Interactions of some peptides with sodium acetate and magnesium acetate in aqueous solutions at 298.15 K: A volumetric approach

Journal of Molecular Liquids 140 (2008) 54 – 60 www.elsevier.com/locate/molliq

Interactions of some peptides with sodium acetate and magnesium acetate in aqueous solutions at 298.15 K: A volumetric approach T.S. Banipal a,⁎, Damanjit Kaur a , P.K. Banipal b , Gagandeep Singh a a

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 25 September 2007; received in revised form 28 December 2007; accepted 9 January 2008 Available online 17 January 2008

Abstract The apparent molar volumes, Vϕ of diglycine, triglycine and glycyl-L-leucine have been determined in water and in aqueous sodium acetate (0.5, 1.0, 2.0, and 4.0 mB) and magnesium acetate (0.5, 1.0, 1.5, and 2.0 mB) solutions at 298.15 K by the measurement of densities using vibrating-tube digital densimeter. The partial molar volumes, V02,m obtained from Vϕ have been used to calculate the partial molar volumes of transfer, ΔtrV02,m for these peptides from water to aqueous solutions of sodium acetate (SA) and magnesium acetate (MA) solutions. The hydration numbers, nH and volumetric interaction coefficients have also been calculated. The ΔtrV02,m data suggest that ion-charged/or peptide group interactions of peptides are stronger with MA in comparison to SA. © 2008 Elsevier B.V. All rights reserved. Keyword: Density; Apparent molar volumes; Hydration number; Interaction coefficients; Peptides; Sodium acetate; Magnesium acetate

1. Introduction The interactions of various functional groups on the proteins with the surrounding environment play an important role in their conformational characteristics. The salt solutions are known to produce large effects on the properties and structure of proteins such as their solubility, stability, denaturation, and dissociation into subunits etc [1]. These effects are sensitive to the nature of salt and may vary over wide range even for salts of same charge type. In order to understand the interactions responsible for these effects, it is necessary to study the low molecular weight model compounds, which contains component of proteins such as peptides, amino acids etc. In this way, it may be possible to sort out the contribution of different components. Further the peptides are most important molecules due to their wide application in drug production and their role as signal transmitters in cell communications [2]. Therefore, the systematic study of peptides can provide valuable information about their behavior in solutions and insight into the con-

⁎ Corresponding author. E-mail address: [email protected] (T.S. Banipal). 0167-7322/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2008.01.004

formational stability of proteins. A variety of thermodynamic and transport studies on peptides are available in simple salt solutions [3–13] but no report has been found in the presence of organic salt solutions particularly acetate of alkali and alkaline earth metals. Hence, in continuation of our studies [14–16] we have undertaken a systematic study on volumetric properties of some peptides in the presence of sodium acetate (SA) and magnesium acetate (MA) solutions. In the present paper, the apparent molar volumes, Vϕ of diglycine, triglycine and glycylL-leucine in water and in different concentration of SA and MA (cosolutes) solutions at 298.15 K have been determined by measuring the densities using vibrating-tube digital densimeter. 0 The calculated partial molar volumes, V2,m at infinite solution have been used to obtain corresponding partial molar volume of 0 transfer, ΔtrV2,m from water to aqueous SA and MA solutions. The hydration numbers, nH, interaction coefficients and cation size effect have also been discussed. 2. Experimental Digycine (G1002), triglycine (G1377) and glycyl-L-leucine (G2002) were obtained from Sigma Chemical Co. and were dried for 24 h in a vacuum oven before use. Analytical

T.S. Banipal et al. / Journal of Molecular Liquids 140 (2008) 54–60 Table 1 Densities, ρ and apparent molar volumes, Vϕ for some peptides in water and in aqueous sodium acetate solutions as a function of molalities of peptides and sodium acetate solutions at 298.15 K a

mA (mol kg− 1)

ρ × 10− 3 (kg m− 3)

Diglycine in water 0.11870 1.003572 0.14835 1.005174 0.17195 1.006440

Vϕ × 106 (m3 mol− 1) 76.69 76.76 76.82

mA (mol kg− 1)

ρ × 10− 3 (kg m− 3)

0.20977 0.25407 0.27432

1.008460 1.010793 1.011853

Vϕ × 106 (m3 mol− 1) 76.88 77.00 77.05

55

Table 1 (continued) a

mA (mol kg− 1)

ρ × 10− 3 (kg m− 3)

