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Enthalpic interactions of some α-amino acids with 1,2-ethanediol in aqueous solutions at 298.15 K
Fluid Phase Equilibria 252 (2007) 28–32
Enthalpic interactions of some ␣-amino acids with 1,2-ethanediol in aqueous solutions at 298.15 K Li Yu a,∗ , Yan Zhu b , Xin-gen Hu c , Xian-hong Pang b , Ming-wei Zhao a a
Key Lab for Colloid and Interface Chemistry of Shandong University, Ministry of Education, Jinan 250100, PR China b Department of Chemistry and Chemical Engineering, Taishan Medical College, Taian 271000, PR China c Chemistry and Material Science College, Wenzhou University, Wenzhou 325027, PR China Received 11 April 2006; received in revised form 1 December 2006; accepted 5 December 2006 Available online 10 December 2006
Abstract The enthalpies of mixing of six kinds of aqueous amino acid solutions (glycine, l-alanine, l-valine, l-serine, l-threonine and l-proline) and aqueous 1,2-ethanediol solution and their respective enthalpies of dilution have been measured at 298.15 K using flow microcalorimetry. The experimental data have been analyzed in terms of the McMillan–Mayer formalism to obtain the heterotactic enthalpic interaction coefficients. The heterotactic enthalpic pairwise interaction coefficients, hxy , have been discussed from the points of view of solute–solute interactions. © 2006 Elsevier B.V. All rights reserved. Keywords: ␣-Amino acid; 1,2-Ethanediol; Heterotactic enthalpic pairwise interaction coefficient; Solute–solute interactions
1. Introduction Studies on various thermodynamic properties of amino acids and simple peptides in aqueous solutions of electrolyte [1,2] or organic substances [3–5] are of current interest due to their importance in the better understanding of the nature and mechanisms taking place in biological cells. Many studies have been done on the effects of polyols on the proteins and it was found that polyols help in stabilizing the native conformations of globular proteins [6–8]. Some authors correlate the stabilizing effect of sugars with the number and position of hydroxyl groups. However, our understanding of the stabilization mechanism of proteins is still incomplete. To understand the nature of interactions between sugars and proteins in aqueous solutions, it is necessary to study biochemical model compounds owing to the complex structure of the biological micromolecules. Amino acids are basic components of protein molecules and are considered to be the model compounds of protein molecules. The native structure of proteins is governed by weak, non-bonding interactions between the amino
∗
Corresponding author. Fax: +86 531 8856 4750. E-mail address: [email protected] (L. Yu).
0378-3812/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2006.12.003
acid residues and between these residues and the aqueous environment [9]. Although the polyols under investigation are not found in cellular or extracellular fluids of living organisms, they find wide application in pharmacoloty and the cosmetics industry. When introduced into a living organism as vehicles for pharmaceuticals or cosmetics, they affect the components of cellular fluids. This has been confirmed by numerous biochemical studies devoted to the interactions between polyols and components of biological cells [10]. In our previous studies, the enthalpies of dilution of amino acids in aqueous solutions of DMF, glucose, sucrose and urea [11–13], and the enthalpies of mixing amino acids with ethanol, chloroethanol, pyridine, methylpyridine, THF and 1,4dioxane [14–18] in water by the method of mixing flow microcalorimetry. The present work reports the results of calorimetric measurements of the mixing enthalpies of amino acids and 1,2-ethanediols aqueous solutions and their respective dilution enthalpies. It is well known that the enthalpic interaction coefficients derived from McMillan–Mayer’s model [19] can be considered as measures of intermolecular interactions in solution, and depend significantly on variation in solvent. This work is aimed at examining the heterotactic enthalpic interaction coefficients between amino acids and 1,2-ethanediol molecules.
L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32
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2. Experimental
To make the calculations easier, an auxiliary function H* has been introduced:
Glycine, l-alanine, l-valine, l-serine, l-threonine and lproline (BR) were purified by means of recrystallization using twice-distilled water. The purified amino acids were used after drying them in vacuum desiccators until their weights became constant. 1,2-Ethanediol (AR grade, from Shanghai Chem. Co.) used in the experiment was purified without further purification. Deionized water was distilled using a quartz sub-boiling purifier and stored in a CO2 -free atmosphere before use. Both the aqueous amino acid solutions and the aqueous 1,2-ethanediol solution were prepared by weight by Mettler AE 200 balance with a precision of ±0.0001 g. All the solutions were degassed and used within 12 h after preparation to minimize decomposition due to bacterial contamination. The calorimetric measurements were performed by a mixing–flow microcalorimeter (2277 Thermal Activity Monitor manufactured in Sweden). All the measurements were carried out at 298.15 K. The solutions were pumped through the mixing vessel of the calorimeter at constant rates using a pair of LKB-2132 microperpex peristaltic pumps. The flow rates were determined by weighing samples delivered in 8 min. The variation in flow rates was less than 0.1% both before and after a complete experiment. The error in measuring of the flow rates of solution in each experiment is less than 0.4%. The relative mean deviation of thermal powers determined three times was 0.3% and at last the relative mean deviations of the molar enthalpies of dilution and mixing were less than 1%. The experimental details have been reported in the earlier publications [14–18].
