Volumetric and ultrasonic studies of some amino acids in binary aqueous solutions of MgCl2·6H2O at 298.15 K

Journal of Molecular Liquids 121 (2005) 148 – 155 www.elsevier.com/locate/molliq

Volumetric and ultrasonic studies of some amino acids in binary aqueous solutions of MgCl2d 6H2O at 298.15 K Amalendu PalT, Suresh Kumar Department of Chemistry, Kurukshetra University, Kurukshetra-136119, India Received 2 August 2004; accepted 20 December 2004 Available online 17 May 2005

Abstract Apparent molar volumes (V / ) and adiabatic compressibilities (K /,s) of glycine, l-alanine and l-valine in binary aqueous solutions of MgCl2d 6H2O (0.1–0.8 mol kg
1) have been determined at 298.15 K from precise density and sound speed measurements. Partial molar 0 volumes (V /0) and partial molar adiabatic compressibilities (K /,s ) of these amino acids at infinite dilution were evaluated. These values are used for calculating the number of water molecules hydrated (n H) to the amino acids. Group contributions for the partial molar volumes and adiabatic compressibilities have been determined from the amino acids. Transfer volumes (DV /0) and transfer adiabatic compressibilities 0 (DK /,s 0) at infinite dilution from water to aqueous magnesium chloride solutions have been calculated. Transfer parameters have been interpreted in terms of solute–cosolute interactions on the basis of a cosphere overlap model. Pair and triplet interaction coefficients have also been calculated from transfer parameters. All these results indicate that hydrophillic–ionic group interactions are predominant over hydrophillic–hydrophobic group interactions over the whole concentration range of magnesium chloride and decrease with the increase in size of the side chain of amino acids. D 2005 Published by Elsevier B.V. Keywords: Amino acids; Transfer functions; Interaction coefficients; Cosphere overlap model

1. Introduction The study of the thermodynamic stability of the native structure of proteins has proved quite challenging [1]. Salt solutions have large effects on the structure and properties of proteins including their solubility, denaturation, dissociation into subunits [2]. In the literature there are some reports [1] about the effect of various neutral salts on the transitition temperatures of proteins. Nagy and Jencks [3] have discussed that electrolytes induce dissociation in the protein without causing any conformational change or denaturation. They have suggested that salts interact directly with the peptide groups of the protein and bring about its dissociation. Due to the complex nature of proteins, direct study is somewhat difficult. Therefore, it is necessary to

T Corresponding author. E-mail address: [email protected] (A. Pal). 0167-7322/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.molliq.2004.12.003

study the low molecular weight model compounds such as amino acids. Literature survey shows that experimental and theoretical work have been reported on thermodynamics of amino acids in aqueous salt solutions [4–8]. Several workers [9–17] have reported volumetric and ultrasonic studies of amino acids in pure and mixed aqueous solutions, but very few in concentrated electrolyte solutions [18,19]. Although, some papers dealing with amino acids in dilute electrolyte solutions [20–22] are also available. Therefore, in order to understand the behavior of proteins in aqueous salt solutions, we have studied the thermodynamic parameters, partial molar volumes and adiabatic compressibilities, for some amino acids in aqueous magnesium chloride solutions at higher concentrations and at 298.15 K. Both these properties are very sensitive to the nature of hydration or interactive changes in solutions. Here we report the apparent molar volumes (V / ) and apparent molar adiabatic compressibilities (K /,s) of glycine, l-alanine and l-valine in binary aqueous solutions of

A. Pal, S. Kumar / Journal of Molecular Liquids 121 (2005) 148–155

magnesium chloride (0.1–0.8 mol kg
1) obtained from experimentally measured densities (q) and sound speeds (u). From these data, the partial molar volumes (V /0) and 0 partial molar adiabatic compressibilities (K /,s ) have been obtained. These are used to calculate the transfer parameters 0 DV /0 and DK /,s , hydration number (n H) and the contributions of the side chain of amino acids. These results and concentration effect of magnesium chloride have also been discussed in terms of solute–solvent and solute–cosolute interactions.

2. Experimental The amino acids used in this study, glycine, l-alanine, and l-valine were analytical reagents and used without further treatment. Analytical reagent grade magnesium chloride was used after drying at 250 8C for 3 h and then in vacuo over P2O5 at room temperature for a minimum of 48 h. Water used in these experiments was deionized and distilled, and was degassed prior to making solutions. Solutions of magnesium chloride (0.1–0.8 mol kg
1) were prepared by mass and used on the day they were prepared. Solutions of amino acids in the concentration range of 0.03– 0.20 mol kg
1 were made by mass on the molality concentration scale with a precision of F1  10
4 g on a Dhona balance (India, Model 200 D). Uncertainties in solution concentration were estimated at F2  10
5 mol kg
1 in calculations. The densities of the solutions were measured using a single stem pycnometer having a total volume of 8 cm3 and an internal diameter of the capillary of about 0.1 cm. The details pertaining to calibration, experimental set up and working procedure have been previously described [23]. An average of triplicate measurements were taken into account. The reproducibility in the density measurements was better than F3  10
2 kg m
3. The speeds of sound were measured at 4 MHZ using a NUSONIC (Mapco, Model 6080) Velocimeter based on the singaround technique [24] with a single transducer cell. The ultrasonic speeds at 298.15 K were directly obtained from the average round trip period of the ultrasonic wave in a fixed path length between the piezoelectric transducer and the reflector. The maximal error of the sound speed relative to water (1496.687 m s
1) [25] is estimated to be less than 0.2 m s
1. The details of the method used to determine speed of sound are given elsewhere [26–28]. A thermostatically controlled well-stirred bath whose temperature was controlled to F 0.01 K was used for all the measurements.

