Effect of magnesium acetate on the volumetric and transport behavior of some amino acids in aqueous solutions at 298.15 K

J. Chem. Thermodynamics 38 (2006) 1214–1226 www.elsevier.com/locate/jct

Effect of magnesium acetate on the volumetric and transport behavior of some amino acids in aqueous solutions at 298.15 K Tarlok S. Banipal a

a,*

, Damanjit Kaur a, Parampaul K. Banipal

b

Department of Applied Chemistry, Guru Nanak Dev University, Amritsar 143 005, India b Department of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India

Received 7 July 2005; received in revised form 3 December 2005; accepted 6 December 2005 Available online 31 January 2006

Abstract Densities, q, and viscosities, g, of glycine, DL-a-alanine, DL-a-amino-n-butyric acid, L-leucine and L-phenylalanine in 0.5, 1.0, 1.5 and 2.0 mB of aqueous magnesium acetate solutions at 298.15 K have been measured as a function of concentration of amino acids using vibrating tube-digital densimeter and Ubbelohde capillary type viscometer, respectively. The apparent molar volumes, V/, and relative viscosities, gr, of amino acids have been derived. The partial molar volume at infinite dilution, V 02 , and viscosity B-coefficient obtained from these data have been used to calculate the corresponding transfer parameters, DtV0, and DtB, for the studied amino acids from water to aqueous magnesium acetate solutions. The activation free energies, Dl062¼ , for the viscous flow of solutions have been obtained by application of the transition-state theory to the viscosity B-coefficient data. The interaction coefficients and hydration number, nH, of amino acids in aqueous solutions have also been calculated to see the effect of magnesium acetate on the hydration of amino acids. The 06¼  0 contribution of the zwitterionic end groups (NHþ 3 , COO ) and (CH2) group of the amino acids to V 2 , viscosity B-coefficient and Dl2 have been calculated. These results have been rationalized in terms of the hydration of hydrophilic and hydrophobic parts of amino acids.  2005 Elsevier Ltd. All rights reserved. Keywords: Apparent molar volume; Viscosity B-coefficient; Interaction coefficients; Activation parameters; Amino acids; Magnesium acetate

1. Introduction The knowledge about the origin of the stability of proteins in aqueous solutions is essential for the understanding of their structure and function. Protein hydration plays a crucial role in this matter. The stability of folded protein structure is marginal (only 20 kJ Æ mol1 to 50 kJ Æ mol1) under physiological conditions, which is due to the delicate balance among various powerful countervailing non-covalent forces such as hydrogen bonding, electrostatic interactions, hydrophobic interactions, etc. Salt solutions are known to produce remarkable effects on the conformation and properties of proteins like denaturation, solubility and

*

Corresponding author. Fax: 91 183 2258819/2258820. E-mail address: [email protected] (T.S. Banipal).

0021-9614/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2005.12.002

dissociation into subunits and the activity of enzymes, etc. [1,2]. Due to complex structure of proteins, a variety of different interactions with salts may occur and it is difficult to resolve straightaway the various interactions participating in protein hydration. Therefore, in order to obtain more insight into the hydration of proteins and non-covalent forces stabilizing their native structure, it is necessary to determine the effects of salts on the model compounds of proteins such as amino acids and peptides. Various workers have studied the interactions between some amino acids and simple salts [3–15] (such as NH4Cl, NaCl, KCl, Na2SO4, KSCN, MgCl2, CaCl2, etc.), which act as stabilizer/destabilizer, but very few studies are available about the behavior of amino acids in the presence of organic salts [16–20]. The volumetric and transport behavior of some amino acids in the presence of sodium acetate [16,17,20], sodium butyrate [18] and sodium caproate [19],

T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

which are known to influence the dissociation of proteins in solutions has been reported. Comparison of results has also shown that the interactions between carboxylate anion and the amino acids are stronger than those between chloride ion and amino acids. Further, the DtV0 values [20] have shown that the acetate ion follows the same order as in the Hofmeister series. The size and nature of anion of organic salt also modulate the interactions. The caproate ions having bigger hydrophobic hydration sphere, have longer destructive effect on the hydration sphere of NHþ 3 group of the amino acids than butyrate and acetate ions. Most of this work on amino acids has been carried out in dilute electrolytes solutions. Recently Badarayani and Kumar [21,22] have studied some amino acids in concentrated ionic solutions and proposed a new method for the determination of ion–amino acid interaction parameters. Although various studies of amino acids are available in the presence of electrolytes having divalent cations [12– 14,21,22], but no report has been found in the presence of organic salts having divalent cations. Therefore, it has been planned to study the effect of carboxylate anion in the presence of a divalent cation on the volumes and viscosities of some amino acids. Magnesium acetate has been chosen as organic salt because magnesium found immense importance in biological chemistry [23]. In the present paper, we report the partial molar volumes, V 02 , and viscosity B-coefficients of glycine, DL-a-alanine, DL-a-amino-nbutyric acid, L-leucine and L-phenylalanine in aqueous 0.5, 1.0, 1.5 and 2.0 mB magnesium acetate (MA) solutions at 298.15 K, where mB, is the molality (mol kg1) of MA in water. From these data, the partial molar volumes of transfer, DtV0, viscosity B-coefficient of transfer, DtB, side chain contributions, hydration number, nH, and volumetric interaction coefficients of amino acids have been calculated. The transition-state theory has been applied to calculate the activation parameters for viscous flow of amino acids in aqueous MA solutions. The results have been discussed in terms of the hydration of different parts of amino acids (hydrophilic and hydrophobic) in aqueous solutions. 2. Experimental Glycine (G-7126), DL-a-alanine (A-7502), DL-a-amino-nbutyric acid (A-1754), L-leucine (L-8000) and L-phenylalanine (P-2126) were obtained from Sigma Chemical Co. and were dried for 24 h in a vacuum oven before use. Analytical reagent grade magnesium acetate tetrahydrate from Sisco Research Laboratory, India was used as such after drying in a vacuum desiccator at room temperature. Deionised, doubly distilled degassed water with specific conductance less than 1.29 · 106 X1 Æ cm1 was used for the measurements. All solutions were prepared afresh by mass using a Mettler Balance having an accuracy of ±0.01 mg. The densities of the solutions were measured using a vibrating tube-digital densimeter (Model DMA 60/602) with precision of ±1 · 106 g Æ cm3 and having accuracy of ±3 · 106 g Æ cm3. The principle and experi-