Vϕ × 106 (m3 mol− 1)

In aqueous sodium acetate solutions mB = 2.0 0.02436 1.087634 144.45 0.03332 1.087933 144.51 mB = 4.0 0.01646 1.123677 146.72 0.03680 1.124209 146.69

mA (mol kg− 1)

ρ × 10− 3 (kg m− 3)

Vϕ × 106 (m3 mol− 1)

0.04770 0.05740

1.088414 1.088740

144.51 144.47

0.04511 0.05820

1.124422 1.124765

146.74 146.68

mA, molality (mol kg− 1) of peptides in solutions, bmB, molality (mol kg− 1) of SA in water, cρ0, density (kg m− 3) of SA solution.

a

In aqueous sodium acetate solutions b mB = 0.5 (cρ0 × 10− 3 = 1.023282) 0.11196 1.029062 79.34 0.17844 1.032456 79.28 0.20664 1.033866 79.36 mB = 1.0 (ρ0 × 10− 3 = 1.047559) 0.11695 1.053333 80.67 0.14063 1.054491 80.65 0.17194 1.056007 80.68 mB = 2.0 (ρ0 × 10− 3 = 1.086810) 0.14252 1.093299 82.51 0.16731 1.094391 82.61 0.18624 1.095258 82.50 mB = 4.0 (ρ0 × 10− 3 = 1.123245) 0.09092 1.126903 85.44 0.15045 1.129258 85.47 0.17206 1.130111 85.45 mB = 5.5 (ρ0 × 10− 3 = 1.139165) 0.11843 1.143606 86.73 0.14762 1.144685 86.73 0.17714 1.145765 86.75 Triglycine in 0.02084 0.03426 0.04612

water 0.998650 0.999676 1.000578

112.20 112.27 112.35

In aqueous sodium acetate solutions mB = 0.5 0.01627 1.024457 115.79 0.02534 1.025113 115.68 0.03567 1.025855 115.71 mB = 1.0 0.02552 1.049313 117.78 0.03458 1.049936 117.70 mB = 2.0 0.01511 1.087771 120.13 0.02560 1.088439 120.03 mB = 4.0 0.02666 1.124800 122.04 0.03504 1.125285 122.07 Glycyl-L-Leucine in water 0.01182 0.997618 140.11 0.01784 0.997907 140.17 0.02490 0.998245 140.22 In aqueous sodium acetate solutions mB = 0.5 0.01403 1.023899 141.86 0.03463 1.024798 141.93 mB = 1.0 0.01972 1.048352 142.93 0.02554 1.048587 142.86

0.22223 0.27879

1.034645 1.037477

79.38 79.37

0.18830 0.23462 0.25898

1.056813 1.059049 1.060192

80.61 80.59 80.68

0.20008 0.22291 0.26698

1.095880 1.096890 1.098841

82.48 82.50 82.48

0.19216 0.21779 0.24745

1.130914 1.131897 1.133027

85.39 85.46 85.53

0.20169 0.21803 0.25618

1.146685 1.147260 1.148648

86.66 86.73 86.71

0.06322 0.10078

1.001869 1.004683

112.49 112.68

reagent grade sodium acetate trihydrate and magnesium acetate tetrahydrate (AR) from SRL were used as such after drying in a vacuum desiccator at room temperature. Doubly distilled, deionised water with specific conductance less than 1.30 × 10− 6 Ω− 1 cm− 1 was used for the measurements. All solutions were prepared afresh on weight basis using a Mettler Balance having accuracy ± 1.0 × 10− 8 kg. Molalities calculated are found to be accurate to 0.000005 M. The densities of solutions were measured using a vibratingtube digital densimeter (Model DMA 60/602, Anton Paar, Austria) with precision of ±1 × 10− 3 kg m− 3 and having accuracy of±3 × 10− 3 kg m− 3. The densimeter was calibrated both with distilled water and dry air, respectively. All the measurements of densities of various solutions were made with reference to pure water. The working of the densimeter was checked by measuring the densities of aqueous sodium chloride solutions, which agreed very well with the literature values [17]. The temperature of water around densimeter cell was controlled within ±0.01 K. 3. Results and discussion

