Solvation of Basic and Neutral Amino Acids in Aqueous Electrolytic

Article pubs.acs.org/jced

Solvation of Basic and Neutral Amino Acids in Aqueous Electrolytic Solutions: Measurements and Modeling Farid I. El-Dossoki* and Maha M. El-Damarany

Downloaded by UNIV OF MICHIGAN ANN ARBOR on September 10, 2015 | http://pubs.acs.org Publication Date (Web): September 3, 2015 | doi: 10.1021/acs.jced.5b00393

Chemistry Department, Faculty of Science, Port-Said University, Port-Said, 42521, Egypt ABSTRACT: The molal solubilities, density, and refractive index of the basic amino acid (L-lysine) and the neutral amino acid (L-cysteine) in aqueous solutions of potassium iodide (KI), sodium bromide (NaBr), sodium chloride (NaCl), sodium sulfate (Na2SO4), and calcium chloride (CaCl2) at 298.15 K were determined experimentally. From the values of the measured refractive indices, molal solubilities, and the densities, the excess refractive indices, the molar refractions, the polarizabilities, and the apparent molar volumes of the systems under study were calculated and discussed. The solvation of the amino acids under study was discussed in terms of their molal solubilities, apparent molar volume, and refractive index data. The phase diagrams of the studied tricomponent systems (water−salt−amino acid) were also determined and discussed. The solubility of L-cysteine and L-lysine were discussed in terms of the nature of the electrolytic salts under study. The effect of the electrolytic solutions under study on the solubility of L-cysteine and L-lysine was compared to that found in the literature of the same electrolytic solutions on the amino acids (DL-valine, DL-serine, glycine, and DL-alanine) at the same temperature. A salting-in effect of the electrolytic solutions under study (potassium iodide, sodium chloride, sodium bromide, calcium chloride, and sodium sulfate) and other electrolytic solutions (potassium fluoride, potassium bromide, sodium iodide, potassium sulfate, and barium chloride) (from literature) was observed for L-cysteine amino acid (present study), DL-valine, DL-serine, glycine, and DL-alanine (from literature) A salting-out effect of the electrolytic solutions under study (potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride) was observed for L-lysine amino acid (present study), and of NaF solution for amino acids; DL-alanine and DL-valine (from literature). The effect of the nature of the electrolytic salts under study agree with that in the literature as in the following order: Na2SO4 > NaCl, NaBr > NaCl, CaCl2 > NaCl.

1. INTRODUCTION As reported earlier,1−4 many interactions in solutions depend on the solvation process of the substances. Also density and refractive index measurements of solutions are used to shed some light on the solute−solvent interactions and configuration of their mixtures.5−14 Crystallization or precipitation methods are often used for the separation of amino acids in aqueous solution. The total production cost includes nearly 50% for the separation process.15 There are two amino acids under study in this work: L-lysine and L-cysteine. L-Cysteine is a precursor used in many industries such as the pharmaceutical, food, flavors and personal-care industries16 and as a processing aid17 for baking. L-Cysteine has been proposed also as a preventative or antidote for some of the negative effects of alcohol, including liver damage and hangover.15 The formation of disulfide bonds in cysteine plays a role in crosslinking proteins, resulting in an increase in the proteins rigidity and proteolytic resistance. Disulfide bridges between cysteine residues, inside the cell, support the protein’s tertiary structure. An example of a protein with cysteine crosslinking is insulin.18 In the body, L-lysine is a considered as a building block for all protein. Also, L-lysine plays a major role in Ca absorption, aiding in recovering from © XXXX American Chemical Society

surgery or sports injuries and building muscle protein, and the body’s production of enzymes, hormones, and antibodies.19 Cysteine as a neutral amino acid comprises a basic group and an acidic group, resulting in zwitterionic molecules in the neutral range of pH. Basic amino acids such as lysine have two amino groups and one carboxyl group, which also give rise to zwitterionic molecules with a basic amino group. Important interactions of individual ions with the zwitterionic amino acid molecules of a large dipole moment were expected. The design of the separation processes of amino acids can be a problem in some cases, therefore studying the solvation of amino acids in different electrolytes is useful for a solution of this problem. Also the effect of different cations and anions is of potential interest in the design of the separation process of amino acids. To aid in the solution of the design of the separation process of amino acids, the solvation of some amino acids, DL-valine, DL-serine, glycine, and DL-alanine in different electrolytic solutions; NaCl, KCl, NaNO3, NaBr, KBr, KNO3, Received: May 5, 2015 Accepted: August 27, 2015

A

DOI: 10.1021/acs.jced.5b00393 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