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mental procedure for the densimeter have been described elsewhere [24]. The temperature of water around densimeter cell was controlled within ±0.01 K. The densimeter was calibrated with dry air and pure water. The working of the densimeter was checked by measuring the densities of aqueous sodium chloride solutions that agree well with the literature values [25]. Viscosity measurements were carried out with a suspended level Ubbelohde type capillary viscometer, mounted in water thermostat which was calibrated with water at four different temperatures (288.15 K to 308.15 K) [9]. The flow time of a constant volume of liquid through capillary was measured with an electronic stopwatch with a resolution of 0.01 s. An average of at least four readings of flow time with a variation not exceeding ±0.1 s was taken for each solution. The temperature of the thermostat bath was controlled within ±0.01 K. The measured viscosities were found to be accurate within ±0.001 mPa s. 3. Results The apparent molar volumes, V/ of amino acids in 0.5, 1.0, 1.5 and 2.0 mB of aqueous MA solutions have been obtained from the solution densities (table 1) using the following relation: V / ¼ ðM=qÞ  ðq  q0 Þ103 =ðm  q  q0 Þ;

ð1Þ

where M and m are, respectively, the molar mass and molality of the amino acids, q0 and q are the densities of solvent (MA and water mixture) and solution, respectively. At infinite dilution, the apparent molar volume, V 0/ becomes same as the standard partial molar volumes V 02 . When V/ show no significant concentration dependence, the average of all the data points has been taken as the standard partial molar volume. However, where finite concentration dependence was observed, V 02 was determined by the method of least squares using V / ¼ V 02 þ S V m;

ð2Þ 0 2

where SV is the experimental slope. The V values along with their standard deviations are listed in table 2. The uncertainty in the determination of V/ occurring because of the measurement of various quantities has been calculated. The uncertainty values for the V/ range from 0.062 cm3 Æ mol1 to 0.009 cm3 Æ mol1 for the lower (60.04m) and higher concentration ranges for glycine, DL-a-alanine and DL-a-amino-n-butyric acid, respectively. The uncertainty for V/ in case of L-leucine and L-phenylalanine are slightly higher because very dilute concentration range has been studied. The uncertainty values in these cases range from 0.331 cm3 Æ mol1 to 0.051 cm3 Æ mol1 for the lower (60.009 m) and higher concentration ranges, respectively. To the best of our knowledge these data for the studied amino acids are being reported for the first time in aqueous MA solutions. The viscosities, g of the solutions were calculated using the following expression:

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TABLE 1 Densities, q, and apparent molar volumes, V/, of some amino acids in aqueous MA solutions as a function of concentration of amino acids and MA at T = 298.15 K m/(mol Æ kg1)

q/(g Æ cm3)

V//(cm3 Æ mol1)

m/(mol Æ kg1)

q/(g Æ cm3)

V//(cm3 Æ mol1)

0.04183 0.13377 0.22182 0.40569

1.034548 1.037170 1.039659 1.044804

Glycine in aqueous MA solutions mB = 0.5 mol Æ kg1 (q0 = 1.033352) 45.82 0.48723 45.75 0.61685 45.74 0.71509 45.70

1.047030 1.050544 1.053207

45.75 45.78 45.76

0.09959 0.17935 0.22347 0.32082

1.066284 1.068379 1.069495 1.072042

46.98 47.01 47.15 47.05

mB = 1.0 mol Æ kg1 (q0 = 1.063639) 0.43229 0.52241 0.68625

1.074906 1.077178 1.081366

47.04 47.07 46.96

0.08343 0.20655 0.27436 0.31153

1.091953 1.094959 1.096651 1.097523

47.98 47.99 47.84 47.92

mB = 1.5 mol Æ kg1 (q0 = 1.089891) 0.38311 0.43374 0.57205

1.099266 1.100478 1.103731

47.87 47.86 47.90

0.06972 0.11056 0.19304

1.114070 1.114999 1.116882

48.88 48.89 48.82

mB = 2.0 mol Æ kg1 (q0 = 1.112471) 0.21005 0.39596

1.117257 1.121453

48.86 48.76

in aqueous MA solutions mB = 0.5 mol Æ kg1 0.40315 0.49772 0.58318

1.043362 1.045598 1.047632

62.36 62.43 62.42

mB = 1.0 mol Æ kg1 0.35327 0.36912 0.45942

1.071594 1.071941 1.073917

63.38 63.38 63.37

mB = 1.5 mol Æ kg1 0.48144 0.50533 0.58097

1.099672 1.100164 1.101579

64.06 64.02 64.12

mB = 2.0 mol Æ kg1 0.28413 0.43964

1.117785 1.120580

64.66 64.71

acid in aqueous MA solutions mB = 0.5 mol Æ kg1 0.20799 0.23731 0.28017

1.038352 1.039039 1.040010

76.89 76.91 77.02

1.067716 1.067971 1.068771

77.76 77.75 77.81

1.093039 1.093739

78.43 78.45

a

DL-a-Alanine

0.05082 0.17623 0.27423 0.33006

1.034643 1.037788 1.040212 1.041593

62.35 62.37 62.37 62.33

0.08023 0.21410 0.26860 0.32929

1.065480 1.068517 1.069701 1.071039

63.37 63.33 63.45 63.45

0.09111 0.25029 0.30202 0.35577

1.091798 1.095046 1.096098 1.097187

64.01 64.10 64.07 64.05

0.04177 0.08804 0.17318

1.113265 1.114145 1.115760

64.68 64.62 64.55

DL-a-Amino-n-butyric

0.03891 0.07487 0.11296 0.17439

1.034297 1.035172 1.036083 1.037552

76.96 76.87 76.93 76.90

0.04248 0.07402 0.12012

1.064551 1.065226 1.066207

77.89 77.86 77.85

mB = 1.0 mol Æ kg1 0.19094 0.20298 0.24199

0.03817 0.10559

1.090622 1.091897

78.42 78.46

mB = 1.5 mol Æ kg1 0.16628 0.20413

T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

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TABLE 1 (continued) m/(mol Æ kg1)

q/(g Æ cm3)

0.14724

1.092671

0.04982 0.05805 0.09559

V//(cm3 Æ mol1)

m/(mol Æ kg1)

q/(g Æ cm3)

0.25537

1.094700

78.40

mB = 2.0 mol Æ kg1 0.23029 0.24189

1.116328 1.116499

78.87 78.93

in aqueous MA solutions mB = 0.5 mol Æ kg1 0.04447 0.04835 0.05419

1.034203 1.034280 1.034391

108.93 108.87 108.88

mB = 1.0 mol Æ kg1 0.04804 0.04903 0.05355

1.064390 1.064405 1.064474

109.44 109.44 109.46

mB = 1.5 mol Æ kg1 0.04444 0.04781 0.05748

1.090428 1.090468 1.090587

110.13 110.14 110.10

in aqueous MA solutions mB = 0.5 mol Æ kg1 0.04090 0.04146 0.05089

1.034925 1.034946 1.035308

123.66 123.67 123.64

mB = 1.0 mol Æ kg1 0.03772 0.04189

1.064941 1.065088

124.65 124.57

78.50

1.113323 1.113456 1.114083

78.80 78.90 78.93

V//(cm3 Æ mol1)

L-Leucine

0.00775 0.01853 0.02757 0.03360

0.01569 0.02300 0.03219 0.04555

0.01489 0.02846 0.03201

1.033501 1.033709 1.033883 1.033998

108.92 108.87 108.85 108.87

1.063885 1.063998 1.064144 1.064353

109.45 109.50 109.41 109.40

1.090073 1.090238 1.090282

110.05 110.06 110.04

L-Phenylalanine

0.01335 0.02070 0.03276

1.033867 1.034152 1.034613

123.68 123.58 123.67

0.01163 0.01871 0.03333

1.064043 1.064286 1.064791

124.56 124.67 124.63

a

mB stands for the molality of MA in water.

