Vaneet Dhir 1,a, Jasbinder Kaur 1, R. P. Singh Grewal 2* Abstract. *Correspondence R. P. Singh Grewal,

Transcription

1 Research Article Volumetric Properties of the Peptide (C 4 H 8 N 2 O 3 ) in Water and in Various Aqueous Solutions of Sodium Nitrate, Potassium Nitrate and Magnesium Nitrate Hexahydrate at different Temperatures ( ) K Vaneet Dhir 1,a, Jasbinder Kaur 1, R. P. Singh Grewal 2* 1 Post Graduate Department of Chemistry, G.H.G Khalsa College, Gurusar Sadhar(Affiliated to Panjab University, Chandigarh) Punjab, India a Punjab Technical University, Jalandhar 2 Department of Applied Chemistry, Guru Nanak Dev Engineering College (GNDEC), Ludhiana, Punjab, 1416, India Abstract Densities,, of peptide (C 4 H 8 N 2 O 3, diglycine) in water and in (.25,.5,.75, 1., 1.5, 2.) mol kg -1 aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate solutions have been determined at various temperatures (288.15, , and ) K. The apparent molar volumes, V 2, Φ of diglycine have been derived. The standard partial molar volumes, V 2, Φ, obtained from V 2, Φ obtained from these data have been used to calculate the corresponding transfer parameter, t V for the studied diglycine from water to aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate. The hydration number, n H, side chain contributions of amino acids, and concentration effect of sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate as well as temperature effects have been discussed in terms of various interactions. The structure making / breaking capacities of diglycine in water and also in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate investigated have been discussed. Keywords: Amino acids; Sodium nitrate; Potassium nitrate; Magnesium nitrate hexahydrate; Partial molar volume; Transfer volumes; Hydration numbers; Interaction coefficients; Partial molar expansibilities. The interaction coefficients and hydration number, n H, of diglycine, partial molar expansibilities in aqueous solutions have also been calculated to see the effect of sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate on the hydration of amino acids. It has been found that diglycine in aqueous KNO 3 solution is behaving like common salt because its partial molar expansibilities decrease with the rise in temperature. *Correspondence R. P. Singh Grewal, rpsinghgrewal@gmail.com Introduction Proteins are large giant molecules. Therefore, direct study of electrolyte-protein interactions is an cumbersome task for biochemist [1]. These interactions are affected by the surrounding solutes and solvent of macromolecules; for this reason, the physicochemical behaviors of proteins are strongly affected by the interactions of solutes [2]. Because of direct solute solvent interactions or alteration of the water structure, these solutes can change many properties of globular proteins such as their hydration, solubility, stability and the activity of enzymes [3-7]. The side chain groups of the amino acids residues provide a very important range of properties, from hydrophilic to hydrophobic groups. The side chain groups are involved in a wide range of interactions. For example; the phenomenon of electrostriction that is caused by the polar end groups; the structure enforcing influences of the hydrophobic alkyl Chem Sci Rev Lett 214, 3(1), Article CS