3. Results and discussion Experimentally measured densities (q) and sound speeds (u) were used to calculate apparent molar volumes

149

(V / ) and adiabatic compressibilities (K /,s) of amino acids in binary aqueous solutions of magnesium chloride (0.1– 0.8 mol kg
1) at 298.15 K by using the following equations. V/ ¼ M =q
f1000ðq
q0 Þ=mqq0 g

ð1Þ


K/;s ¼ M bs =q
1000 bs;0 q
bs q0 =mqq0

ð2Þ

where m is the molality (mol kg
1) of the solution, M is the relative molar mass of the solute (kg mol
1), and q 0, q, b S,0, and b S are the densities and coefficients of adiabatic compressibilities of pure solvent and solution, respectively. The values of density, sound speeds and resulting values of apparent molar volumes (V / ) and adiabatic compressibilities (K /,s) for amino acids in different concentrations of magnesium chloride at 298.15 K are reported in Table 1. All the plots of densities and sound velocities were found to be linear. Figs. 1 and 2 show the sample plots of density and sound velocity of glycine in different concentrations of magnesium chloride at 298.15 K whereas Figs. 3 and 4 show the plots of density and sound velocity of different amino acids in 0.4 mol kg
1 magnesium chloride solution at 298.15 K. The coefficients of adiabatic compressibility (b S ) of the solutes were determined from the sound speed (u) and density (q) data by using the relation
bs ¼ 1= u2 q : ð3Þ The variation of apparent molar quantities with molality can be adequately represented by the relation Y/ ¼ Y/0 þ SQ m

ð4Þ

where Y /0 ( Y /0 denotes V /0 or K 0/,s) is the infinite dilution value that is equal to the partial molar property at infinite dilution and S Q (S Q denotes S V or S K ) is the experimental or limiting slope. Eq. (4) was fitted to our experimental V / and K /,s values in different electrolyte solutions by the method of least squares to evaluate the infinite dilution partial molar volume V /0 and the adiabatic compressibility 0 K 0/,s. The values of V /0, K /,s , S V, and S K for the amino acids at 298.15 K are given in Table 2 together with the standard errors. The experimental values of V /0 and K 0/,s for the amino acids in water reported in the literature [9,11,29– 33] are compared with our results in Table 2. All the amino acids studied have positive V /0 and negative K 0/,s values in binary aqueous solutions of magnesium chloride at 298.15 K. It is found that both V /0 and K 0/,s increase linearly with the size of the alkyl chain of amino acids and increase with the increase in the concentration of magnesium chloride in solutions. It indicates that the cosolute–solvent interactions increase both on increasing concentration of magnesium chloride and the size of the alkyl side chain of amino acids. The

150

A. Pal, S. Kumar / Journal of Molecular Liquids 121 (2005) 148–155

Table 1 Densities (q), speeds of sound (u), apparent molar volumes (V / ) and apparent molar adiabatic compressibilities (K /,s) of glycine, l-alanine, and l-valine in aqueous solutions of MgCl2d 6H2O at 298.15 K m q  10
3 (mol kg
1) (kg m
3) glycine + 0.2 0.00000 0.04834 0.08076 0.11320 0.13697 0.17036 0.19902

glycine + 0.4 0.00000 0.04823 0.07854 0.10722 0.13901 0.16759 0.19940

glycine + 0.6 0.00000 0.04862 0.08084 0.11001 0.13854 0.16719 0.20051

glycine + 0.8 0.00000 0.05102 0.08153 0.11237 0.13923 0.17120 0.19851

V /  106 m u K /,s  106 (m3 mol
1) (mol kg
1) (m s
1) (m3 mol
1 GPa
1)

mol kg
1 MgCl 2 d 6H2 O 1.01111 1.01256 44.84 1.01353 44.83 1.01451 44.72 1.01522 44.71 1.01622 44.68 1.01708 44.64

mol kg
1 MgCl 2 d 6H2 O 1.02696 1.02831 46.50 1.02916 46.44 1.02996 46.43 1.03085 46.39 1.03165 46.35 1.03254 46.31

mol kg
1 MgCl 2 d 6H2 O 1.04009 1.04142 46.83 1.04230 46.81 1.04310 46.75 1.04388 46.72 1.04466 46.70 1.04557 46.67

mol kg
1 MgCl 2 d 6H2 O 1.05202 1.05335 47.74 1.05415 47.66 1.05496 47.58 1.05567 47.51 1.05651 47.46 1.05722 47.45

l-alanine + 0.2 mol kg 0.00000 1.01111 0.05072 1.01252 0.07625 1.01323 0.11026 1.01417 0.13997 1.01500 0.17044 1.01585 0.19958 1.01666