H ∗ = Hmix − Hdil (x) − Hdil (y) = H E (mx , my ) − H E (mx ) − H E (my )
(2)
where Hmix denotes the mixing enthalpy of the ternary solution, Hdil (x) and Hdil (y) are the dilution enthalpies of the corresponding binary solutions. The mixing enthalpy Hmix (J kg−1 ) of aqueous x solution and aqueous y solution is calculated from the equation: Hmix =
P∗ fx + fy − mx,i Mx fx − my,i My fy
(3)
where P* is the mixing thermal power (W) and fx , fy the flow rates of solutions x and y, respectively, and mx,i , my,i are the initial molalities of solutions x and y before mixing. The dilution enthalpies Hdil (J kg−1 ) are obtained by measuring thermal power P (W) and flow rates of solution and solvent (fA and fB , mg s−1 ): Hdil =
P fA + fB − mx,i Mx fA
(4)
where Mx is the molar mass of solute (kg mol−1 ) and mx,i is initial molality (mol g−1 ). The final molality mx (mol kg−1 ) may be calculated by using the equation: mx =
mx,i fA fB (mx,i Mx + 1) + fA
(5)
and combining Eqs. (1) and (2): 3. Results and discussion The enthalpies of mixing of aqueous amino acids solutions and aqueous 1,2-ethanediol solutions and their respective dilution enthalpies are given in Table 1 together with the initial and final molalities of the two solutes. According to the McMillian–Mayer formalism [19], the excess enthalpy of the ternary solution per 1 kg of water can be expressed as a power series in the molalities: H E (mx , my ) H(mx , my ) ∞ ∞ = − h∗w − mx Hx,m − my Hy,m w1 w1 = hxx m2x + 2hxy mx my + hyy m2y + hxxx m3x + 3hxxy m2x my + 3hxyy mx m2y + hyyy m3y + · · · (1) where H E (mx , my )/w1 and H(mx , my )/w1 represent the excess and the absolute enthalpy, respectively, of a solution containing 1 kg of water, mx mol of x and my mol of y, h∗w the stan∞ and H ∞ are dard enthalpy of 1 kg of pure water and Hx,m y,m the limiting partial molar enthalpies of species x and y, respectively. hij and hiij are the enthalpic pair, triplet, etc., interaction parameters. mx and my are the molalities of the solutes x and y, respectively.
H ∗ = 2hxy mx my + 3hxxy m2x my + 3hxyy mx m2y + · · · w1
(6)
The pairwise and triplet enthalpic interaction coefficients for the solutions studied are reported in Table 2. Since the triplet enthalpic interaction coefficients contain some contributions from the pairwise interaction terms, they are not discussed and only the pairwise coefficients hxy are considered in the present paper. The enthalpic interaction coefficients describe the total energetic effect of the interaction of heterotactic amino acid–1,2-ethanediol pair including the contribution of the solvent molecules surrounding the interacting solute molecules. The process of interaction of two solvated species can be represented as two successive stages: the partial dehydration of the solutes and the further direct interaction caused by the short-range molecular forces [20,21]. By and large, the interactions between ␣-amino acids and 1,2ethanediol in the aqueous solutions reflect three superimposed processes: the first is the partial dehydration of hydration shell of the ␣-amino acid zwitterions (making positive contributions to hxy ); the second is the partial dehydration of hydration shell of 1,2-ethanediol (making positive contributions to hxy ); and the third is the direct interaction, which plays the dominant role in the processes of interaction between ␣-amino acid and 1,2ethanediol molecules.
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L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32
Table 1 Enthalpies of dilution and mixing of aqueous amino acid solutions and aqueous 1,2-ethanediol solutions at 298.15 K mx,i (mol kg−1 )
my,i (mol kg−1 )
mx,f (mol kg−1 )
my,f (mol kg−1 )
Hdil(x) /w1 (J kg−1 )
Hdil(y) /w1 (J kg−1 )
Hmix /w1 (J kg−1 )
H ∗ /w1 (J kg−1 )
Glycine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0541 0.0811 0.0972 0.1079 0.1186 0.1347 0.1507 0.1613 0.1719 0.1879 0.2038 0.2144 0.2249 0.2407 0.2670
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
0.74 2.62 3.61 4.27 5.54 6.99 8.12 9.35 10.87 12.95 14.82 15.80 17.94 20.05 22.72
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
1.21 2.45 2.67 3.78 4.01 5.26 7.66 8.10 10.00 10.20 12.13 13.46 14.70 16.84 20.33
1.54 2.08 2.11 3.27 3.64 4.74 7.23 7.14 8.55 9.41 11.38 13.08 13.32 15.24 20.84
l-Alanine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0541 0.0810 0.0971 0.1078 0.1185 0.1344 0.1504 0.1716 0.1875 0.2033 0.2138 0.2243 0.2401 0.2662
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
−0.44 −1.43 −2.08 −2.27 −2.67 −3.13 −3.89 −5.76 −6.49 −8.20 −8.38 −9.55 −10.93 −14.28
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