2 groups; the interactions in between the hydrophilic and hydrophobic groups, hydrogen bonds that provides the peptide bonds in the polypeptide backbone. All these wide ranges of interactions are being are being affected with the change in the concentration of aqueous electrolyte that is being studied. Attempts are being made for the systematic study of the volumetric in aqueous solutions of sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate at various temperatures (288.15, , and ) K. Salt induced electrostatic forces are known to play an very significant role in modifying the protein structure by affecting the properties like solubility, denaturation of enzymes [8-9]. Remarkable experimental work has been reported on thermodynamics of amino acids in aqueous alkali metal salts [1-23]. Despite the ample use and importance of amino acids in many industries, their interactions with electrolytes and physicochemical properties in electrolyte solutions have been the subject of few investigations [23-27]. The role of Mg 2+ ions as components of all body fluids are indispensable, among others (Ca 2+, Na + and K + ), for a correct functioning of the nervous system. In addition Mg 2+ ion is one amongst the enzyme activators [28]. Consequently, in the present paper, the apparent molar volumes, V 2, Φ, of peptide (diglycine) in water and in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate (.25,.5,.75, 1., 1.5, 2.) mol kg -1 have been determined by measuring the densities using a vibrating-tube digital densimeter at various temperatures (288.15, , and ) K. The standard partial molar volumes, V 2, Φ, obtained from V 2, Φ obtained from these data have been used to calculate the corresponding transfer parameters, t V, for the studied peptide from water to aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate. From these data, the partial molar volumes, V 2, Φ and the hydration number, n H, side chain contributions of peptide (diglycine) and concentration effect of sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate have been discussed in terms of various interactions. The structure making / breaking capacities of these amino acids in water and also in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate investigated have been discussed. The interaction coefficients and hydration number, n H, of diglycine in aqueous solutions have also been calculated to see the effect of sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate on the hydration of diglycine. Experimental section The diglycine (G-12) selected for the present study was obtained from sigma chemicals co. These along with sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate (AR, S.d. fine chemicals Ltd., India), were used without further purification and dried over anhydrous CaCl 2 in a vacuum desiccator before use. Deionized, doubly distilled degassed water with a specific conductance of less than Ωm -1 was used for all of the measurements. All solutions were prepared by mass using a mettler balance with an accuracy of ±.1 mg. The solution densities were measured using a vibrating tube digital densimeter (model DMA 6/62, Anton paar), accuracy of 3 x1-9 kg m -3. Densities function was checked by measuring the densities of aqueous sodium chloride solutions [29], which agreed well with the literature values [literature values of density are: (15.571, , , , and ) kg m -3 at.2918,.5653, , , , and ) mol kg -1 of sodium chloride, respectively] at K. The temperature of water around the densimeter cell was controlled to within.1k. Results and discussion The densities, ρ, and apparent molar volumes, V 2,Φ of diglycine in water and in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate solutions of various molalities (m s, molality of sodium nitrate and potassium nitrate solutions, mol kg -1 ) at (288.15, , and ) K are given in Table 1. Chem Sci Rev Lett 214, 3(1), Article CS

3 Table 1. Solutions densities,, and apparent molar volumes, V 2,, for the diglycine in aqueous solutions of sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate at different concentrations and at various temperatures ( )K. m s (mol kg 1 ) 1-3 (kg m -3 ) V 2, 1 6 (m 3 mol -1 ) m s (mol kg -1 ) 1-3 (kg m -3 ) V 2, 1 6 (m 3 mol -1 ) m s (mol kg -1 ) 1-3 (kg m -3 ) V 2, 1 6 (m 3 mol -1 ) T=288.15K Diglycine m s = m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg T=298.15K Diglycine m s = m s =.25 mol kg m s =.5 mol kg Chem Sci Rev Lett 214, 3(1), Article CS

4 m s =.75 mol kg m s =1. mol kg m s =2. mol kg T=38.15K Diglycine m s = m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg T=318.15K Diglycine m s = Chem Sci Rev Lett 214, 3(1), Article CS

5 m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg KNO 3 T=288.15K m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg -1 Chem Sci Rev Lett 214, 3(1), Article CS

6 T=298.15K m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg T=38.15K m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg Chem Sci Rev Lett 214, 3(1), Article CS

7 T=318.15K m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg MgNO 3 T=288.15K m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg -1 Chem Sci Rev Lett 214, 3(1), Article CS

8 T=298.15K m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg T=38.15K m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg Chem Sci Rev Lett 214, 3(1), Article CS

