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Solubility and Chemical Thermodynamics of D,L-Alanine and D,L-Serine in Aqueous NaCl and KCl Solutions Sanjay Roy,† Aslam Hossain,‡ and Bijoy Krishna Dolui*,‡ †

Department of Chemistry, Shibpur Dinobundhoo Institution (College), Howrah 711102, West Bengal, India Department of Chemistry, Visva-Bharati, Santiniketan, Birbhum 731235, West Bengal, India

‡

S Supporting Information *

ABSTRACT: The solubilities of D,L-alanine and D,L-serine in aqueous sodium chloride (NaCl) and potassium chloride (KCl) solutions are determined at five equidistant temperatures by using “formol titrimetry”. The thermodynamic parameters such as standard transfer Gibbs energies, entropies, and enthalpies have been evaluated at 298.15 K. Other important related parameters like molar mass, density, molar volume, dipole moment, and solvent diameter, etc., of the experimental solutions are also determined in the present study. ΔG0t,ch(i), i.e., chemical effects of the transfer Gibbs energies, and TΔS0t,ch(i), i.e., chemical effects of the transfer entropy of these amino acids, are also evaluated and discussed, the factors which are dependent on mainly nature of the solute, solvent, interactions between solute and solvent, etc. The nature and the extent of the involved factors, which are influencing the solvation of the amino acids in aqueous NaCl and KCl solutions, are also explained by the different physical and analytical approaches.

1. INTRODUCTION

In considering these points of views and to explore the solvation mechanisms of various amino acids, the solubility and thermodynamic studies were also carried out by different group of researchers in different aquo-organic20−27 and nonaqueous solvent systems.28−30 The results in terms of solvation thermodynamics reflected that difference in proticity, dipolar aproticity, acid−base property, hydrophilicity, hydrophobicity, and chemical nature of reference and cosolvents of such binary solvent systems20−30 influence the relative stability as well as solvation mechanism of amino acids. On the other hand various efforts were made by Pitzer, Khoshkbarchi, Held, and others to achieve a quantitative knowledge on the variation of amino acid solubility and other thermodynamic parameters in aqueous electrolyte mixtures with various experimental conditions by using different standard equations and models to corroborate the theoretical results with the experimental findings.1−6,33,34 But a detailed study on the solubility in different experimental condition and solvation mechanisms in the light of transfer energetics of the amino acids in aqueous electrolyte solution is still scarce. Therefore, in the present article efforts are concentrated on the thermodynamic point of views to enrich the knowledge of solvation chemistry of D,L-alanine and D,L-serine in aqueous NaCl and in KCl solution. The effect of aqueous solution of NaCl and KCl on the solubility of D,L-alanine and D,L-serine have been analyzed as a part of our systematic study.

For aqueous electrolyte solutions containing amino acids, experimental works for a long time have been most focused on the study of the electrolyte effect on the solubility of different amino acids.1−6 The different factors such as chemical structure, the polarity of the solvents,7 ionic strength,8 acidity or alkalinity of the media,9,10 experimental temperatures,11 and the concentration of the electrolyte present in aqueous amino acid solution12 can affect the solubility of the biomolecular species and hence the thermodynamics of solvation significantly. Earlier studies13,14 have shown that the solubility as well as the thermodynamics of solvation of these molecules determines their biochemical and biophysical mode of actions. This is why the thermodynamical studies on solvation of amino acids in different electrolyte solvent systems under different experimental conditions are important. This type of studies may also help in understanding the solubility behavior and solvation chemistry of related biomolecules. Previous studies1−6 showed that the addition of salts such as NaCl, KCl, and so forth, causes a change in actual orientation of amino acids and the solubility properties. The change in orientation of amino acids leads to a change in hydrophobic interactions and dipole−dipole interactions between solvent−solvent, solute− solvent, and solute−solute, thereby modifying the structure of proteins.15 In order to understand the effect of electrolytes on the thermodynamic properties, the study of transfer energetics is very much essential. It is of vital importance because these studies give valuable information regarding the protein folding and unfolding process15−18 and the extent of hydrophobic interaction of amino acids.19 © XXXX American Chemical Society

Received: April 17, 2015 Accepted: November 19, 2015

A

DOI: 10.1021/acs.jced.5b00351 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Specification of Chemical Samples

a

Stated by the supplier.

In this regard a reliable as well as convenient method, “formol titrimetry”,29−31,43,44 is used to measure the solubility of D,L-alanine and D,L-serine in different molal concentrations (mol·kg−1) of aqueous mixtures of NaCl and KCl at 288.15, 293.15, 298.15, 303.15, and 308.15 K. Then different thermodynamic properties have also been evaluated at these temperatures. Finally transfer energetics of D,L-alanine and D,L-serine in such aquo-ionic solvent mixtures are computed, discussed, and compared at 298.15 K.

