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Jul 24, 2015 - determined for glycylglycine. All of the ΔtrH0 values are negative in the three salt solutions. The relative order of ΔtrH0 in the sa...
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Transfer Properties of Glycylglycine from Water to Aqueous NaNO3, NaClO4, and Na2SO4 Solutions Chunli Liu,* Li Zhou, Liqun Fan, and Chonggui Ren Department of Chemistry and Chemistry Engineering, Zao Zhuang University, Zao Zhuang 277160, People’s Republic of China S Supporting Information *

ABSTRACT: Enthalpies of solution and densities of glycylglycine in aqueous solutions of NaNO3, NaClO4, and Na2SO4 were measured at 298.15 K. Standard transfer enthalpies (ΔtrH0), standard transfer partial molar volume (ΔtrV0ϕ), and hydration numbers (Nh) have been determined for glycylglycine. All of the ΔtrH0 values are negative in the three salt solutions. The relative order of ΔtrH0 in the same concentration of 1−1 type salts is NaNO3 > NaClO4. The ΔtrH0 in low concentration of Na2SO4 solutions is between the corresponding values in NaNO3 and NaClO4 solutions. With increasing Na2SO4 concentration, the ΔtrH0 values exceed that in NaNO3 solutions. The ΔtrV0ϕ values are positive and vary in the sequence Na2SO4 > NaNO3 > NaClO4. The results were discussed in terms of various interactions.

1. INTRODUCTION The stabilization of native conformations of proteins is closely related to weak interactions such as hydrogen bonding, electrostatic and hydrophobic interactions, and so on. The physicochemical behaviors of protein are strongly influenced by its interaction with the surrounding solvent molecules. The effect of solvent on the properties of proteins depends on the nature of the interactions between its functional groups and solvent molecules. So the behaviors of protein model compounds such as amino acids,1−3 amides,4,5 and peptides6−9 have been extensively investigated. Salts have large effects on the structure and properties of proteins. Some salts tend to disrupt protein structures whereas others protect them.10 Many studies of thermodynamic properties of protein model compounds have been carried out in aqueous alkali metal and alkaline earths halide solutions.11−14 Santosh et al. have studied the thermodynamic properties of glycylglycine in aqueous FeCl2, MnCl2, NiCl2, and Mn(COOCH3)2 solutions15−18 and found that cation and anion play a significant role in influencing the behavior of glycylglycine. In Venkatesu et al.’s study, salt effects of KCl, KBr, and KAc on the peptide backbone unit were investigated by transfer free energies.19 However, few authors have studied the behavior of protein model compounds in aqueous oxacid salts solutions.20−22 As a part of the continuation of our studies on the thermodynamics properties of amino acids and dipeptides in aqueous oxacid salts solutions,23,24 this work reports the transfer properties of glycylglycine from water to aqueous NaNO3, NaClO4, and Na2SO4 solutions.

chemicals are given in Table 1. The water was deionized and distilled twice. All solutions were prepared freshly by mass on a Table 1. Specications of Chemical Samples mass fraction purity CAS no.

source

%

glycylglycine

556-50-3

Aldrich

⩾ 99.0

NaNO3 NaClO4 Na2SO4

7631-99-4 7601-89-0 7757-82-6

Aladdin Aladdin Aladdin

99.99 99.99 99.99

purification method dried at room temperature dried at 423 K dried at 423 K dried at 423 K

METTLER AE200 balance with a sensitivity of ± 0.0001 g. The uncertainty of the molalities is (2 × 10−4) mol·kg−1. The measurements of enthalpies of solution (ΔsolH0) were carried out on a RD496-II microcalorimeter at 298.15 K as described in the literature.7 The uncertainty of ΔsolH0 was estimated within ± 0.09 kJ·mol−1. The final molality of glycylglycine is 0.0500 mol·kg−1. The densities of the solutions were measured at 298.15 K with an Anton Paar DMA 5000 M digital densimeter (Graz, Austria) precise to (1 × 10−6) g·cm−3. The uncertainty was within (5 × 10−6) g·cm−3. The temperature of the measuring cell was kept constant to (298.15 ± 0.01) K. All of the measurements were carried out under atmospheric pressure. The densimeter was calibrated with twice distilled water and dried air just prior to each series of ρ measurements.