same range of the electrolyte concentrations of the same studied salts in the literature.20−23 The procedure of solution preparation and measurements are as described in the references22,23 as follow: “Vials of 24 mm outer diameter and 95 mm height were used as sample bottles. The amino acids were added in excess of the amount required for saturation. The electrolyte was then added, as a 20 mL solution, into the sample bottles. The sample bottles were sealed using Parafilm and kept in a thermostatic water bath at 298.15 ± 0.1 K. The solutions were agitated for 48 h using Teflon-coated magnetic stir bars. The mixing was then stopped, and the solutions were allowed to settle for 7 h. Samples were taken of the supernatant liquid phase using a plastic syringe and filtered through 0.22 μm fine filter paper. A glass dish was weighed empty and with the filtered solution. The dish and its weighed cap were put into an oven for 48 h at about 380 K and weighed again with the dry sample. The solubility of the sample was calculated from knowledge of the mass of the empty dish, the mass of the cap, the mass of the dish with the solution and the cap, the dry mass of the solid, and the electrolyte concentration of the sample. Humphrey et al.24 reported the measurements for the aqueous cysteine solutions of molalities significantly comparable to the saturated molality obtained in this work. The reason for this inconsistency is unknown. The values reported in this work are the average of at least three replicates. In the replicates, different quantities of the amino acid were taken in excess of that at saturation. The results were found to differ by < 0.5 % molal. To check the possibility of adsorption or incorporation of the used electrolytes on the solid-phase amino acid, atomic absorption was used to analyze the cations in the solution. The concentrations of the cations in the electrolyte−water system and in the amino acid−electrolyte−water system were compared. Quantities of amino acid 5 %, 10 %, and 50 % in excess to saturation were added for these comparisons, and the cation concentration was measured in the supernatant phase. The maximum difference in the results was ± 0.0005 molal, implying that, even with different quantities of the amino acid added, no appreciable amount of electrolyte was precipitated or adsorbed on the amino acid in the solid phase”. The refractive indices of the prepared systems were measured ± 0.0001 using an Abbe refractometer connected with an ultrathermostat of type Kottermann 4130.

NaF, KI, NaI, K2SO4, Na2SO4, BaCl2, and CaCl2 was reported.20−23 The present study aims to report the solvation behavior of the zwitterionic amino acids: L-cysteine and L-luciene in aqueous solutions of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride at 298.15 K. Also the present study aims to compare the obtained results with that found in the literature for amino acids glycine, DL-alanine, DL-valine, and DL-serine in the same electrolytic solutions and other electrolytic solutions at the same temperature (298.15 K).


2. EXPERIMENTAL SECTION 2.1. Materials. Amino acids L-cysteine and L-lysine of 99 % purity, were obtained from LOBA Chemie Company (India) and used without further purification (Scheme 1). Scheme 1

Sodium sulfate (98.0 %), and calcium chloride (98 %), potassium iodide (98 %), sodium bromide (99 %), and sodium chloride (98.5 %) were obtained from Fisher Scientific Company (England). The salts under study were dried at 393 K for 72 h, then cooled using a desiccator. The description of the chemicals used have also been provided in Table 1. 2.2. Solutions, Apparatus, and Procedure. Five different molal concentrations of the electrolyte solutions under study were prepared using bidistilled water. The bidistilled water has a conductivity of < 0.8 μS cm−1. The effect of the anion and the cation was compared with that in the literature by using the

3. RESULTS AND DISCUSSION 3.1. Molal Solubility. The determined molal solubilities of L-lysine and L-cysteine in water and in aqueous solutions of different molar concentration of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) are reported in Table 2, and shown in Figures 1 and 2. The obtained data show that the molal solubility’s of L-lysine are decreased in the presence of different molar concentration of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride indicating a salting-out effect of the studied salts on the molal solubility’s of L-lysine and the effect increase as the concentration of the studied salts increase. On the other hand the results show that the molal solubility’s of L-Cysteine are increased in the presence of different molar concentration of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride indicating a salting-in effect of the studied salts on the molal solubility’s of Lcysteine and the effect increase as the concentration of the studied salts increase.

Table 1. Description of Chemicals Used

chemical name sodium sulfate calcium chloride potassium iodide sodium bromide sodium chloride cysteine lysine

source Fisher Scientific Company (England)

LOBA Chemie Company (India)

initial mole fraction purity 0.980 0.980

purification method only the salts were dried at 393 K for 72 h

final mole fraction purity 0.980 0.980

0.980

0.980

0.990

0.990

0.985

0.985

0.990 0.990

0.990 0.990 B



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Table 2. Molal Solubility (mol kg−1) of L-Lysine (Lys) and L-Cysteine (Cys) in Saturated Aqueous Electrolytic Salt Solutions at 298.15 K and Pressure (p = 0.1 MPa)a NaBr

a

NaCl

KI

Na2SO4

CaCl2

[salt]mol kg−1

Lys

Cys

Lys

Cys

Lys

Cys

Lys

Cys

Lys

Cys

0.00 0.05 0.10 0.50 1.00

4.7339 4.7326 4.7202 4.4835 4.1721

1.8212 2.4437 2.6963 2.7115 2.7906

4.7339 4.1851 4.1646 4.0479 3.9724

1.8212 1.8626 2.1035 2.2020 2.4689

4.7339 4.4163 4.4142 4.3119 4.1087

1.8212 2.2078 2.2081 2.3597 2.5836

4.7339 4.7127 4.6791 4.4664 4.1389

1.8212 2.5182 2.5979 2.6007 2.6222

4.7339 4.0575 4.0335 3.9759 3.8361

1.8212 3.0166 3.0953 3.1006 3.1051

Relative standard uncertainty in the solubilities ur(m) = 1.05 %.

cation (C+ or C2+) and the anion (A− or A−2) of the electrolyte, of the form:


In the case of 1:1 electrolytes −

AA+ + C+A− ↔ C+(−AA+)A−

In the case of 2:1 electrolytes −

+

+−

AA+ + C + (A−)2 ⇌ C +

AA+ A− − + A− AA

or 2(−AA+) + C++A−2←→......(+AA−)C++(−AA+)A−......