0.04627 0.05852

1.026618 1.027492

115.66 115.71

0.04596 0.05564

1.050712 1.051374

117.74 117.70

0.03634 0.05632

1.089118 1.090376

120.06 120.09

0.04657 0.05748

1.125954 1.126585

122.04 122.02

0.03172 0.04020

0.998569 0.998971

140.31 140.37

0.04496 0.05610

1.025245 1.025723

141.98 142.05

0.03249 0.04005

1.048865 1.049164

142.88 142.95

3.1. Results The apparent molar volumes, Vϕ of diglycine, triglycine and glycyl-L-leucine in water and in (0.5, 1.0, 2.0, and 4.0 mB, where mB is the molality of cosolutes, mol kg− 1) aqueous sodium acetate (SA) and in (0.5, 1.0, 1.5, and 2.0 mB) aqueous magnesium acetate (MA) solutions at 298.15 K were calculated using following equation: V/ ¼ M =q  ½1000ðq  q0 Þ=ðmA qq0 Þ

ð1Þ

where M is the molar mass of peptides, mA is the molality of peptides in solution, ρ and ρ0 are the densities of solution and solvent, respectively. The Vϕ and densities as a function of molality are given in Tables 1–2. The uncertainty in the determination of Vϕ occurring because of the measurement of various quantities has been calculated. In case of diglycine, the uncertainty values for Vϕ range from 0.024 × 10− 6 to 0.010 × 10− 6 m3 mol− 1 for the lower (≤ 0.09 mA) and higher concentration ranges, respectively. The uncertainty values in triglycine and glycyl-L-leucine range from 0.214 × 10− 6 to 0.039 × 10− 6 m3 mol− 1 for the lower (≤0.012 mA) and higher concentration ranges, respectively in both the cosolutes.

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T.S. Banipal et al. / Journal of Molecular Liquids 140 (2008) 54–60

Table 2 Densities, ρ and apparent molar volumes, Vϕ for some peptides in aqueous magnesium acetate solutions as a function of the molalities of peptides and magnesium acetate solutions at 298.15 K a

mA (mol kg− 1)

ρ × 10− 3 (kg m− 3)

Vϕ × 106 (m3 mol− 1)

mA (mol kg− 1)

Diglycine In aqueous magnesium acetate solutions b mB = 0.5 (cρ0 × 10− 3 = 1.023282) 0.11685 1.039269 79.96 0.19832 0.14008 1.040443 79.88 0.22452 0.17527 1.042194 79.91 0.25788 mB = 1.0 (ρ0 × 10− 3 = 1.063639) 0.11356 1.069048 81.68 0.19911 0.14529 1.070552 81.61 0.21269 0.17363 1.071889 81.56 0.25301 mB = 1.5 (ρ0 × 10− 3 = 1.089891) 0.12881 1.095677 82.95 0.19707 0.15284 1.096761 82.84 0.23170 0.17857 1.097890 82.89 0.25809 mB = 2.0 (ρ0 × 10− 3 = 1.112471) 0.12828 1.117943 83.86 0.21204 0.15464 1.119041 83.92 0.24026 0.17908 1.120068 83.89 0.26848 Triglycine mB = 0.5 0.01518 0.02477 0.03456 mB = 1.0 0.01646 0.02691 mB = 1.5 0.02497 0.03541 mB = 2.0 0.01584 0.02588 0.03561

ρ × 10− 3 (kg m− 3)

Vϕ × 106 (m3 mol− 1)

1.043346 1.044642 1.046293

79.87 79.87 79.84

1.073080 1.073691 1.075590

81.56 81.65 81.53

1.098716 1.100214 1.101378

82.83 82.91 82.86

1.121434 1.122598 1.123770

83.91 83.92 83.89

0 values were found to be concentration dependent the V2,m at infinite dilution were determined by least-squares fitting using the equation:

0 V/ ¼ V2;m þ SV mA

0 where SV is the experimental slope. The V2,m values along with their standard deviation are summarized in Tables 3–4. The 0 V2,m values for the studied peptides in water agree well with the 0 literature values[3,6,18–20]. The V2,m values for the peptides are higher in aqueous SA/MA solutions than in water and these values increase with the concentration of both the cosolutes. 0 Further the V2,m values increase from diglycine to triglycine to 0 glycyl-L-leucine. The increase of the V2,m values from diglycine to triglycine is due to the increase in the number of peptide units, whereas from diglycine to glycyl-L-leucine, the increase in 0 V2,m values is due to increase in the size of alkyl side chain of amino acids constituting the peptides. 0 The partial molar volumes of transfer, ΔtrV2,m at infinite dilution from water to aqueous SA and MA solutions have been evaluated as follows:

0 0 0 Dtr V2;m ¼ V2;m ðin aqueous SA=MAÞ  V2;m ðin waterÞ:

1.034425 1.035102 1.035791

116.78 116.73 116.73

0.04282 0.05498

1.036370 1.037224

116.75 116.70

1.064734 1.065426

118.95 118.98

0.03782 0.04609

1.066148 1.066699

118.96 118.85

1.091458 1.092107

120.59 120.67

0.04748 0.05868

1.092864 1.093560

120.55 120.55

1.113423 1.114026 1.114605

121.40 121.35 121.42

0.04754 0.05683

1.115318 1.115876

121.37 121.29

142.29 142.25 142.22

0.04472 0.05729

1.035245 1.035772

142.25 142.26

143.70 143.63

0.03686 0.04700

1.065019 1.065395

143.69 143.71

144.47 144.50

0.04457 0.05480

1.091376 1.091709

144.46 144.53

145.36 145.41

0.04566 0.05667

1.113806 1.114123

145.40 145.43

Glycyl-L-Leucine mB = 0.5 0.01198 1.033861 0.02029 1.034214 0.03221 1.034719 mB = 1.0 0.02036 1.064403 0.02956 1.064749 mB = 1.5 0.02469 1.090716 0.03509 1.091060 mB = 2.0 0.01519 1.112918 0.02763 1.113281

mA, molality (mol kg− 1) of peptides in solutions, bmB, molality (mol kg− 1) of MA in water, cd0, density (kg m− 3) of MA solution.

a

At infinite dilution, Vϕ becomes the same as the partial molar 0 volume, Vϕ0 i.e. Vϕ0 = V2,m . In most of the cases, where Vϕ values show no significant concentration dependence, the average of all the data points which are within the uncertainty limits has 0 been taken as the V2,m . However in the case, where V ϕ values

ð2Þ

ð3Þ

0 The ΔtrV2,m values for the studied peptides have been 0 illustrated in Figs. 1–2. The magnitude of ΔtrV2,m is positive and increases with the concentration of both the cosolutes. The 0 more positive ΔtrV2,m values for diglycine in comparison to glycyl-L-leucine suggest that the charged ends as well as peptide groups in diglycine are more effective than in case of glycyl-L0 leucine for the increase of ΔtrV2,m values. This may be attributed to the large non-polar side chain of leucine unit in glycyl-L-leucine. Similar behavior has also been observed in case of amino acids studied in aqueous SA/MA solutions 0 [14,15], where ΔtrV2,m values decrease with the increase in the non-polar side chain of amino acids. Further, the higher 0 ΔtrV2,m values for triglycine than for diglycine suggest that the polar-hydrophilic groups are more effective for the increase of 0 ΔtrV2,m values than the non-polar side chain groups of amino acids/peptides. 0 According to Shrinkage model [21], the positive ΔtrV2,m values in the presence of SA and MA, can be attributed to the decrease in electrostriction of water in the vicinity of charged centers/or peptides backbone units which results in the decrease in volume of shrinkage. In ternary systems of peptides + SA/MA + water the following type of interactions are occurring:

(i) Ion-charged group interactions occurring between ions of SA/MA (Na+/Mg+2, CH3COO−) and charged end groups (NH3+, COO−) in peptides. (ii) Ion-peptide group interactions occurring between ions of SA/MA and peptide backbone units (–CH2CONH) of peptides (iii) Ion-non-polar (hydrophobic) group interactions occurring between ions of SA/MA and non-polar groups of peptides.

T.S. Banipal et al. / Journal of Molecular Liquids 140 (2008) 54–60

57

Table 3 Partial molar volumes, V02,m at infinite dilution for some peptides in water and in aqueous sodium acetate solutions at 298.15 K Peptides

V02,m × 106 (m3 mol− 1) 0.5 amB

Water b

c

d

e

f

1.0 mB

2.0 mB

4.0 mB

5.5 mB

g

Diglycine 76.42 ± 0.01 (2.27) 76.23, 76.27, 76.43, 76.63, 76.76 79.35 ± 0.04 80.65 ± 0.04 82.51 ± 0.05 85.46 ± 0.05 86.72 ± 0.03 Triglycine 112.07 ± 0.02 (6.15) c112.11, d111.81, e, g112.51 115.71 ± 0.05 117.73 ± 0.04 120.08 ± 0.04 122.04 ± 0.02 Glycyl-L-Leucine 140.00 ± 0.01 (9.32) c139.70, f139.69 141.79 ± 0.01 (4.43) 142.90 ± 0.04 144.49 ± 0.03 146.71 ± 0.03 a b c d e f g

mB, molality (mol kg− 1) of SA in water. Standard deviation, parentheses contain experimental slope, Sv × 106 (m3 mol− 2 kg). Ref. [17]. Ref. [18]. Ref. [6]. Ref. [3]. Ref. [19].