2. EXPERIMENTAL SECTION Glycylglycine (>99%, Aldrich) was dried under reduced pressure at room temperature for 48 h. The salts, NaNO3, NaClO4, and Na2SO4, were of 99.99% purity. All of the salts were obtained from Aladdin and were dried at 423 K for 5 h and then stored over P2O5 prior to use. The details of the © 2015 American Chemical Society

chemical name

Received: March 31, 2015 Accepted: July 14, 2015 Published: July 24, 2015 2420

DOI: 10.1021/acs.jced.5b00298 J. Chem. Eng. Data 2015, 60, 2420−2425

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Table 2. Standard Enthalpya of Solution (ΔsolH0) of Glycylglycine in Aqueous NaNO3, NaClO4, and Na2SO4 Solutions and the Corresponding Standard Enthalpy of Transfer (ΔtrH0) of Glycylglycine from Water to These Aqueous Solutions at T = 298.15 K at p = 105 Pa mNaNO3

ΔsolH0

ΔtrH0

mNaClO4

ΔsolH0

ΔtrH0

mNa2SO4

ΔsolH0

ΔtrH0

(mol·kg−1)

(kJ·mol−1)

(kJ·mol−1)

(mol·kg−1)

(kJ·mol−1)

(kJ·mol−1)

(mol·kg−1)

(kJ·mol−1)

(kJ·mol−1)

0.0200 0.0500 0.1000 0.2000 0.4000 0.6000 0.8000

11.29 11.09 10.97 10.65 9.97 9.44 8.85

−0.45 −0.65 −0.77 −1.09 −1.77 −2.30 −2.89

0.0200 0.0500 0.1000 0.2000 0.4000 0.6000 0.8000

10.95 10.82 10.65 10.33 9.35 8.75 7.88

−0.79 −0.92 −1.09 −1.41 −2.39 −2.99 −3.86

0.0200 0.0500 0.1000 0.2000 0.4000 0.6000 0.8000

11.12 10.92 10.80 10.49 9.86 9.61 9.30

−0.62 −0.82 −0.94 −1.25 −1.88 −2.13 −2.44

Standard uncertainties u are u(T) = 0.01 K, u(m) = (2 × 10−4) mol·kg−1, u(ΔsolH0) = 0.09 kJ·mol−1, and the combined expanded uncertainties of ΔtrH0 at a confidence level of 0.95 (k = 2) are within 0.15 kJ·mol−1. a

form (−OOC−CH2−NHCO−CH2−NH3+) in aqueous solutions. So the interactions between glycylglycine and salts can be separated into three types: (a) ion−ion interaction of Na+ with the COO− group and anions of salts with the NH3+ group; (b) ion−dipole interactions between salt ions and peptide group (-NHCO-); (c) hydrophilic−hydrophobic interactions between salt ions and the apolar group (-CH2-). According to the cosphere overlap model,28,29 a and b lead to a negative contribution to ΔtrH0, but c leads to a positive contribution to ΔtrH0. The negative ΔtrH0 values indicate that interactions of types a and b dominate the interactions of type c. As seen from Figure 1, the relative order of ΔtrH0 is NaNO3 > NaClO4 in the same concentration of 1−1 type salt solutions. In low concentration Na2SO4 solutions, ΔtrH0 is between the corresponding values in NaNO3 and NaClO4 solutions, but with the increasing concentration the ΔtrH0 values exceed that in NaNO3 solutions. The difference of ΔtrH0 comes from the different interactions between glycylglycine zwitterion and anion. According to Lilley et al.,30 the interaction between glycylglycine and salts can be separated into electrostatic interaction and structural interaction. Structural interaction includes partial desolvation of solutes and solvent reorganization. Electrostatic interaction mainly depends on the ionic radius and charge which gives a negative contribution to the transfer enthalpy.30 Obviously, electrostatic interaction between zwitterions and salts should be in the following sequence: 1−2 type (Na2SO4) > 1−1 type (NaNO3, NaClO4). For 1−1 type salts, the radius of NO3− is smaller than ClO4− and the charge surface density is higher than ClO4−. So the electrostatic interaction between zwitterion and anion which give negative contribution to ΔtrH0 should be in the following sequence: ClO4− < NO3−. If one just considers electrostatic interaction, the negative contribution to ΔtrH0 should be Na2SO4 > NaNO3 > NaClO4. But the experimental results are not the case. Na2SO4 is a structure-maker of water because of the high charge density of SO42−.31 The dehydration processes of SO42− absorb more heat, and the trend is more obvious with the increasing concentration of Na2SO4. For 1−1 type NaNO3 and NaClO4, NO3− is a more structure-making ion than ClO4− due to its smaller size in comparison to ClO4−; in dehydration processes, more water molecules are removed from the hydration sheath of NO3− than ClO4−32 which lead to less negative ΔtrH0 values in NaNO3 solutions. We can conclude that structure interaction between anion and the electropositive fragment of glycylglycine zwitterion is responsible for the observed results. In spite of this, the negative ΔtrH0 values still