In the case of 1:2 electrolytes C+2 (−AA+)2X−2 −

AA + (C+)2A−− ⇌

C+

−

+

−

C Figure 1. Molal solubility of L-lysine in aqueous solutions of ▲, NaCl; △, NaBr; ●, CaCl2; ○, Na2SO4; ■, KI.

AA+ +

A−−

AA

or 2(−AA+) + C+2A−− ↔ ....... C+(−AA+)A−−(+AA−)C+........″

This interpretation can explain the salting-in effect as in the case of L-cysteine in the presence of different molar concentrations of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride. The molal solubilities of L-cysteine in water is less than that in the presence of the studied salts which may be due to the ability of L-cysteine to form dimers through the SH group which can decrease the solvation of L-cysteine in water as in Scheme 2.18 Scheme 2

Figure 2. Molal solubility of L-cysteine in aqueous solutions of NaCl; △, NaBr; ●, CaCl2; ○, Na2SO4; ■, KI.

▲,

The presence of salts may prevent or lower the formation of these dimers, and so enhance the solvation of L-cysteine in water in addition to the formation of a soluble complexes as described above. On the other hand, in the case of L-lysine, where the saltingout effect was observed in the presence of different molar concentrations of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride, we think that there are two opposite factors affecting the molal solubilities of

The effect of the studied salt on the molal solubility’s of Llysine and L-cysteine can be interpreted as described in the reference23 as follows: “since the experiments described in the previous section showed that no salt precipitates with the amino acid, one concludes that the anions retain the amino acid in the aqueous phase. The amino acids, existing as zwitterions − AA+ in the system, may form soluble complexes with the C



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L-lysine,

which has basic properties, as a result of the second amino group in its chemical structure as shown in Scheme 1. The first factor which may increase the molal solubilities of Llysine is the presence of the zwitterion ion formed between the carboxylic group and the neighboring amino group (salting-in effect) as described above by formation of soluble complexes with the salts under study, where L-lysine plays a major role in calcium absorption in the human body.19 The second opposite factor which may decrease the molal solubilities of L-lysine, in the presence of the studied salts (salting-out effect), is the presence of the second amino group. In this case there is a solvation competition between the second amino group, the water molecules, and the salt. This solvation competition is considered positive (high) between the salt and the water molecules and so is negative (low) between the water and the second amino group. This will result in a decrease in the molal solubility of L-lysine (decrease in the solvation process), in the presence of the studied salts (salting-out effect). The net result is the salting-out effect is then due to the higher effect of the second factor than that of the first factor. The sudden change in the molal solubility of the studied amino acids in the presence of different molar concentration of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride may be related to the change in the activity coefficient of the ions (of salts and zwitterion amino acid ions) in solution. The molal solubility of the studied amino acids in water and in the presence of different molar concentration of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride was also analyzed in term of the Setchenow relationship25 (converted to a molality basis) as in the following equation: log10(m /mo) = kC

Figure 3. log(m/mo) of L-lysine as a function of salt concentration: ▲, NaCl, △, NaBr; ●, CaCl2; ○, Na2SO4; ■, KI.

(1)

where mo and m are the molal solubility of the amino acid in water in the absence and in the presence of an electrolyte, respectively, and C is the salt concentration. Examination of the plots in the present study for the amino acids under study, in water and in the presence of different molar concentrations of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride, suggest that a quadratic term was added, leading to the following extended Setchenow equation: log10(m /mo) = k 2C 2 + k1C + k 0

Figure 4. log(m/mo) of L-cysteine as a function of salt concentration: ▲, NaCl, △, NaBr; ●, CaCl2; ○, Na2SO4; ■, KI.

in the presence of different molar concentrations of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride, were compared with that for the molal solubilities of the amino acids: DL-valine, DL-serine, glycine, and DL-alanine, in aqueous solution of the same molar concentrations of the same salts.23 A salting-in effect of the electrolytic solutions under study (potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride) and other electrolytic solutions (potassium fluoride, potassium bromide, sodium iodide, potassium sulfate, and barium chloride)23 was observed for the cysteine amino acid (present study), glycine, DL-alanine, DL-valine, DL-serine.23 A salting-out effect of the electrolytic solutions under study (potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride) was observed for L-lysine amino acid (present study), and of the NaF solution for amino acids DL-alanine and 23 DL-valine. The effect of the nature of the electrolytic salts under study agree with that in the literature21−23 as in the following order: Na2SO4 > NaCl, NaBr > NaCl, CaCl2 > NaCl, NaNO3 > KNO3, BaCl2 > CaCl2. The amino acids under study have higher solubility in the presence of K+ than in the presence of Na+ and in the presence

(2)

The relation of log10(m/mo) versus C for the studied systems are shown in Figures 3 and 4. The parameters; ko, k1, and k2 of eq 2 were evaluated and reported in Table 3 and shown in Figure 5. The positive values of k1 (Figure 3) in the case of Lcysteine indicate a salting-in effect of the salts under study on the molal solubility of L-cysteine, while the negative values of k1 (Figure 4) in case of L-lysine indicate a salting-out effect of the salts under study on the molal solubility of L-lysine. Figure 5 shows that the values of k1 adequately represent the experimental molal solubility of the amino acids under study where the same trend of the change in the solubility was observed. The determined parameters were examined to see whether they are additive, and it was found that the difference between the parameters are not sensibly constant. This indicates that the determined parameters are not additive. Depending on the values of k1 and k2 of the extended Setchenow relationship, the molal solubilities of the amino acids studied here (L-lysine and L-cysteine) in aqueous solution D