9 T=318.15K m s =.25 mol kg m s =.5 mol kg m s =.75 mol kg m s =1. mol kg m s =2. mol kg Apparent molar volumes of amino acids have been calculated as follows: V 2, = M / ( o ) 1/m o (1) Where M is the molar mass of diglycine, m (mol.kg -1 ) is the molality, and ρ is the densities of solution and solvent, respectively. At infinite dilution, the apparent molar volumes, and partial molar volumes, are identical (V 2,Φ = V 2, Φ ). In the case of negligible concentration dependence of V 2,Φ, V 2, Φ was determined by taking the average of all the data points. However, where finite concentration dependence was observed, V 2, Φ was determined by least-squares fitting of the data using the following equation. V 2, =V2, Φ + S v m (2) Where V 2, Φ is the infinite dilution apparent molar volume and has the same meaning as the standard partial molar volume, and S v is the experimental slope. The V 2, Φ values along with their standard deviations are summarized in Table 2 at T = (288.15, , and ) K. The experimental values of for the diglycine in water agreed well with those reported in the literature[3-31]. The V 2,Φ values for the diglycine increases with the increase in the concentration of sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate. Chem Sci Rev Lett 214, 3(1), Article CS

10 Table 2. Standard Partial molar volumes, V 2, 1 6 at infinite dilution for the peptide in water and in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate solutions at ( ) K V 2, 1 6 /m 3 mol -1 at various m s /mol kg -1a Peptide Water a m s =.25 m s =.5 m s =.75 m s = 1. m s = 1.5 m s = 2. Diglycine ±.33 (9.32) a b Diglycine 76.12±.15 (1.21) a b Diglycine 76.95±.2 (1.23) a 77.1 b Diglycine 77.74±.8 (8.22) Diglycine ±.33 (9.32) Diglycine ±.51 (1.21) Diglycine ±.2 (1.23) Diglycine ±.8 (8.22) Diglycine ±.33 (9.32) Diglycine ±.51 (1.21) Diglycine 76.95±.2 (1.23) Diglycine ±.8 (8.22) ±.16 (5.72) 78.5±.14 (11.35) 79.65±.7 (5.11) 8.84±.11 (7.84) ±.18 (7.21) 78.4 ±.9 (5.74) 79.6 ±.28 (6.67) 8.51 ±.24 (6.6) 77.4 ±.17 (6.15) ±.33 (6.62) ±.26 (6.3) 81.5 ±.13 (8.8) ±.36 (1.85) 78.52±.7 (12.22) 79.78±.13 (8.18) 81.5±.8 (8.4) NaNO 3 T = K 78.1 ±.11 (7.41) T = K T=38.15K T=318.15K 79.21±.14 (4.52) 8.11±.8 (6.73) 81.54±.6 (5.56) KNO 3 T=288.15K 77.4 ± ±.9 (8.4) (6.4) T=298.15K ± ±.15 (8.62) (8.64) T=38.15K ± ±.7 (7.85) (6.58) T=318.15K 8.78 ±.6 (4.71) 8.84 ±.16 (6.23) MgNO 3 T=288.15K ± ±.17 (8.33) (9.45) T=298.15K ± ±.11 (9.49) (9.25) T=38.15K ± ±.19 (7.84) (3.35) T=318.15K ±.7 (9.98) ±.21 (8.76) ±.8 (8.18) 79.34±.8 (6.69) 8.44±.7 (5.69) 81.69±.17 (1.17) ±.8 (6.4) ±.15 (8.13) 8.4 ±.2 (7.1) ±.9 (6.23) ±.14 (8.2) ±.7 (6.93) 8.71 ±.17 (3.76) ±.13 (9.96) ±.12 (7.73) 8.25±.5 (5.53) 81.11±.14 (6.67) 82.51±.13 (5.) ±.7 (7.68) ±.16 (7.86) ±.23 (5.33) ±.9 (5.59) ±.5 (6.75) 8.35 ±.3 (8.22) ±.8 (8.95) ±.13 (6.39) ±.24 (6.88) 81.41±.12 (4.5) 82.36±.24 (8.34) 83.83±.7 (6.5) 79.8 ±.14 (3.94) 8.23 ±.24 (7.31) ±.15 (6.61) ±.17 (8.21) 8.4 ±.13 (5.38) ±.14 (6.44) ±.19 (7.41) ±.1 (4.98) a Ref. 3, b Ref. 31. From the V 2,Φ data, m the standard partial molar volumes of transfer, Δ t V, at infinite dilution from water to aqueous sodium nitrate, potassium nitrate solutions and magnesium nitrate hexahydrate solutions have been evaluated as follows: Chem Sci Rev Lett 214, 3(1), Article CS