3. RESULTS AND DISCUSSION 3.1. Enthalpy of Transfer. The ΔsolH0 of glycylglycine in aqueous NaNO3, NaClO4, and Na2SO4 solutions are presented in Table 2. The ΔsolH0 of glycylglycine in water is (11.74 ± 0.08) kJ·mol−1 and agrees well with the reported data (see Table 3). The dissolution process of glycylglycine is Table 3. Comparisons of Obtained Standard Enthalpy of Solution (ΔsolH0), Standard Partial Molar Volume(V0ϕ), and Hydration Number (Nh) for Glycylglycine in Water at 298.15 K with Those Given in Literature ΔsolH0 V0ϕ Nh

this work

literature

11.74 76.22 5.72

11.56,25 11.8426 76.39,12 76.23,33 76.2934 5.67,12 5.69,37 5.7038

endothermic because the destruction of its crystal lattice needs to absorb energy. The ΔsolH0 values decrease with the increasing salts concentration, indicating that the interaction between glycylglycine and salts is increased. Standard transfer enthalpies, ΔtrH0, were derived by eq 1; the results were also listed in Table 2. Δtr H 0 = Δsol H 0(in salt solutions) − Δsol H 0(in water) (1)

Figure 1 shows the variation of ΔtrH0 with the molality of oxacid salts. The ΔtrH0 values of glycylglycine are negative, which suggests that the transfer process of glycylglycine from water to these three salts solutions is exothermic. Edsall and Blanchard27 confirmed that glycylglycine is the major charged

Figure 1. Enthalpies of transfer of glycylglycine from water to aqueous NaNO3, NaClO4, and Na2SO4 solutions at T = 298.15 K. 2421

DOI: 10.1021/acs.jced.5b00298 J. Chem. Eng. Data 2015, 60, 2420−2425

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Table 4. Densities (ρ) and Apparent Molar Volumesa (Vϕ) of Glycylglycine in Aqueous Solutions of NaNO3, NaClO4, and Na2SO4 at T = 298.15 K at p = 105 Pa msolute

ρ

Vϕ

msolute

ρ

Vϕ

msolute

ρ

Vϕ

(mol·kg−1)

(g·cm−3)

(cm3·mol−1)

(mol·kg−1)

(g·cm−3)

(cm3·mol−1)

(mol·kg−1)

(g·cm−3)

(cm3·mol−1)