TABLE 2 Partial molar volumes, V 02 , at infinite dilution of some amino acids in aqueous MA solutions at T = 298.15 K Amino acids

Glycine DL-a-Alanine DL-a-Amino-n-butyric acid L-Leucine L-Phenylalanine a

V 02 =ðcm3  mol1 Þ mBa = 0.5 mol Æ kg1

mB = 1.0 mol Æ kg1

mB = 1.5 mol Æ kg1

mB = 2.0 mol Æ kg1

45.76 ± 0.04 62.37 ± 0.04 76.92 ± 0.05 108.88 ± 0.03 123.65 ± 0.05

47.04 ± 0.06 63.39 ± 0.04 77.82 ± 0.06 109.44 ± 0.03 124.62 ± 0.05

47.91 ± 0.06 64.06 ± 0.04 78.44 ± 0.03 110.09 ± 0.04

48.84 ± 0.05 64.64 ± 0.06 78.89 ± 0.05

mB stands for the molality of MA in water.

g=q ¼ at  b=t;

ð3Þ

where q is the density of the solution, t is the flow time and a and b are the viscometer constants. The values of the viscometer constants a and b used in present work are: a = 5.9697 · 106 mPa Æ m3 Æ kg1 and b = 7.3591 · 103 mPa Æ s2 Æ m3 Æ kg1, except in case of glycine in 1.0 and 2.0 mB of aqueous MA solution where a = 6.1339 · 106 mPa Æ m3 Æ kg1 and b = 5.2218 · 103 mPa Æ s2 Æ m3 Æ kg1, respectively.

The relative viscosities, gr (gr = g/g0 where g0 and g are the viscosities of solvent and solution, respectively) are given in table 3. The relative viscosities, gr were fitted by the method of least squares to obtain the viscosity B-coefficients using Jones–Dole equation as follows: gr ¼ 1 þ Bc;

ð4Þ

where c is the molarity (calculated from molality). The viscosity B-coefficient values along with their standard deviations are summarized in table 4.

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TABLE 3 Relative viscosities, gr, of some amino acids in aqueous MA solutions as a function of concentration of amino acids and MA at T = 298.15 K c/(mol Æ dm3)

0.09144 0.14551

0.10540 0.18913

0.05807 0.11028

0.05636 0.10083

gr

c/(mol Æ dm3)

c/(mol Æ dm3)

gr

1.0139 1.0215

Glycine in aqueous MA solutions mBa = 0.5 mol Æ kg1 (g0 = 1.3550) 0.23235 1.0389 0.31655 1.0487

0.41508 0.45641

1.0599 1.0679

1.0165 1.0317

mB = 1.0 mol Æ kg1 (g0= 1.9922) 0.23507 1.0369 0.33584 1.0507

0.45006 0.54150

1.0692 1.0838

1.0103 1.0173

mB = 1.5 mol Æ kg1 (g0 = 2.8740) 0.16180 1.0268 0.22334 1.0371

0.29278 0.34806

1.0457 1.0533

1.0087 1.0157

mB = 2.0 mol Æ kg1 (g0 = 4.0278) 0.27464 1.0432 0.35479 1.0556

0.41505

1.0699

0.30377 0.35702

1.0834 1.0942

1.0389 1.0528

0.26529 0.32444

1.0737 1.0908

1.0638 1.0808

0.34017

1.0940

1.0548 1.0698

0.29804

1.0879

0.19330 0.22503

1.0717 1.0842

1.0397 1.0555

0.17592 0.20460

1.0656 1.0772

1.0589 1.0742

0.22863

1.0877

1.0431 1.0573

0.16981

1.0675

0.03896 0.04406

1.0213 1.0243

1.0133 1.0156

0.03451 0.04027

1.0190 1.0223

1.0152 1.0189

0.04080

1.0230

gr

DL-a-Alanine

0.04703 0.10051

1.0128 1.0280

in aqueous MA solutions mB = 0.5 mol Æ kg1 0.14860 1.0417 0.18425 1.0522

0.05531 0.09295

1.0157 1.0250

0.14252 0.19933

0.10898 0.15752

1.0296 1.0447

0.22304 0.27996

0.10791 0.15694

1.0337 1.0461

0.20104 0.24765

mB = 1.0 mol Æ kg1

mB = 1.5 mol Æ kg1

mB = 2.0 mol Æ kg1

DL-a-Amino-n-butyric

0.03630 0.07878

1.0136 1.0286

acid in aqueous MA solutions mB = 0.5 mol Æ kg1 0.11296 1.0389 0.15883 1.0605

0.03984 0.07667

1.0147 1.0298

0.11039 0.14121

0.08353 0.12044

1.0332 1.0468

0.15669 0.19122

0.03322 0.07618

1.0132 1.0290

0.11493 0.14743

mB = 1.0 mol Æ kg1

mB = 1.5 mol Æ kg1

mB = 2.0 mol Æ kg1

L-Leucine

0.01002 0.01616

1.0055 1.0088

in aqueous MA solutions mB = 0.5 mol Æ kg1 0.02192 1.0120 0.03061 1.0167 mB = 1.0 mol Æ kg1

0.01109 0.01679

1.0061 1.0093

0.02401 0.02832 mB = 1.5 mol Æ kg1

0.01102 0.02229

1.0061 1.0125

0.02731 0.03418

T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

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TABLE 3 (continued) c/(mol Æ dm3)

c/(mol Æ dm3)

gr

gr

gr

0.03191 0.03723

1.0193 1.0223

0.03298 0.03733

1.0200 1.0227

L-Phenylalanine

0.01035 0.01636

1.0063 1.0098

0.02177 0.02773

0.01017 0.01586

1.0061 1.0095

0.02262 0.02678

in aqueous MA solutions mB = 0.5 mol Æ kg1 1.0129 1.0165

c/(mol Æ dm3)

mB = 1.0 mol Æ kg1

a

1.0138 1.0163

mB stands for the molality of MA in water.