standardized (0.05M) NaOH (GR, E Merck; >99.0%) solution and 1% alcoholic phenolphthalein solution as indicator. Excess freshly neutralized formaldehyde (GR, E Merck; >99%) solution was used in the method to mask α-amino group of amino acids. Standardized (0.05 M) NaOH solution from buret was added to 5 mL of formaldehyde with one drop of indicator to get freshly neutralized formaldehyde. The end point was indicated by the appearance of pink color. The neutral formaldehyde solution was then added to the pre neutralized amino acid solution followed by titration with NaOH until the color changed to a pale pink color. A solution was considered to attain saturation when successive molal concentration measurements at five day intervals agreed within the experimental error of ±1%. Attainment of saturation usually took 6−8 days at each temperature. Fresh solvents had been used at different temperatures to avoid the effect of change of composition of solvents. These measurements were made at five equidistant temperatures ranging from 288.15 to 308.15 K. Four sets of measurements were made for the experimental amino acids at all temperatures by equilibrating the solutions from both above and below (±0.10 K) the required temperatures, and the solubilities were found to agree mostly to within 3.0%.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Their Purifications. D,L-alanine and D,L-serine (>99.8%, GR, E. Merck) were used as received without further purification and kept in a dehydrator with silica gel to avoid water contamination. Sodium chloride (NaCl) and potassium chloride (KCl) of purity 99.9% obtained from E. Merck, Bombay, India. The salts were oven-dried at 370 K for 3−4 days to remove any water content, then cooled in a vacuum desiccator for 3 days and weighed. After 48 h, no change in its weight was observed. For formol titration29−31,43,44 standardized NaOH (E Merck) solution and phenolphthalein indicator (LR, BDH) were used. Neutral formaldehyde (E Merck) was used as amino group (−NH2) blocking agent before titration. Triple distilled water was used for whole experimental work. 2.2. Determination of Saturated Solubility. The aqueous solvent mixtures of NaCl and KCl of the concentrations, 0, 0.5, 1.5 2.0, 2.5, 3.0, and 3.5 in mol·kg−1 were made. The solvent mixtures (H2O/H2O + NaCl/H2O + KCl) and excess amount of each amino acid were placed in well fitted stoppered glass tubes. Glass tubes were incompletely filled to facilitate shaking and good mixing. A low-cum-high temperature thermostat was used for all measurements which is capable of registering temperatures having an accuracy of ±0.10 K. A known mass (at about 0.2−0.3 g) of filtered saturated solution was transferred to a dry conical flask from solution thermostated at all experimental temperatures. The saturated solubility measurements of D,L-alanine and D,L-serine were made by the “formol titrimetric method”29−31,43,44 using freshly

3. THEORETICAL SECTION 3.1. Calculation of Standard Transfer Gibbs Free Energy and Entropy. The specifications of used chemicals are presented in Table 1. The involved important parameters of D,L-alanine and D,L-serine in aqueous NaCl and KCl solutions are presented in Table S1. The data quality of saturated solubility of the amino acids on molal scale (mol·kg−1) are checked by comparing the values obtained in this work to those already published and summarized in Table 2, and the nature of variation of the obtained values at 298.15 K is represented by Figure S1. In order to examine the trustworthiness of the present method used to determine the saturated solubility, the same of the experimental amino acid, i.e., D,L-alanine, D,L-serine, and other amino acids under different experimental conditions were evaluated by B



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Table 2. Comparison of the Solubility Data (mol·kg−1) for Glycine, D,L-Alanine, and D,L-Serine by Formol Titrimetry Method and with Earlier Results by Different Analytical Technique in Aqueous Media in the Absence and Presence of NaCl and KCl at 298.15 Ka solubility in presence of electrolyte(s) amino acid(s) glycine

D,L-alanine

D,L-serine

solubility in pure water 27,c

3.339 3.320 ± 0.0733 3.3292,47 3.3343,c 3.3334 3.33927,44,c 1.84 ± 0.0633 1.861 1.8882,47 1.85043c 1.8914 1.88944,c 0.47945 0.4782 0.4764 0.529246 0.529 ± 0.004b

−1

NaCl (3.0 mol·kg−1)

NaCl (1.0 mol·kg ) 3.33 ± 0.12 3.3712 3.40944,c

3.60 ± 0.11 3.68644,c

33

33

KCl (1.0 mol·kg−1) 3.27 ± 0.10 3.2811 3.4962 3.37244,c

KCl (3.0 mol·kg−1) 3.38 ± 0.1233 3.40444,c

33

1.87 ± 0.0933 1.8592 1.85944,c 1.859 ± 0.003b

1.75 ± 0.0933 1.78644,c 1.806 ± 0.013b

1.85 ± 0.0833 1.8411 2.012 1.89644,c 1.896 ± 0.010b

1.67 ± 0.0933 1.794 ± 0.013b

0.5692 0.570 ± 0.011b

0.672 ± 0.013b

0.7252 0.726 ± 0.009b

1.056 ± 0.010b

In refs 1, 2, 4, 33, 45, 46, and 47, solubilities are measured by “analytical gravimetric method” or others. bThe solubility values were taken from present study. cThe results were shown from earlier literature by using “formol titrimetry” method.

a

well established “formol titrimetry method”29−31,43,44 and compared with the earlier results determined by this as well as other like analytical “gravimetric methods”1−5,8−10,33,36,45,46 (Table 2). Also a comparison of the solubility data of the involved amino acids of our present experiment and other literature4,5,10,14,19,27,47−51 in pure water in different temperatures is presented by Figures 1 and 2.

Figure 2. Mole fraction solubility of D,L-serine in pure water at different temperatures: red ●, this study; black ●, from literature.10,48

thereby justifying the analytical technique of measurement as used in the present study. Like previous studies,21,27−32,36,43,44,46 the standard Gibbs energies of solutions (ΔG0s ) on molal scale at a particular temperature can be calculated in the present study by using eq 1, ΔGs0(i) = −RT ln Sγ ≈ −RT ln S

Figure 1. Mole fraction solubility of D,L-alanine in pure water at different temperatures: red ●, this study; black ●, from literature.4,5,10,14,19,27,47−51

(1)

where γ is the molal activity coefficient and “S” is the saturated solubility of the amino acids in mol·kg−1. Amino acids are likely to be zwitterions in solutions. So they are expected to have large dipole−dipole interaction among themselves. Therefore, the activity coefficient factor −RT lnγ may contribute to ΔG0s (i). In this regard Khoshkbarchi, Held, Breil, Mollerup and their coworkers2,8,33−35 had concentrated their efforts for the measurement of activity coefficients of some amino acids and dipeptides in aqueous electrolyte mixed solvent systems. They had computed values of activity coefficient (γ) nearly unity for such biomolecules in lower concentrations. It is to be mentioned here that the mole fractions of D,L-alanine and D,L-serine estimated in different compositions of these aqueous NaCl and KCl solutions,