0.19 0.17 2.67 1.55 −0.76 −1.12 −0.87 −0.74 −1.24 −0.30 0.03 −0.22 −0.19 −0.16
1.70 3.85 7.81 7.59 7.09 8.48 10.71 14.43 17.41 21.98 23.83 25.89 29.19 37.35
l-Valine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500
0.0540 0.0808 0.0968 0.1075 0.1181 0.1340 0.1499 0.1604 0.1709 0.1866 0.2023 0.2127 0.2231 0.2387
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016
−1.19 −4.42 −6.39 −8.03 −10.24 −12.55 −15.67 −18.07 −21.60 −24.16 −28.14 −30.07 −34.48 −43.14
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44
−0.33 −1.21 −1.77 −2.39 −3.60 −4.86 −5.08 −5.10 −5.90 −7.73 −9.97 −10.10 −10.74 −12.42
2.04 5.45 7.67 9.39 11.80 14.16 18.27 21.36 25.11 28.60 32.24 35.39 40.30 49.17
l-Serine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0541 0.0809 0.0969 0.1076 0.1183 0.1342 0.1501 0.1607 0.1712 0.1870 0.2027 0.2132 0.2236 0.2393 0.2653
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
1.01 4.34 5.81 7.84 8.76 10.73 12.22 15.05 17.21 19.98 24.15 26.00 30.10 33.10 36.37
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
2.30 4.54 5.73 6.83 8.78 12.02 15.27 15.58 18.70 20.79 24.40 26.73 29.49 33.29 39.33
2.35 2.44 2.97 2.75 5.19 7.77 10.73 8.92 10.90 12.97 14.33 16.15 15.95 18.63 26.19
L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32
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Table 1 (Continued ) mx,i (mol kg−1 )
my,i (mol kg−1 )
mx,f (mol kg−1 )
my,f (mol kg−1 )
Hdil(x) /w1 (J kg−1 )
Hdil(y) /w1 (J kg−1 )
Hmix /w1 (J kg−1 )
H ∗ /w1 (J kg−1 )
l-Threonine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0540 0.0808 0.0968 0.1075 0.1181 0.1340 0.1498 0.1604 0.1709 0.1866 0.2022 0.2127 0.2231 0.2386 0.2644
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
0.25 0.82 1.05 1.65 1.71 1.95 2.13 2.53 3.01 2.35 3.19 3.35 3.62 3.09 4.71
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
1.33 2.25 2.70 3.40 3.53 4.59 5.75 7.11 8.68 9.04 10.77 12.05 12.91 15.15 17.88
2.15 3.68 4.71 5.51 6.99 9.12 11.30 12.96 15.09 18.85 21.65 24.12 25.86 30.50 36.41
l-Proline + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0540 0.0808 0.0969 0.1075 0.1181 0.1340 0.1499 0.1604 0.1710 0.1867 0.2024 0.2128 0.2232 0.2388 0.2647
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
−0.93 −2.02 −2.59 −3.16 −3.87 −5.09 −5.96 −6.69 −7.27 −8.37 −13.94 −15.03 −17.98 −18.92 −23.54
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
0.53 0.79 0.84 1.25 0.79 1.23 1.96 2.47 2.23 2.71 4.19 4.35 4.62 5.51 6.97
2.52 5.05 6.49 8.16 9.83 12.79 15.60 17.55 18.92 23.25 32.19 34.80 39.17 42.87 53.73
mx,i and my,i are the initial molalities of solutes x and y; m,x,f and my,f are the final molalities of solutes x and y.
The relative magnitude of hxy coefficients studied in this work mainly depends on the direct interactions between ␣-amino acids and 1,2-ethanediol. Since both the amino acid and 1,2ethanediol molecules have hydrophobic and hydrophilic groups, the direct interaction between the two kinds of molecules can be summarized as: (a) the hydrophobic–hydrophobic interaction (an endothermic process leading to a positive contribution to hxy ); (b) the hydrophobic–hydrophilic interaction (an endothermic process leading to a positive contribution to hxy ); (c) the hydrophilic–hydrophilic interaction (an exothermic process leading to a negative contribution to hxy ).
The resulting sign and magnitude of hxy would be a consequence of the competitive equilibrium between the above effects. For chain amino acids studied, the experimentally observed positive values of hxy testify to the predominance of endothermic processes over the effect of exothermic direct interaction of amino acid with 1,2-ethanediol. While for cyclic l-proline, the negative value of hxy indicates that the exothermic effect plays a significant role during the process of the interaction between it and 1,2-ethanediol. The differences of hxy coefficients of different kinds of ␣amino acids with 1,2-ethanediol are dramatically contingent on
Table 2 Heterotactic enthalpic interaction coefficients between amino acids and 1,2-ethanediol in aqueous solutions at 298.15 K Solutes x + y
hxy (J kg mol−2 )
hxxy × 10−4 (J kg2 mol−3 )
hxyy × 10−4 (J kg2 mol−3 )
S.D.a
R2b
Glycine + 1,2-ethanediol l-Alanine + 1,2-ethanediol l-Valine + 1,2-ethanediol l-Serine + 1,2-ethanediol l-Threonine + 1,2-ethanediol l-Proline + 1,2-ethanediol
221 (129c ) 272 (244) 992 (280) 544 (276) 37.4 (66.5) −45.7 (168)
−131 (171) −5.20 (21.4) 21.9 (10.6) 15.3 (11.9) 8.08 (1.99) 12.7 (8.80)
156 (204) 6.21 (2.54) 25.7 (12.4) 18.1 (14.1) −9.48 (2.33) −14.8 (10.3)
0.59 1.08 0.92 1.24 0.30 1.21
0.9916 0.9921 0.9968 0.9752 0.9994 0.9953
a b c
Standard deviation. Square of correlation coefficient. The estimated deviation.