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Table 3. Parameters K0, K1, and K2 of the Studied Amino Acids in Saturated Aqueous Electrolytic Salt Solutions at 298.15 K acid L-cysteine


L-lysine

K0 K1 K2 K0 K1 K2

NaBr

NaCl

Na2SO4

CaCl2

KI

0.1400 0.1033 −0.0589 0.0029 −0.0477 −0.0102

0.0232 0.1491 −0.0413 −0.0516 −0.0407 0.0161

0.1446 0.0321 −0.0187 −0.0001 −0.0425 −0.0156

0.2221 0.0304 −0.0211 −0.0672 −0.0112 −0.0129

0.0793 0.0589 −0.0138 −0.0292 −0.0127 −0.0196

< I−, with both Na and K cations, may be the reason for the change in the solubility of the amino acids under study in the same order: F− < Cl− < Br− < I−.23 The chemical structure of the studied amino acids (Schemes 1 and 3) was found to have an effect on the amino acids solubility in aqueous solution. The determined molal solubility of the amino acids under study in the presence and in the absence of the same used salts at the same temperature21−23 has the order glycine > DL-alanine > L-lysine > DL-valine > DL-serine > L-cysteine. This can be explained in term of the chemical structure of DL-valine which has two CH2 groups more than DLalanine. Hence, the hydrophobic interactions in DL-alanine and DL-valine are larger than those in glycine, resulting in a saltingout effect, even with the chloride and fluoride anion.7 The solubility of DL-serine, in the presence of the studied salts, is almost twice its solubility in pure water, if compared to that for DL-valine. This may be related to the presence of an OH group in DL-serine (Scheme 3). The presence of the OH group, resulting in a salting-in effect with all anions studied in ref 23, with chloride anions,21 and with nitrate anions.22 The lower solubility of L-cysteine may be related to its hydrophobic properties as a result of the presence of SH group and so its ability to form dimers through disulfide bonds (Scheme 2),18 while the relatively higher solubility of L-lysine may be related to its basic properties as a result of two amino groups (Scheme 1). 3.2. Apparent Molar Volume. The density of the L-lysine and the L-cysteine in aqueous solutions of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride at 298.15 K was determined experimentally and reported in Tables 4 and 5 (± 0.0005). From the molal solubilities and density values the apparent molal volumes, φV, of L-lysine and L-cysteine in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, were calculated using following:26

Figure 5. K1 for the used electrolytes; 1 = NaCl, 2 = NaBr, 3= CaCl2, 4 = Na2SO4, 5 = KI, with ▲, L-cysteine; △, L-lysine; ●, glycine; ○, DLalanine; □, DL-valine; and ■, DL-serine.

of Ba2+ than in the presence of Ca2+. This may be related to the different abilities of the cations to shield the hydrophobic interactions of these amino acids and also may be related to the kind of the formed complexes in the aqueous phase between the amino acid and the cations and.23 The higher electrolytic properties of K and Ba salts than that of Na and Ca salts respectively, as a result of the higher ionic radius of K+ and Ba2+, may also be used to explain the higher solubility of the amino acids under study in the presence of K+ than in the presence of Na+ and in the presence of Ba2+ than in the presence of Ca2+.23 The anion charge effect on the solubility of the amino acids under study, for the amino acids studied here, L-lysine, Lcysteine, and that in the literature, glycine, DL-alanine, DL-valine, and DL-serine21−23 in the presence of potassium and sodium as cations, are in the following order: F− < Cl− < Br− < I− < NO3− < SO4. The difference in the solubility trends was related to the different abilities of the cations to shield the hydrophobic interactions of these amino acids and also may be related to the kind of formed complexes in the aqueous phase between the amino acid and the cations. The difference in the electrolytic properties of the halogenated salts in the order, F− < Cl− < Br−

φV =

M 1000 ⎡ 1 1⎤ − − ⎥ ⎢ ρ m ⎣ ρ° ρ⎦

(3)

where M is the molar mass of L-lysine and L-cysteine, m1 is the molal concentration of L-lysine and L-cysteine in solution, ρ and ρ° are the densities of solution and solvent, respectively. The calculated apparent molal volumes φV of L-lysine and L-cysteine in water and in potassium iodide, sodium bromide, sodium

Scheme 3

E



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Table 4. Density, ρ (g cm−3) and the Apparent Molal Volumes, φV (cm3 mol−1) of L-Lysine in Saturated Aqueous Electrolytic Salt Solutions at 298.15 K and Pressure (p = 0.1 MPa)a NaBr [salt] mol kg−1 0.00 0.05 0.10 0.50 1.00 a

NaCl

KI

Na2SO4

CaCl2

ρ

φV

ρ

φV

ρ

φV

ρ

φV

ρ

φV

1.2421 1.2419 1.2401 1.2306 1.2150

75.9716 76.0062 76.3153 76.4188 77.2822

1.2421 1.1435 1.1400 1.1316 1.1221

75.9716 97.2359 98.1226 99.8150 102.2336

1.2421 1.1958 1.1855 1.1752 1.1647

75.9716 84.5865 87.2765 89.2164 90.4653

1.2421 1.2390 1.2341 1.2131 1.1961

75.9716 76.2031 77.3608 80.5953 81.9800

1.2421 1.1621 1.1591 1.1421 1.1311

75.9716 90.7772 91.4472 96.0521 98.3518

Relative standard uncertainty in the density ur(ρ) = 0.04 %.