1

0.00000 0.06275 0.08073 0.10080 0.12460 0.14372 0.16777 0.18956

0.00000 0.05876 0.07356 0.09285 0.11552 0.13850 0.16341 0.18993

0.00000 0.05975 0.07375 0.09455 0.11688 0.13974 0.16172 0.18849

0.00000 0.05980 0.07435 0.09345 0.11683 0.13777 0.16448 0.18997

MgCl 2 d 6H2 O 0.00000 60.83 0.05888 60.79 0.07370 60.78 0.09402 60.69 0.11754 60.62 0.14072 60.58 0.16335 0.18923

l-alanine + 0.4 mol kg
1 MgCl 2 d 6H2 O 0.00000 1.02696 0.00000 0.05021 1.02831 61.18 0.06003 0.07757 1.02905 61.08 0.07370 0.10877 1.02989 61.04 0.09349 0.13700 1.03065 60.99 0.11655 0.16862 1.03150 60.95 0.13885 0.19939 1.03233 60.90 0.16447 0.19026

1518.6 1522.0 1523.0 1524.1 1525.4 1526.4 1527.7 1528.9

1540.9 1544.5 1545.4 1546.6 1548.0 1549.4 1550.9 1552.5

1560.7 1564.6 1565.5 1566.8 1568.2 1569.6 1571.0 1572.7

1577.4 1581.1 1582.2 1583.6 1585.3 1586.9 1588.9 1590.8

1518.6 1522.4 1523.3 1524.6 1526.1 1527.6 1529.1 1530.8

1540.9 1544.8 1545.7 1546.9 1548.4 1549.8 1551.5 1553.1


25.02
23.81
24.15
23.92
23.76
23.60
24.01


24.18
23.46
23.09
23.33
23.45
23.62
23.41


23.45
23.41
22.41
22.09
22.42
22.31
22.24


18.44
20.28
20.63
21.93
22.14
22.65
23.29


22.82
21.11
21.53
21.42
21.46
21.68
21.59


18.92
18.72
18.97
18.74
18.85
18.67
18.51

Table 1 (continued) m q  10
3
1 (mol kg ) (kg m
3)

V /  106 m u K /,s  106 3
1
1
1 (m mol ) (mol kg ) (m s ) (m3 mol
1 GPa
1)

l-alanine + 0.6 mol kg
1 MgCl 2 d 6H2 O 0.00000 1.04009 0.00000 0.04799 1.04127 62.86 0.06296 0.08005 1.04206 62.79 0.07445 0.10837 1.04276 62.72 0.09480 0.13901 1.04351 62.71 0.11807 0.16853 1.04423 62.70 0.13929 0.20158 1.04505 62.61 0.16344 0.19089

1560.7 1565.1 1566.0 1567.4 1569.1 1570.6 1572.4 1574.4


18.15
18.03
18.07
18.71
18.44
18.66
19.12

l-alanine + 0.8 mol kg
1 MgCl 2 d 6H2 O 0.00000 1.05202 0.00000 0.04947 1.05319 63.24 0.06014 0.07982 1.05391 63.18 0.07365 0.10829 1.05458 63.17 0.09380 0.13907 1.05531 63.11 0.11461 0.16925 1.05603 63.04 0.13701 0.19954 1.05674 63.03 0.16266 0.19147

1577.4 1581.4 1582.2 1583.5 1584.9 1586.3 1588.0 1589.9


13.92
13.46
13.60
14.32
13.69
13.71
14.13

0.00000 0.05961 0.07329 0.09241 0.11583 0.13834 0.16262 0.18923

1509.2 1515.0 1516.3 1518.2 1520.4 1522.6 1524.9 1527.5


30.86
30.41
30.49
30.72
30.67
30.58
30.87

0.00000 0.05865 0.07296 0.09195 0.11470 0.13795 0.16273 0.18772

1518.6 1524.5 1526.0 1527.9 1530.3 1532.7 1535.2 1537.6


29.83
30.28
30.40
30.95
31.11
30.67
30.89

0.00000 0.05960 0.07278 0.09210 0.11516 0.13726 0.15926 0.19099

1530.5 1536.6 1537.9 1540.0 1542.3 1544.6 1546.9 1550.2


26.89
26.18
27.90
26.98
27.53
27.35
27.64

0.00000 0.05911 0.07289 0.09188 0.11443 0.13588 0.15825 0.18876

1540.9 1546.9 1548.3 1550.2 1552.6 1554.7 1557.0 1560.1


24.23
24.86
24.98
25.82
25.47
25.55
25.67

l-valine + 0.1 mol kg
1 0.00000 1.00391 0.03019 1.00481 0.04397 1.00522 0.05733 1.00562 0.07249 1.00607 0.08573 1.00647 0.10005 1.00690

MgCl 2 d 6H2 O

l-valine + 0.2 mol kg
1 0.00000 1.01111 0.03013 1.01195 0.04380 1.01233 0.05816 1.01273 0.07187 1.01311 0.08596 1.01350 0.10057 1.01391

MgCl 2 d 6H2 O

l-valine + 0.3 mol kg
1 0.00000 1.02049 0.03016 1.02127 0.04432 1.02164 0.05708 1.02197 0.07258 1.02237 0.08608 1.02272 0.10009 1.02308