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L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32
the discrepancies of the structure of the studied ␣-amino acids. Glycine, which has no side-chain, is the simplest amino acid in nature. Compared to glycine, l-alanine has one hydrogen atom on the ␣-carbon replaced by a methyl. For the above two kinds of amino acids, there are marked discriminations in the interaction (a) and (b) concerning the non-polar groups of ␣-amino acid, which make positive contributions to hxy . As the alkyl sidechain of amino acid is lengthened, the positive contributions made to the hxy coefficients of the interaction between amino acids and 1,2-ethanediol in aqueous solutions become larger. Therefore, the hxy coefficients are in the following sequence: hxy (l-alanine) > hxy (glycine). l-Serine is similar to l-alanine except that it has an OH group replacing a hydrogen atom of the methyl group, which strengthens its hydrophilic properties and enhances interactions (b) and (c). The relative magnitudes of the values of hxy for l-serine can be treated as a result of competition between interactions (b) and (c). The dominant role played by interaction (b) causes hxy (l-serine) > hxy (l-alanine). As to l-valine and l-serine, the side chain of l-valine can be considered as a substitute for one methyl group of the hydroxyl group and a replacement of one hydrogen atom by another methyl group for l-serine. The main differences of the interactions of l-valine and l-serine with 1,2-ethanediol lie in the following: there exist hydrophobic–hydrophobic and hydrophobic–hydrophilic interactions (both of them making positive contributions to hxy ) between the methyl groups of l-valine with the 1,2-ethanediol molecule, and there also exist hydrophilic–hydrophobic (making positive contributions to hxy ) and hydrophilic–hydrophilic interactions (making negative contributions to hxy ) between the hydroxyl group on the side chain of l-serine with the 1,2-ethanediol molecule. The comparative magnitude of the heterotactic enthalpic pairwise interaction coefficients of l-valine and l-serine with 1,2-ethanediol molecule depends on the competitive balance of the above varied interactions. In aqueous 1,2-ethanediol solutions, there exists hxy (l-valine) > hxy (l-serine), which shows the former yields very strong interaction effects. Thus, for the above unbranched long-chain amino acids, there exists the following rule: hxy (l-valine) > hxy (l-serine) > hxy (lalanine) > hxy (glycine). Since there is one methyl on the side chain of l-threonine as compared to l-serine, interactions (a) and (b) (making positive contribution to hxy ) are reinforced. But in the meantime, the steric effect, which can weaken the interaction between l-threonine and 1,2-ethanediol, becomes more remarkable because of the existence of the methyl. As a result of the balance between the above effects, the hxy values increase in the following order: hxy (l-serine) > hxy (l-threonine). l-Proline is a natural amino acid that has one pyrrole ring. Its special structure makes it important to the properties of polypeptide. On one hand, the cyclic structure weakens the attracting interactions between l-proline molecules due to the steric effect, which makes solvation easy. This indicates the absorbing heat of the dehydration process and the positive contribution to the
enthalpic pairwise interaction coefficient are relatively little. On the other hand, although both of l-proline and l-valine contain four carbon atoms, the cyclic structure of the former can weaken the interaction between it and 1,2-ethanediol molecule. As a result of the above two effects, the exothermic process dominates over the overall interaction process. And consequently, the value of hxy of l-proline with 1,2-ethanediol is negative. 4. Conclusions In aqueous solutions, the calorimetric measurements of the mixing enthalpies of six kinds of native ␣-amino acids (glycine, l-alanine, l-valine, l-serine, l-threonine and l-proline) and 1,2ethanediol and their respective dilution enthalpies were carried out at 298.15 K. These results served as a basis for calculations of the enthalpic coefficients of interactions between amino acids and 1,2-ethanediol in aqueous solution, according to McMillan–Mayer’s model. The heterotactic enthalpic pairwise interaction coefficients between amino acids and 1,2ethanediol molecules have been interpreted from the point of view of solute–solute interactions. Acknowledgements The authors are grateful to the Natural Science Foundation of Shandong Province of China (No. Q2003B01) and Excellent Young Scientist Awarded Fund of Shandong Province (No. 2005BS08009) for financial support. References [1] B. Pałecz, H. Piekarski, Fluid Phase Equilib. 164 (1999) 257–265. [2] B. Pałecz, Fluid Phase Equilib. 167 (2000) 253–261. [3] G. Barone, G. Castronuovo, P. Del Vecchio, C. Giancola, J. Chem. Soc., Faraday Trans. I 85 (1989) 2087–2097. [4] J. Fernandez, T.H. Lilley, J. Chem. Soc., Faraday Trans. 88 (1992) 2503–2509. [5] B. Pałecz, Fluid Phase Equilib. 126 (1996) 299–303. [6] R.B. Simpson, W. Kauzmann, J. Am. Chem. Soc. 75 (1953) 5139–5152. [7] P.H. Yancey, M.E. Clark, S.C. Hand, R.D. Bowlus, G.N. Somero, Science 217 (1982) 1214–1220. [8] T.S. Lakshmi, P.K. Nandi, J. Phys. Chem. 80 (1976) 249–252. [9] G. Castronuovo, V. Elia, F. Velleca, J. Chem. Soc., Faraday Trans. I 92 (1996) 4215–4219. [10] F. Fujita, Y. Noda, Bull. Chem. Soc. Jpn. 57 (1984) 1891–1896. [11] X.L. Ren, Y.M. Ni, R.S. Lin, Thermochim. Acta 348 (2000) 19–24. [12] S.Q. Li, X.G. Hu, R.S. Lin, Thermochim. Acta 342 (2000) 1–6. [13] S. Shao, X.G. Hu, R.S. Lin, Thermochim. Acta 360 (2000) 93–100. [14] L. Yu, R.S. Lin, X.G. Hu, G.Y. Xu, J. Solution Chem. 32 (2003) 273–281. [15] L. Yu, X.G. Hu, R.S. Lin, G.Y. Xu, J. Solution Chem. 33 (2004) 131–141. [16] L. Yu, X.G. Hu, R.S. Lin, G.Y. Xu, J. Chem. Thermodyn. 36 (2004) 483–490. [17] L. Yu, R.S. Lin, X.G. Hu, H.L. Zhang, G.Y. Xu, J. Chem. Eng. Data 48 (2003) 990–994. [18] L. Yu, S.L. Yuan, X.G. Hu, R.S. Lin, Chem. Eng. Sci. 61 (2006) 794–801. [19] W.G. MacMillan, J.E. Mayer, J. Chem. Phys. 13 (1945) 276–305. [20] K.G. Davis, T.H. Lilley, Thermochim. Acta 107 (1986) 267–276. [21] J.J. Savage, R.H. Wood, J. Solution Chem. 5 (1976) 733–750.