The saturated solubility of D,L-serine in both aqueous NaCl and KCl solutions is also evaluated specially using the “gravimetric” method1−5,8−10,33,36,45,46 to compare with the present “formol titrimetric” method. The solubility data of the same in aqueous KCl determined by “gravimetric” method are presented in Table S2 for instance. It is to be noted that the saturated solubility data of D,L-serine in both aqueous NaCl and KCl solutions, determined by both “gravimetric” and “formol titrimetric” methods are mainly concordant and found to agree to within 3.0%. These results clearly show a reasonable agreement among the values, C



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Table 3. Experimental Solubilities (S) (mol·kg−1) of D,L-Alanine and D,L-Serine in Aqueous Solvent Mixtures of NaCl/KCl at Different Temperatures and under Atmospheric Pressure, p = 0.1 MPaa solubility (S) −1

solute/solvent system D,L-alanine

D,L-serine

NaCl−water

D,L-alanine

D,L-serine

a

NaCl−water

KCl−water

KCl−water

T/K

solvent molality (m) (mol·kg )

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

mol % NaCl/KCl

0.0

0.89

1.77

2.63

3.48

4.31

5.13

5.93

1.828 1.850 1.889 1.930 1.958 0.460 0.496 0.529 0.559 0.586 1.828 1.850 1.889 1.930 1.958 0.460 0.496 0.529 0.559 0.586

1.816 1.846 1.874 1.907 1.936 0.504 0.518 0.544 0.570 0.600 1.836 1.858 1.902 1.934 1.969 0.583 0.592 0.609 0.650 0.670

1.806 1.832 1.859 1.894 1.918 0.537 0.554 0.570 0.588 0.618 1.842 1.860 1.896 1.938 1.975 0.658 0.704 0.726 0.754 0.828

1.790 1.824 1.844 1.880 1.902 0.580 0.592 0.608 0.616 0.630 1.830 1.844 1.885 1.902 1.954 0.706 0.768 0.829 0.866 0.902

1.782 1.811 1.830 1.866 1.883 0.610 0.620 0.628 0.638 0.644 1.780 1.792 1.856 1.873 1.898 0.742 0.840 0.925 0.945 1.004

1.769 1.800 1.821 1.836 1.864 0.622 0.636 0.651 0.666 0.683 1.758 1.769 1.836 1.859 1.876 0.824 0.888 0.978 1.044 1.122

1.756 1.788 1.806 1.820 1.847 0.647 0.654 0.672 0.680 0.694 1.739 1.748 1.794 1.822 1.852 0.868 0.960 1.056 1.170 1.234

1.740 1.774 1.794 1.810 1.829 0.669 0.678 0.692 0.699 0.712 1.716 1.728 1.768 1.788 1.802 0.930 1.070 1.138 1.260 1.364

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

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) = 0.013.

as calculated from saturated solubility values in Table 3 is negligibly small. Therefore, “γ” has also been assumed here as unity for the present experimental solvent systems in calculating of ΔG0s (i). Moreover, in this study our main objective is to determine the standard total transfer free energies [ΔG0t (i)] of the amino acids in the experimental aqueous electrolyte binary solvent mixtures. The same objective was also reflected in the previous studies.25−32,36,43,44,46 The ΔG0t (i) is related as [ΔG0t (i) = ΔG0s (i) − ΔG0R(i)], where ΔG0s (i) and ΔG0R(i) represent the involved free energy of the amino acids in cosolvent (i.e., mixed aqueous NaCl or KCl solvent mixtures) and reference solvent, water, respectively. Therefore, ΔG0t (i) contains the ratio of activity coefficient factor, −RT ln γs/γR (“s” for aq-NaCl/KCl and “R” for reference solvent, H2O), which is likely to be negligibly small. So the assumption of ignoring the contribution of activity coefficient in this context will not be too hard here. The evaluated free energies of solutions, ΔG0s at the experimental five equidistant different temperatures were fitted by the method of least-squares using eq 2 as,31 ΔGs0 = a + bT + cT ln T

and ΔSt0(i) = (bR − bs) + (c R − cS)(1 + ln T ) + R ln(Ms /MR )

(4)

here the subscript s stands for aqueous NaCl and KCl mixtures, while R refers to the reference solvent (H2O). MR and Ms are the molar masses of the pure and mixed solvent, respectively. ΔG0t (i) and TΔS0t (i) values of the amino acids are evaluated and presented in Tables 4 and 5. The standard uncertainties in determining the ΔG0t (i) and ΔS0t (i) values are found be 0.05 kJ·mol−1and 2 kJ·K−1 mol−1, respectively. 3.2. Computation of Standard Transfer Gibbs Free Energies and Entropy Due to Cavity, Dipole−Dipole, and Chemical Interactions and Standard Transfer Enthalpy due to Cavity Formation. The term ΔP0t (i) (where P = G or S) can be defined as the sum of the following terms (assuming dipole induced dipole term to be negligibly small).31,36, i.e. 0 0 0 ΔPt0(i) = ΔPt,cav (i) + ΔPt,d − d(i) + ΔPt,ch(i)

(2)

(5)

Here, ΔP0t,cav(i) is the standard transfer energy contribution for the cavity effect which is owing to the creation of cavities for the species, D,L-alanine and D,L-serine in H2O and aqueous 0 (i) is for to the dipole−dipole NaCl and KCl mixtures. ΔPt,d−d interaction involving the interaction between dipolar zwitterionic amino acid and solvent molecules. On the other hand, ΔP0t,ch(i) includes all other effects such as those arising from acid−base or short-range dispersion interaction, hydrophilic or hydrophobic hydration and structural effects, etc. In the present study scaled particle theory (SPT)27,28 is used for computation of ΔP0t,cav(i) by assuming the fact that solutes and solvent molecules are equivalent as hard-sphere models as are dictated by their respective diameters (Table S1). In this

where T is the absolute temperature in Kelvin, while a, b, and c are the coefficients, values, and units of which are summarized in Table 4. The values are found to reproduce the experimental data within ±0.04 units. Standard transfer Gibbs energies ΔG0t and entropies ΔS0t of the amino acids in aqueous to aqueous NaCl and KCl mixtures were determined at 298.15 K on mole fraction scale by using eqs 3 and 4, respectively, 0 0 ΔGt0(i) = sΔGsol (i) − RΔGsol (i)

i.e. ΔGt0(i) = (as − aR ) + (bs − bR )T + (cs − c R )T ln T − RT ln(Ms/MR )

