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Fluid Phase Equilibria 252 (2007) 28–32

Enthalpic interactions of some �-amino acids with1,2-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 Chinab 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 2006Available online 10 December 2006

bstract

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

queous 1,2-ethanediol solution and their respective enthalpies of dilution have been measured at 298.15 K using flow microcalorimetry. Thexperimental data have been analyzed in terms of the McMillan–Mayer formalism to obtain the heterotactic enthalpic interaction coefficients. Theeterotactic enthalpic pairwise interaction coefficients, hxy, have been discussed from the points of view of solute–solute interactions.

2006 Elsevier B.V. All rights reserved.

eywords: �-Amino acid; 1,2-Ethanediol; Heterotactic enthalpic pairwise interaction coefficient; Solute–solute interactions

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. Introduction

Studies on various thermodynamic properties of amino acidsnd simple peptides in aqueous solutions of electrolyte [1,2]r organic substances [3–5] are of current interest due to theirmportance in the better understanding of the nature and mech-nisms taking place in biological cells. Many studies have beenone on the effects of polyols on the proteins and it was foundhat polyols help in stabilizing the native conformations of glob-lar proteins [6–8]. Some authors correlate the stabilizing effectf sugars with the number and position of hydroxyl groups.owever, our understanding of the stabilization mechanism ofroteins is still incomplete.

To understand the nature of interactions between sugars androteins in aqueous solutions, it is necessary to study biochem-cal model compounds owing to the complex structure of theiological micromolecules. Amino acids are basic components

f protein molecules and are considered to be the model com-ounds of protein molecules. The native structure of proteins isoverned by weak, non-bonding interactions between the amino

∗ Corresponding author. Fax: +86 531 8856 4750.E-mail address: ylmlt@sdu.edu.cn (L. Yu).

atcctifi

378-3812/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.fluid.2006.12.003

cid residues and between these residues and the aqueous envi-onment [9].

Although the polyols under investigation are not found inellular or extracellular fluids of living organisms, they find widepplication in pharmacoloty and the cosmetics industry. Whenntroduced into a living organism as vehicles for pharmaceuticalsr cosmetics, they affect the components of cellular fluids. Thisas been confirmed by numerous biochemical studies devoted tohe interactions between polyols and components of biologicalells [10].

In our previous studies, the enthalpies of dilution of aminocids in aqueous solutions of DMF, glucose, sucrose andrea [11–13], and the enthalpies of mixing amino acids withthanol, chloroethanol, pyridine, methylpyridine, THF and 1,4-ioxane [14–18] in water by the method of mixing flowicrocalorimetry. The present work reports the results of calori-etric measurements of the mixing enthalpies of amino acids

nd 1,2-ethanediols aqueous solutions and their respective dilu-ion enthalpies. It is well known that the enthalpic interactionoefficients derived from McMillan–Mayer’s model [19] can be

onsidered as measures of intermolecular interactions in solu-ion, and depend significantly on variation in solvent. This works aimed at examining the heterotactic enthalpic interaction coef-cients between amino acids and 1,2-ethanediol molecules.

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L. Yu et al. / Fluid Phase

. Experimental

Glycine, l-alanine, l-valine, l-serine, l-threonine and l-roline (BR) were purified by means of recrystallization usingwice-distilled water. The purified amino acids were used afterrying them in vacuum desiccators until their weights becameonstant. 1,2-Ethanediol (AR grade, from Shanghai Chem. Co.)sed in the experiment was purified without further purification.eionized water was distilled using a quartz sub-boiling puri-er and stored in a CO2-free atmosphere before use. Both thequeous amino acid solutions and the aqueous 1,2-ethanediololution were prepared by weight by Mettler AE 200 balanceith a precision of ±0.0001 g. All the solutions were degassed

nd used within 12 h after preparation to minimize decomposi-ion due to bacterial contamination.

