sulfate (SO42−) anion with the same K+ ion for the amino acid Lserine. The different trend in solubility in aqueous electrolytic solutions having different cations and anions might be due to size-dependent dispersion and hydrogen bonding interactions with amino acid molecules. In the present study, it is important to note that the sizes of NaNO3 (0.628 nm)18,35 and the amino acid L-serine (0.593 nm)22 are very much similar. This induces stronger hydrogen bonding and size-dependent dispersion interactions. For comparison, sizes of NaNO3 and L-serine facilitate in forming the ion−pair complex with the zwitterionic species of amino acids.15,38,39 Amino acid molecules form intermolecular hydrogen bonding with the electrolytes and water molecules (Scheme 1). By hydrogen bonding interaction,

(14)

C means the molal concentration of the electrolyte in solution. Now, plotting log(Ss/SR) versus C (concentration) (Figure 4)

Scheme 1. Cavity-forming interaction through H bonding within amino acids and solvated electrolyte ions.

Figure 4. Plot of the logarithm of the ratio of solubilities Ss with and SR without electrolytes for L-isoleucine and L-serine in aqueous electrolyte (NaNO3/KNO3) mixtures in different compositions at 298.15 K.

and from the linear relationship,48,49 the values of Ksi at 298.15 K were evaluated and are given in Table 5. The Ksi values presented in Table 5 for the present amino acids in electrolyte mixtures abide the solubility trends for the same given in Table 3. Whether the salting-in or salting-out interaction is really acting in the experimental solution might be explain in terms of positive and negative values of the Ksi constant. Positive values of the Ksi constant suggest the salting-in effect, while the negative values justify the salting-out effect. These values are summarized in Table 5. The difference in the solubility and salting-in/out effects in the presence of NaNO3 and KNO3 is most likely due to the unlikely nature of complex arrangements in aqueous media by the various-sized amino acids with the anion of electrolytes, which was established previously by Held and co-workers.47 Results suggest that there is an increase in solubility by a maximum of 1−7% for KNO3 compared to NaNO3 in different electrolyte concentrations for L-isoleucine. However, a reverse trend is observed in the case of L-serine (Table 3). From Table 3, it is clear that the solubility is affected considerably by the temperature of the solution. The variation in solubility under different experimental conditions arises due to the interactions involving ions produced from electrolytic salt and water with the hydrocarbon backbone as well as charged functional groups of the zwitterionic molecule. The noticeably different trend in solubility of the present amino acids in solution is possibly due to the difference in structure and hard sphere diameter of solute amino acid molecules and solvated electrolyte ions. The difference of solubility and relative solubility of L- isoleucine and L-serine at a certain temperature raise a summarizing report on the salting-in/out effect at a particular concentration of the electrolyte. At each point of electrolyte content, the solubility of L-serine is much more than that of L-isoleucine. L-Serine contains one −OH group, whereas L-isoleucine does not have any −OH

they form ion−pair complexes in mixed solvent systems.50 The ion−pair complex is a complex that is formed in between the zwitterion of the amino acid and the electrolyte anion and cation. The zwitterionic amino acid (A−A+) in the ternary system (water + amino acid + salt) may possibly create soluble ion−pair complexes (due to cavity-forming interaction) like A−A+ + C+X− ↔ C+(A−A+)X−21,22,38,39 with the electrolyte cation C+ (here, Na+/K+) and electrolyte anion X− (here, NO3−). The difference in the solubility behavior of amino acids in solution is observed as the salting-in or salting-out effect. Such an effect varies from electrolyte to electrolyte because it depends on the nature of the ions produced from the electrolyte and the nature of cationic and anions parts of the zwitterions. This is why the solubility of L-serine was highest for the water−NaNO3−L-serine ternary system. However, in the case of L-isoleucine, the solubility was slightly higher in the KNO3− water system than in the water−NaNO3 system. The result of higher solubility might be due to the higher dispersion interaction with the larger K+ ion and L-isoleucine molecule. 4.2. Solute−Solvent Interaction and Chemical Stability (via Transfer Gibbs Free Energetics). The knowledge of solute−solvent interactions in electrolyte media especially for amino acid like biomolecules is very much essential for biological and pharmaceutical applications. In the present study, we discussed it through transfer Gibbs energetics arising from solute and solvent interaction. For precise assessment of standard transfer free energies (ΔG0t (i)), we used the least H