(3) D



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Table 4. Coefficients a, b, and c, Gibbs Energies ΔG0t , and Entropies TΔS0t of Transfer of D,L-Alanine and D,L-Serine on Mole Fraction Scale from H2O to H2O−NaCl and H2O−KCl Mixture at 298.15 Ka molality (mol·kg−1) D,L-alanine

NaCl−H2O 0.0 [water] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 DL-alanine KCl−H2O 0.0 [water] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 DL-serine NaCl−H2O 0.0 [water] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 DL-serine KCl−H2O 0.0 [water] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 a

a (kJ·mol−1)

b (kJ·mol−1·K‑1)

c (kJ·mol−1·K−1)

ΔG0t (i) (kJ·mol−1)

TΔS0t (i) (kJ·mol−1)

−9.02 −2.09 −2.91 5.13 4.41 7.83 9.27 23.35 −9.02 −9.07 −28.96 −34.87 7.29 22.09 14.18 12.45 73.59 −63.35 −55.79 8.92 11.67 −10.11 −7.36 −2.13 73.59 −88.34 −267.33 141.32 241.83 35.17 98.20 87.55

0.2482 0.0872 0.1036 −0.0777 −0.0645 −0.1458 −0.1797 −0.4957 0.2482 0.2493 0.6966 0.8242 −0.1217 −0.4509 −0.2749 −0.2474 −1.4782 1.5542 1.3547 −0.1384 −0.2197 0.2967 0.2192 0.0947 −1.4782 2.0942 6.1599 −3.0021 −5.2299 −0.5700 −1.9529 −1.7063

−0.03919 −0.01499 −0.01738 0.00971 0.00784 0.02010 0.02522 0.07241 −0.03919 −0.03936 −0.10614 −0.12505 0.01618 0.06525 0.03902 0.03526 0.21705 −0.23460 −0.20410 0.01976 0.03237 −0.04549 −0.03356 −0.01482 0.21705 −0.31485 −0.92324 0.44399 0.77570 0.07939 0.28486 0.24772

0 −0.009 −0.049 −0.091 −0.096 −0.132 −0.143 −0.154 0 −0.078 −0.114 −0.165 −0.173 −0.224 −0.272 −0.244 0 −0.112 −0.271 −0.495 −0.608 −0.729 −0.861 −0.956 0 −0.446 −0.808 −1.305 −1.585 −1.802 −2.099 −2.369

0 −0.276 −0.343 −0.339 −0.496 −0.696 −0.771 −0.749 0 0.079 0.134 −0.085 −0.025 0.195 0.154 −0.483 0 −2.169 −3.543 −5.353 −6.250 −4.695 −5.369 −5.632 0 −2.898 −0.134 1.373 3.264 4.425 6.492 7.185

u(T) = 0.10 K.

regard the following equations are essential in computing the necessary parameters: 0 ΔGcav (i) = GC + RT ln(RT /VS)

0 ΔGt,cav (i) = sΔG 0 cav (i) − RΔG 0 cav (i) = RGC − sGC

+ RT ln(VR /Vs)

(6)

(7)

Again

where

0 0 0 ΔGt,d − d (i) = ( sΔGd − d (i) − RΔGd − d (i))

GC = RT[−ln(1 − Z) + {3X /(1 − Z)}σx

ΔS0t,d−d(i)

(8)

and = − calculated by means of the Keesom-orientation expression, for sΔG0d−d(i) in a solvent S, as given below:

+ {3Y /(1 − Z)}σx 2 + {9X2 /2(1 − Z)2 }σx 2] Z = πNA /6Vs(z R σR 3 + zsσs 3)

0 sΔGd − d (i)

(sΔS0d−d(i)

0 RΔSd−d(i)) are 36

= −(8Π/9)NA 2μs 2 μx 2 σs − x −3(kT )−1vs−1 = A /TVs (9)

X = πNA /6Vs(z R σR 2 + zsσs 2)

−1

−3

where A = −(8Π/9)NA μs μx σs−x (k) that of ΔS0d−d(i) as follows: 2

Y = πNA /6Vs(z R σR + zsσs)

0 s ΔSd − d(i)

Vs = Ms /ds

2

2

and Vs = Ms/ds and

= −{δ sΔGd0− d(i)/δT }p

i.e.

Here NA is the Avogadro number, while zR and zs are the mole fraction of reference solvent water and cosolvent, respectively. σx, σR, and σs are the hard sphere diameters of amino acid, water, and aqueous NaCl/KCl mixtures, respectively. Ms and ds are the molar mass and molar density of the solvents, respectively. The required ΔG0t,cav(i) can be represented as below:

T sΔSd0− d(i) = sΔGd0− d(i)[1 + Tα]

(10)

where NA is the Avogadro number, while μs and μx representing the dipole moment of aqueous NaCl/KCl mixtures and amino acid respectively (Table S1). σs−x is the distance at which the attractive and repulsive interactions between the solvent and solute molecules38 are equal which is generally equal to 1/2(σs + σx), E



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Table 5. Gibbs Energies of Transfera ΔG0t (i), ΔG0t,cav(i), ΔG0t,dd(i), ΔG0t,ch(i), and Enthalpy of Transfer, ΔH0t,cav(i) and Entropies of Transfer TΔS0t (i), TΔS0t,cav(i), TΔS0t,dd(i), and TΔS0t,ch(i) of D,L-Alanine and D,L-Serine from H2O to H2O−NaCl and H2O−KCl Mixtures at 298.15 K (on Mole Fraction Scale) in kJ·mol−1 molality (mol·kg−1)