mNaNO3 = 0.0200 mol·kg−1 0 0.1027 0.1514 0.1979 0.2556 0.3009 mNaNO3

0.998191 1.003840 1.006476 1.008966 1.012019 1.014391 = 0.8000 mol·kg−1

mNaClO4

1.040022 1.045540 1.048352 1.050809 1.053482 1.056247 = 0.0201 mol·kg−1

76.62 76.80 76.93 77.07 77.14

mNaClO4

0.998644 1.004532 1.010029 1.012459 1.015327 1.017320 = 0.8002 mol·kg−1

78.46 78.55 78.62 78.70 78.76

0 0.1100 0.1702 0.2179 0.2894 0.3445

mNa2SO4

1.055678 1.061023 1.065443 1.067740 1.070259 1.073265 = 0.0201 mol·kg−1

76.85 77.03 77.14 77.24 77.36

0 0.1047 0.2045 0.2569 0.3101 0.3624

mNa2SO4

0.999638 1.005526 1.010619 1.013316 1.015815 1.018790 = 0.8001 mol·kg−1

81.89 81.98 82.03 82.09 82.13

0 0.1049 0.2029 0.2493 0.2941 0.3529

0 0.1062 0.1614 0.2103 0.2641 0.3204 0 0.1069 0.2090 0.2551 0.3101 0.3485 0 0.1036 0.1910 0.2372 0.2884 0.3501 0 0.1074 0.2024 0.2536 0.3016 0.3594 0 0.1059 0.2055 0.2648 0.3051 0.3607

mNaNO3 = 0.1001 mol·kg−1

1.090934 1.095830 1.100330 1.102967 1.104718 1.107142

76.72 76.80 76.89 77.00 77.07

0 0.1058 0.1605 0.2116 0.2609 0.3207

78.58 78.69 78.80 78.90 78.99

0 0.1059 0.1549 0.2011 0.2503 0.3000 0 0.1053 0.2024 0.2507 0.3005 0.3486

mNaNO3 = 0.4001 mol·kg−1

mNaNO3

1.002651 1.008444 1.011384 1.014097 1.016681 1.019785 = 1.2010 mol·kg−1

mNaClO4

1.060253 1.065586 1.068013 1.070269 1.072663 1.075042 = 0.1000 mol·kg−1

76.77 77.02 77.14 77.24 77.35

mNaClO4

1.004815 1.010585 1.015773 1.018309 1.020899 1.023367 = 1.1999 mol·kg−1

79.21 79.25 79.28 79.32 79.34

0 0.1055 0.1658 0.2260 0.2856 0.3341

mNa2SO4

1.083488 1.088954 1.091894 1.094200 1.097617 1.100220 = 0.1001 mol·kg−1

77.84 78.00 78.08 78.15 78.22

0 0.1029 0.2058 0.2572 0.3120 0.3642

mNa2SO4

1.009692 1.015310 1.020536 1.023239 1.025955 1.028597 = 1.0002 mol·kg−1

82.50 82.63 82.69 82.74 82.80

0 0.1051 0.2008 0.2547 0.3093 0.3589

1.112491 1.117154 1.121399 1.123372 1.125255 1.127704

76.86 76.98 77.08 77.20 77.30

0 0.1045 0.2109 0.2652 0.3208 0.3738

79.41 79.47 79.53 79.57 79.65

0 0.1048 0.2091 0.2622 0.3198 0.3736 0 0.1051 0.1904 0.2433 0.2935 0.3491

77.76 77.91 78.01 78.12 78.19

mNaNO3

1.019017 1.024602 1.030162 1.032945 1.035755 1.038415 = 1.6016 mol·kg−1

79.97 80.12 80.17 80.24 80.29

mNaClO4

1.079669 1.084802 1.089784 1.092281 1.094953 1.097422 = 0.4001 mol·kg−1

77.61 77.76 77.83 77.91 78.02

mNaClO4

1.027273 1.032883 1.037337 1.040063 1.042615 1.045403 = 1.6001 mol·kg−1

79.64 79.72 79.82 79.89 79.97

mNa2SO4

1.111935 1.116998 1.119835 1.122617 1.125340 1.127522 = 0.4002 mol·kg−1

79.98 80.13 80.20 80.27 80.33

mNa2SO4

1.045749 1.050921 1.055973 1.058454 1.061066 1.063530 = 1.2000 mol·kg−1 1.133412 1.137873 1.141826 1.144005 1.146183 1.148137

83.20 83.33 83.41 83.49 83.55

a Standard uncertainties u are u(T) = 0.01 K, u(m) = (2 × 10−4) mol·kg−1, u(ρ) = (5 × 10−6) g·cm−3, and the combined expanded uncertainty of the Vϕ is within 0.12 cm3·mol−1.