Table 5. Density, ρ (g cm−3) and the Apparent Molal Volumes, φV (cm3 mol−1) of L-Cysteine in Saturated Aqueous Electrolytic Salt Solutions at 298.15 K and Pressure (p = 0.1 MPa)a


NaBr [salt] mol kg 0.00 0.05 0.10 0.50 1.00 SD a

−1

NaCl

KI

Na2SO4

CaCl2

ρ

φV

ρ

φV

ρ

φV

ρ

φV

ρ

φV

1.0741 1.1072 1.1287 1.1305 1.1401 0.0263

73.4864 68.7397 64.0867 63.6369 61.2992 4.8626

1.0741 1.0900 1.1122 1.1290 1.1328 0.0252

73.4864 65.4235 59.7368 54.2414 54.4153 8.1401

1.0741 1.0797 1.0887 1.0980 1.1160 0.0165

73.4864 73.3998 73.2087 71.4153 67.3238 2.6252

1.0741 1.0982 1.1074 1.1775 1.1881 0.0507

73.4864 73.3779 70.8659 70.4153 66.3238 16.6761

1.0741 1.1674 1.1769 1.1875 1.1981 0.0498

73.4864 55.3846 53.5426 50.2619 47.0354 10.3134

Relative standard uncertainty in the density ur(ρ) = 0.04 %. SD = standard deviation.

chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, are given in Tables 4 and 5. The apparent molal volume versus the molar concentration of potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions for L-lysine and L-cysteine are shown in Figures 6 and

Figure 7. Apparent molar volume of L-cysteine in aqueous solutions of ▲, NaCl; △, NaBr; ●, CaCl2; ○, Na2SO4; ■, KI.

in the apparent molal volume of L-cysteine with the concentration of the salts may be related to the decrease in the density of L-lysine and increase in the density of L-cysteine with the concentration of the salts, respectively, in which the volume has an inverse proportion change with the density (d = m/v). Also it was found that the density of L-lysine and L-cysteine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions has the following order: Na2SO4 > NaCl, NaBr > NaCl, CaCl2 > NaCl. On the other hand the apparent molal volume of L-lysine and L-cysteine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions has the following reverse order: Na2SO4 < NaCl, NaBr < NaCl, CaCl2 < NaCl (Figures 6 and 7).

Figure 6. Apparent molar volume of L-lysine in aqueous solutions of ▲, NaCl; △, NaBr; ●, CaCl2; ○, Na2SO4; ■, KI.

7. As shown from Figures 6 and 7, the apparent molal volume of L-lysine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, increases as the concentration of the salts increase. On the other hand the apparent molal volume of L-cysteine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, decreases as the concentration of the salts increase. This increase in the apparent molal volume of L-lysine and decrease F



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Table 6. Refractive Indices of L-Lysine (Lys) and L-Cysteine (Cys) in Saturated Aqueous Electrolytic Salt Solutions at T = 298.15 K and Pressure (p = 0.1 MPa)a NaBr mol kg−1 0.00 0.05 0.10 0.50 1.00 a

NaCl

KI

Na2SO4

CaCl2

Lys

Cys

Lys

Cys

Lys

Cys

Lys

Cys

Lys

Cys

1.4155 1.4155 1.4176 1.4139 1.4134

1.3439 1.3467 1.3495 1.3559 1.3608

1.4155 1.4126 1.4124 1.4114 1.4114

1.3439 1.3457 1.3485 1.3553 1.3594

1.4155 1.4174 1.4176 1.4210 1.4238

1.3439 1.3493 1.3522 1.3619 1.3704

1.4155 1.4171 1.4147 1.4163 1.4202

1.3439 1.3486 1.3511 1.3588 1.3682

1.4155 1.4172 1.4146 1.4172 1.4203

1.3439 1.3497 1.3494 1.3577 1.3667

Relative standard uncertainty in the refractive indices ur(n) = 0.007 %.

Table 7. Excess Refractive Indices of L-Lysine (Lys) and L-Cysteine (Cys) in Saturated Aqueous Electrolytic Salt Solutions at 298.15 K