The calorimetric measurements were performed by aixing–flow microcalorimeter (2277 Thermal Activity Monitoranufactured in Sweden). All the measurements were carried

ut at 298.15 K. The solutions were pumped through the mix-ng vessel of the calorimeter at constant rates using a pair ofKB-2132 microperpex peristaltic pumps. The flow rates wereetermined by weighing samples delivered in 8 min. The vari-tion in flow rates was less than 0.1% both before and after aomplete experiment. The error in measuring of the flow ratesf solution in each experiment is less than 0.4%. The rela-ive mean deviation of thermal powers determined three timesas 0.3% and at last the relative mean deviations of the molar

nthalpies of dilution and mixing were less than 1%. The exper-mental details have been reported in the earlier publications14–18].

. Results and discussion

The enthalpies of mixing of aqueous amino acids solutionsnd aqueous 1,2-ethanediol solutions and their respective dilu-ion enthalpies are given in Table 1 together with the initial andnal molalities of the two solutes.

According to the McMillian–Mayer formalism [19], thexcess enthalpy of the ternary solution per 1 kg of water cane expressed as a power series in the molalities:

HE(mx, my)

w1= H(mx, my)

w1− h∗

w − mxH∞x,m − myH

∞y,m

= hxxm2x + 2hxymxmy + hyym

2y + hxxxm

3x

+ 3hxxym2xmy + 3hxyymxm

2y + hyyym

3y + · · ·

(1)

here HE(mx, my)/w1 and H(mx, my)/w1 represent the excessnd the absolute enthalpy, respectively, of a solution containingkg of water, mx mol of x and my mol of y, h∗

w the stan-ard enthalpy of 1 kg of pure water and H∞

x,m and H∞y,m are

he limiting partial molar enthalpies of species x and y, respec-ively. hij and hiij are the enthalpic pair, triplet, etc., interactionarameters. mx and my are the molalities of the solutes x and y,espectively.

otie

libria 252 (2007) 28–32 29

To make the calculations easier, an auxiliary function �H*

as been introduced:

H∗ = �Hmix − �Hdil(x) − �Hdil(y)

= HE(mx, my) − HE(mx) − HE(my) (2)

here �Hmix denotes the mixing enthalpy of the ternary solu-ion, �Hdil(x) and �Hdil(y) are the dilution enthalpies of theorresponding binary solutions.

The mixing enthalpy �Hmix (J kg−1) of aqueous x solutionnd aqueous y solution is calculated from the equation:

Hmix = P∗

fx + fy − mx,iMxfx − my,iMyfy

(3)

here P* is the mixing thermal power (�W) and fx, fy the flowates of solutions x and y, respectively, and mx,i, my,i are thenitial molalities of solutions x and y before mixing.

The dilution enthalpies �Hdil (J kg−1) are obtained by mea-uring thermal power P (�W) and flow rates of solution andolvent (fA and fB, mg s−1):

Hdil = P

fA + fB − mx,iMxfA(4)

here Mx is the molar mass of solute (kg mol−1) and mx,i isnitial molality (mol g−1).

The final molality mx (mol kg−1) may be calculated by usinghe equation:

x = mx,ifA

fB(mx,iMx + 1) + fA(5)

nd combining Eqs. (1) and (2):

�H∗

w1= 2hxymxmy + 3hxxym

2xmy + 3hxyymxm

2y + · · · (6)

The pairwise and triplet enthalpic interaction coefficients forhe solutions studied are reported in Table 2.

Since the triplet enthalpic interaction coefficients containome contributions from the pairwise interaction terms, theyre not discussed and only the pairwise coefficients hxy areonsidered in the present paper. The enthalpic interaction coef-cients describe the total energetic effect of the interaction ofeterotactic amino acid–1,2-ethanediol pair including the con-ribution of the solvent molecules surrounding the interactingolute molecules.

The process of interaction of two solvated species can beepresented as two successive stages: the partial dehydrationf the solutes and the further direct interaction caused by thehort-range molecular forces [20,21].