ΔG0t (i)

ΔG0t,cav(i)

ΔG0t,dd(i)

ΔG0t,ch(i)

TΔS0t (i)

ΔH0t,cav(i)

TΔS0t,cav(i)

TΔS0t,dd(i)

TΔS0t,ch(i)

D,L-alanine NaCl−H2O 0.0 [water] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 D,L-alanine KCl−H2O 0.0 [water] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 D,L-serine NaCl−H2O 0.0 [water] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 D,L-serine KCl−H2O 0.0 [water] 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0 −0.009 −0.049 −0.091 −0.096 −0.132 −0.143 −0.154 0 −0.078 −0.114 −0.165 −0.173 −0.224 −0.272 −0.244 0 −0.112 −0.271 −0.495 −0.608 −0.729 −0.861 −0.956 0 −0.446 −0.808 −1.305 −1.585 −1.802 −2.099 −2.369

0 −0.081 −0.176 −0.256 −0.378 −0.407 −0.475 −0.545 0 −0.179 −0.347 −0.503 −0.650 −0.787 −0.917 −1.040 0 −0.082 −0.178 −0.260 −0.343 −0.413 −0.481 −0.553 0 −0.181 −0.351 −0.510 −0.659 −0.797 −0.929 −1.050

0 −0.082 −0.335 −0.772 −1.400 −2.260 −3.290 −4.530 0 −0.084 −0.352 −0.820 −1.500 −2.400 −3.500 −4.860 0 −0.037 −0.151 −0.353 −0.648 −1.050 −1.560 −2.180 0 −0.039 −0.166 −0.386 −0.705 −1.130 −1.660 −2.300

0 0.154 0.462 0.937 1.682 2.535 3.622 4.921 0 0.185 0.585 1.158 1.977 2.963 4.145 5.656 0 0.007 0.058 0.118 0.423 0.734 1.180 1.777 0 −0.226 −0.291 −0.409 −0.221 0.125 0.490 0.981

0 −0.276 −0.343 −0.339 −0.496 −0.696 −0.771 −0.749 0 0.079 0.134 −0.085 −0.025 0.195 0.154 −0.483 0 −2.169 −3.543 −5.353 −6.250 −4.695 −5.369 −5.632 0 −2.898 −0.134 1.373 3.264 4.425 6.492 7.185

0 −0.130 −0.273 −0.379 −0.503 −0.627 −0.712 −0.817 0 −0.239 −0.458 −0.638 −0.819 −1.000 −1.140 −1.280 0 −0.134 −0.281 −0.390 −0.518 −0.646 −0.734 −0.841 0 −0.247 −0.472 −0.657 −0.844 −1.030 −1.170 −1.320

0 −0.049 −0.097 −0.123 −0.125 −0.220 −0.237 −0.272 0 −0.060 −0.111 −0.135 −0.169 −0.213 −0.223 −0.240 0 −0.052 −0.103 −0.130 −0.178 −0.233 −0.253 −0.288 0 −0.066 −0.121 −0.147 −0.185 −0.233 −0.241 −0.270

0 −0.087 −0.357 −0.823 −1.490 −2.400 −3.500 −4.820 0 −0.089 −0.375 −0.875 −1.600 −2.550 −3.730 −5.170 0 −0.039 −0.161 −0.377 −0.690 −1.120 −1.660 −2.320 0 −0.042 −0.177 −0.412 −0.752 −1.200 −1.770 −2.450

0 −0.140 0.111 0.607 1.119 1.924 2.966 4.343 0 0.225 0.620 0.925 1.744 2.958 4.107 4.927 0 −2.078 −3.279 −4.846 −5.382 −3.342 −3.456 −3.024 0 −2.790 0.164 1.932 4.201 5.858 8.503 9.905

a Dipole moments of D,L-alanine and D,L-serine are 15.9 D27 and 11.10 D,41 respectively. The sizes of D,L-alanine43 and D,L-serine46 are 5.83 and 5.93 Å, respectively.

where σs and σx are the hard sphere diameter of cosolvent and solute molecules, respectively (Table S1). The “α” is the isobaric thermal expansibility constant of the solvent and is given by eq 11. α = δ(ln Vs/δT )P = − (δ ln ds/δT )P

and Y = (ΠNA /6Vs)(Z R σR + ZSσS)

Following Marcus37 and Kim et al.39 in order to obtain this ΔP0t,d−d(i) term on mole fraction scale the quantity should be further multiplied by the term Xs1.

(11)

Now the standard transfer enthalpy contribution owing to the cavity forming interaction in water to aqueous NaCl/KCl mixtures is measured by the eq 12. 0 ΔHt,cav (i )

0 ΔHcav (i )

0 RΔHcav(i)

(12)

= (A + H + K + E ) × B

(13)

=

0 sΔHcav(i)

−

Xs1 = Xs(μs /σs 3)/(μR /σR 3)

(14)

This represents the real mole fraction contribution owing to the dipole−dipole interaction.30 Subtraction of ΔP0t,cav(i) and ΔP0t,d−d(i) from the total one can get ΔP0t,ch(i) of the solute amino acids. The values of ΔP0t,cav(i), ΔP0t,d−d(i), and ΔP0t,ch(i) are summarized in Table 5.

where A = (ΠNA /6Vs)(Z R σR 3 + ZSσS3)

4. RESULT AND DISCUSSION 4.1. Solubility. Here it is important to note that the most of the solubility data of the experimental amino acids in pure water excellently corroborate with the previous literature data.4,5,10,14,19,27,47−51 The comparison study (Figures 1 and 2) reflects that a very few data are distorted slightly which may be due to the experimental error or may be due to disciplined error of the previous4,5,10,14,19,27,47−51 or present experimental conditions.