NaBr mol kg 0.00 0.05 0.10 0.50 1.00 SD

−1

NaCl

KI

Na2SO4

CaCl2

Lys

Cys

Lys

Cys

Lys

Cys

Lys

Cys

Lys

Cys

0.0821 0.0810 0.0777 0.0740 0.0700 0.0050

0.0105 0.0122 0.0140 0.0160 0.0174 0.0078

0.0821 0.0781 0.0769 0.0716 0.0680 0.0056

0.0105 0.0112 0.0130 0.0154 0.0160 0.0025

0.0821 0.0819 0.0799 0.0766 0.0726 0.0040

0.0105 0.0138 0.0145 0.0175 0.0192 0.0034

0.0821 0.0805 0.0780 0.0730 0.0690 0.0054

0.0105 0.0120 0.0144 0.0155 0.0170 0.0026

0.0821 0.0817 0.0790 0.0760 0.0720 0.0042

0.0105 0.0132 0.0138 0.0165 0.0184 0.0031

charges on the ions and the relative permittivity of the solvent) there is usually little difficulty in separating the properties of the ion pair from the long-range nonspecific ion−ion interactions that exist in all electrolyte solutions. However, when the ion association is weak, there is a strong correlation between these nonspecific ion−ion interactions (characterized in terms of activity coefficients) and ion pair formation (characterized in terms of an association constant).″ The ion pair was expressed in terms of the ion pair association constant which can be calculated by applying many theories, all of them depending on the activity coefficients of ions in solutions. The change in the apparent molal volume is usually accompanied by the ion pair formation.28 3.3. Refractive Index, Molar Refraction, and Polarizability. The refractive indices of L-lysine and L-cysteine in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, were measured at 298.15 K and the values are reported in Table 6. The excess refractive indices (nE) as a result of the presence of L-lysine and L-cysteine in solutions were calculated according eq 4, reported in Table 7, and shown in Figures 8 and 9.

The interaction between the studied amino acids and the cosolutes in the ternary systems of (amino acid + salts + water) is one of the following types of interaction:27 (i) ion−charged group interactions occurring between ions of salts (Na+, K+, Ca2+, Cl−, Br−, and SO4−2) and charged end groups (NH3+, COO−) in the amino acid which leads to positive change in the apparent molal volume; (ii) ion−nonpolar (hydrophobic) group interactions occurring between ions of salts (Na+, K+, Ca2+, Cl−, Br−, and SO4−2) and nonpolar groups of the amino acids which leads to negative change in the apparent molal volume. The positive change in the apparent molal volume values observed for L-lysine throughout the concentration range of salt solutions may be due to the dominance of the ion− charged group interactions over the ion−nonpolar group interactions. On the other hand, the negative change in the apparent molal volume values observed for L-cysteine throughout the concentration range of salt solutions may be due to the dominance of the ion−nonpolar group interactions over the ion−charged group interactions. This decrease in the apparent molal volume observed for L-cysteine may also explain the increase in the molal solubilities of L-cysteine in the presence of the studied salts which may be due to the ability of salts to prevent or lower the formation of L-cysteine dimers. As shown above, the interactions are dealt with in terms of ionic strength and ionic atmospheres (Debye−Hückel or other theories of activity coefficients). But at high concentration of the cosolutes as in the present case (0.5 and 1.0 mol·L−1) the interactions cannot be seen only in terms of ionic strength and ionic atmospheres (Debye−Hückel or other theories of activity coefficients) but also in terms of ion pairing.28 Ion pairing was reported by Marcus28 as follows: “Ion pairing describes the (partial) association of oppositely charged ions in electrolyte solutions to form distinct chemical species called ion pairs. Ion pair formation is invoked as the most plausible explanation either of certain types of direct experimental evidence or of deviations observed at moderate concentrations from predictions of electrolyte theories that accurately describe the properties of very dilute electrolyte solutions. If the ion association is reasonably strong (the value depends on the

n E = (namino acid in salt solution) − (nsolvent or nsalt solution)

(4)

As shown from Figures 8 and 9, the excess refractive indices of L-lysine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, decrease as the concentration of the salts increase. On the other hand the excess refractive indices of L-cysteine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, increase as the concentration of the salts increase. This decrease in the excess refractive indices of L-lysine and increase in the excess refractive indices of L-cysteine with the concentration of the salts may be related to the decrease in the molal solubility of L-lysine (salting-out effect) and increase in the molal G



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bromide, sodium chloride, sodium sulfate and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, the molar refraction (Rm) can be calculated5 using eq 5. Rm =

n2 − 1 φV n2 + 2

(5)

where φV is the apparent molar volume of L-lysine and Lcysteine in solutions, n is the refractive index of L-lysine and Lcysteine in water after subtraction of the values of the refractive indices of the salts under study. The relation between the polarizability (α) of the molecules and the refractive index of the substance was reported by the Lorenz−Lorenz formula29 in eq 6:


(n2 − 1)/(n2 + 2) = (4παN /3φV )

(6)

where φV is the apparent molar volume and N is Avogadro’s number. The polarizabilities of L-lysine and L-cysteine in water were calculated applying eq 6. The values of the calculated molar refraction (Rm) and polarizability (α) were recorded in Tables 8 and 9, respectively. The molar refraction and the polarizability are directly proportional to the apparent molal volume., As shown from Tables 8 and 9, the molar refraction and the polarizability of Llysine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions, increase as the concentration of the salts increase. On the other hand the molar refraction and the polarizability of L-cysteine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions decrease as the concentration of the salts increase. This increase in the molar refraction and the polarizability of L-lysine and decrease in the molar refraction and the polarizability of L-cysteine with the concentration of the salts may be related to the increase in the apparent molar volume of L-lysine and decrease in the apparent molar volume of L-cysteine with the concentration of the salts, respectively. Also it was found that the molar refraction and the polarizability of L-lysine and L-cysteine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions has the following reverse order: Na2SO4 < NaCl, NaBr < NaCl, CaCl2 < NaCl. 3.4. Phase Diagram. The percentage of each component in the tricomponent systems (amino acid−salt−water) was calculated, and the phase diagrams of these systems was drawn as shown in Figures 10 and 11 as examples. As shown from Figure 10, for L-lysine, the liquid−liquid phase (undashed area) in the presence of sodium sulfate is greater than that in the presence of sodium chloride (Na2SO4 > NaCl) while the solid−liquid phase (dashed area) in the presence of sodium

Figure 8. Excess refractive index of L-lysine in aqueous solutions of ▲, NaCl; △, NaBr; ●, CaCl2; ○, Na2SO4; ■, KI.