Enthalpic interactions of some ␣-amino acids with 1,2-ethanediol in aqueous solutions at 298.15 K Li Yu a,∗ , Yan Zhu b , Xin-gen Hu c , Xian-hong Pang b , Ming-wei Zhao a a
Key Lab for Colloid and Interface Chemistry of Shandong University, Ministry of Education, Jinan 250100, PR China b Department of Chemistry and Chemical Engineering, Taishan Medical College, Taian 271000, PR China c Chemistry and Material Science College, Wenzhou University, Wenzhou 325027, PR China Received 11 April 2006; received in revised form 1 December 2006; accepted 5 December 2006 Available online 10 December 2006
Abstract The enthalpies of mixing of six kinds of aqueous amino acid solutions (glycine, l-alanine, l-valine, l-serine, l-threonine and l-proline) and aqueous 1,2-ethanediol solution and their respective enthalpies of dilution have been measured at 298.15 K using flow microcalorimetry. The experimental data have been analyzed in terms of the McMillan–Mayer formalism to obtain the heterotactic enthalpic interaction coefficients. The heterotactic enthalpic pairwise interaction coefficients, hxy , have been discussed from the points of view of solute–solute interactions. © 2006 Elsevier B.V. All rights reserved. Keywords: ␣-Amino acid; 1,2-Ethanediol; Heterotactic enthalpic pairwise interaction coefficient; Solute–solute interactions
1. Introduction Studies on various thermodynamic properties of amino acids and simple peptides in aqueous solutions of electrolyte [1,2] or organic substances [3–5] are of current interest due to their importance in the better understanding of the nature and mechanisms taking place in biological cells. Many studies have been done on the effects of polyols on the proteins and it was found that polyols help in stabilizing the native conformations of globular proteins [6–8]. Some authors correlate the stabilizing effect of sugars with the number and position of hydroxyl groups. However, our understanding of the stabilization mechanism of proteins is still incomplete. To understand the nature of interactions between sugars and proteins in aqueous solutions, it is necessary to study biochemical model compounds owing to the complex structure of the biological micromolecules. Amino acids are basic components of protein molecules and are considered to be the model compounds of protein molecules. The native structure of proteins is governed by weak, non-bonding interactions between the amino
∗
Corresponding author. Fax: +86 531 8856 4750. E-mail address: [email protected] (L. Yu).
0378-3812/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2006.12.003
acid residues and between these residues and the aqueous environment [9]. Although the polyols under investigation are not found in cellular or extracellular fluids of living organisms, they find wide application in pharmacoloty and the cosmetics industry. When introduced into a living organism as vehicles for pharmaceuticals or cosmetics, they affect the components of cellular fluids. This has been confirmed by numerous biochemical studies devoted to the interactions between polyols and components of biological cells [10]. In our previous studies, the enthalpies of dilution of amino acids in aqueous solutions of DMF, glucose, sucrose and urea [11–13], and the enthalpies of mixing amino acids with ethanol, chloroethanol, pyridine, methylpyridine, THF and 1,4dioxane [14–18] in water by the method of mixing flow microcalorimetry. The present work reports the results of calorimetric measurements of the mixing enthalpies of amino acids and 1,2-ethanediols aqueous solutions and their respective dilution enthalpies. It is well known that the enthalpic interaction coefficients derived from McMillan–Mayer’s model [19] can be considered as measures of intermolecular interactions in solution, and depend significantly on variation in solvent. This work is aimed at examining the heterotactic enthalpic interaction coefficients between amino acids and 1,2-ethanediol molecules.
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2. Experimental
To make the calculations easier, an auxiliary function H* has been introduced:
Glycine, l-alanine, l-valine, l-serine, l-threonine and lproline (BR) were purified by means of recrystallization using twice-distilled water. The purified amino acids were used after drying them in vacuum desiccators until their weights became constant. 1,2-Ethanediol (AR grade, from Shanghai Chem. Co.) used in the experiment was purified without further purification. Deionized water was distilled using a quartz sub-boiling purifier and stored in a CO2 -free atmosphere before use. Both the aqueous amino acid solutions and the aqueous 1,2-ethanediol solution were prepared by weight by Mettler AE 200 balance with a precision of ±0.0001 g. All the solutions were degassed and used within 12 h after preparation to minimize decomposition due to bacterial contamination. The calorimetric measurements were performed by a mixing–flow microcalorimeter (2277 Thermal Activity Monitor manufactured in Sweden). All the measurements were carried out at 298.15 K. The solutions were pumped through the mixing vessel of the calorimeter at constant rates using a pair of LKB-2132 microperpex peristaltic pumps. The flow rates were determined by weighing samples delivered in 8 min. The variation in flow rates was less than 0.1% both before and after a complete experiment. The error in measuring of the flow rates of solution in each experiment is less than 0.4%. The relative mean deviation of thermal powers determined three times was 0.3% and at last the relative mean deviations of the molar enthalpies of dilution and mixing were less than 1%. The experimental details have been reported in the earlier publications [14–18].