B = σSRT 2/1 − A H = σx 3Y /1 − A

K = σx 3X /1 − A E = 9σx 2X2 /(1 − A)2

X = (ΠNA /6Vs)(ZRσR 2 + ZSσS2) F



Article

Now the effect of electrolytes on solubilities of amino acids or the other biomolecules in general strongly depends on change in temperature as well as the specific nature of both biochemicals and electrolytes.1 Here the presented solubility data in Table 3 shows that the solubility of D,L-alanine and D,L-serine increases with increasing temperature in all involved compositions for both the NaCl and KCl aqueous electrolyte solvent systems. However, solubility of D,L-alanine mainly decreases with the increase of concentration of NaCl and KCl (Table 3). But a slight increment in solubility of D,L-alanine in KCl-water system up to 1 molal concentration was observed. Thereafter solubility decreases gradually as the concentration of KCl is increased. While the solubility of the same in NaCl water system shows a linear decrement (Table 3; Figure S1). This phenomenon can be explained with the “salting-out” effect. For D,L-alanine the presence of both NaCl and KCl electrolytes in water mainly leads to “salting-out” effect due to the presence of hydrophobic −CH3 group in D,L-alanine.44 But the extent of solubility of D,L-alanine in aqueous KCl solution is somehow affected by mixed as well as competitive behavior of size, polarity, hydrophilicity, and hydrophobicity originated in such solution.1 On the other hand for the amino acid, D,L-serine in both the above solvent systems, it is observed that solubility increases with the increased concentration of electrolytes (Table 3; Figure S1). But the extent of solubility of D,L-serine is greater in aqueous KCl than in aqueous NaCl mixed solvent system. This phenomenon can be explained with the “salting-in”-effect for D,L-serine, in the presence of NaCl and KCl over the whole region of water−electrolyte concentrations. This “salting-in”effect may be due to the structural effects of the amino acid. The amino acid, D,L-serine, contains a polar hydrophilic hydroxyl group (−OH) replacing one H atom of −CH3 group in D,L-alanine. This −OH group increases the polarity of the hydrocarbon backbone of D,L-serine and thereby helps to increase its tendency to be dissolved in ionic solutions. This may support the “salting-in” effect on D,L-serine in the presence of NaCl and KCl over the whole range of electrolyte concentration studied here. The extent of “salting-in” effect is more pronounced in aqueous KCl due to the greater stability of the [K+(−OOCCH(NH3+)CH2OH]Cl− “ion-pair complex”2,44 formed due to interaction among K+, Cl− and zwitterionic amino acid, D,L-serine having −CH2OH side chain with polar as well as apolar parts. The larger size of K+ (1.33 Å)44 probably imparts proper matching to form such ion-pair complex. 4.2. Transfer Free Energetics Due to Solute−Solvent Interactions. Figure 3 represents the variation of ΔG0t (i) (Table 4 or 5) for D,L-alanine and DL-serine against the mol % of NaCl/KCl at 298.15 K. The nature of variation of ΔG0t (i) is guided by the combined as well as gradual change in cavity interaction (ΔG0t,cav(i)), dipole−dipole interaction (ΔG0t,d−d(i)) and all types of chemical interactions (ΔG0t,ch(i)). It is found that for both the amino acids, the ΔG0t (i) value shows a decreasing order with increasing mol % of electrolytes which indicates that the solutes are stabilized with the increased concentration of the electrolytes. Here it is to be noted that the both amino acids are more stabilized in aqueous KCl system than in aqueous NaCl solvent system and the extent of stability of DL-serine is greater than D,L-alanine. The ΔG0t,cav(i) values are found in gradually decreasing order with mol % of NaCl/KCl content (Table 5; Figure S2) which indicates that the involved amino acids acquire more stability with the increased mol % of NaCl or KCl content in the solvent

Figure 3. Variation of ΔG0t (i) in kJ·mol−1 of D,L-alanine and D,L-serine in NaCl + water (dash lines) and in KCl + water (solid lines) mixtures at 298.15 K.

system, i.e., it should be easily accommodated in NaCl or KCl water system than in pure H2O with release of concerned energy because the amino acids molecules are more easily accommodated in larger size of NaCl (2.83 Å)40,41 and KCl (3.14 Å)40,41 when these are transferred from smaller H2O (2.74 Å).37 The ΔG0t,d−d(i) (Table 5; Figure S3) values for the same solutes are decreased gradually with increased mol % of NaCl/ KCl imparting higher stability. The dipole moments of NaCl (9.0 D)41 and KCl (10.27 D)41 are higher than H2O (1.83 D)37 which may support such type of variation. The chemical transfer of Gibbs energies, i.e., ΔG0t,ch(i) values for D,L-alanine and D,L-serine have been evaluated after subtraction of ΔG0t,cav(i) and ΔG0t,d−d(i) from ΔG0t (i) (Table 5). Figure 4 shows the variation of ΔG0t,ch(i) with mol % of aqueous NaCl/KCl solutions. The chemical interaction in term

Figure 4. Variation of ΔG0t,ch(i) in kJ·mol−1 of D,L-alanine and D,L-serine in NaCl + water (dashed lines) and in KCl + water (solid lines) mixtures at 298.15 K.

of hydrophilic interaction is manifested among the charged electrolyte ions and charged amino and carboxyl group of the zwitterionic amino acids in such aqueous NaCl and aqueous KCl mixed binary solvent systems. On the other hand hydrophobic interaction is manifested between hydrophobic backbone of amino acids and water in the presence of such electrolytes. The ΔG0t,ch(i) values (Figure 4) gradually become positive with the increased concentration of NaCl/KCl in the present system indicating clearly destabilization of D,L-alanine and D,L-serine due to chemical interactions between such involved solutes and G