By and large, the interactions between �-amino acids and 1,2-thanediol in the aqueous solutions reflect three superimposedrocesses: the first is the partial dehydration of hydration shellf the �-amino acid zwitterions (making positive contributionso hxy); the second is the partial dehydration of hydration shell

f 1,2-ethanediol (making positive contributions to hxy); and thehird is the direct interaction, which plays the dominant rolen the processes of interaction between �-amino acid and 1,2-thanediol molecules.

30 L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32

Table 1Enthalpies 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-ethanediol0.1000 0.1000 0.0541 0.0453 0.74 −1.07 1.21 1.540.1500 0.1500 0.0811 0.0679 2.62 −2.24 2.45 2.080.1800 0.1800 0.0972 0.0814 3.61 −3.06 2.67 2.110.2000 0.2000 0.1079 0.0904 4.27 −3.76 3.78 3.270.2200 0.2200 0.1186 0.0993 5.54 −5.17 4.01 3.640.2500 0.2500 0.1347 0.1128 6.99 −6.47 5.26 4.740.2800 0.2800 0.1507 0.1262 8.12 −7.68 7.66 7.230.3000 0.3000 0.1613 0.1351 9.35 −8.38 8.10 7.140.3200 0.3200 0.1719 0.1440 10.87 −9.41 10.00 8.550.3500 0.3500 0.1879 0.1574 12.95 −12.16 10.20 9.410.3800 0.3800 0.2038 0.1707 14.82 −14.07 12.13 11.380.4000 0.4000 0.2144 0.1795 15.80 −15.42 13.46 13.080.4200 0.4200 0.2249 0.1884 17.94 −16.57 14.70 13.320.4500 0.4500 0.2407 0.2016 20.05 −18.44 16.84 15.240.5000 0.5000 0.2670 0.2237 22.72 −23.23 20.33 20.84

l-Alanine + 1,2-ethanediol0.1000 0.1000 0.0541 0.0453 −0.44 −1.07 0.19 1.700.1500 0.1500 0.0810 0.0679 −1.43 −2.24 0.17 3.850.1800 0.1800 0.0971 0.0814 −2.08 −3.06 2.67 7.810.2000 0.2000 0.1078 0.0904 −2.27 −3.76 1.55 7.590.2200 0.2200 0.1185 0.0993 −2.67 −5.17 −0.76 7.090.2500 0.2500 0.1344 0.1128 −3.13 −6.47 −1.12 8.480.2800 0.2800 0.1504 0.1262 −3.89 −7.68 −0.87 10.710.3200 0.3200 0.1716 0.1440 −5.76 −9.41 −0.74 14.430.3500 0.3500 0.1875 0.1574 −6.49 −12.16 −1.24 17.410.3800 0.3800 0.2033 0.1707 −8.20 −14.07 −0.30 21.980.4000 0.4000 0.2138 0.1795 −8.38 −15.42 0.03 23.830.4200 0.4200 0.2243 0.1884 −9.55 −16.57 −0.22 25.890.4500 0.4500 0.2401 0.2016 −10.93 −18.44 −0.19 29.190.5000 0.5000 0.2662 0.2237 −14.28 −23.23 −0.16 37.35