TABLE 4 Viscosity B-coefficients of some amino acids in aqueous MA solutions at T = 298.15 K Amino acids

B-coefficient/(dm3 Æ mol1) a

Glycine DL-a-Alanine DL-a-Amino-n-butyric acid L-Leucine L-Phenylalanine a

mB = 0.5 mol Æ kg1

0.150 ± 0.008 0.271 ± 0.008 0.371 ± 0.013 0.548 ± 0.002 0.599 ± 0.006

mB = 1.0 mol Æ kg1

mB = 1.5 mol Æ kg1

mB = 2.0 mol Æ kg1

0.155 ± 0.006 0.276 ± 0.007 0.377 ± 0.011 0.552 ± 0.002 0.607 ± 0.005

0.158 ± 0.009 0.282 ± 0.006 0.385 ± 0.007 0.559 ± 0.008

0.162 ± 0.006 0.288 ± 0.014 0.389 ± 0.009

mB stands for the molality of MA in water.

The MA being a salt of weak acid and strong base undergoes hydrolysis and gives basic solution. By using the following equation, the pH of the solutions at different concentrations of MA can be calculated as pH ¼ 14  1=2pK w þ 1=2 log c þ 1=2pK a ;

ð5Þ

where Kw is the ionic product of water, Ka is the dissociation constant of acetic acid, and c (mol Æ dm3) is the molarity of the MA solution. Although this equation is applicable in the case where strong base is produced, e.g., in sodium acetate. This equation has been applied in case of MA by multiplying the concentration by two, as there are two acetate groups present in MA. The values of pH for various solutions are 9.36, 9.49, 9.57, 9.62 at (0.5, 1.0, 1.5, 2.0) mol kg1 of MA, respectively (molality was converted to molarity). The measured values of pH of aqueous MA solutions are 8.64, 8.59, 8.57, 8.58 at (0.5, 1.0, 1.5, 2.0) mol Æ kg1, respectively. This may be due to less basic nature of MA (acetate of alkaline earth metals) in comparison to acetates of alkali metals (sodium acetate) which gives strong base, e.g., sodium hydroxide, which is completely ionizable and the above equation is based on this fact. Although the pH values are less than the pKa values for amino acids studied (pKa = glycine 9.78, DL-a-alanine 9.87, L-leucine 9.74 and L-phenylalanine 9.31), the amino acids will be present as an equilibrium mixture of different forms, however the proportion of these will vary with the concentration of MA. For a particular pH, the hydroxyl ion concentration can be calculated. Now if we take the extreme case that if all the [OH] ions furnished by Mg(OH)2 (although weak base) are neutralized by the zwitterions to give deprotonated form of amino acids then the concentration of zwitterionic forms of amino acids in solutions of MA may be obtained by subtracting the [OH] from the

actual molality of amino acids. By considering this, the molality of amino acids in solution changes by about two units at the fifth place, which results in the change of V 02 by only 0.02 cm3 Æ mol1 (at lower concentration of amino acids) in case of glycine, DL-a-alanine, DL-amino-a-n-butyric acid, respectively, which is within the uncertainty limits in the measurements. However, in case of L-leucine and L-phenylalanine where studied concentrations are very small, the change in V 02 is slightly higher, i.e., 0.05 cm3 Æ mol1. The change is still very small at higher concentration of amino acids in all the cases. Thus, there is a fraction of fully deprotonated amino acids, but it is very small and its effect can be neglected. Therefore, mainly zwitterionic species are present and the results have been discussed in terms of these. The partial molar volumes of transfer, DtV0 at infinite dilution and viscosity B-coefficients of transfer, DtB from water to aqueous MA solutions have been evaluated by combining the present data with that reported earlier in water [20] as follows: Dt V 0 ¼ V 02 ðin aqueous MA solutionsÞ  V 02 ðin waterÞ; ð6Þ Dt B ¼ B-coefficient ðin aqueous MA solutionsÞ B-coefficient ðin waterÞ.

ð6aÞ

A reasonable linear relation has been observed between V 02 =B-coefficient of amino acids and the number of carbon atoms, nC in their alkyl side chains. A similar linear correlation has also been reported for some amino acids in aqueous potassium thiocyanate (KSCN) [11] and sodium acetate (SA) [16,17,20] solutions. The linear relationship can be represented by

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 0 V 02 ¼ V 02 ðNHþ 3 ; COO Þ þ nC V 2 ðCH2 Þ;

B-coefficient ¼

ð7Þ

 B-coefficientðNHþ 3 ; COO Þþ

nC B-coefficientðCH2 Þ;  where V 02 ðNHþ 3 ; COO Þ  þ cient(NH3 , COO ) and

ð7aÞ

0 2 ðCH2 Þ

and V and B-coeffiB-coefficient(CH2) represent the zwitterionic end group and methylene group contributions to V 02 and B-coefficient, respectively. According to Eyring simple model [26], activation free energy of viscous flow of a single solute in a free solvent, Dl061¼ can be calculated as follows: go ¼ ðhN A =V 01 Þ expðDl061¼ =RT Þ;

ð8Þ

where h is the Plank’s constant, NA is the Avagadro’s number, R is the gas constant and V 01 is the average molar volume of aqueous MA solution. The values of Dl061¼ in water and in (0.5, 1.0, 1.5, 2.0) mol Æ kg1 aqueous MA solutions are (9.16, 10.34, 11.44, 12.47, 13.43) kJ Æ mol1, respectively. The activation free energy of viscous flow, Dl062¼ for studied amino acids in aqueous MA solutions were evaluated by using Feakin’s extension of Eyring transition-state theory [27] by combining B-coefficient and partial molar volume data: B ¼ ðV 01  V 02 Þ=1000 þ ðV 01 =1000ÞðDl062¼  Dl061¼ Þ=RT .

ð9Þ

The above equation can be rearranged as Dl062¼ ¼ Dl061¼ þ ðRT =V 01 Þ½1000B  ðV 01  V 02 Þ;

ð10Þ

0 2

where V is the partial molar volume of amino acids in aqueous MA solutions. 4. Discussion The DtV0 values are positive and increase with the increase in the concentration of MA (figure 1). The increase is linear initially and tends to level off almost in all the cases with the increase in the concentration of MA, which is indicative of the start of saturation of various interactions. The DtV0 values are being influenced by the size of side chain of amino acids as the magnitude of DtV0 decreases with the increase in the length of alkyl side chain of amino acid, i.e., from glycine to L-leucine. It may also be observed that the decrease in DtV0 becomes smaller and smaller with the increase in the size of side chain of the amino acids. This behavior may be explained on the basis that with the increase in the size of side chain of amino acids, the electrostriction of the charged end groups will decrease and this will lead to decrease in the DtV0. Hence L-leucine has smaller positive DtV0 values as compared to glycine. However in case of L-phenylalanine the DtV0 values are more in comparison to DL-a-amino-n-butyric acid and L-leucine, which may be attributed to the difference in the hydration behavior of aliphatic and aromatic side chains as reported earlier in the presence of SA [20] also. In the ternary system (amino acids + magnesium acetate + water), the interactions can be classified into: (1) Ion-charged group interactions occurring between Mg2+