Figure 9. Excess refractive index of L-cysteine in aqueous solutions of ▲, NaCl; △, NaBr; ●, CaCl2; ○, Na2SO4; ■, KI.

solubility of L-cysteine (salting-in effect) with the concentration of the salts, respectively. Also it was found that the excess refractive indices of L-lysine and L-cysteine, in water and in potassium iodide, sodium bromide, sodium chloride, sodium sulfate and calcium chloride ((0.05, 0.1, 0.5, and 1.0) mol·L−1) solutions has the following order: Na2SO4 > NaCl, NaBr > NaCl, CaCl2 > NaCl (Figures 8 and 9). Also from the values of the measured refractive indices, of Llysine and L-cysteine, in water and in potassium iodide, sodium

Table 8. Molar Refraction (Rm) of L-Lysine (Lys) and L-Cysteine (Cys) in saturated aqueous electrolytic salt solutions at 298.15 K NaBr mol kg 0.00 0.05 0.10 0.50 1.00

−1

NaCl

KI

Na2SO4

CaCl2

Lys

Cys

Lys

Cys

Lys

Cys

Lys

Cys

Lys

Cys

19.045 19.054 19.216 19.092 19.287

15.563 14.665 13.772 13.901 13.555

19.045 24.226 24.436 24.805 24.406

15.563 13.921 12.804 11.830 12.873

19.045 21.289 21.976 22.625 23.075

15.563 16.667 15.842 15.835 15.243

19.045 19.167 19.356 20.238 20.755

15.563 15.818 15.291 09.666 09.144

19.045 22.838 22.880 24.165 24.905

15.563 11.908 11.503 11.029 10.554

H



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Table 9. Polarizability (α) of L-Lysine (Lys) and L-Cysteine (Cys) in Saturated Aqueous Electrolytic Salt Solutions at 298.15 K NaBr mol kg−1 0.00 0.05 0.10 0.50 1.00

NaCl

KI

Na2SO4

CaCl2

Lys·10−25

Cys·10−25

Lys·10−25

Cys·10−25

Lys·10−25

Cys·10−25

Lys·10−25

Cys·10−25

Lys·10−25

Cys·10−25

75.418 75.452 76.096 75.604 76.377

61.631 58.074 54.538 55.048 53.680

75.418 95.935 96.766 98.228 100.61

61.631 55.128 50.705 46.849 50.977

75.418 84.306 87.023 89.594 91.376

61.631 66.002 60.726 62.709 60.361

75.418 75.904 76.649 80.142 82.190

61.631 62.640 60.554 38.277 36.212

75.418 90.439 90.607 95.695 98.624

61.631 47.156 45.553 43.675 41.793

equilibrium with an aqueous electrolyte solution saturated in amino acid ( fAA/f °AA) is equal to unity, or equivalently:


mAA =

◦ ◦ mAA γAA

γAA

(7)

As the solubility of the amino acid in pure water and its activity coefficient at saturation in pure water and in the waterelectrolyte systems are known, the solubility of the amino acid in an aqueous solution containing an electrolyte can be calculated. The activity coefficients in eq 7 are represented here by a model proposed earlier.30 The model considers the contributions of long- and short-range interactions to the excess Gibbs free energy of the system. The long-range interaction term takes into account all interactions in a binary water−electrolyte system, whereas the short-range interaction terms account for all interactions between water and amino acid, amino acid and electrolyte, and also for the change in the interaction between the electrolyte and water, because of the presence of the amino acid. The structure of the model is such that the excess Gibbs free energy of the system reduces to the excess Gibbs free energy of the binary water−electrolyte system in the absence of amino acid and to the excess Gibbs free energy of the binary amino acid−water system in the absence of electrolyte. The Wilson31 model is employed to represent the shortrange interaction term. This model is written as

Figure 10. Phase diagram of the tricomponent system of (L-lysine− H2O−NaCl/Na2SO4).

ln γi = 1 − ln(∑ xi Λji) − j=1

Figure 11. Phase diagram of the tricomponent system of (L-cysteine− H2O−NaCl/Na2SO4/CaCl2).