indicate that electrostatic interaction between glycylglycine and salts is predominant. Comparing the ΔtrH0 data of glycylglycine with that of glycine23,24 in the same concentration salt solutions, the more negative ΔtrH0 values of glycylglycine reflect the stronger interaction between glycylglycine and salts than that of glycine. 3.2. Partial Molar Volumes of Transfer. The measured density of glycylglycine in aqueous NaNO3, NaClO4, and Na2SO4 solutions at 298.15 K are given in Table 4. Apparent molar volumes (Vϕ) of glycylglycine were calculated from the densities of solution (ρ) by using the equation

Vϕ = M /ρ − 1000(ρ − ρ0 )/mρρ0

(2)

where M is the molar mass of glycylglycine, ρ0 is the density of solvent, and m is the molality of glycylglycine in the solvent. The calculated results are also listed in Table 4. The apparent molar volumes can be fit by the equation Vϕ = V ϕ0 + Bv m

(3)

V0ϕ

where is the infinite dilution apparent molar volume that equals the standard partial molar volume and Bv is an experimentally determined parameter. Values of V0ϕ are listed in Table 5. The V0ϕ value of glycylglycine in water is (76.22 ± 2422

DOI: 10.1021/acs.jced.5b00298 J. Chem. Eng. Data 2015, 60, 2420−2425

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Table 5. Standard Partial Molar Volumes (V0ϕ) and Hydration Numbersa (Nh) of Glycylglycine in Aqueous NaNO3, NaClO4, and Na2SO4 Solutions at 298.15 K mNaNO3

V0ϕ

(mol·kg−1)

(cm3 ·mol−1)

0.0200 0.1001 0.4001 0.8000 1.2010 1.6016

76.52 76.64 77.57 78.39 79.29 79.86

mNaClO4

V0ϕ

Nh

(mol·kg−1)

(cm3 ·mol−1)

5.63 5.59 5.31 5.06 4.79 4.62

0.0201 0.1000 0.4001 0.8002 1.1999 1.6001

76.37 76.54 77.43 78.32 79.15 79.49

mNa2SO4

V0ϕ

Nh

(mol·kg−1)

(cm3 ·mol−1)

Nh

5.67 5.62 5.35 5.08 4.83 4.73

0.0201 0.1001 0.4002 0.8001 1.0002 1.2000

76.63 77.69 79.85 81.79 82.37 83.06

5.59 5.27 4.62 4.03 3.85 3.65

a Standard uncertainties u are u(T) = 0.01 K, u(m) = (2 × 10−4) mol·kg−1, and the combined expanded uncertainties of V0ϕ and Nh at a confidence level of 0.95 (k = 2) are within 0.23 cm3·mol−1 and 0.13, respectively.

0.06) cm3·mol−1 and agrees well with reference values (see Table 3). The partial molar volumes of transfer (ΔtrV0ϕ) from water to aqueous NaNO3, NaClO4 and Na2SO4 solutions have been calculated by eq 4 and are illustrated in Figure 2.

3.3. Hydration Number. The hydration numbers (Nh) of glycylglycine in water can be evaluated according to the following equation:36 0 Nh = V elect /(V e0 − V b0)

(6)

V0elect

where is the electrostriction partial molar volume of glycylglycine, V0e is the molar volume of electrostricted water, and V0b is the molar volume of bulk water. The value of (V0e − V0b) is approximately −3.3 cm3·mol−1 at 298.15 K.36 The V0elect can be estimated from the V0ϕ values of glycylglycine by the following equation.36

Δtr V ϕ0 = V ϕ0(in aqueous salt solutions) − V ϕ0(in water) (4)

0 0 V elect = V ϕ0 − V int

The intrinsic molar volume 0 = V int

⎛ 0.7 ⎞ M ⎜ ⎟ ⎝ 0.634 ⎠ ρ c

(7)

V0int

can be estimated by eq 8:

36

(8)

where ρc is the density of the glycylglycine crystal. The hydration number of glycylglycine in water was compared with that in the literature (see Table3). The hydration numbers of glycylglycine in aqueous salt solutions are given in Table 5 and are illustrated in Figure 3.

Figure 2. Transfer partial molar volumes of glycylglycine from water to aqueous NaNO3, NaClO4, and Na2SO4 solutions at T = 298.15 K.