FIGURE 1. Partial molar volumes of transfer Dt V 02 of some amino acids vs concentration of aqueous magnesium acetate solutions mB: ( j) glycine; (d) DL-a-alanine; (m) DL-a-amino-n-butyric acid; (.) L-leucine; (r) L-phenylalanine.

ions and COO groups of amino acid and between CH3COO ions of MA and NHþ 3 groups of amino acids, (2) Ion-non-polar group interactions occurring between ions of MA (Mg2+, CH3COO) and non-polar groups of amino acids. According to cosphere overlap model [28], the ion–charged group interactions contribute positively to the DtV0 values, whereas ion–non-polar group interactions contribute negatively. Therefore, the presently observed positive DtV0 values for the studied amino acids throughout the concentration range of MA indicate that ion–charged group interactions are dominating over the ion–non-polar group interactions. Thus, due to the stronger interactions between the ions of MA and zwitterionic  centers (NHþ 3 , COO ) of the amino acids, the electrostriction of water molecules lying in the vicinity of these charged centers will be reduced, which give rise to a positive DtV0. Further the interactions between the ions of MA and non-polar side chain of amino acids contribute negatively to DtV0 due to structure-breaking effect of MA ions on the hydrophobic hydration sphere of non-polar side chains. The contribution of this effect to DtV0 increases with the increase in the size of non-polar side chain of amino acids, which may be attributed to the decrease in DtV0 from glycine to L-leucine. As mentioned earlier that the decrease in DtV0 becomes smaller and smaller with the increase in the size of the side chain of amino acids, which may be due to fact that the structure-breaking effect of MA on hydrophobic hydration sphere of apolar group reaches to a limiting value and/or the effect of non-polar side chain reaches to a limiting value. The contribution of zwitterionic end groups (NHþ 3, COO) to V 02 is more than the contribution of (CH2) group (table 5) and the contribution increases with the concentration of MA. This also indicates that the interactions between ions of MA and charged centers of amino acids are stronger than those between ions of MA and CH2 group in the non-polar side chain of amino acids. Similar

T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

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TABLE 5  Contribution to the partial molar volumes, V 02 from zwitterionic end groups (NHþ 3 , COO ), CH2 and other alkyl side chains of some amino acids in aqueous MA solutions at T = 298.15 K V 02 =ðcm3  mol1 Þ

Group NHþ 3,



COO –CH2 –CHCH3 –CHCH2CH3 –CHCH2CH(CH3)2 a

mBa = 0.5 mol Æ kg1

mB = 1.0 mol Æ kg1

mB = 1.5 mol Æ kg1

mB = 2.0 mol Æ kg1

30.31 ± 0.64 15.70 ± 0.21 31.40 ± 0.31 47.10 ± 0.36 78.50 ± 0.47

31.73 ± 0.61 15.52 ± 0.19 31.04 ± 0.27 46.56 ± 0.33 77.60 ± 0.42

32.56 ± 0.59 15.48 ± 0.19 30.96 ± 0.27 46.44 ± 0.33 77.40 ± 0.42

34.07 ± 0.97 15.02 ± 0.45 30.04 ± 0.64 45.06 ± 0.78

mB stands for the molality of MA in water.

conclusions have also been drawn from the studies of some amino acids in other organic salt solutions [16,18,20]. From the table 5, it has been seen that V 02 ðCH2 Þ values for the amino acids are insensitive to the concentration of MA and similar trend have also been observed in aqueous KSCN [11] and SA [16,20] solutions. Further the average value (15.43 cm3 Æ mol1) of the contribution of (CH2) group agrees well with the values (15.50 cm3 Æ mol1, 15.40 cm3 Æ mol1) reported in other salts [11,16] by various workers. The contribution of zwitterionic end groups (NHþ 3, COO) and (R) groups [R = –CH2, –CHCH3, –CH CH2CH3, –CHCH2CH(CH3)2] to DtV0 for the amino acids from water [20] to aqueous MA solutions have been calculated using the equation analogous to equation (6) and the results are illustrated in figure 2. The contribution of  0 (NHþ is positive and almost 3 , COO ) groups to DtV increases linearly with the concentration of MA. The contribution of (R) groups to DtV0 is negative and magnitude increases with the concentration of MA and increase becomes sharp at higher concentration. Further the magnitude of the contribution of the (R) groups also increases with increase in the size of alkyl side chain of the amino acids. These observations support the above conclusions. Although the direct comparison of the partial molar quantities of solutes having different molecular sizes is

rather difficult, but partial specific quantities are primarily independent of the size of the solute [20]. As to the solute– solvent interactions operating in aqueous solutions, the ratio of the effect of hydrophilic to the hydrophobic hydration is reflected in the partial specific quantities, while partial molar quantities are the reflection of the net changes in both types of hydrations. The molar specific volumes, m02 (m02 ¼ V 02 =M, where M is the molar mass of the amino acids) for the studied amino acids in aqueous MA solutions are illustrated in figure 3. The m02 values regularly increase from glycine to L-leucine with the increase in the size of side chain of amino acids. It may be seen that the concentration effect of MA is more in case of glycine, which decreases with the increase of alkyl side chain of amino acids and m02 is almost independent of the concentration of MA in case of L-leucine. This may be due to the competition between the positive effect due to the interactions of the ions of MA with the zwitterionic end groups of amino acids and the negative effect due to interactions of the ions of MA with the non-polar side chain of amino acids. These results are in line with the observations that interactions among ions of MA and charged centers of amino acids dominate over the other interactions and the effect gets reduced with the increase in the non-polar side chain of amino acids. Like DtV0 (figure 1) the m02 values for L-phenylalanine lie again in between DL-a-alanine and DL-a-

 0 FIGURE 2. DtV0(NHþ 3 , COO )/DtV (R) vs concentration of aqueous  magnesium acetate solutions mB: (j) NHþ 3 , COO ; (d) –CH2; (m) – CHCH3; (.) –CHCH2CH3; (r) –CHCH2CH(CH3)2.

FIGURE 3. Partial specific volumes m02 vs concentration of aqueous magnesium acetate solutions mB: (j) glycine; (d) DL-a-alanine; (m) DL-aamino-n-butyric acid; (.) L-leucine; (r) L-phenylalanine.

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T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

amino-n-butyric acid due to distinct hydration behavior of aromatic and aliphatic side chains. The number of water molecules, nH hydrated to the amino acids can be estimated from the V 02 by using the method of Millero et al. [29] as follows: nH ¼ V 02 ðelectÞ=ðV 0E  V 0B Þ;

ð11Þ

0 E

where V is the molar volume of electrostricted water and V 0B is the molar volume of bulk water. According to Millero et al. [29] the value of ðV 0E  V 0B Þ can be taken equal to 3.3 cm3 Æ mol1 at 298.15 K. The V 02 ðelectÞ can be calculated from experimentally measured V 02 values by the following equation: V 02 ðelectÞ ¼ V 02 ðamino acidÞ  V 02 ðintÞ.