H ∗ = Hmix − Hdil (x) − Hdil (y) = H E (mx , my ) − H E (mx ) − H E (my )
(2)
where Hmix denotes the mixing enthalpy of the ternary solution, Hdil (x) and Hdil (y) are the dilution enthalpies of the corresponding binary solutions. The mixing enthalpy Hmix (J kg−1 ) of aqueous x solution and aqueous y solution is calculated from the equation: Hmix =
P∗ fx + fy − mx,i Mx fx − my,i My fy
(3)
where P* is the mixing thermal power (W) and fx , fy the flow rates of solutions x and y, respectively, and mx,i , my,i are the initial molalities of solutions x and y before mixing. The dilution enthalpies Hdil (J kg−1 ) are obtained by measuring thermal power P (W) and flow rates of solution and solvent (fA and fB , mg s−1 ): Hdil =
P fA + fB − mx,i Mx fA
(4)
where Mx is the molar mass of solute (kg mol−1 ) and mx,i is initial molality (mol g−1 ). The final molality mx (mol kg−1 ) may be calculated by using the equation: mx =
mx,i fA fB (mx,i Mx + 1) + fA
(5)
and combining Eqs. (1) and (2): 3. Results and discussion The enthalpies of mixing of aqueous amino acids solutions and aqueous 1,2-ethanediol solutions and their respective dilution enthalpies are given in Table 1 together with the initial and final molalities of the two solutes. According to the McMillian–Mayer formalism [19], the excess enthalpy of the ternary solution per 1 kg of water can be expressed as a power series in the molalities: H E (mx , my ) H(mx , my ) ∞ ∞ = − h∗w − mx Hx,m − my Hy,m w1 w1 = hxx m2x + 2hxy mx my + hyy m2y + hxxx m3x + 3hxxy m2x my + 3hxyy mx m2y + hyyy m3y + · · · (1) where H E (mx , my )/w1 and H(mx , my )/w1 represent the excess and the absolute enthalpy, respectively, of a solution containing 1 kg of water, mx mol of x and my mol of y, h∗w the stan∞ and H ∞ are dard enthalpy of 1 kg of pure water and Hx,m y,m the limiting partial molar enthalpies of species x and y, respectively. hij and hiij are the enthalpic pair, triplet, etc., interaction parameters. mx and my are the molalities of the solutes x and y, respectively.
H ∗ = 2hxy mx my + 3hxxy m2x my + 3hxyy mx m2y + · · · w1
(6)
The pairwise and triplet enthalpic interaction coefficients for the solutions studied are reported in Table 2. Since the triplet enthalpic interaction coefficients contain some contributions from the pairwise interaction terms, they are not discussed and only the pairwise coefficients hxy are considered in the present paper. The enthalpic interaction coefficients describe the total energetic effect of the interaction of heterotactic amino acid–1,2-ethanediol pair including the contribution of the solvent molecules surrounding the interacting solute molecules. The process of interaction of two solvated species can be represented as two successive stages: the partial dehydration of the solutes and the further direct interaction caused by the short-range molecular forces [20,21]. By and large, the interactions between ␣-amino acids and 1,2ethanediol in the aqueous solutions reflect three superimposed processes: the first is the partial dehydration of hydration shell of the ␣-amino acid zwitterions (making positive contributions to hxy ); the second is the partial dehydration of hydration shell of 1,2-ethanediol (making positive contributions to hxy ); and the third is the direct interaction, which plays the dominant role in the processes of interaction between ␣-amino acid and 1,2ethanediol molecules.
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L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32
Table 1 Enthalpies of dilution and mixing of aqueous amino acid solutions and aqueous 1,2-ethanediol solutions at 298.15 K mx,i (mol kg−1 )
my,i (mol kg−1 )
mx,f (mol kg−1 )
my,f (mol kg−1 )
Hdil(x) /w1 (J kg−1 )
Hdil(y) /w1 (J kg−1 )
Hmix /w1 (J kg−1 )
H ∗ /w1 (J kg−1 )
Glycine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0541 0.0811 0.0972 0.1079 0.1186 0.1347 0.1507 0.1613 0.1719 0.1879 0.2038 0.2144 0.2249 0.2407 0.2670
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
0.74 2.62 3.61 4.27 5.54 6.99 8.12 9.35 10.87 12.95 14.82 15.80 17.94 20.05 22.72
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
1.21 2.45 2.67 3.78 4.01 5.26 7.66 8.10 10.00 10.20 12.13 13.46 14.70 16.84 20.33
1.54 2.08 2.11 3.27 3.64 4.74 7.23 7.14 8.55 9.41 11.38 13.08 13.32 15.24 20.84
l-Alanine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0541 0.0810 0.0971 0.1078 0.1185 0.1344 0.1504 0.1716 0.1875 0.2033 0.2138 0.2243 0.2401 0.2662
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
−0.44 −1.43 −2.08 −2.27 −2.67 −3.13 −3.89 −5.76 −6.49 −8.20 −8.38 −9.55 −10.93 −14.28
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
0.19 0.17 2.67 1.55 −0.76 −1.12 −0.87 −0.74 −1.24 −0.30 0.03 −0.22 −0.19 −0.16
1.70 3.85 7.81 7.59 7.09 8.48 10.71 14.43 17.41 21.98 23.83 25.89 29.19 37.35