The Δ tr V ϕ0 values are positive, indicating that the coordination of the hydration spheres of Na+ with that of the -COO− group and anion with the hydration spheres of the -NH3+ group make water molecules relaxed to bulk water, and the volume of the solution increases. According to cosphere overlap model,28,29 the major contribution to ΔtrVϕ0 is interactions of type a and type b which both lead to positive ΔtrV0ϕ values. This tendency can also be explained by eq 5.35 V ϕ0 = VVW + Vvoid − Vshrinkage

(5)

where VVW is the van der Waals volume, Vvoid is the contribution associated with the voids and empty volume, and Vshrinkage is the shrinkage in volume due to solute−solvent interaction. VVW and Vvoid are generally assumed approximately the same in water and in aqueous solutions,35 so the positive ΔtrV0ϕ values mainly come from the decrease of Vshrinkage. Some water molecules around glycylglycine may be released to bulk water in the presence of oxacid salts and give a positive change in volume. The relative order of ΔtrV0ϕ in the same concentration of oxacid salts is Na2SO4 > NaNO3 > NaClO4. This tendency is consistent with electrostatic interaction between glycylglycine and salts. The results suggest that electrostatic interaction between the zwitterionic headgroup of glycylglycine and ions to ΔtrV0ϕ is predominate.

Figure 3. Hydration number of glycylglycine in aqueous NaNO3, NaClO4, and Na2SO4 solutions at T = 298.15 K.

Hydration number reflects the electrostriction effect of the charge center of glycylglycine on the vicinity water moleculars. In the three salts solutions, the Nh values exhibit a decreasing trend with the increasing salt concentration which suggests that interactions between salts and glycylglycine become stronger and reduce the electrostriction effect of glycylglycine. We can conclude that the three salts have a dehydration effect on glycylglycine. Similar results were observed of some peptides in sodium butyrate,37 KCl,39 sodium halide,12 sodium acetate, and magnesium acetate solutions.40 In the same molality of salt 2423