ð12Þ

0 2 ðintÞ

Using the values of V for the amino acids from their molar crystal volumes [29] the calculated nH values for the amino acids are illustrated in figure 4 (except for DL-a-amino-n-butyric acid whose crystal molar volume data is not available), which decrease with the increase in the concentration of MA. It may be noted that the values of nH in aqueous MA solutions are less than in water as reported earlier [20]. This may be attributed to the decrease of the electrostriction of water lying in the vicinity of charged centers of the amino acids due to the solute (amino acids)– cosolute (MA) interactions. Further, nH values for these amino acids in the presence of MA are also smaller than in the presence of sodium acetate (SA) [20], which is indic-

FIGURE 4. Hydration number nH of some amino acids vs concentration of aqueous magnesium acetate solutions mB: (j) glycine; (d) DL-aalanine; (m) L-leucine; (.) L-phenylalanine.

ative of stronger dehydration effect of MA than that of SA, and various workers have reported similar views for amino acids in the presence of electrolytes [16,18,20]. The transfer volumes, DtV0 of amino acids can be expressed by the McMillan Mayer theory [30] of solutions that permits the formal separation of the effects due to interactions between pairs of the solute molecules and those due to interactions between three or more solute molecules by the following equation: Dt V 0 ¼ 2V XY mB þ 3V XYY m2B þ   

ð13Þ

where X stands for amino acids and Y stand for MA. VXY and VXYY are the pair and triplet volumetric interaction coefficients, respectively and are given in table 6. The VXY and VXYY are positive and negative, respectively, and magnitude decreases as the alkyl side chain of amino acids increases. The large positive VXY values suggest the domination of pair interactions for the studied amino acids. The decrease in VXY values from glycine to L-leucine comes from the difference in the interactions of the alkyl side chains of amino acids with MA, as the interactions of zwitterionic end groups for different amino acids with MA are almost same. This suggests that the alkyl side chains of amino acids play an important role in modulating the volumes of transfer. Therefore the amino acid with longer hydrophobic alkyl side chain may undergo stronger dehydration effect in the presence of MA. Due to this fact that L-leucine having longer alkyl chain has smaller values of DtV0 and consequently the smaller values of VXY, whereas reverse is true in case of glycine which has shorter alkyl chain. The contribution of the relative mass of the pair and triplet interaction coefficients to DtV0 may be better judged at various molalities of MA by plotting the contribution of the interaction coefficients vs mB [only representative plot of contribution of pair and triplet interaction coefficients vs mB is shown in figure 5 for glycine]. Similar trends have been observed for VXY and VXYY in other cases. The relative mass for VXY is positive and increases linearly with the increase in the concentration of MA in all the cases. The relative masses for VXYY are negative and almost zero upto .0.5 mol Æ kg1 and after this magnitude increases sharply with the increase in the concentration of MA for the studied amino acids. Using the literature data, volumetric interaction coefficients for amino acids in aqueous NH4Cl [4], NaCl [5] and MgCl2 [12] solutions have been calculated using

TABLE 6 Pair and triplet interaction coefficients, VXY, and, VXYY, of some amino acids in aqueous MA solutions at T = 298.15 K Amino acids Glycine DL-a-Alanine DL-a-Amino-n-butyric acid L-Leucine a b

VXY/(cm3 Æ mol2 Æ kg) 2.5481 ± 0.2487 1.9817 ± 0.1538 1.3570 ± 0.0338 1.1545 ± 0.1718

VXYY/(cm3 Æ mol2 Æ kg2) a

(1.3686) (0.9298) (0.7909) (0.9941)

Reference [20]: Parentheses contain the values in sodium acetate solutions. Correlation coefficient.

0.3882 ± 0.0965 0.3155 ± 0.0597 0.1849 ± 0.0131 0.1716 ± 0.0866

Rb a

(0.0990) (0.0596) (0.0480) (0.0796)

0.989 0.992 0.999 0.992

0.995 1.3342 ± 0.7622 7.0670 ± 0.8567

g

f

e

d

c

0.991 0.0697 ± 0.0309 0.2204 ± 0.0822

Reference [4]. Reference [5]. Reference [12]. VXY (cm3 Æ mol2 Æ kg). VXYY (cm3 Æ mol2 Æ kg2). Correlation coefficient. Standard deviations have not been included due to two data points only. b

a

R

0.989 0.999 0.1309 ± 0.3005 0.5863 ± 0.1447

VXYY VXY R

0.964 1 1 1 0.0622 ± 0.0175 0.0711g 0.0172g 0.0433g

VXYY VXY

0.6409 ± 0.0900 0.4967g 0.2158g 0.2850g

R

0.970 0.998 0.982 0.0918 ± 0.1028 0.1135 ± 0.0284 0.0915 ± 0.0502

f

0.4272 ± 0.2841 0.8882 ± 0.0694 0.8722 ± 0.1349

Glycine DL-a-Alanine DL-a-Amino-n-butyric acid L-Leucine L-Phenylalanine

In magnesium chloridec In sodium chlorideb

VXYd

VXYYe

In ammonium chloridea Amino acids

equation (13). From table 7, it is clear that VXY are positive whereas VXYY are negative for the above amino acids. Further, the magnitude of VXY is higher than VXYY, which shows that the interactions between amino acids and various salts are mainly pair. Comparison of volumetric pair interaction parameters (tables 6 and 7) for various amino acids particularly from glycine to L-leucine in different salts shows that pair interaction parameters are sensitive to both cation and anion of the salts. VXY decreases in case of NaCl, sodium acetate (SA) [20], MA and increases in case of NH4Cl, MgCl2 from glycine to L-leucine. VXY values are higher in case of SA in comparison to NaCl, which indicate that interactions between acetate (CH3COO) ions and amino acids are stronger than between chloride (Cl) ions. Similar observation has been reported earlier also [16,20]. Further, the higher VXY values in the presence of MA than in case of SA show that the order of interactions between amino acids and cations is: Mg2+ > Na+ (although ionic strength in case of MA can also contribute to some extent). This is the same order as in the Hofmeister series [31]. By comparing the data in the presence of NH4Cl and MgCl2, the order of interactions between amino acids and cations will be: Mgþ2 > NHþ 4 . However, it is difficult to compare the VXY values in the presence of NH4Cl and MgCl2 with other salts as VXY values increase in these cases from glycine to L-leucine. The positive viscosity B-coefficients (table 4) for the studied amino acids increase with the concentration of MA, which suggest the net structural increase. Further the viscosity B-coefficients of amino acids in aqueous MA solutions increase in the order of glycine < DL-a-alanine < DL-a-amino-n-butyric acid < L-leucine < L-phenylalanine, which is also the order of the molar mass of amino acids. It is well known that size, shape and charge of solute molecules are important in determining the viscosity B-coefficients [32]. In case of presently studied amino acids, the charged groups of amino acids are same, hence observed order of viscosity B-coefficients may be rationalized

TABLE 7 Pair and triplet interaction coefficients, VXY, and, VXYY, of some amino acids in aqueous ammonium, sodium and magnesium chloride solutions at T = 298.15 K

FIGURE 5. Contribution of various interaction coefficients to partial molar volumes of transfer Dt V 02 of glycine vs concentration of aqueous magnesium acetate solutions mB: (n) VXY; (d) VXYY.