According to cosphere overlap model [22], the ion-charged group and ion-peptide group interactions will lead to positive 0 ΔtrV2,m values, whereas the ion-non-polar group interactions will 0 0 lead to negative ΔtrV2,m values. The overall positive ΔtrV2,m values observed for the presently studied peptides throughout concentration range of SA/MA solutions are due to dominance of the ioncharged group/or ion-peptide group interactions over the ion-non0 polar group interactions. The higher magnitude of ΔtrV2,m values in case of MA than in case of SA, indicate that stronger interactions are occurring between ions of MA and charged end

groups/peptide groups or the dehydration effect of MA is more than SA on the studied peptides. Ionic strength will also show its effect. Bhat and Ahluwalia have reported [3] positive partial molar 0 values for some peptides heat capacities, ΔtrC 02,p and ΔtrV2,m from water to aqueous sodium chloride (NaCl) solutions at 0 298.15 K. The ΔtrC 02,p and ΔtrV2,m values decrease from diglycine to glycyl-L-leucine with the increase in the size of alkyl side chain of peptides. This behavior is similar to volumetric behavior of presently studied peptides in the presence of SA/MA solutions. 0 To analyse the results further, the ΔtrV2,m values have been plotted against the number of glycyl-units, ng (Figs. 3–4) in 0 aqueous SA/MA solutions which show that ΔtrV2,m values decrease as follows: triglycine N diglycine N glycine. In the pres0 ence of SA, the ΔtrV2,m values vary almost linearly with the ng up to ∼ 2.0 mB and the behavior become slightly non-linear at 0 higher concentrations. In case of MA, the ΔtrV2,m values vary linearly throughout the studied concentration range. The in0 crease in ΔtrV2,m values with the concentration of SA and MA suggests that the interactions between these glycyl-peptides and salts are being strengthened. Bhat and Ahluwalia have also observed [3] similar trends for some glycyl-peptides in aqueous NaCl solutions.

Fig. 1. Partial molar volumes of transfer, ΔtrV 02,m for some peptides vs different molalities, mB of aqueous sodium acetate solutions at 298.15 K: (■) diglycine; (●) triglycine; (▲) glycyl-L-leucine.

Fig. 2. Partial molar volumes of transfer, ΔtrV 02,m for some peptides vs different molalities, mB of aqueous magnesium acetate solutions at 298.15 K: (■) diglycine; (●) triglycine; (▲) glycyl-L-leucine.

Table 4 Partial molar volumes, V02,m at infinite dilution for some peptides in aqueous magnesium acetate solutions at 298.15 K Peptides

Diglycine Triglycine Glycyl-LLeucine a b

V02,m × 106 (m3 mol− 1) 0.5 amB

1.0 mB

1.5 mB

2.0 mB

79.89 ± 0.04 b 116.74 ± 0.03 142.25 ± 0.02

81.60 ± 0.06 118.94 ± 0.06 143.68 ± 0.04

82.88 ± 0.05 120.59 ± 0.06 144.49 ± 0.03

83.90 ± 0.02 121.37 ± 0.05 145.40 ± 0.03

mB, molality (mol kg− 1) of MA in water. Standard deviation.

58

T.S. Banipal et al. / Journal of Molecular Liquids 140 (2008) 54–60 Table 5 Contribution of peptide backbone units (–CH2CONH) to the partial molar volumes of transfer, ΔtrV02,m for some peptides in aqueous sodium acetate solutions at 298.15 K Methods

Eq. (5) Eq. (6) Eq. (7) a

Fig. 3. Partial molar volumes of transfer, ΔtrV 02,m for glycyl-peptides in aqueous sodium acetate solutions vs number of glycyl-units, ng at 298.15 K: (■) 0.5 mB; (●) 1.0 mB; (▲) 2.0 mB; (▼) 4.0; (♦) 5.5 mB. 0 The ΔtrV2,m of peptide backbone units (–CH2CONH) from water to different concentrations of aqueous SA/MA solution have been calculated as follow:

0 0 ðCH2 CONHÞ ¼ Dtr V2;m  ðdiglycineÞ Dtr V2;m 0 Dtr V2;m ðglycineÞ

ð4Þ

or 0 0 Dtr V2;m ðCH2 CONHÞ ¼ Dtr V2;m ðtriglycineÞ 0 ðdiglycineÞ  Dtr V2;m

ð5Þ

or 0 0 Dtr V2;m ðCH2 CONHÞ ¼ Dtr V2;m ð glycylQLQleucineÞ 0  Dtr V2;m ð LQleucineÞ

ð6Þ

0 0 (–CH2CONH) group contributions to ΔtrV2,m valThe ΔtrV2,m ues are positive for the studied peptides in both the cosolutes 0 (Tables 5–6) and the variation in ΔtrV2,m (–CH2CONH) values for these peptide backbone units is due to the difference in the electrostriction effects of homologous glycyl-peptides. These contributions increase with the increase in concentration of SA

ΔtrV02,m × 106 (m3 mol− 1) 0.5 amB

1.0 mB

2.0 mB

4.0 mB

5.5 mB

1.26 0.72 1.37

1.33 1.43 1.88

1.71 1.92 2.47

3.45 0.92 3.89

3.94

mB, molality (mol kg− 1) of SA in water.

(except at ∼4.0 mB calculated as Gly3-gly2). However, in case of MA, the behavior becomes more irregular than observed in case of SA that may be indicative of cation specific interactions. 0 The partial specific volumes [23], ν02,m (ν02,m = V 2,m / M, where M is the molar mass of the peptides) for the studied peptides in water and in aqueous SA/MA solutions are illustrated in Figs. 5–6. The ν02,m values increase from diglycine to glycyl-L-leucine with the increase in size of side chain of peptide and also increase with the concentration of both the cosolutes. The concentration effect of SA/MA is more in case of diglycine, which decrease with the increase in alkyl side chain. This may be due to the negative contribution from alkyl side chain of peptide in case of glycyl-L-leucine, which is similar to the behavior that has been observed in case of amino acids with the increase in non-polar side chain [14,15]. The hydration numbers, nH for the studied peptides were calculated using the method reported by Millero et al. [24].
0 nH ¼ V2;m ðelectÞ= VE0  VB0 ð7Þ where VE0 is the molar volume of electrostricted water and VB0 is the molar volume of bulk water. According to Millero et al. [24] the value of (VE0 − VB0) can be taken equal to ∼ −3.3 ×10– 6 m3 mol− 1 at 298.15 K. The V20,m(elect) can be calculated from experimentally measured Vϕ0 values by the following equation: 0 0 0 V2;m ðelectÞ ¼ V2;m ðamino acidÞ  V2;m ðintÞ:

ð8Þ

The calculated nH values are given in Table 7. A good agreement has been found for nH values for diglycine with the literature values [25] in case of water. The nH values for the studied peptides in aqueous SA and MA solutions are less than in water and decrease with the concentration of SA/MA, which suggest that interactions between ions of SA or MA with the

Table 6 Contribution of peptide backbone units (–CH2CONH) to the partial molar volumes of transfer, ΔtrV02,m for some peptides in aqueous magnesium acetate solutions at 298.15 K Methods

Fig. 4. Partial molar volumes of transfer, ΔtrV 02,m for glycyl-peptides in aqueous magnesium acetate solutions vs number of glycyl-units, ng at 298.15 K: (■) 0.5 mB; (●) 1.0 mB; (▲) 1.5 mB; (▼) 2.0 mB.

Eq. (5) Eq. (6) Eq. (7) a

ΔtrV02,m × 106 (m3 mol− 1) 0.5 amB

1.0 mB

1.5 mB

2.0 mB

0.87 1.20 3.38

1.30 1.69 1.99

1.71 2.06 2.15

1.80 1.82

mB, molality (mol kg− 1) of MA in water.