MgCl 2 d 6H 2 O

l-valine + 0.4 mol kg
1 0.00000 1.02696 0.03092 1.02773 0.04461 1.02807 0.05894 1.02843 0.07184 1.02875 0.08556 1.02909 0.09939 1.02944

MgCl 2 d 6H2 O

87.04 87.02 86.95 86.94 86.84 86.78

88.52 88.51 88.48 88.47 88.46 88.39

89.90 89.78 89.77 89.76 89.73 89.72

90.39 90.38 90.30 90.29 90.28 90.20

A. Pal, S. Kumar / Journal of Molecular Liquids 121 (2005) 148–155

Fig. 1. Plots of q vs. m for glycine in different molal concentration of cosolute magnesium chloride: 0.2 m B, o; 0.4 m B, 4; 0.6 m B, 5; 0.8 m B,  at 298.15 K.

trends in the variations of V /0 and K 0/,s values for lalanine in aqueous magnesium chloride are compared with literature [18] and show satisfactory. The partial molar volumes of transfer (DV /0) and partial molar adiabatic compressibilities of transfer (DK 0/,s) from

Fig. 2. Plots of u vs. m for glycine in different molal concentration of cosolute magnesium chloride: 0.2 m B, o; 0.4 m B, 4; 0.6 m B, 5; 0.8 m B,  at 298.15 K.

151

Fig. 3. Plots of q vs. m for glycine, o; l-alanine, 4; l-valine, 5 in 0.4 m B magnesium chloride at 298.15 K.

water to aqueous magnesium chloride solutions have been calculated by the equation. DY/0 ¼ Y/0 ðin aqueous magnesium chloride solutionÞ
Y/0 ðin waterÞ

ð5Þ

where Y /0 denotes V /0 or K 0/,s and the values are listed in Table 2 and are illustrated in Fig. 5. The DK 0/,s values increase with the concentration of magnesium chloride in all cases. The effect is more pronounced at higher

Fig. 4. Plots of u vs. m for glycine, o; l-alanine, 4; l-valine, 5 in 0.4 m B magnesium chloride at 298.15 K.

152

A. Pal, S. Kumar / Journal of Molecular Liquids 121 (2005) 148–155

Table 2 Limiting partial molar volumes (V /0), limiting partial molar adiabatic compressibilities (K 0/,s), experimental slopes (S V ) and (S K ), transfer volumes (DV /0) and transfer adiabatic compressibilities (DK 0/,s) of glycine, l-alanine, and l-valine in aqueous solutions of MgCl2d 6H2O at 298.15 K m B (mol kg
1)

V /0  106 (m3 mol
1)

S V  106 (m3 L1/2 mol
3/2)

0  106 K /,s (m3 mol
1 GPa
1)

S K  106 (kg m3 mol
2 GPa
1)

DV /0  106 (m3 mol
1)

0  106 DK /,s (m3 mol
1 GPa
1)

glycine 0.0 0.2 0.4 0.6 0.8

43.19 44.91 46.55 46.88 47.82

(F0.04) (F0.03) (F0.01) (F0.01) (F0.03)

2.54
1.40
1.20
1.11
2.05

(F 0.32) (F 0.19) (F 0.08) (F 0.10) (F 0.21)


25.97
24.81
23.76
23.72
17.40

(F0.20) (F0.47) (F0.38) (F0.44) (F0.64)


1.62 6.22 2.17 9.25
32.98

(F1.50) (F3.58) (F3.00) (F3.45) (F5.06)

1.72 3.36 3.69 4.63

1.16 2.21 2.25 8.57

l-alanine 0.0 0.2 0.4 0.6 0.8

60.43 60.93 61.24 62.91 63.31

(F0.03) (F0.02) (F0.02) (F0.03) (F0.02)


2.05
1.75
1.75
1.44
1.46

(F 0.20) (F 0.18) (F 0.16) (F 0.20) (F 0.14)


24.63
22.09
19.06
17.53
13.58

(F0.45) (F0.61) (F0.13) (F0.22) (F0.34)


0.12 3.63 2.41
7.68
2.13

(F3.46) (F4.82) (F1.01) (F1.69) (F2.71)

0.50 0.81 2.48 2.88

2.54 5.57 7.10 11.05

l-valine 0.0 0.1 0.2 0.3 0.4

90.87 87.17 88.58 89.92 90.48

(F0.02) (F0.02) (F0.02) (F0.04) (F0.03)


1.99
3.77
1.65
2.16
2.64

(F 0.17) (F 0.43) (F 0.30) (F 0.60) (F 0.39)


28.53
30.52
29.74
26.41
24.08

(F0.64) (F0.20) (F0.34) (F0.57) (F0.39)


1.96
1.13
7.24
6.75
9.76

(F4.97) (F1.58) (F2.72) (F4.52) (F3.14)


3.70
2.29
0.95
0.39


1.99
1.21 2.12 4.45

Parentheses indicate standard errors; V /0 and K 0/,s values for (amino acids + water) are taken from Refs. [23,42].