Article

solvent mixtures. Although the cavity forming and dipole−dipole interactions impart stability to the amino acid but the other factors such as chemical interactions associated with total transfer free energy, ΔG0t (i) make ΔG0t,ch(i) value as a positive increment, i.e., D,L-alanine and D,L-serine becomes destabilized due to involved chemical interaction with the increased concentration of NaCl/KCl in NaCl/KCl−water solvent system. Results indicate that the extent of relative stability of D,L-serine is higher in the aqueous KCl and in aqueous NaCl solvent system than D,L-alanine in the same experimental condition. Here it is important to note that the degree of stability of D,L-alanine in NaCl−water electrolyte system is more than in the KCl -water system but the stability of D,L-serine is just reverse in the same solvent systems. The solute molecules tend to form “ion-pair complex” with the charged ions of electrolytes in the solution with the increased concentration of the NaCl/KCl contents in their aqueous mixtures. Therefore, interaction between electrolyte and nonpolar hydrocarbon backbone (−CH3/−CH2OH) of the solute molecules is increased. This phenomenon disrupts the hydrophobic hydration cosphere between the water and solute molecules; consequently the hydrophobic interaction is decreased in a greater extent resulting in positive increment of ΔG0t,ch(i). The positive increment of ΔG0t,ch(i) for D,L-alanine in KCl−water is comparatively higher than in NaCl−water; i.e., the chemical stability of DL-alanine is higher in NaCl−water system. Here comparatively smaller Na+ (0.97 Å)44 and amino acid, D,L-alanine (5.83 Å)43 form stable “ion-pair complex” [Na+{−OOCCH(NH3+)CH3}Cl−] due to proper matching.2,44 Whereas D,L-serine shows more positive increment of ΔG0t,ch(i) in NaCl−water than in KCl−water system which indicates D,L-serine is more stable in KCl−water system than in aqueous NaCl system. Here comparatively larger K+ (1.33 Å)44 and amino acid, D,L-serine (5.93 Å)46 make more stable “ion-pair complex” [K+{−OOCCH(NH3+)CH2OH}Cl−] due to proper matching2,44 which are more pronounced to screen the hydrophobic and electrostatic interactions here than the ion-pair formed between the smaller Na+(0.97 Å)44 and the same. From this observation it be may concluded that KCl−water system would be more stabilizers than NaCl−water solvent system for the larger amino acid. On the hand, the amino acid D,L-serine is relatively more stable than D,L-alanine in both the electrolyte solvent systems. The reasons behind such stability may be due to chemical structural effect of D,L-serine. It contains −CH2OH group as its side chain. The presence of hydroxyl group may impart more polarity as well as the cationophilicity and hydrophilicity to the D,L-serine molecule. Therefore, in respect of polarity, cationophilicity and hydrophilicity D,L-serine is more potential than D,L-alanine. The comparatively larger size serine molecule having higher polarity index allow it to form more stable “ion-pair complex” which are more pronounced to screen the hydrophobic and electrostatic interactions which may help to increase its tendency to be dissolved in ionic solutions due to “salting-in” phenomenon. Consequently it gains higher stability in the electrolyte solvent systems studied here. 4.3. Standard Transfer Entropy of Solutions. Figure 5 represents the variations of total transfer entropy [TΔS0t (i)] with mol % NaCl/KCl. Results show that the TΔS0t (i) values mainly show a positive increment for aqueous KCl solution and a negative increment in aqueous NaCl solution for both the amino acids. Actually TΔS0t (i) is a combined effect of transfer entropies

Figure 5. Variation of TΔS0t (i) in kJ·mol−1 of D,L-alanine and D,L-serine in NaCl + water (dashed lines) and in KCl + water (solid lines) mixtures at 298.15 K.

due to cavity effect, dipole−dipole, and chemical interaction effects, i.e. 0 0 0 T ΔSt0(i) = T ΔSt,cav (i) + T ΔSt,d − d(i) + T ΔSt,ch(i)

(15)

TΔS0t,ch(i)

The values are obtained after the subtraction of TΔS0t,cav(i) and TΔS0t,d−d(i) from TΔS0t (i). Figure 6 represents the

Figure 6. Variation of TΔS0t,ch(i) in kJ·mol−1 of D,L-alanine and D,L-serine in NaCl + water (dashed lines) and in KCl + water (solid lines) mixtures at 298.15 K.

variation of TΔS0t,ch(i) in aqueous NaCl/KCl solvent system. The figure shows similar trend of variation as in Figure 5. The nature of variation is found in increasing order for the water−KCl solvent system in the presence of both the amino acids with a distortion for D,L-serine in the lower concentration of electrolyte. In the water-NaCl solvent system D,L-serine shows negative increment whereas in the same solvent system D,L-alanine shows a positive increment. On the other hand in KCl−water system both the experimental amino acids exhibit positive change of TΔS0t,ch(i) with increased concentration of the electrolyte. At initial electrolytes concentration zwitterionic amino acid molecules are associated with water molecules through hydrogen bonding. As the concentration of NaCl/KCl content increases the hydrogen bonds between the water molecules and amino acid molecules are broken down, consequently the hydrophobic hydration cosphere is disrupted thereby generating a large number of free water molecules, cation and anion of the electrolyte, and amino acid molecules. The cations, K+ and Na+, generated from H