l-Valine + 1,2-ethanediol0.1000 0.1000 0.0540 0.0453 −1.19 −1.07 −0.33 2.040.1500 0.1500 0.0808 0.0679 −4.42 −2.24 −1.21 5.450.1800 0.1800 0.0968 0.0814 −6.39 −3.06 −1.77 7.670.2000 0.2000 0.1075 0.0904 −8.03 −3.76 −2.39 9.390.2200 0.2200 0.1181 0.0993 −10.24 −5.17 −3.60 11.800.2500 0.2500 0.1340 0.1128 −12.55 −6.47 −4.86 14.160.2800 0.2800 0.1499 0.1262 −15.67 −7.68 −5.08 18.270.3000 0.3000 0.1604 0.1351 −18.07 −8.38 −5.10 21.360.3200 0.3200 0.1709 0.1440 −21.60 −9.41 −5.90 25.110.3500 0.3500 0.1866 0.1574 −24.16 −12.16 −7.73 28.600.3800 0.3800 0.2023 0.1707 −28.14 −14.07 −9.97 32.240.4000 0.4000 0.2127 0.1795 −30.07 −15.42 −10.10 35.390.4200 0.4200 0.2231 0.1884 −34.48 −16.57 −10.74 40.300.4500 0.4500 0.2387 0.2016 −43.14 −18.44 −12.42 49.17

l-Serine + 1,2-ethanediol0.1000 0.1000 0.0541 0.0453 1.01 −1.07 2.30 2.350.1500 0.1500 0.0809 0.0679 4.34 −2.24 4.54 2.440.1800 0.1800 0.0969 0.0814 5.81 −3.06 5.73 2.970.2000 0.2000 0.1076 0.0904 7.84 −3.76 6.83 2.750.2200 0.2200 0.1183 0.0993 8.76 −5.17 8.78 5.190.2500 0.2500 0.1342 0.1128 10.73 −6.47 12.02 7.770.2800 0.2800 0.1501 0.1262 12.22 −7.68 15.27 10.730.3000 0.3000 0.1607 0.1351 15.05 −8.38 15.58 8.920.3200 0.3200 0.1712 0.1440 17.21 −9.41 18.70 10.900.3500 0.3500 0.1870 0.1574 19.98 −12.16 20.79 12.970.3800 0.3800 0.2027 0.1707 24.15 −14.07 24.40 14.330.4000 0.4000 0.2132 0.1795 26.00 −15.42 26.73 16.150.4200 0.4200 0.2236 0.1884 30.10 −16.57 29.49 15.950.4500 0.4500 0.2393 0.2016 33.10 −18.44 33.29 18.630.5000 0.5000 0.2653 0.2237 36.37 −23.23 39.33 26.19

L. Yu et al. / Fluid Phase Equilibria 252 (2007) 28–32 31

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-ethanediol0.1000 0.1000 0.0540 0.0453 0.25 −1.07 1.33 2.150.1500 0.1500 0.0808 0.0679 0.82 −2.24 2.25 3.680.1800 0.1800 0.0968 0.0814 1.05 −3.06 2.70 4.710.2000 0.2000 0.1075 0.0904 1.65 −3.76 3.40 5.510.2200 0.2200 0.1181 0.0993 1.71 −5.17 3.53 6.990.2500 0.2500 0.1340 0.1128 1.95 −6.47 4.59 9.120.2800 0.2800 0.1498 0.1262 2.13 −7.68 5.75 11.300.3000 0.3000 0.1604 0.1351 2.53 −8.38 7.11 12.960.3200 0.3200 0.1709 0.1440 3.01 −9.41 8.68 15.090.3500 0.3500 0.1866 0.1574 2.35 −12.16 9.04 18.850.3800 0.3800 0.2022 0.1707 3.19 −14.07 10.77 21.650.4000 0.4000 0.2127 0.1795 3.35 −15.42 12.05 24.120.4200 0.4200 0.2231 0.1884 3.62 −16.57 12.91 25.860.4500 0.4500 0.2386 0.2016 3.09 −18.44 15.15 30.500.5000 0.5000 0.2644 0.2237 4.71 −23.23 17.88 36.41