concentration than at lower concentration of magnesium chloride. The more positive values of DV /0 and DK 0/,s for glycine and l-alanine indicate the dominance of the charged end groups, NH3+ and COO
, while negative DV /0 and DK 0/,s (at lower concentration of magnesium chloride) in case of l-valine indicate the effect of hydrophobic parts. That is, the interactions between the magnesium chloride and the zwitterionic center of amino

acids (in case of glycine and l-alanine) increase with increasing magnesium chloride concentration. For l-valine, the interactions between the non-polar group of l-valine and magnesium chloride are predominant. The overall effect is that the charged end group of glycine and lalanine influences electrostatically the surrounding water molecules, the so-called electrostriction. In other words, the hydration cospheres of NH3+COO
, which are more hydrated than that of magnesium chloride, will be effected to a greater extent than the latter, resulting in higher values of DV /0. With the increase in concentration of magnesium chloride, electrostriction decreases and structure-making tendency of the ion increases. As a result, the electrostricted water is much less compressible than bulk water and leads to a large decrease in the compressibility with increase in magnesium chloride concentration. Thus, K 0/,s values are negative and DK 0/,s values are highly positive for all the amino acids. These observations again support the positive DV /0 and DK 0/,s results that the dehydration of solute and cosolute occurs more in the case of glycine and l-alanine and it is increased with the increase in the concentration of magnesium chloride. Li et al. [34] also reported positive DV /0 values for different amino acids from water to aqueous glucose solutions. Franks and collaborators [35] reported that the partial molar volume at infinite dilution of a nonelectrolyte is a combination of two factors by the following equation V/0 ¼ Vint þ VS

0 Fig. 5. Plots of DV /0 and DK /,s vs. m B for glycine, o and and E; l-valine, 5 and n, respectively at 298.15 K.

.; l-alanine, D

ð6Þ

where Vint is the intrinsic molar volume of the non-hydrated solute, and V S is the contribution due to the interaction of the

A. Pal, S. Kumar / Journal of Molecular Liquids 121 (2005) 148–155

solute with water. Some workers [36,37] have suggested that the Vint is made up of the following types of contributions Vint ¼ Vv;w þ Vvoid

ð7Þ

where V v,w is the van der Waals volume [38a] and V void is the volume associated with the void or empty spaces [38b]. For electrolytes and zwitterionic solutes, this equation was modified by Shahidi et al. [36] to find the contribution of one molecule to the partial molar volume of a hydrophobic solute as V/0

¼ Vv;w þ Vvoid
Vshrinkage

ð8Þ

where V shrinkage is the volume due to shrinkage. This is due to the interaction of hydrogen bonding sites with water molecules. Assuming that V v,w and V void have the same magnitudes in water and aqueous magnesium chloride solutions, the positive DV /0 values of glycine and l-alanine might arise from the decrease in V shrinkage in magnesium chloride solutions. The interactions of the magnesium chloride with the zwitterionic center of the amino acids (glycine and l-alanine) reduces the effect of electrostriction of water, thereby causing a decrease in V shrinkage. In other words, some water molecules may be released as bulk water in the presence of magnesium chloride. It brings about the increase in volume of the solvent [39], thereby reducing the strong interactions between amino acids and water. This results in a positive volume of transfer from water to aqueous magnesium chloride solutions as observed in glycine and l-alanine. Thus, a positive DV /0 for glycine and l-alanine results from the decreased effect of magnesium chloride and glycine or lalanine on water structure which arises due to glycine or l-alanine–magnesium chloride interactions. The negative DV /0 values of l-valine might arise from the smaller decrease in V shrinkage in magnesium chloride solutions. The DV /0 values can also be explained on the basis of the cosphere overlap model [40,41] in terms of solute–cosolute interactions. According to this model, hydrophilic–ionic group interactions contribute positively, whereas hydrophilic–hydrophobic group interactions contribute negatively to the DV /0 values. In case of glycine and l-alanine, the former type of interactions are predominant over the latter, and for l-valine, hydrophilic–hydrophobic group interactions are dominating over the hydrophilic–ionic group interactions. It may be noted from our previous finding [42] that DV /0 values of l-alanine are less than those of glycine and l-valine in presence of sucrose. As can be seen from Fig. 5, the increasing magnitude of DV /0 with the increasing concentration of magnesium chloride, may also be attributed to the greater ion–hydrophilic interactions. The overall effect is that the solute–cosolute interactions are predominant over the solute–solvent interactions throughout the concentration range of magnesium chloride as obtained in glycine and l-alanine.

153

Table 3 Contribution of (NH+3 COO
) and (R) groups to the limiting partial molar volumes (V /0) and limiting partial molar adiabatic compressibilities (K 0/,s) of glycine, l-alanine, and l-valine in aqueous solutions of MgCl2d 6H2O at 298.15 K m B (mol kg
1)

V /0  106 (m3 mol
1) NH+3 COO


–CH2

–CHCH3

–CHCH(CH3)2

27.97 (F1.32) 31.09 (F1.44) 31.93 (F0.05)

15.80 14.45 14.64

31.59 28.90 29.28

63.19 57.81 58.56

K 0/,s  10 6 (m 3 mol
1 GPa
1 ) 0.0
24.02 (F2.15) 0.2
20.99 (F4.28) 0.4
21.25 (F4.72)


1.01
1.96
0.45


2.02
3.91
0.90


4.04
7.82
1.80

0.0 0.2 0.4

Parentheses indicate standard errors.