∑ j=1

xi Λji ∑k = 1 xk Λjk

(8)

where Λji is the binary energy parameter with Λii = 0. The subscripts i, j, and k indicate the amino acid, electrolyte, and water, respectively. In this framework, the activity coefficient of the amino acid is obtained from the following relation:

sulfate is less than that in the presence of sodium chloride (Na2SO4 < NaCl). This is related to the high salting-out effect of sodium sulfate than that of sodium chloride. On the other hand as shown from Figure 11, for L-cysteine, the liquid−liquid phase (undashed area) in the presence of calcium chloride is greater than that in the presence of sodium sulfate and in the presence of sodium chloride (CaCl2 > Na2SO4 > NaCl), while the solid−liquid phase (dashed area) in the presence of calcium chloride is less than that in the presence of sodium sulfate and in the presence of sodium chloride (CaCl2 < Na2SO4 < NaCl). This is related to the higher salting-in effect of calcium chloride than those of sodium sulfate and sodium chloride. 3.5. Modeling of the Solubility Data. The modeling of the resulting data was done as described in the reference23 as follows: “the solubility of an amino acid in an aqueous solution containing an electrolyte has been modeled and discussed.21 Depending on these models, the solubility of an amino acid can be written in the following simple form, after we assume that the ratio of the fugacity of the amino acid in the solid phase in

m x x ln γAA = ln γAA − lim xAA → o, xS→ 0 ln γAA

− ln[1 + 0.001MW (mAA + mS)]

(9)

where the superscripts (m) and (x) indicate the activity coefficient of the amino acid normalized based on the molality and the mole fraction scale, respectively. MW is the molecular weight of water. The activity coefficients given by eq 9, are normalized in the unsymmetric convention, that is, with γi → 1 as xi → 0. The reference state for water is its pure state, while that of the electrolyte and amino acid are their states at infinite dilution in water. The Wilson model for a ternary amino acid− electrolyte−water system contains three sets of binary interaction parameters, that is, water−amino acid, amino acid−amino acid, and electrolyte−water parameters. The two binary interaction parameters for the water−electrolyte pair included in the Wilson model account for the change in the I




Article

carboxyl groups of the amino acid become more strong and the ion-pair complexes act as independent molecules. This renders the activity coefficient model less accurate and therefore reduces the accuracy of the solubility model. High electrolyte concentrations also increase the adsorption of ions on the solid amino acid phase and therefore change the crystallographic shape of the crystalline in the solid phase. This, in turn, changes the fugacity of the solid phase compared to a solid phase in equilibrium with a water−amino acid solution.27 As a result of this phenomenon, the accuracy of the parameters of the model decreases for systems with high electrolyte concentrations. This phenomenon has been discussed in detail earlier.30

interactions between the electrolyte and water due to the presence of amino acid molecules. This change in interactions of the water−electrolyte pair is mainly caused by the formation of weak physical bonds between charged groups of amino acid molecules and the free ions of the electrolyte. The model used here assumes that the weak physical bonds between charged groups of amino acids and the ions do not form new molecules, and that the activity coefficients of all amino acid molecules are the same.30 The model was employed to correlate the solubilities of Llysine and L-cysteine in the solutions of the five electrolytes studied here. The binary electrolyte−amino acid and electrolyte−water parameters were considered to be symmetric, Λji = Λij, Λkj = Λjk, and were treated as two adjustable parameters.27 These parameters were evaluated by fitting the parameters of the model to the experimental data. The lists of the parameters, along with the root-mean-square deviation error, are presented in Table 10.

■

CONCLUSIONS The molal solubilities, density, and refractive index of the basic amino acid (L-lysine) and the neutral amino acid (L-cysteine) in aqueous solutions of potassium iodide (KI), sodium bromide (NaBr), sodium chloride (NaCl), sodium sulfate (Na2SO4), and calcium chloride (CaCl2) at 298.15 K was determined experimentally. A salting-in effect of the electrolytic solutions under study (potassium iodide, sodium bromide, sodium chloride, sodium sulfate, and calcium chloride) and others electrolytic solutions (potassium fluoride, potassium bromide, sodium iodide, potassium sulfate, and barium chloride) (from literature) was observed for L-cysteine amino acid (present study), glycine, DLalanine, DL-valine, DL-serine (from literature). A salting-out effect of the electrolytic solutions under study (potassium iodide, sodium bromide, sodium chloride, sodium sulfate and calcium chloride) was observed for L-lysine amino acid (present study), and of NaF solution for amino acids DLalanine and DL-valine (from literature). The effect of the nature of the electrolytic salts under study agree with that in the literature and occurs in the following order: Na2SO4 > NaCl, NaBr > NaCl, CaCl2 > NaCl. The proposed model for modeling the solubility data can accurately represent the experimental data for solubility of Llysine and L-cysteine for all electrolytes studied here.

Table 10. Binary Energy Interaction Parameters and the Root Mean Square Deviation Error (rmsd) for the Solubility of the Studied Amino Acids in Saturated Aqueous Electrolytic Salt Solutions amino acid L-lysine

L-cysteine

ΛAA‑W ΛAA‑S rmsd ΛAA‑W ΛAA‑S rmsd

NaBr

KI

Na2SO4

NaCl

CaCl2

0.8979 1.0453 0.0047 0.8754 1.1131 0.0255

0.4276 0.3709 0.0052 0.8215 1.1293 0.0345

0.6107 0.9113 0.0057 0.8153 1.1109 0.0169

0.3898 0.3987 0.0049 0.7941 1.0781 0.0261

0.6207 0.9215 0.0043 0.8060 1.1103 0.0141

The modeling results are shown in Figure 12 for L-cysteine as an example. From this figure, the model can accurately

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

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Figure 12. Molal solubility of L-cysteine in aqueous solutions of ▲, NaCl; △, NaBr; ●, CaCl2; ○, Na2SO4. The solid lines are for experimental results while dotted lines are for modeling results.

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Solvation of Basic and Neutral Amino Acids in Aqueous Electrolytic

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