11 t V 2 [water aqueous NaNO 3 /KNO 3 / Mg (NO 3 ) 2 ] = V 2 [in aqueous NaNO 3 /KNO 3 / Mg (NO 3 ) 2 V 2 (in water)] (3.3) The Δ t V values for diglycine in NaNO 3 / KNO 3 / Mg (NO 3 ) 2 6H 2 O are summarized in Table 3 and illustrated in Figures 1a-c. Table 3. Standard Partial molar volumes of transfer, t V o 1 6, at infinite dilution for the peptide in water and in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate solutions at ( ) K t V o x 1 6 /m 3 mol -1 at various m S / mol kg -1a T= K NaNO 3 Peptide.25 m.5 m.75 m 1. m 1.5 m 2. m Diglycine 1.41± ± ± ± ± ±.41 T= K Diglycine 1.93±.2 2.4± ± ± ± ±.19 T=38.15 K Diglycine 2.7± ± ± ± ± ±.31 T= K Diglycine 3.2± ± ± ± ± ±.11 KNO 3 T= K Diglycine.91± ± ± ± ± ±.36 T= K Diglycine 1.92± ± ± ± ± ±.56 T=38.15 K Diglycine 2.65± ± ± ± ±.3 4.5±.25 T= K Diglycine 2.77± ±.1 3.1± ± ± ±.19 Mg(NO 3 ) 2 T= K Diglycine 1.66± ± ± ± ± ±.35 T= K Diglycine 2.52± ± ± ± ± ±.53 T=38.15 K Diglycine 2.68 ± ± ± ± ± ±.28 T= K Diglycine 3.31 ± ± ± ± ± ±.13 Chem Sci Rev Lett 214, 3(1), Article CS

12 Figure 1a. Standard partial molar volumes of transfer, t V, of diglycine vs. different molalities, m s, of sodium nitrate solutions at (a), K; (b), K; (c), K; (d), K. Figure 1b. Standard partial molar volumes of transfer, t V, of diglycine vs. different molalities, m s, of potassium nitrate solutions at (a), K; (b), K; (c), K; (d), K. Figure 1c. Standard partial molar volumes of transfer, t V, of diglycine vs. different molalities, m s, of magnesium nitrate hexahydrate solutions at (a), K; (b), K; (c), K; (d), K. Chem Sci Rev Lett 214, 3(1), Article CS