Article

squares method27,29 for calculation. In this method, some coefficients, that is, a, b, and c, were calculated first, and then using them, the accurate values of ΔG0t (i) were evaluated (Table 6). The variations of total transfer Gibbs free energies (ΔG0t (i)) from water to electrolyte mixtures of the experimental important amino acids are shown in Figure 5. L-Isoleucine shows a positive

Figure 6. Variation of ΔG0t, ch(i) for L-isoleucine and L-serine in aqueous electrolyte (NaNO3/KNO3) mixtures in different compositions at 298.15 K.

ion−pair complex formed in the water−NaNO3 mixture is more important to save the hydrophobic and electrostatic type of interaction than the ion−pair complex formed between water− KNO3 and L-serine. This is why L-serine is more stable in the water−NaNO3 mixture than in water−NaNO3. On the other hand, L-isoleucine shows less stability in both the electrolyte solvent systems due to poorer interaction for a greater size difference between amino acid and mixed solvent molecules. 4.3. Solvent−Solvent Interactions via Transfer Entropies. The solvent−solvent interaction was described in this study by evaluating the transfer entropies of the amino acids from aqueous to aqueous electrolyte solvent mixture. For getting more precise results, we used the least squares method26,28 for calculation of standard total transfer entropy TΔS0t (i)(Tables 6 and 7). The values of TΔS0t (i) are plotted against molal concentrations of electrolytes in Figure 7. TΔS0t (i) is a joint

Figure 5. Variation of ΔG0t (i) for L-isoleucine and L-serine in aqueous electrolyte (NaNO3/KNO3) mixtures in different compositions at 298.15 K.

increment in both the experimental water−electrolyte solvent systems. A comparatively higher increment was found in water− NaNO3 solution, which resembles relatively less solubility and stability of L-isoleucine. In contrast, L-serine shows a gradual negative increment in both the water−electrolyte mixtures. This specifies that L-serine is comparatively more stable in water− electrolyte systems than L-isoleucine. The major solute−solvent interaction expressed in total transfer Gibbs free energies (ΔG0t (i)) is a combined energy arising due to cavity creation free energy ΔG0t, cav(i), free energy for dipole−dipole interaction between solute and solvent molecules ΔGt,0d − d(i) (Table 7), and the free energy of some unique categories of chemical interactions like dispersion, acid−base, hydrophilic, and hydrophobic interactions. The term chemical transfer Gibbs free energy (ΔG0t, ch(i)) is a measure of the specific chemical stabilities of the amino acids in electrolyte solutions. It was obtained after subtracting energies due to cavity-forming energy and dipole−dipole interactions from the entire transfer free energy. ΔG0t, ch(i) increases by chemical interactions, which take place in between solute and solvent molecules. The results confirm that L-isoleucine is less stable than L-serine in terms of chemical types of interactions in both the electrolyte mixtures. It was found that the values of ΔG0t, ch(i) show a gradual increasing trend (positive) for Lisoleucine, whereas a gradual decreasing trend (negative) for Lserine was observed. This indicates higher solubility as well as stability for L-serine than L-isoleucine in the experimental solution (Table 7, Figure 6). The results suggest that L-serine is more stable in aqueous electrolyte systems than in pure water, while L-isoleucine is more chemically stable in pure water. Yet, Lserine is comparatively more stable in the water−NaNO3 system than in the water−KNO3 system. This type of variation of stability arises due to the difference in size of both solute and solvated ions. In the water−NaNO3 solvent mixture, L-serine (hard sphere diameter: 0.593 nm)22 forms a more stable ion− pair complex due to suitable matching in size between electrolyte (NaNO3; 0.628 nm)35 ions and L-serine. The stable