l-Proline + 1,2-ethanediol0.1000 0.1000 0.0540 0.0453 −0.93 −1.07 0.53 2.520.1500 0.1500 0.0808 0.0679 −2.02 −2.24 0.79 5.050.1800 0.1800 0.0969 0.0814 −2.59 −3.06 0.84 6.490.2000 0.2000 0.1075 0.0904 −3.16 −3.76 1.25 8.160.2200 0.2200 0.1181 0.0993 −3.87 −5.17 0.79 9.830.2500 0.2500 0.1340 0.1128 −5.09 −6.47 1.23 12.790.2800 0.2800 0.1499 0.1262 −5.96 −7.68 1.96 15.600.3000 0.3000 0.1604 0.1351 −6.69 −8.38 2.47 17.550.3200 0.3200 0.1710 0.1440 −7.27 −9.41 2.23 18.920.3500 0.3500 0.1867 0.1574 −8.37 −12.16 2.71 23.250.3800 0.3800 0.2024 0.1707 −13.94 −14.07 4.19 32.190.4000 0.4000 0.2128 0.1795 −15.03 −15.42 4.35 34.800.4200 0.4200 0.2232 0.1884 −17.98 −16.57 4.62 39.170.4500 0.4500 0.2388 0.2016 −18.92 −18.44 5.51 42.87

4

m al mo

maets(hmtl

qFipan

TH

S

Glllll

0.5000 0.5000 0.2647 0.2237 −23.5

x,i and my,i are the initial molalities of solutes x and y; m,x,f and my,f are the fin

The relative magnitude of hxy coefficients studied in this workainly depends on the direct interactions between �-amino

cids and 1,2-ethanediol. Since both the amino acid and 1,2-thanediol molecules have hydrophobic and hydrophilic groups,he direct interaction between the two kinds of molecules can beummarized as: (a) the hydrophobic–hydrophobic interactionan endothermic process leading to a positive contribution to

xy); (b) the hydrophobic–hydrophilic interaction (an endother-ic process leading to a positive contribution to hxy); (c)

he hydrophilic–hydrophilic interaction (an exothermic processeading to a negative contribution to hxy).

ai

a

able 2eterotactic enthalpic interaction coefficients between amino acids and 1,2-ethanedio

olutes x + y hxy (J kg mol−2) hxxy × 10−4 (J kg2

lycine + 1,2-ethanediol 221 (129c) −131 (171)-Alanine + 1,2-ethanediol 272 (244) −5.20 (21.4)-Valine + 1,2-ethanediol 992 (280) 21.9 (10.6)-Serine + 1,2-ethanediol 544 (276) 15.3 (11.9)-Threonine + 1,2-ethanediol 37.4 (66.5) 8.08 (1.99)-Proline + 1,2-ethanediol −45.7 (168) 12.7 (8.80)

a Standard deviation.b Square of correlation coefficient.c The estimated deviation.

−23.23 6.97 53.73

lalities of solutes x and y.

The resulting sign and magnitude of hxy would be a conse-uence of the competitive equilibrium between the above effects.or chain amino acids studied, the experimentally observed pos-

tive values of hxy testify to the predominance of endothermicrocesses over the effect of exothermic direct interaction ofmino acid with 1,2-ethanediol. While for cyclic l-proline, theegative value of hxy indicates that the exothermic effect plays

significant role during the process of the interaction between

t and 1,2-ethanediol.The differences of hxy coefficients of different kinds of �-

mino acids with 1,2-ethanediol are dramatically contingent on

l in aqueous solutions at 298.15 K

mol−3) hxyy × 10−4 (J kg2 mol−3) S.D.a R2b

156 (204) 0.59 0.99166.21 (2.54) 1.08 0.9921

25.7 (12.4) 0.92 0.996818.1 (14.1) 1.24 0.9752−9.48 (2.33) 0.30 0.9994

−14.8 (10.3) 1.21 0.9953

3 Equi

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2 L. Yu et al. / Fluid Phase

he discrepancies of the structure of the studied �-amino acids.lycine, which has no side-chain, is the simplest amino acid inature. Compared to glycine, l-alanine has one hydrogen atomn the �-carbon replaced by a methyl. For the above two kindsf amino acids, there are marked discriminations in the inter-ction (a) and (b) concerning the non-polar groups of �-aminocid, which make positive contributions to hxy. As the alkyl side-hain of amino acid is lengthened, the positive contributionsade to the hxy coefficients of the interaction between amino

cids and 1,2-ethanediol in aqueous solutions become larger.herefore, the hxy coefficients are in the following sequence:xy (l-alanine) > hxy (glycine).