Article

(3) Soto, A.; Arce, A.; Khoshkbarchi, M. K.; Vera, J. H. Effect of the Cation and the Anion of an Electrolyte on the Solubility of DLAminobutyric Acid in Aqueous Solutions: Measurement and Modelling. Biophys. Chem. 1998, 73, 77−83. (4) Pradhan, A. A.; Vera, J. H. Effect of Anions on the Solubility of Zwitterionic Amino Acids. J. Chem. Eng. Data 2000, 45, 140−143. (5) Ramasami, P. Solubilities of Amino Acids in Water and Aqueous Sodium Sulfate and Related Apparent Transfer Properties. J. Chem. Eng. Data 2002, 47, 1164−1166. (6) Soto-Campos, A. M.; Khoshkbarchi, M. K.; Vera, J. H. Interaction of DL-Threonine with NaCl and NaNO3 in Aqueous Solutions: E.M.F. Measurements with Ion-Selective Electrodes. J. Chem. Thermodyn. 1997, 29, 609−622. (7) Mandal, U.; Bhattacharya, S.; Das, K.; Kundu, K. K. Medium effects on deprotonation of mono- and di-protonated piperazines in binary aqueous mixtures of some protic, aprotic and dipolar aprotic cosolvents. Z. Phys. Chem. 1988, 159, 21−36. (8) Held, C.; Cameretti, L. F.; Sadowski, G. Measuring and Modeling Activity Coefficients in Aqueous Amino-Acid Solutions. Ind. Eng. Chem. Res. 2011, 50, 131−141. (9) Lu, J.; et al. Solubilities of Glycine and Its Oligopeptides in Aqueous Solutions. J. Chem. Eng. Data 2006, 51, 1593−1596. (10) Pradhan, A. A.; Vera, J. H. Effect of acids and bases on the solubility of amino acids. Fluid Phase Equilib. 1998, 152, 121−132. (11) Romero, C. M.; Oviedo, C. D. Effect of Temperature on The Solubility of alpha-Amino Acids and alpha, omega - Amino Acids in Water. J. Solution Chem. 2013, 42, 1355−1362. (12) Kamali-Ardakani, M.; Modarress, H.; Taghikhani, V.; Khoshkbarchi, M. K. Activity Coefficients of Glycine in Aqueous Electrolyte Solutions: Experimental Data for (H2O + KCl + Glycine) at 298.15 K and (H2O + NaCl + Glycine) at 308.15 K. J. Chem. Thermodyn. 2001, 33, 821−836. (13) Tomé, L. I. N.; Pinho, S. P.; Jorge, M.; Gomes, J. R. B.; Coutinho, A. P. Salting-in with a Salting-out Agent: Explaining the Cation Specific Effects on the Aqueous Solubility of Amino Acids. J. Phys. Chem. B 2013, 117, 6116−6128. (14) Ferreira, L. A.; Macedo, E. A.; Pinho, S. P. The effect of ammonium sulfate on the solubility of amino acids in water at (298.15 and 323.15) K. J. Chem. Thermodyn. 2009, 41, 193−196. (15) Von Hippel, P.; Schleich, T. Structure of stability of biological macromolecules; Marcel Dekker: New York, 1969. (16) Anfinsen, C. B.; Scheraga, H. A. Experimental and Theoretical Aspects of Protein Folding. Adv. Protein Chem. 1975, 29, 205−300. (17) Reading, J. F.; Watson, I. D.; Hedwig, G. R. Thermodynamic properties of peptide solutions 5. Partial molar volumes of glycylglycine, glycyl-DL-leucine, and glycyl-DL-serine at 308.15 and 318.15 K. J. Chem. Thermodyn. 1990, 22, 159−165. (18) Lapamje, S. In physico-chemical Aspects of proteins denaturation; Wiley Interscience: New York, 1978. (19) Islam, M. N.; Wadi, R. P. Thermodynamics of Transfer of Amino Acids from Water to Aqueous Sodium Sulfate. Phys. Chem. Liq. 2001, 39, 77−84. (20) Bani pal, T.-S.; Singh, G.; Lark, B.-S. Partial molar volumes of transfer of some amino acids from water to aqueous glycerol solutions at 25 °C. J. Solution Chem. 2001, 30, 657−670. (21) Das, P.; Chatterjee, S.; Basu Mallick, I. Thermodynamic studies on amino acid solvation in some aqueous alcohols. J. Chin. Chem. Soc. 2004, 51, 1−6. (22) Koseoglu, F.; Kilic, E.; Dogan, A. Studies on the Protonation Constants and Solvation of α-Amino Acids in Dioxan-Water Mixtures. Anal. Biochem. 2000, 277, 243−246. (23) Nozaki, Y.; Tanford, C. The Solubilities of Amino acids and related compounds in Aqueous Urea Solutions. J. Boil. Chem. 1963, 238, 4074−4081. (24) Abu-Hamdiyyah, M.; Shehabuddin, A. Transfer enthalpies and entropies of amino acids from water to urea solutions. J. Chem. Eng. Data 1982, 27, 74−76.

the electrolytes possess a different charge density, size, and polarizing capacity. Due to higher positive charge density on the comparatively smaller Na+ cation (0.97 Å),44 it can be associated more strongly with the free water molecules as well as amino acid molecules, and hence the entropy of the NaCl + water + amino acid system will be diminished comparatively more than of the KCl + water + amino acid system. For these reasons amino acid induced disorderness, i.e., higher entropy, in the case of KCl− water solvent system will be greater than in the other aqueous NaCl solution. On the other hand it is to be noted that D,L-serine induces greater association thereby lesser disorderness in NaCl + water system due to its more pronounced hydrophilic character toward water molecules in solutions.

■

CONCLUSION Solubility and thermodynamics of solvation of the amino acids in aqueous solution may play important role in their biochemical and biophysical actions in human physiology. The solubility study of D,L-alanine and D,L-serine under different experimental conditions shows that electrolytes affect the solubility of D,L-alanine and D,L-serine. The dipole−dipole and cavity forming interactions in such electrolyte system stabilize the amino acids. The chemical stability of D,L-alanine is higher in NaCl water system than in KCl−water system, and D,L-serine is more stable in the KCl−water system than the NaCl−water system. The electrolytes sodium chloride and potassium chloride disrupt the hydrophobic hydration cosphere between the zwitterionic amino acids (i.e., D,L-alanine and D,L-serine) and water molecules. The amino acids induce more disorderness in the case of KCl−water solvent system than the NaCl-water. Amino acid D,L-serine induces lesser disorderness than D,L-alanine in the NaCl + water solvent system due to the stronger hydrophilic nature of the former.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00351. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

The authors are thankful to the Department of Chemistry, VisvaBharati for financial assistance. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the Department of Chemistry, Visva-Bharati for use of computational facilities and Shibpur Dinobundhoo Institution (college) for encouragement in this study.

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REFERENCES

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