Table 3 shows the contributions of the various groups to the V /0 and K 0/,s of glycine, l-alanine, and l-valine in aqueous magnesium chloride solutions at 298.15 K. The values of V /0 and K 0/,s for CH2– (glycine), CH3CH– (lalanine), –CHCH(CH3)2 (l-valine) and the zwitterionic end groups (NH3+COO
) were estimated by least squares fitting 0 of V /0 and K /,s of the amino acids containing the group vs. the molecular weight of the hydrocarbon portion of the amino acid. Values for V /0 of –CH2, –CHCH3, and NH3+COO
agree well with the literature values [17,43– 45] in case of water. The contribution of (NH3+COO
) to V /0 is larger than the –CH2 group and increases with the increase in the concentration of magnesium chloride. It indicates that the interactions between magnesium chloride and charged end groups (NH3+COO
) of amino acids are much stronger than those between magnesium chloride and –CH2 groups. However, the V /0 values for non-polar group increase with the increase in the size of side chain of amino acids but decrease initially and then increase with the concentration of magnesium chloride. It indicates that the increased side chain of amino acids leads to an increase in the hydrophillic–hydrophobic group interactions. This is very important in case of l-valine. Kozak et al. [46] proposed a theory based on the McMillan–Mayer [47] theory of solutions. This approach has further been discussed by Friedman and Krishnan [48] and Frank et al. [49] in order to include solute–cosolute interactions in the solvation spheres. According to this treatment, a thermodynamic transfer function at infinite dilution can be expressed as DY/0 ðwater aqueous magnesium chloride solutionÞ ¼ 2yAB mB þ 3yABB m2B þ N

ð9Þ

where A stands for amino acids and B stands for magnesium chloride, and m B is the molality of magnesium chloride (cosolute). The constants, y AB and y ABB are pair and triplet interaction coefficients and are obtained by fitting DY /0 data to Eq. (9). The corresponding parameters, K AB and K ABB for adiabatic compressibilities, and VAB and VABB for

154

A. Pal, S. Kumar / Journal of Molecular Liquids 121 (2005) 148–155

Table 4 Pair and triplet interaction coefficients for glycine, l-alanine, and l-valine in aqueous solutions of MgCl2d 6H2O at 298.15 K Amino acid

From volume

glycine l-alanine l-valine

From compressibility

VAB  106 (m3 mol
2 kg)

VABB  106 (m3 mol
3 kg2)

K AB  106 (m3 mol
2 kg GPa
1)

K ABB  106 (m3 mol
3 kg2 GPa
1)

4.9098 1.0013
13.9295


1.7406 0.7554 23.6290


0.9559 5.9585
10.9153

4.8401 0.6398 28.4140

Further, the number of water molecules n H hydrated to the amino acids were calculated using the method given by Millero et al. [30,53] 0 0 0 nH ¼
K/;s ðelectÞ=V/;b Ks;b

ð14Þ

where K 0s,b is the compressibility of bulk water. The value of V 0/,b K s0 b is c0.81  10
5 m3 mol
1 GPa
1. The electrostriction partial molar compressibility K 0/,s(elect) can be estimated from the values of K 0/,s (amino acid) from 0 0 0 ðelectÞ ¼ K/;s ðamino acidÞ
K/;s ðintÞ K/;s

0 DK /,s

DV /0,

volumes, estimated from and respectively, are listed in Table 4. The triplet interaction coefficients, K ABB are positive, whereas pair and triplet interaction parameters, VAB, VABB, and K AB contribute positively as well as negatively. Pair and triplet interaction coefficients, VAB, and VABB vary linearly with the size of the alkyl side chain of amino acids. The positive values of the pair interaction coefficients, VAB in case of glycine and l-alanine suggest that interactions occur due to the overlap of hydration spheres of the solute–cosolute molecules, which again supports the conclusion drawn from the cosphere overlap model. The number of water molecules n H hydrated to the amino acids were calculated using the method given by [50,51]   0 0 nH ¼ V/0 ðelectÞ= V/;e
V/;b

ð11Þ

where V /0(int) [30] has been calculated from the following expressions V/0 ðintÞ ¼ ð0:7=0:6ÞV/0 ðcrystÞ

ð12Þ

and V/0 ðintÞ

¼

ð0:7=0:634ÞV/0 ðcrystÞ

ð15Þ

0 /,s (isomer)

where [30,31] = K for glycine (2.7  10 m mol
1 GPa
1), alanine (3.35  10
6 m3 mol
1 GPa
1),and valine (3.0  10
6 m3 mol
1 GPa
1). Since one would expect K 0/,s(int) to be small. It is less than 5  10
6 m3 mol
1 GPa
1 for ionic crystal and many organic solutes in water [30]. So, one can assume K 0/,s (int) c 0. Therefore, for K /,s0 (int) c 0, Eq. (15) becomes 0 0 K/;s ðelectÞ ¼ K/;s ðamino acidÞ

ð16Þ

where the values of K 0/,s(elect) are those given in Table 2. The values of n H calculated from Eq. (14) using the K 0/,s(elect) values determined by these two methods are also listed in Table 5. The values of n H (in water) calculated from the volume and compressibility data by several methods are