Figure 7. Variation of TΔS0t (i) for L-isoleucine and L-serine in aqueous electrolyte (NaNO3/KNO3) mixtures in different compositions at 298.15 K.

effect of cavity-forming transfer entropy TΔS0t, cav(i) and transfer entropy due to dipole−dipole interaction TΔSt,0 d − d(i), and the kind of interaction, namely, hydrogen bonding and acid−base, TΔS0t, ch(i), in the solvent molecules by the influence of amino acid molecules, shows a complicated nature of variation (Table 7). I



Article

slightly higher stability was found for L-isoleucine in the KNO3− water system, which is probably due to a good agreement of the size factor of L-isoleucine and K+ of the electrolyte. The chemical entropy results suggested that the solvent−solvent interaction is more favorable for L-isoleucine in the KNO3−water system and less favorable for L-serine in the NaNO3−water system, which indicates that L-isoleucine induces less disorderness between solvent molecules, whereas L-serine induces more disorderness between solvent molecules.

The variation of TΔS0t, ch(i) for L-serine and L-isoleucine in electrolyte solutions is shown in Figure 8. L-Serine shows a

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00363. Standard Gibbs energies of solutions of the amino acids in aqueous electrolytes at different temperatures and cosolvent parameters (PDF)

■

Figure 8. Variation of TΔS0t, ch(i) for L-isoleucine and L-serine in aqueous electrolyte (NaNO3/KNO3) mixtures in different compositions at 298.15 K.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.R.). *E-mail: [email protected] (K.M.).

positive as well as nonlinear nature of variation in both aqueous electrolyte mixtures. It is found that TΔS0t, ch(i) values are found to have comparatively less positive values (more ordered) for Lserine in KNO3 than in NaNO3 solution. The result suggests that the L-serine induces more solvent−solvent interaction between electrolyte KNO3 ions (K+ and NO3−) and water molecules rather than the interaction of NaNO3 (Na+ and NO3−) water molecules. In this case, Na+ interacts more strongly with the water molecules in the presence of L-serine in the waterrich region of electrolyte solution. In the water-rich region, Lserine encourages to form a first hydration zone around the cation. The cation Na+ has relatively smaller size and higher cationic charge density than the larger K+. That is why the hydrated Na+ is more stable than K+ in the first hydration zone.18 In the case of L-isoleucine, the change of the TΔS0t, ch(i) value shows a similar trend like in L-serine but shows a lower positive value. In this case, the positive trend of TΔS0t, ch(i) is more in NaNO3 solution than that in KNO3 solution. Results suggest that L-isoleucine interacts strongly with the cation, anion, and water molecules, and therefore, it encourages less disorderness rather than L-serine in both the electrolyte solutions. In aqueous KNO3 solution, TΔS0t, ch(i) results are more negative as the larger-sized KNO3 molecules convey more organization around the water molecules due to the inducing effect of L-isoleucine.

ORCID

Sanjay Roy: 0000-0001-6841-4961 Funding

K.M. is thankful to Project No. ST/P/S&T/15 G-5/2018 Science & Technology and Biotechnology (W.B.) and DSTPURSE for financial support at the Department of Chemistry, Kalyani University. Notes

The authors declare no competing financial interest.

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5. CONCLUSIONS The study on the solubilities of amino acids such as L-serine and L-isoleucine under various experimental situations established that electrolytes influence the solubilities of L-serine and Lisoleucine considerably. Both amino acids showed higher solubilities in aqueous NaNO3 solution than in aqueous KNO3 solution. Various interactions like dipole−dipole and cavity creation, dispersion, hydrogen bonding, and acid−base interaction affected by the size factor of the cations of electrolytes are mainly responsible for the solubilities and stabilities of amino acids in aqueous electrolytes systems. The chemical stability of L-serine is higher in aqueous NaNO3 solution than in aqueous KNO3 solution, whereas L-isoleucine is unstable in both electrolyte solvent systems. However, a J



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