13 The Δ t V values for the studied diglycine are positive and increase almost linearly with the increase in concentration of aqueous NaNO 3 / KNO 3 / Mg(NO 3 ) 2 solutions. The more positive Δ t V values in the case of diglycine indicate the dominance of the effect of charged end groups (NH 3 + and COO - ). The cosphere model [32] can be utilized to rationalize the Δ t V values in terms of solute-cosolute interactions. According to this model, when two solute particles come close enough together so that their cosphere overlap some cosphere material is displaced, and this is accompanied by the change in thermodynamic parameters. The following types of interactions can occur between amino acids and NaNO 3 / KNO 3 / Mg (NO 3 ) 2 in solutions: i. Ion-ion interactions occurring between Mg 2+ / Na + / K + ions and COO - groups of amino acids and - between NO 3 ions of NaNO 3 / KNO 3 / Mg(NO 3 ) 2 and NH + 3 groups of diglycine. ii. Interactions between ions (Mg 2+ / Na + / K +, NO - 3 ) of NaNO 3 / KNO 3 / Mg(NO 3 ) 2 and nonpolar (hydrophobic) part of diglycine. The ion-ion interactions dominate the ion- hydrophobic group interactions. Because of the first types of interactions, the electrostriction of water in the neighborhood of charged end groups (NH +, COO - ) of diglycine gets reduced, and this will contribute positively to the volumes of transfers. Furthermore, the increase in Δ t V values with increasing NaNO 3 / KNO 3 / Mg (NO 3 ) 2 concentration strengthens this view. This is a qualitative interpretation of the results. Franks et al.[33] have shown that the partial molar volume of non-electrolyte is a combination of two types of contributions given by the following equation: V 2 = V int + V s (4) where, V int is the intrinsic volume of non-electrolyte and V s volume due to it s interactions with the solvent. It has been considered [36,37] that V int is made up of two types of contributions. V int = V vw + V void (5) where, V vw, is the van der Waals volume and V void is the associated void or empty volume. Shahidi et al.[34] further modified Eq. (5) to include the contribution of interactions of a non-electrolyte solute with the solvent. V 2 = V vw + V void - n s (6) Where s is the shrinkage in volume produced by the interactions of hydrogen bonding groups present in the solute with water molecules and n is the potential number of hydrogen bonding sites in the molecule. For non-electrolytes and zwitterionic solutes the shrinkage is caused by electrostriction and finally V 2 can be evaluated as: V 2=V vw + V void V shrinkage (7) Where, V shrinkage is the volume due to shrinkage caused by the interaction of hydrogen bonding groups present in the solute with water molecules. It has been reported that V vw and V void have the same magnitude in water and in mixed solvents for the same class of compounds. Therefore positive t V values can be attributed to a decrease in the shrinkage volume in the presence of aqueous solutions of Mg (NO 3 ) 2 / NaNO 3 / KNO 3. Because of the stronger interactions of Mg 2+, Na + and NO 3 with COO and NH 3 + in the diglycine, the electrostriction of neighboring water molecules due to these charged centers / peptide backbone will be reduced which will result into a reduction in the shrinkage volume. The increase of V 2 values with the increase in temperature may also be attributed to the reduction in electrostriction with temperature. This also gets support from the volumetric and compressibility studies of glycine and DL- -alanine in aqueous sodium sulfate solutions by Wadi and Ramasami, who reported that the hydration number of amino acids decreases with increasing temperature and concentration of sodium sulfate[35]. The increase in positive V 2 values of Chem Sci Rev Lett 214, 3(1), Article CS

14 studied diglycine with the increase in the concentration of Mg (NO 3 ) 2 / NaNO 3 / KNO 3 may further be attributed to the formation of non-covalent ion-pairs between the charged groups of the diglycine and the cation Mg 2+ / Na + / K + and the anion (NO 3 ) of the electrolyte. This increases the apparent molar volume of diglycine and decreases electrostriction of water around diglycine in the presence of Mg (NO 3 ) 2 / NaNO 3 / KNO 3. The decrease in the magnitude of t V values with the increase of temperature can be attributed to the enhanced thermal agitation and weakening of various interactions involving ions resulting in the relaxation to the bulk more of the electrostricted water molecules from the mutually interacting regions Kozak et al. [36] have proposed formalism on the basis of McMillan-Mayer [37] theory of solutions which permits the formal separation of the effects due to interactions between pairs of solute molecules and those due to interactions involving three or more solute molecules. Friedman and Krishnan [38] and Franks et al. [39] have further discussed this approach to include the solute-cosolute interactions in the solvation sphere. Various workers [4,41] used this approach to study the interactions of amino acids and cosolutes in aqueous medium. The various transfer functions of the diglycine can be expressed by the following equation: t V = 2V AB.m S + 3V ABB.m 2 S + 4V ABBB.m 3 S+.. (8) Where, V AB, 3V ABB and V ABBB are respectively, pair, triplet and quartet interaction coefficients (where A stands for the diglycine, B stands for [Mg (NO 3 ) 2 /NaNO 3 / KNO 3 ]. The data reveal that all pair volumetric interaction parameters V AB are positive for the diglycine are larger than corresponding V ABB values. This shows that the interactions between the diglycine and Mg (NO 3 ) 2 / NaNO 3 / KNO 3 are mainly pair interactions. The pair, triplet and quartet interactions coefficients from volumes in case of diglycine are summarized in Table 4 and illustrated in Figure 2. Table 4. Hydration number, n H 1 6 for the peptide in water and in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate solutions at ( ) K Peptide Water m s =.25 m s =.5 m s =.75 m s = 1. m s =1.5 m s =2. NaNO 3 T= K Diglycine 4.23± ± ± ± ± ± ±.24 T= K Diglycine 4.3± ± ± ± ± ±.5 3.8±.12 T=38.15 K Diglycine 3.88±.2 3.1± ± ± ±.7 3.2± ±.24 KNO 3 T= K Diglycine 4.98± ± ± ± ±.8 4.4± ±.14 T= K Diglycine 4.56± ±.9 4.5± ± ± ± ±.24 T=38.15 K Diglycine 4.36± ± ± ± ± ± ±.15 Mg(NO 3 ) 2 T= K Diglycine 6.67± ± ± ± ± ± ±.13 T= K Diglycine 5.74± ± ± ± ± ± ±.14 T=38.15 K Diglycine 4.53± ± ± ± ± ±.8 3.8±.19 Chem Sci Rev Lett 214, 3(1), Article CS