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59

Table 7 Hydration numbers, nH for diglycine and triglycine in water and in aqueous sodium acetate and magnesium acetate solutions at 298.15 K Peptides

nH 0.5 amB

Water

1.0 mB

In aqueous sodium acetate solutions Diglycine 6.01 5.13 4.73 b 5.67 Triglycine 8.55 7.45 6.84 0.5 mB

Fig. 5. Partial specific volumes, ν02,m for some peptides vs different molalities, mB of aqueous sodium acetate solutions at 298.15 K: (■) diglycine; (●) triglycine; (▲) glycyl-L-leucine.

charged end/peptide groups become stronger with the concentration of cosolutes. Lin et al. have reported [26] nH for diglycine in aqueous KCl solutions at 298.15 and 308.15 K. They have also observed that nH values decrease with the increase in the concentration of KCl due to stronger interactions between KCl and diglycine. Kozak et al. proposed [27] a formalism based on the McMillan and Mayer theory of solutions which is further discussed by Friedman and Krishnan [28] and Franks et al. [29] in order to include the solute–cosolute interactions in the solvation spheres. According to this treatment, at infinite dilution 0 ΔtrV2,m can be expressed as: 0 Dtr V2;m ¼ 2VXY mB þ 3VXYY m2B þ N

ð9Þ

where X stands for peptides and Y stands for SA/MA, respectively. The VXY and VXYY are the pair and triplet volumetric interaction coefficients, respectively for the studied peptides. These results are given in Table 8. The VXY and VXYY are positive and negative, respectively for the studied peptides in both the cosolutes. The magnitude of both interaction coefficients decreases with the increase in the size of side chain of the amino acid constituting the peptides. Further the magnitude of VXY is greater than VXYY, which suggest that interactions be-

2.0 mB

4.0 mB

5.5 mB

4.17

3.27

2.89

6.12

5.53

1.0 mB

In aqueous magnesium acetate solutions Diglycine 4.96 4.44 Triglycine 7.14 6.47 a b

1.5 mB

2.0 mB

4.06 5.97

3.75 5.73

mB, molality (mol kg− 1) of SA/MA in water. Ref. [24].

tween peptides and SA/MA are mainly pair wise. However the magnitude of VXY and VXYY increases from diglycine to triglycine in both the cosolutes. The higher VXY values in aqueous MA solutions for the studied peptides than in SA solutions reveal that stronger interactions occur between peptides and MA. Similarly, the higher 0 ΔtrC 02,p and ΔtrV2,m values for some peptides have been reported [30] in aqueous calcium chloride (CaCl2) solutions than in aqueous sodium chloride (NaCl) solutions. Comparison of volumetric properties of studied peptides in 0 aqueous NaCl and SA reveals that ΔtrV2,m values decrease in the order: NaCOOCH3 N NaCl, which suggest that interactions between CH3COO− ions of SA and peptides are much stronger than Cl− ions of NaCl and peptides. Further, for the presently studied peptides in aqueous SA and MA solutions, the higher 0 ΔtrV2,m values and interaction coefficients in case of MA than in SA solutions show that stronger interactions occur between MA and peptides than between SA and peptides. This may be attributed partially to higher ionic strength for MA as compared to SA at the same concentration (at 0.5 mB the ionic strengths for SA and MA are 0.5 and 1.5, respectively) and to the smaller size and higher charge density of Mg+2 ion in comparison to 0 Na+ ion. Similar behavior for ΔtrV2,m values has also been observed for the studied amino acids in the presence of these

Table 8 Pair, VXY and triplet, VXYY interaction coefficients for some peptides in aqueous sodium acetate and magnesium acetate solutions at solutions at 298.15 K Peptides

Fig. 6. Partial specific volumes, ν02,m for some peptides vs different molalities, mB of aqueous magnesium acetate solutions at 298.15 K: (■) diglycine; (●) triglycine; (▲) glycyl-L-leucine.

VXY × 106 (m3 mol− 2 kg)

VXYY × 106 (m3 mol− 3 kg2)

In aqueous sodium acetate solutions Diglycine 1.9874 ± 0.2273 a Triglycine 3.0959 ± 0.2980 Glycyl-L-leucine 1.5468 ± 0.1260

− 0.1308 ± 0.0316 − 0.3110 ± 0.0556 − 0.1191 ± 0.0234

In aqueous magnesium acetate solutions Diglycine 3.4629 ± 0.2804 Triglycine 4.7922 ± 0.3247 Glycyl-L-leucine 2.3205 ± 0.1660

− 0.5434 ± 0.1089 − 0.8346 ± 0.1260 − 0.3325 ± 0.0645

a

Standard deviation.

60

T.S. Banipal et al. / Journal of Molecular Liquids 140 (2008) 54–60

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