l-Valine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500
0.0540 0.0808 0.0968 0.1075 0.1181 0.1340 0.1499 0.1604 0.1709 0.1866 0.2023 0.2127 0.2231 0.2387
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016
−1.19 −4.42 −6.39 −8.03 −10.24 −12.55 −15.67 −18.07 −21.60 −24.16 −28.14 −30.07 −34.48 −43.14
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44
−0.33 −1.21 −1.77 −2.39 −3.60 −4.86 −5.08 −5.10 −5.90 −7.73 −9.97 −10.10 −10.74 −12.42
2.04 5.45 7.67 9.39 11.80 14.16 18.27 21.36 25.11 28.60 32.24 35.39 40.30 49.17
l-Serine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0541 0.0809 0.0969 0.1076 0.1183 0.1342 0.1501 0.1607 0.1712 0.1870 0.2027 0.2132 0.2236 0.2393 0.2653
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
1.01 4.34 5.81 7.84 8.76 10.73 12.22 15.05 17.21 19.98 24.15 26.00 30.10 33.10 36.37
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
2.30 4.54 5.73 6.83 8.78 12.02 15.27 15.58 18.70 20.79 24.40 26.73 29.49 33.29 39.33
2.35 2.44 2.97 2.75 5.19 7.77 10.73 8.92 10.90 12.97 14.33 16.15 15.95 18.63 26.19
L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32
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Table 1 (Continued ) mx,i (mol kg−1 )
my,i (mol kg−1 )
mx,f (mol kg−1 )
my,f (mol kg−1 )
Hdil(x) /w1 (J kg−1 )
Hdil(y) /w1 (J kg−1 )
Hmix /w1 (J kg−1 )
H ∗ /w1 (J kg−1 )
l-Threonine + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0540 0.0808 0.0968 0.1075 0.1181 0.1340 0.1498 0.1604 0.1709 0.1866 0.2022 0.2127 0.2231 0.2386 0.2644
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
0.25 0.82 1.05 1.65 1.71 1.95 2.13 2.53 3.01 2.35 3.19 3.35 3.62 3.09 4.71
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
1.33 2.25 2.70 3.40 3.53 4.59 5.75 7.11 8.68 9.04 10.77 12.05 12.91 15.15 17.88
2.15 3.68 4.71 5.51 6.99 9.12 11.30 12.96 15.09 18.85 21.65 24.12 25.86 30.50 36.41
l-Proline + 1,2-ethanediol 0.1000 0.1000 0.1500 0.1500 0.1800 0.1800 0.2000 0.2000 0.2200 0.2200 0.2500 0.2500 0.2800 0.2800 0.3000 0.3000 0.3200 0.3200 0.3500 0.3500 0.3800 0.3800 0.4000 0.4000 0.4200 0.4200 0.4500 0.4500 0.5000 0.5000
0.0540 0.0808 0.0969 0.1075 0.1181 0.1340 0.1499 0.1604 0.1710 0.1867 0.2024 0.2128 0.2232 0.2388 0.2647
0.0453 0.0679 0.0814 0.0904 0.0993 0.1128 0.1262 0.1351 0.1440 0.1574 0.1707 0.1795 0.1884 0.2016 0.2237
−0.93 −2.02 −2.59 −3.16 −3.87 −5.09 −5.96 −6.69 −7.27 −8.37 −13.94 −15.03 −17.98 −18.92 −23.54
−1.07 −2.24 −3.06 −3.76 −5.17 −6.47 −7.68 −8.38 −9.41 −12.16 −14.07 −15.42 −16.57 −18.44 −23.23
0.53 0.79 0.84 1.25 0.79 1.23 1.96 2.47 2.23 2.71 4.19 4.35 4.62 5.51 6.97
2.52 5.05 6.49 8.16 9.83 12.79 15.60 17.55 18.92 23.25 32.19 34.80 39.17 42.87 53.73
mx,i and my,i are the initial molalities of solutes x and y; m,x,f and my,f are the final molalities of solutes x and y.
The relative magnitude of hxy coefficients studied in this work mainly depends on the direct interactions between ␣-amino acids and 1,2-ethanediol. Since both the amino acid and 1,2ethanediol molecules have hydrophobic and hydrophilic groups, the direct interaction between the two kinds of molecules can be summarized as: (a) the hydrophobic–hydrophobic interaction (an endothermic process leading to a positive contribution to hxy ); (b) the hydrophobic–hydrophilic interaction (an endothermic process leading to a positive contribution to hxy ); (c) the hydrophilic–hydrophilic interaction (an exothermic process leading to a negative contribution to hxy ).
The resulting sign and magnitude of hxy would be a consequence of the competitive equilibrium between the above effects. For chain amino acids studied, the experimentally observed positive values of hxy testify to the predominance of endothermic processes over the effect of exothermic direct interaction of amino acid with 1,2-ethanediol. While for cyclic l-proline, the negative value of hxy indicates that the exothermic effect plays a significant role during the process of the interaction between it and 1,2-ethanediol. The differences of hxy coefficients of different kinds of ␣amino acids with 1,2-ethanediol are dramatically contingent on
Table 2 Heterotactic enthalpic interaction coefficients between amino acids and 1,2-ethanediol in aqueous solutions at 298.15 K Solutes x + y
hxy (J kg mol−2 )
hxxy × 10−4 (J kg2 mol−3 )
hxyy × 10−4 (J kg2 mol−3 )
S.D.a
R2b
Glycine + 1,2-ethanediol l-Alanine + 1,2-ethanediol l-Valine + 1,2-ethanediol l-Serine + 1,2-ethanediol l-Threonine + 1,2-ethanediol l-Proline + 1,2-ethanediol
221 (129c ) 272 (244) 992 (280) 544 (276) 37.4 (66.5) −45.7 (168)
−131 (171) −5.20 (21.4) 21.9 (10.6) 15.3 (11.9) 8.08 (1.99) 12.7 (8.80)
156 (204) 6.21 (2.54) 25.7 (12.4) 18.1 (14.1) −9.48 (2.33) −14.8 (10.3)
0.59 1.08 0.92 1.24 0.30 1.21
0.9916 0.9921 0.9968 0.9752 0.9994 0.9953
a b c
Standard deviation. Square of correlation coefficient. The estimated deviation.