ð10Þ

where V 0/,e is the molar volume of electrostricted water and V 0/,b is the molar volume of bulk water (18.069  10
6 m3 mol
1 at 298.15 K). The reported values [30] of 0 0 (V /,e
V /,b ) are c
3.3  10
6 m3 mol
1 at 298.15 K. The electrostriction partial molar volume, V /0(elect) can be estimated from the values of V /0(amino acid) V/0 ðelectÞ ¼ V/0 ðamino acidÞ
V/0 ðintÞ

K 0/,s (int)
6 3

ð13Þ

where V /0(cryst) (=mol. wt./d cryst) is the crystal molar volume, 0.7 is the packing density for molecules in the organic crystal and 0.634 is the packing density for random packing spheres. The values of V /0(int) for the amino acids have been estimated from Eqs. (12) and (13) using d cryst values for glycine, alanine, and valine (1.598, 1.371, and 1.267) g cm
3 taken from the work of Berlin and Pallansch [52]. Using the value of (V 0/,e
V 0/,b)and the values of V /0(elect), calculated from both the two methods, the n H values were estimated from Eq. (10) and are given in Table 5.

Table 5 Hydration number (n H) for glycine, l-alanine, and l-valine in aqueous solutions of MgCl2d 6H2O at 298.15 K mB (mol kg
1)

nH From volume

From compressibility

Using Eq. (12)

Using Eq. (13)

Using Eq. (15)

Using Eq. (16)

glycine 0.0

3.52

0.2 0.4 0.6 0.8

3.00 2.50 2.40 2.12

2.63 2.63 [30] 2.11 1.61 1.51 1.23

3.56 3.67 [30] 3.42 3.29 3.28 2.50

3.23 3.23 [30] 3.08 2.95 2.94 2.16

l-alanine 0.0

4.66

0.2 0.4 0.6 0.8

4.51 4.42 3.91 3.79

3.43 3.41 [30] 3.28 3.18 2.68 2.55

3.48 3.57 [30] 3.16 2.78 2.59 2.10

3.06 3.16 [30] 2.74 2.37 2.18 1.69

l-valine 0.0

5.15

0.1 0.2 0.3 0.4

6.27 5.85 5.44 5.27

3.40 3.43 [30] 4.52 4.09 3.69 3.52

3.92 4.16 [30] 4.16 4.07 3.65 3.36

3.54 3.78 [30] 3.79 3.69 3.28 2.99

The values of n H for (amino acids + water) are taken from Ref. [42].

A. Pal, S. Kumar / Journal of Molecular Liquids 121 (2005) 148–155

in good agreement. These values decrease with the increase in the concentration of magnesium chloride for all the amino acids, which again indicates the increase in solute–cosolute interactions with the increase in magnesium chloride concentration. Furthermore, the values of n H for glycine and l-alanine in the presence of magnesium chloride are less than in water and reverse is the case with l-valine. However, when one H-atom of l-alanine is replaced by the –C(CH3)2 group as in l-valine, the hydrophillic–hydrophobic group interactions increase, and as a result, greater electrostriction of the solvent water is produced, leading to high values of n H and smaller values of DV /0 as compared to glycine and l-alanine.

4. Conclusion In summary, volume and compressibility data have been determined for aqueous solutions of glycine, l-alanine, and l-valine in different concentrations of aqueous magnesium chloride solutions and the results have been used to estimate the number of water molecules hydrated to the amino acids. The electrostriction partial molar volume and the compressibility were determined from the measured values of V /0 and K 0/,s. Group contributions for V /0 and K 0/,s have also been determined. The results are in good agreement with the values calculated by other workers. This approach of relating the volume and compressibility behavior seems successful in obtaining credible values for apparent hydration numbers when applied to amino acids in aqueous salt solutions.

Acknowledgment One of the authors (S.K.) is thankful to the University authorities for awarding a University Research Fellowship.

References [1] (a) S.N. Timasheff, G.D. Fasman, in: Structure and Stability of Biological Macromolecules, vol. 2, Marcel Dekker, New York, 1969, p. 65, Ch. 2; (b) S.N. Timasheff, G.D. Fasman, Structure and Stability of Biological Macromolecules, 1969, p. 213, Ch. 3. [2] P.H. Von Hippel, T. Schleich, in: S.N. Timasheff, G.D. Fasman (Eds.), Structure and Stability of Biological Macromolecules, vol. 2, Marcel Dekker, New York, 1969, p. 417. [3] B. Nagy, W.P. Jencks, J. Am. Chem. Soc. 87 (1965) 2480. [4] R. Carta, G. Tola, J. Chem. Eng. Data 41 (1996) 414. [5] M.K. Khoshkbarchi, J.H. Vera, Ind. Eng. Chem. Res. 36 (1997) 2445. [6] R.R. Raposo, G.E. Garcia-Garcia, L.F. Merida, M.A. Esteso, J. Electroanal. Chem. 454 (1998) 59. [7] A. Pradhan, J.H. Vera, J. Chem. Eng. Data 45 (2000) 140. [8] Z. Yan, J. Wang, J. Lu, J. Chem. Eng. Data 46 (2001) 217.