1223

2.0720 ± 0.3620 3.9899 ± 0.1525

T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

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T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

in terms of shape and size of the alkyl side chain of amino acids. Hence the higher values of viscosity B-coefficient for the L-phenylalanine as compared to other studied amino acids agree well with the above statement. Similar trends of viscosity B-coefficients for some amino acids in aqueous SA [20] solutions have also been reported. The plot of DtB vs mB (figure 6) shows that the DtB values are positive throughout the concentration range of MA, which may be attributed to the promotion of structure of the liquid in the presence of MA. Further the DtB values increase in all the cases, with the concentration of MA. It may be noted that the order of variation of DtV0 (as already mentioned in case of volume of transfer) and viscosity B-coefficient for the presently studied amino acids is not same, which indicates that the dependence of these parameters for the studied amino acids is characteristically different. This further gets the support from plot of DtB vs mB where the order of variation of DtB for the amino acids becomes more complicated. A comparison with our recent results for these amino acids in aqueous SA solutions [20] reveals that partial molar volumes of transfer DtV0 and viscosity B-coefficients of transfer DtB in aqueous MA solutions are larger than the corresponding values in aqueous SA solutions. The magnesium ion having smaller size and higher charge density as compared to sodium ion (ionic ˚ and 1.02 A ˚, radii [33] for Mg2+ and Na+ ions are 0.72 A

FIGURE 6. Viscosity B-coefficient of transfer DtB of some amino acids vs concentration of aqueous magnesium acetate solutions mB: (j) glycine; (d) DL-a-alanine; (m) DL-a-amino-n-butyric acid; (.) L-leucine; (r) L-phenylalanine.

respectively), may interact more strongly with COO terminus of amino acids and thus the interactions between MA and amino acids are stronger than between amino acids and SA. This may also be attributed partially to higher ionic strength for MA as compared to SA at the same concentration (at 0.5 mB the ionic strength for SA and MA are 0.5 and 1.5, respectively).  Although, the contributions of (NHþ 3 , COO ) groups to viscosity B-coefficients systematically increase with the increase in the concentration of MA, but increase is small and within the experimental error and the contribution of (CH2) group is almost insensitive to the concentration of MA (table 8). Similar relations for some amino acids in aqueous SA [17,20] and sodium butyrate [18] have also been observed. It is interesting to note that there is a linear correlation [17,18,20] between viscosity B-coefficient and V 02 values for the studied amino acids in aqueous MA solutions, i.e., B ¼ A1 þ A2 V 02 ;

ð14Þ

where A1 and A2 are the coefficients obtained by a least square analysis and are reported in table 9 along with correlation coefficients. The A2 value reflects the size and shape of the solute. It can be noted that as A2 value is greater than 2.5 and increase as the concentration of MA increases, which indicates that the amino acids are strongly solvated in aqueous MA solutions. The Dl062¼ values for amino acids in water and in aqueous MA solutions have been summarized in table 10 and good agreement has been observed with the literature [10,34] values in case of water. The Dl062¼ values are positive and larger than Dl061¼ indicate stronger ion–solvent interactions. The magnitude of Dl062¼ decreases with the increase in the concentration of MA except in case of glycine where values increase with the concentration. Further, the magnitude of Dl062¼ increases from glycine to L-phenylalanine with the increase in the size of alkyl side chain of amino acids. This can be explained that more energy is needed for the amino acids with longer alkyl side chains for the transferring from ground-state solvent to transition-state solvent. As discussed in case of volumetric interaction coefficients, the interactions of charged end groups of different amino acids with MA are almost same and similar is the case with water. Therefore, it can be concluded that the increase in Dl062¼ values results from the difference in the interactions of alkyl groups of the amino acids with MA and water. Thus, the gradual increase of Dl062¼ from

TABLE 8  Contribution to the viscosity B-coefficient from zwitterionic end groups (NHþ 3 , COO ) and CH2 group of some amino acids in aqueous MA solutions at T = 298.15 K B-coefficient/(dm3 Æ mol1)

Group (NHþ 3, –CH2 a



COO )

mBa = 0.5 mol Æ kg1

mB = 1.0 mol Æ kg1

mB = 1.5 mol Æ kg1

0.065 ± 0.016 0.098 ± 0.005

0.070 ± 0.017 0.098 ± 0.005

0.074 ± 0.019 0.099 ± 0.006

mB stands for the molality of MA in water.

T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

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TABLE 9 Parameters A1 and A2 of equation (14) for some amino acids in water and in aqueous MA solutions at T = 298.15 K mBa/(mol Æ kg1)

A1/(dm3 Æ mol1)

A2

Rb

Standard deviation

water 0.5 1.0 1.5 2.0

0.098 ± 0.019 0.095 ± 0.030 0.099 ± 0.031 0.134 ± 0.031 0.205 ± 0.017

5.7 ± 0.22 5.8 ± 0.34 5.8 ± 0.35 6.4 ± 0.40 7.6 ± 0.25

0.998 0.995 0.994 0.996 0.999

0.014 0.022 0.022 0.018 0.005

a b

mB stands for the molality of MA in water. Correlation coefficient.

TABLE 10 Activation free energy, Dl062¼ , for viscous flow of some amino acids in water and in aqueous MA solutions at T = 298.15 K Dl062¼ =ðk J  mol1 Þ

Amino acids

Water Glycine DL-a-Alanine DL-a-Amino-n-butyric L-Leucine DL-Leucine L-Phenylalanine a b c

acid

32.22 ± 0.69 32.20,b 32.7c 49.41 ± 1.37 49.8,b 50.4c 63.43 ± 1.09 65.4c 95.27 ± 0.82 88.3c 103.64 ± 0.96

mBa = 0.5 mol Æ kg1

mB = 1.0 mol Æ kg1

mB = 1.5 mol Æ kg1

mB = 2.0 mol Æ kg1

33.26 ± 1.04

33.76 ± 0.74

34.00 ± 1.05

34.37 ± 0.67

51.11 ± 1.04

50.63 ± 0.86

50.34 ± 0.70

50.12 ± 1.55

65.97 ± 1.69

64.80 ± 1.35

64.03 ± 0.82

62.92 ± 1.00

93.08 ± 0.26

90.17 ± 0.25

88.01 ± 0.93

101.61 ± 0.78

98.79 ± 0.61

mB stands for the molality of MA in water. Reference [10]. Reference [34].