l-Serine is similar to l-alanine except that it has an OH groupeplacing a hydrogen atom of the methyl group, which strength-ns its hydrophilic properties and enhances interactions (b) andc). The relative magnitudes of the values of hxy for l-serine cane treated as a result of competition between interactions (b)nd (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

e considered as a substitute for one methyl group of theydroxyl group and a replacement of one hydrogen atom bynother methyl group for l-serine. The main differences ofhe interactions of l-valine and l-serine with 1,2-ethanediolie in the following: there exist hydrophobic–hydrophobicnd hydrophobic–hydrophilic interactions (both of them mak-ng positive contributions to hxy) between the methyl groupsf l-valine with the 1,2-ethanediol molecule, and there alsoxist hydrophilic–hydrophobic (making positive contributionso hxy) and hydrophilic–hydrophilic interactions (making neg-tive contributions to hxy) between the hydroxyl group onhe side chain of l-serine with the 1,2-ethanediol molecule.he comparative magnitude of the heterotactic enthalpic pair-ise interaction coefficients of l-valine and l-serine with,2-ethanediol molecule depends on the competitive balance ofhe above varied interactions. In aqueous 1,2-ethanediol solu-ions, there exists hxy (l-valine) > hxy (l-serine), which showshe former yields very strong interaction effects.

Thus, for the above unbranched long-chain amino acids, therexists the following rule: hxy (l-valine) > hxy (l-serine) > hxy (l-lanine) > hxy (glycine).

Since there is one methyl on the side chain of l-threonines compared to l-serine, interactions (a) and (b) (making pos-tive contribution to hxy) are reinforced. But in the meantime,he steric effect, which can weaken the interaction between-threonine and 1,2-ethanediol, becomes more remarkableecause of the existence of the methyl. As a result of the bal-nce between the above effects, the hxy values increase in theollowing order: hxy (l-serine) > hxy (l-threonine).

l-Proline is a natural amino acid that has one pyrrole ring. Itspecial structure makes it important to the properties of polypep-

ide. On one hand, the cyclic structure weakens the attractingnteractions between l-proline molecules due to the steric effect,hich makes solvation easy. This indicates the absorbing heatf the dehydration process and the positive contribution to the

[[[[

libria 252 (2007) 28–32

nthalpic pairwise interaction coefficient are relatively little. Onhe other hand, although both of l-proline and l-valine con-ain four carbon atoms, the cyclic structure of the former caneaken the interaction between it and 1,2-ethanediol molecule.s a result of the above two effects, the exothermic process dom-

nates over the overall interaction process. And consequently, thealue of hxy of l-proline with 1,2-ethanediol is negative.

. Conclusions

In aqueous solutions, the calorimetric measurements of theixing enthalpies of six kinds of native �-amino acids (glycine,

-alanine, l-valine, l-serine, l-threonine and l-proline) and 1,2-thanediol and their respective dilution enthalpies were carriedut at 298.15 K. These results served as a basis for calcu-ations of the enthalpic coefficients of interactions betweenmino acids and 1,2-ethanediol in aqueous solution, accord-ng to McMillan–Mayer’s model. The heterotactic enthalpicairwise interaction coefficients between amino acids and 1,2-thanediol molecules have been interpreted from the point ofiew of solute–solute interactions.

cknowledgements

The authors are grateful to the Natural Science Foundationf Shandong Province of China (No. Q2003B01) and Excel-ent Young Scientist Awarded Fund of Shandong Province (No.005BS08009) for financial support.

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