155

[9] D.P. Kharakoz, Biophys. Chemist. 34 (1989) 115. [10] M.M. Duke, A.W. Hakin, R.M. Mckay, K.E. Preuss, Can. J. Chem. 72 (1994) 1489. [11] M. Kikuchi, M. Sakurai, K. Nitta, J. Chem. Eng. Data 40 (1995) 935. [12] T.V. Chalikian, A.P. Sarvazyan, K.J. Breslauer, J. Phys. Chem. 97 (1993) 13017. [13] R.K. Wadi, P. Ramaswami, J. Chem. Soc., Faraday Trans. 93 (1997) 243. [14] R.G. Clarke, L. Hnedkovsky, P.R. Tremaine, V. Majer, J. Phys. Chem., B 104 (2000) 11781. [15] J.-Lin Shen, Z.-Fen Li, B.-Huai Wang, Y.-Min Zhang, J. Chem. Thermodyn. 32 (2000) 805. [16] B. Palcez, Fluid Phase Equilib. 167 (2000) 253. [17] T.S. Banipal, D. Kaur, P. Lal, G. Singh, P.K. Banipal, J. Chem. Eng. Data 47 (2002) 1391. [18] A. Kumar, R. Badarayani, Fluid Phase Equilib. 201 (2002) 321. [19] T.S. Banipal, G. Sehgal, Thermochim. Acta 262 (1995) 175. [20] R. Bhat, J.C. Ahluwalia, J. Phys. Chem. 89 (1985) 1099. [21] I.N. Basumallick, R.K. Mohanty, Indian J. Chem. 25A (1986) 1089. [22] T.S. Banipal, P. Kapoor, J. Indian Chem. Soc. 76 (1999) 431. [23] A. Pal, S. Kumar, J. Indian Chem. Soc. 79 (2002) 866. [24] R. Garnsey, R.J. Boe, R. Mahoney, T.A. Litovitz, J. Chem. Phys. 50 (1969) 5222. [25] V.A. Del Grosso, C.W. Mader, J. Acoust. Soc. Am. 52 (1972) 1442. [26] Y.P. Singh, PhD Thesis, University of Kurukshetra, India (1996). [27] A. Pal, G. Dass, J. Indian Chem. Soc. 76 (1999) 446. [28] A. Pal, H. Kumar, Indian J. Phys. 75B (5) (2001) 419. [29] D.P. Kharakoz, J. Phys. Chem. 95 (1991) 5634. [30] F.J. Millero, A.L. Surdo, C. Shin, J. Phys. Chem. 82 (1978) 784. [31] F.T. Gucker Jr., R.M. Hagg, J. Acoust. Soc. Am. 25 (1953) 470. [32] T. Ogawa, M. Yasuda, K. Mizutani, Bull. Chem. Soc. Jpn. 57 (1984) 662. [33] C. Jolicoeur, J. Boileau, Can. J. Chem. 56 (1978) 2707. [34] S. Li, W. Sang, R. Lin, J. Chem. Thermodyn. 34 (2002) 1761. [35] F. Frank, M.A.J. Quickenden, D.S. Reid, B. Watson, Trans. Faraday Soc. 66 (1970) 582. [36] F. Shahidi, P.G. Farrell, J.T. Edwards, J. Solution Chem. 5 (1976) 807. [37] S. Terasawa, H. Itsuki, S. Arakawa, J. Phys. Chem. 79 (1975) 2345. [38] (a) A. Bondi, J. Phys. Chem. 68 (1964) 441; (b) A. Bondi, J. Phys. Chem. 58 (1959) 929. [39] J.L. Fortier, P.R. Philip, J.E. Desnoyers, J. Solution Chem. 3 (1974) 323. [40] R.W. Gurney, Ionic Process in Solution, vol. 3, McGraw Hill, New York, 1953, Ch. 1. [41] H.S. Frank, M.W. Evans, J. Chem. Phys. 13 (1945) 507. [42] A. Pal, S. Kumar, Z. Phys. Chem. 218 (2004) 1169. [43] R.K. Wadi, R.K. Goyal, J. Solution Chem. 21 (1992) 163. [44] Z. Yan, J. Wang, W. Liu, J. Lu, Thermochim. Acta 334 (1999) 17. [45] S. Cabani, V. Mollica, L. Lepori, T.S. Lobo, J. Phys. Chem. 81 (1977) 987. [46] J.J. Kozak, W.S. Knight, W. Kauzman, J. Chem. Phys. 48 (1968) 675. [47] W.G. McMillan Jr., J.E. Mayer, J. Chem. Soc. 13 (1945) 276. [48] H.L. Friedman, C.V. Krishnan, J. Solution Chem. 2 (1973) 37. [49] F. Franks, M. Pedley, D.S. Reid, J. Chem. Soc., Faraday Trans. I 72 (1976) 359. [50] H.S. Harned, B.B. Owen, The Physical Chemistry of Electrolyte Solutions, ACS Monograph Series, vol. 137, Reinhold, New York, 1958. [51] (a) J. Padova, J. Chem. Phys. 39 (1963) 1552; (b) J. Padova, J. Chem. Phys. 40 (1964) 691. [52] E. Berlin, M.J. Pallansch, J. Phys. Chem. 72 (1968) 1887. [53] F.J. Millero, G.K. Ward, F.K. Lepple, E.V. Haff, J. Phys. Chem. 78 (1974) 1636.