15 Figure 2. Pair interaction coefficient, V XY, of diglycine at various temperatures in the aqueous solutions of different nitrates;, NaNO 3 ;, KNO 3 ;, Mg(NO 3 ) 2. The pair and quartet coefficients are all positive while triplet negative. This shows that the pair and quartet interactions in the case of two thermodynamic properties contribute positively, while triplet contribution is negative which is falling in magnitude with the rise in temperature. The positive values of pair interactions occur due to the overlap of hydration spheres of diglycine with Mg (NO 3 ) 2 / NaNO 3 / KNO 3 molecules, which supports the conclusion drawn earlier from the cosphere overlap model. V 2,Φ = V 2,Φ (int) + n H (V E V B) (9) Where V 2,Φ (int) is the intrinsic volume of a solute molecule, V E and V B are the partial molar volumes of water in the bulk state and in the hydration shell of a solution. The standard partial molar volume of a non-electrolyte can be divided into two parts [42]: V 2,Φ = V int + V elect (1) V where int is the intrinsic molar volume of the non-electrolyte solute and Velect the electrostriction partial molar volume due to the hydration of the non-electrolyte. Millero et al. [27] reported that the intrinsic molar volume could be estimated from the molar volume of non-electrolyte crystal: V int = (.7 /.634) V cryst (11) where.7 is the packing density for molecules in organic crystals,.634 is the packing density for random packing Vcryst spheres, and is the molar volume of the non-electrolyte crystal. The values of crystal densities for diglycine are g.cm -3 at K [42] respectively, and crystal density values for these diglycine at and K were assumed to be same as at K. Following the procedure described by Millero et al. [27] (V E V B) = -2.9, -3.3, -4. cm 3.mol -1 at K, K and K, respectively. Therefore, as an approximation, the hydration number n H can be obtained by Eq. (9), and the results are listed in Table 5 and illustrated in figures 3a-c. Chem Sci Rev Lett 214, 3(1), Article CS

16 Table 5. Pair and Triplet Interactions coefficients for the peptide in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate solutions at ( ) K Peptide V XY 1 6 (m 3 mol -2 kg) V XYY 1 6 (m 3 mol -2 kg 2 ) NaNO 3 T = K Diglycine ± ±.7 T = K Diglycine ± ±.1566 T = K Diglycine ± ±.2182 T = K Diglycine KNO 3 T = K Diglycine ± ±.862 T = K Diglycine ± ±.1169 T = K Diglycine ± ±.1169 T = K Diglycine ± ±.26 Mg(NO 3 ) 2 T = K Diglycine ± ±.126 T = K Diglycine ± ±.1944 T = K Diglycine ± ±.24 T = K Diglycine ± ±.2785 Figure 3a. Hydration number, n H 1 6 for the diglycine in water and in aqueous sodium nitrate, solutions at different temperatures,, ;, ;, K. Chem Sci Rev Lett 214, 3(1), Article CS