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L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32
the discrepancies of the structure of the studied ␣-amino acids. Glycine, which has no side-chain, is the simplest amino acid in nature. Compared to glycine, l-alanine has one hydrogen atom on the ␣-carbon replaced by a methyl. For the above two kinds of amino acids, there are marked discriminations in the interaction (a) and (b) concerning the non-polar groups of ␣-amino acid, which make positive contributions to hxy . As the alkyl sidechain of amino acid is lengthened, the positive contributions made to the hxy coefficients of the interaction between amino acids and 1,2-ethanediol in aqueous solutions become larger. Therefore, the hxy coefficients are in the following sequence: hxy (l-alanine) > hxy (glycine). l-Serine is similar to l-alanine except that it has an OH group replacing a hydrogen atom of the methyl group, which strengthens its hydrophilic properties and enhances interactions (b) and (c). The relative magnitudes of the values of hxy for l-serine can be treated as a result of competition between interactions (b) and (c). The dominant role played by interaction (b) causes hxy (l-serine) > hxy (l-alanine). As to l-valine and l-serine, the side chain of l-valine can be considered as a substitute for one methyl group of the hydroxyl group and a replacement of one hydrogen atom by another methyl group for l-serine. The main differences of the interactions of l-valine and l-serine with 1,2-ethanediol lie in the following: there exist hydrophobic–hydrophobic and hydrophobic–hydrophilic interactions (both of them making positive contributions to hxy ) between the methyl groups of l-valine with the 1,2-ethanediol molecule, and there also exist hydrophilic–hydrophobic (making positive contributions to hxy ) and hydrophilic–hydrophilic interactions (making negative contributions to hxy ) between the hydroxyl group on the side chain of l-serine with the 1,2-ethanediol molecule. The comparative magnitude of the heterotactic enthalpic pairwise interaction coefficients of l-valine and l-serine with 1,2-ethanediol molecule depends on the competitive balance of the above varied interactions. In aqueous 1,2-ethanediol solutions, there exists hxy (l-valine) > hxy (l-serine), which shows the former yields very strong interaction effects. Thus, for the above unbranched long-chain amino acids, there exists the following rule: hxy (l-valine) > hxy (l-serine) > hxy (lalanine) > hxy (glycine). Since there is one methyl on the side chain of l-threonine as compared to l-serine, interactions (a) and (b) (making positive contribution to hxy ) are reinforced. But in the meantime, the steric effect, which can weaken the interaction between l-threonine and 1,2-ethanediol, becomes more remarkable because of the existence of the methyl. As a result of the balance between the above effects, the hxy values increase in the following order: hxy (l-serine) > hxy (l-threonine). l-Proline is a natural amino acid that has one pyrrole ring. Its special structure makes it important to the properties of polypeptide. On one hand, the cyclic structure weakens the attracting interactions between l-proline molecules due to the steric effect, which makes solvation easy. This indicates the absorbing heat of the dehydration process and the positive contribution to the
enthalpic pairwise interaction coefficient are relatively little. On the other hand, although both of l-proline and l-valine contain four carbon atoms, the cyclic structure of the former can weaken the interaction between it and 1,2-ethanediol molecule. As a result of the above two effects, the exothermic process dominates over the overall interaction process. And consequently, the value of hxy of l-proline with 1,2-ethanediol is negative. 4. Conclusions In aqueous solutions, the calorimetric measurements of the mixing enthalpies of six kinds of native ␣-amino acids (glycine, l-alanine, l-valine, l-serine, l-threonine and l-proline) and 1,2ethanediol and their respective dilution enthalpies were carried out at 298.15 K. These results served as a basis for calculations of the enthalpic coefficients of interactions between amino acids and 1,2-ethanediol in aqueous solution, according to McMillan–Mayer’s model. The heterotactic enthalpic pairwise interaction coefficients between amino acids and 1,2ethanediol molecules have been interpreted from the point of view of solute–solute interactions. Acknowledgements The authors are grateful to the Natural Science Foundation of Shandong Province of China (No. Q2003B01) and Excellent Young Scientist Awarded Fund of Shandong Province (No. 2005BS08009) for financial support. References [1] B. Pałecz, H. Piekarski, Fluid Phase Equilib. 164 (1999) 257–265. [2] B. Pałecz, Fluid Phase Equilib. 167 (2000) 253–261. [3] G. Barone, G. Castronuovo, P. Del Vecchio, C. Giancola, J. Chem. Soc., Faraday Trans. I 85 (1989) 2087–2097. [4] J. Fernandez, T.H. Lilley, J. Chem. Soc., Faraday Trans. 88 (1992) 2503–2509. [5] B. Pałecz, Fluid Phase Equilib. 126 (1996) 299–303. [6] R.B. Simpson, W. Kauzmann, J. Am. Chem. Soc. 75 (1953) 5139–5152. [7] P.H. Yancey, M.E. Clark, S.C. Hand, R.D. Bowlus, G.N. Somero, Science 217 (1982) 1214–1220. [8] T.S. Lakshmi, P.K. Nandi, J. Phys. Chem. 80 (1976) 249–252. [9] G. Castronuovo, V. Elia, F. Velleca, J. Chem. Soc., Faraday Trans. I 92 (1996) 4215–4219. [10] F. Fujita, Y. Noda, Bull. Chem. Soc. Jpn. 57 (1984) 1891–1896. [11] X.L. Ren, Y.M. Ni, R.S. Lin, Thermochim. Acta 348 (2000) 19–24. [12] S.Q. Li, X.G. Hu, R.S. Lin, Thermochim. Acta 342 (2000) 1–6. [13] S. Shao, X.G. Hu, R.S. Lin, Thermochim. Acta 360 (2000) 93–100. [14] L. Yu, R.S. Lin, X.G. Hu, G.Y. Xu, J. Solution Chem. 32 (2003) 273–281. [15] L. Yu, X.G. Hu, R.S. Lin, G.Y. Xu, J. Solution Chem. 33 (2004) 131–141. [16] L. Yu, X.G. Hu, R.S. Lin, G.Y. Xu, J. Chem. Thermodyn. 36 (2004) 483–490. [17] L. Yu, R.S. Lin, X.G. Hu, H.L. Zhang, G.Y. Xu, J. Chem. Eng. Data 48 (2003) 990–994. [18] L. Yu, S.L. Yuan, X.G. Hu, R.S. Lin, Chem. Eng. Sci. 61 (2006) 794–801. [19] W.G. MacMillan, J.E. Mayer, J. Chem. Phys. 13 (1945) 276–305. [20] K.G. Davis, T.H. Lilley, Thermochim. Acta 107 (1986) 267–276. [21] J.J. Savage, R.H. Wood, J. Solution Chem. 5 (1976) 733–750.