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(9) Smirnov, V. I.; Badelin, V. G. Enthalpies of β-Alanine Dissolution in Some Water + Organic Mixtures at 298.15 K. J. Chem. Eng. Data 2014, 59, 1774−1780. (10) Von Hippel, P. H.; Schleich, T. Ion Effects on the Solution Structure of Biological Macromolecules. Acc. Chem. Res. 1969, 2, 257− 265. (11) Soto, A.; Arce, A.; Khoshkbarchi, M. K. Thermodynamics of Diglycine and Triglycine in Aqueous NaCl Solutions: Apparent Molar Volume, Isentropic Compressibility and Refractive Index. J. Solution Chem. 2004, 33, 11−21. (12) Lin, G. M.; Bian, P. F.; Lin, R. S. The Limiting Partial Molar Volume and Transfer Partial Molar Volume of Glycylglycine in Aqueous Sodium Halide Solutions at 298.15 and 308.15 K. J. Chem. Thermodyn. 2006, 38, 144−151. (13) Badarayani, R.; Satpute, D. B.; Kumar, A. Effect of NaBr, KCl, KBr, and MgCl2 on Viscosities of Aqueous Glycine and L-alanine Solutions at 298.15 K. J. Chem. Eng. Data 2005, 50, 1083−1086. (14) Lark, B. S.; Patyar, P.; Banipal, T. S.; Kishore, N. Densities, Partial Molar Volumes, and Heat Capacities of Glycine, L-Alanine, and L-leucine in Aqueous Magnesium Chloride Solutions at Different Temperatures. J. Chem. Eng. Data 2004, 49, 553−565. (15) Santosh, M. S.; Bhat, D. K. Refractive Indices and Isentropic Compressibilities of Glycylglycine-FeCl2 in Aqueous Ethanol Mixtures. J. Chem. Eng. Data 2010, 55, 5365−5369. (16) Santosh, M. S.; Bhat, D. K.; Bhat, A. S. Molecular Interactions in Glycylglycine-MnCl2 Aqueous Solutions at (288.15, 293.15, 298.15, 303.15, 308.15, 313.15, and 318.15) K. J. Chem. Eng. Data 2009, 54, 2813−2818. (17) Santosh, M. S.; Bhat, D. K. Molar Volume, Compressibility and Excess Properties of Glycylglycine in Aqueous NiCl2 Solutions. Fluid Phase Equilib. 2010, 299, 102−108. (18) Santosh, M. S.; Bhat, D. K.; Bhatt, A. S. Molecular Interactions between Glycylglycine and Mn(COOCH3)2 in Aqueous and Aqueous Ethanol Mixtures. J. Chem. Eng. Data 2011, 56, 768−782. (19) Venkatesu, P.; Lee, M. J.; Lin, H. M. Transfer Free Energies of Peptide Backbone Unit from Water to Aqueous Electrolyte Solutions at 298.15K. Biochem. Eng. J. 2006, 32, 157−170. (20) Riyazuddeen; Altamash, T.; Coronas, A. Interactions in LHistidine/L-Glutamic acid/L-Tryptophan/Glycylglycine+2mol•L−1 Aqueous KCl/KNO3 Systems at Different Temperatures: An Isothermal Compressibility Study. Thermochim. Acta 2012, 543, 313−317. (21) Ramasami, P.; Kakkar, R. Partial Molar Volumes and Adiabatic Compressibilities at Infinite Dilution of Amino Carboxylicacids and Glycylglycine in Water and Aqueous Solutions of Sodium Sulphate at (288.15, 298.15 and 308.15) K. J. Chem. Thermodyn. 2006, 38, 1385− 1395. (22) Riyazuddeen; Bansal, G. K. Intermolecular/Interionic Interactions in L-Leucine-, L-Asparagine-, and Glycylglycine-Aqueous Electrolyte Systems. Thermochim. Acta 2006, 445, 40−48. (23) Liu, C. L.; Ren, C. G. Transfer Properties of Amino Acids from Water to Aqueous Sodium Sulfate Solutions at 298.15 K. J. Chem. Eng. Data 2009, 54, 3296−3299. (24) Liu, C. L.; Lin, R. S. Enthalpies of Transfer of Amino Acids from Water to Aqueous Solutions of Sodium Nitrate and Sodium Perchlorate at T=298.15K. Thermochim. Acta 2006, 440, 57−59. (25) Nowicka, B.; Piekarski, H. Calorimetric Studies of Interactions between Simple Peptides and Electrolytes in Water at 298.15 K. J. Mol. Liq. 2002, 95, 323−328. (26) Davis, K. G.; Gallardo-Jiménez, M. A.; Lilley, T. H. Aqueous Solutions Containing Amino Acids and Peptides-26. The Enthalpies of Interaction of Some Glycyl and Alanyl Peptides with Sodium Chloride and Potassium Chloride in Water at 25°C. Fluid Phase Equilib. 1990, 57, 191−204. (27) Edsall, J. T.; Blanchard, M. H. The Activity Ratio of Zwitterions and Uncharged Molecules in Ampholyte Solutions. The Dissociation Constants of Amino Acid Esters. J. Am. Chem. Soc. 1933, 55, 2337− 2353.

solutions, the Nh of glycylglycine varies in the following order: Nh (Na2SO4) < Nh (NaNO3) < Nh (NaClO4). This tendency is consistent with the result of transfer partial molar volume.

4. CONCLUSIONS Enthalpy of solution and density of glycylglycine in aqueous solutions of NaNO3, NaClO4, and Na2SO4 were measured at 298.15 K. The influence of different interactions between glycylglycine and salts on transfer enthalpies and transfer partial molar volume is discussed. The results show that electrostatic interaction between zwitterions and salts has great contributions to ΔtrH0 and ΔtrV0ϕ. But structural interaction makes a large contribution to ΔtrH0 and is almost responsible for the difference of ΔtrH0 in different salts solutions.

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing speciations of chemical samples, standard enthalpies of solution and corresponding enthalpies of transfer, and comparisons of obtained standard enthalpies of solution, standard partial molar volumes, and hydration numbers and figures showing enthalpies of transfer, transfer partial molar volumes, and hydration numbers. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00298.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 632 3785967. Fax: +86 632 3786819. E-mail: liu2000419@126. com. Funding

This work is supported by the Science Technology Foundation for Middle Aged and Young Scientists of Shandong Province (Grant BS2011SF026). Notes

The authors declare no competing financial interest.

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REFERENCES

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