17 Figure 3b. Hydration number, n H 1 6 for the diglycine in water and in aqueous potassium nitrate, solutions at different temperatures,, ;, ;, K. Figure 3c. Hydration number, n H 1 6 for the diglycine in water and in aqueous magnesium nitrate hexahydrate, solutions at different temperatures,, ;, ;, K. As explained earlier, addition of Mg (NO 3 ) 2 / NaNO3 / KNO 3 decreases the electrostriction of water and this also means that n H decreases as the electrostricted water becomes more like bulk water. The number of water molecules bound to the amino acids was calculated using the method reported by Millero et al [27]. n H = V 2 (elect) / V E V B (12) Where V E is the molar volume of electrostricted water and V B is the molar volume of bulk water. The electrostriction partial molar volume, V 2 (elect) can be estimated from experimentally measured V 2 using following equation: V 2 (elect) = V 2 (amino acid) - V 2 (int) (13) The intrinsic volume, V 2 (int), consists of two terms: the vander Waals volume and the void volume. V 2 (int) can be obtained from crystal volume data [42]. The n H values for the diglycine in the presence of Mg (NO 3 ) 2 / NaNO 3 / KNO 3 are less than in water and decrease with increasing concentration of Mg(NO 3 ) 2 /NaNO 3 / KNO 3, which suggests that Mg (NO 3 ) 2 / NaNO 3 / KNO 3 has a dehydration effect on the diglycine. As shown in Table 2 the V 2,Φ increases with increasing temperature from to K. An increase in temperature reduces the electrostriction and hence Chem Sci Rev Lett 214, 3(1), Article CS

18 increases V 2,Φ. The reduction in the electrostriction with increasing temperature is confirmed by the decreased n H. Therefore, temperature effect on the electrostriction of water by the zwitterionic centers of the diglycine is a predominant factor in determining temperature dependence of V 2,Φ of the diglycine. The temperature dependence of V 2 for various diglycine in water and in aqueous Mg (NO 3 ) 2 / NaNO 3 / KNO 3 solutions, studied here can be expressed by the general equation as follows: V 2 = + T+ T 2 (14) Values of various coefficients of the equation for diglycine in water and in aqueous Mg (NO 3 ) 2 / NaNO 3 / KNO 3 solutions are recorded in Table 6. The partial molar (limiting apparent molar) expansibilities, E = ( V / T) p, calculated from the general equation are given in Table 7. it is found that E values for diglycine in aqueous solutions of NaNO 3 increases in magnitude with increase in temperature while in aqueous solutions of KNO 3, the values for E of diglycine decrease with rise in temperature indicating thereby that the behavior of diglycine is just like that of common salts, because in the case of common salts the molar expansibility should decrease with rise in temperature. It is also evident from Table 6 that the E values increase in temperature for diglycine in water. Table 6. Partial Apparent Molar Expansibility E 1-6 /m 3 mol -1 K -1 and ( 2 V / T 2 ) p 1-6 for the peptide in aqueous sodium nitrate, potassium nitrate and magnesium nitrate hexahydrate solutions at ( ) K E 1-6 (m 3 mol -1 K -1 ) ( 2 V / T 2 ) p 1-6 (m 3 mol -1 K -2 ) Peptide K K 38.15K K Diglycine Diglycine in water NaNO 3.25 m s m s m s m s m s KNO 3.25 ms ms ms ms ms Mg(NO 3 ) 2.25 ms m s m s m s m s During the past few years it has been emphasized by different workers that S v * is not the sole criterion for determining the structure making or structure breaking nature of any solute. Hepler [43] developed a technique of examining the Chem Sci Rev Lett 214, 3(1), Article CS

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