glycine to L- phenylalanine may be attributed to the interactions of alkyl groups with MA and water molecules, which increase with the increasing alkyl side chain length of amino acids. Like, standard partial molar volumes and viscosity Bcoefficients of amino acids, Dl062¼ also varies linearly [10,17] with nC. The regression analysis of Dl062¼ values as a function of nC has been carried out using 06¼  Dl062¼ ¼ Dl062¼ ðNHþ 3 ; COO Þ þ nC Dl2 ðCH2 Þ;

ð15Þ

 where Dl062¼ ðNHþ and Dl062¼ ðCH2 Þ are the 3 ; COO Þ zwitterionic end groups and methylene group contributions 06¼  to Dl062¼ . The contribution from (NHþ 3 , COO ) to Dl2 in 1 water is (17.10 ± 0.97 kJ Æ mol ) and in aqueous MA solutions are (20.19 ± 2.09, 21.49 ± 2.06 and 22.39 ± 2.16) kJ Æ mol1 at 0.5, 1.0 and 1.5 mB, respectively. Similarly contribution from (CH2) group to Dl062¼ in water is (15.63 ± 0.31 kJ Æ mol1) and in aqueous MA solutions are (14.79 ± 0.67, 13.95 ± 0.66, 13.35 ± 0.69) kJ Æ mol1, respectively. In spite of uncertainties, there is a distinct trend of these contributions with concentration of MA. 06¼  The Dl062¼ ðNHþ 3 ; COO Þ values increase, while Dl2 ðCH2 Þ values decrease with the increase in the concentration of MA.

5. Conclusion In the present work, partial molar volumes and viscosity B-coefficients of the some amino acids in different concen-

trations of MA solutions were obtained using density and viscosity data. The partial molar volumes of transfer, DtV0 and viscosity B-coefficients of transfer, DtB data suggest that ion–charged group interactions are dominating over the ion–non-polar group interactions throughout the MA concentrations. The DtV0 as well as VXY values decrease from glycine to L-leucine as the size of alkyl side chain of amino acids increases, which shows that more hydrophobic amino acids undergo more dehydration effect of MA. The larger DtV0 and DtB values in aqueous MA solutions than in aqueous SA solutions reveal that stronger interactions occur between MA and amino acids, which may be due to the smaller size and higher charge density of magnesium cation as compared to sodium cation. Comparison of the volumetric properties of amino acids in aqueous NaCl, MgCl2 and SA, MA shows that the stabilizing effect of these cations, i.e. (Na+ and Mg2+) on the proteins also appears in the same order as in the Hofmeister series. References [1] P.H. Von Hippel, T. Schleich, in: S.N. Timasheff, G.D. Fasman (Eds.), Structure and Stability of Biological Macromolecules, Marcel Dekker, New York, 1969, pp. 417–574. [2] W.P. Jencks, Catalysis in Chemistry and Enzymology, McGraw Hill, New York, 1969, p. 351. [3] T. Owaga, K. Mizutani, M. Yasuda, Bull. Chem. Soc. Jpn. 57 (1984) 2064–2068. [4] M. Natarajan, R.K. Wadi, H.C. Gaur, J. Chem. Eng. Data 35 (1990) 87–93.

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T.S. Banipal et al. / J. Chem. Thermodynamics 38 (2006) 1214–1226

[5] R. Bhat, J.C. Ahluwalia, J. Phys. Chem. 89 (1985) 1099–1105. [6] H. Rodriguez, A. Soto, A. Arce, M.K. Khoshkbarchi, J. Solution Chem. 32 (2003) 53–63. [7] T.S. Banipal, G. Sehgal, Thermochim. Acta 262 (1995) 175–183. [8] R. Badarayani, A. Kumar, J. Chem. Eng. Data 48 (2003) 664–668. [9] R.K. Wadi, P. Ramasami, J. Chem. Soc., Faraday Trans. 93 (1997) 243–247. [10] R.K. Wadi, R.K. Goyal, J. Chem. Eng. Data 37 (1992) 377–386. [11] R.K. Wadi, R.K. Goyal, J. Solution Chem. 21 (1992) 163–170. [12] B.S. Lark, P. Patyar, T.S. Banipal, N. Kishore, J. Chem. Eng. Data 49 (2004) 553–565. [13] R. Bhat, J.C. Ahluwalia, Int. J. Peptide Protein Res. 30 (1987) 145–152. [14] Z. Yan, J. Wang, W. Kong, J. Lu, Fluid Phase Equilib. 215 (2004) 143–150. [15] S.K. Singh, N. Kishore, J. Solution Chem. 32 (2003) 117–135. [16] J. Wang, Z. Yan, K. Zhuo, D. Liu, Z. Phys. Chem. 214 (2000) 333– 345. [17] Z. Yan, J. Wang, J. Lu, Biophys. Chem. 99 (2002) 199–207. [18] Z. Yan, J. Wang, J. Lu, J. Chem. Eng. Data 46 (2001) 217–222. [19] J. Wang, Z. Yan, J. Lu, J. Chem. Thermodyn. 36 (2004) 281–288. [20] T.S. Banipal, D. Kaur, P.K. Banipal, J. Chem. Eng. Data 49 (2004) 1236–1246. [21] R. Badarayani, A. Kumar, Fluid Phase Equilib. 201 (2001) 321–333. [22] R. Badarayani, A. Kumar, J. Chem. Thermodyn. 35 (2003) 897–908.

[23] J.A. Cowan, Biological Chemistry of Magnesium, VCH, New York, 1995. [24] T.S. Banipal, D. Kaur, P. Lal, G. Singh, P.K. Banipal, J. Chem. Eng. Data 47 (2002) 1391–1395. [25] A.L. Surdo, E.M. Alzola, F.J. Millero, J. Chem. Thermodyn. 14 (1982) 649–662. [26] S. Glasstone, K.J. Laidle, H. Eyring, The Theory of Rate Processes, McGraw Hill, New York, 1941, p. 477. [27] D. Feakins, W.E. Waghorne, K.G. Lawrence, J. Chem. Soc. Faraday Trans. 1 82 (1986) 563–568. [28] R.W. Gurney, Ionic Processes in Solutions, vol. 3, McGraw Hill, New York, 1953 (Chapter 1). [29] F.J. Millero, A.L. Surdo, C. Shin, J. Phys. Chem. 82 (1978) 784–792. [30] W.G. McMillan, J. Mayer, J. Chem. Phys. 13 (1945) 276–305. [31] B.T. Nall, K.L. Dill, Conformations and Forces in Protein Folding, American Association for the Advancement of Science, Washington DC, 1991, p. 169. [32] R.H. Stokes, R. Mills, International Encyclopedia of Physical Chemistry and Chemical Physics, Pergamon Press, New York, 1965. [33] J.D. Lee, Concise Inorganic Chemistry, Chapman and Hall Ltd., London, 1996. [34] Z. Yan, J. Wang, W. Liu, J. Lu, Thermochim. Acta 334 (1999) 17–27.

JCT 05-177