Temperature Dependences and Enthalpic Discrimination of

Article pubs.acs.org/jced

Temperature Dependences and Enthalpic Discrimination of Homochiral Pairwise Interactions of Six α‑Amino Acid Enantiomers in Pure Water Jia-min Liu, Li-yuan Zhu, Hua-qin Wang, Nan Chen, and Xin-gen Hu* College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China S Supporting Information *

ABSTRACT: Dilution enthalpies of six α-amino acid enantiomers (L-alanine vs D-alanine, L-serine vs D-serine, and L-proline vs D-proline) in pure water were determined at T = (288.15 to 323.15) K by isothermal titration calorimetry (ITC). Homochiral enthalpic pairwise interaction coefficients (hxx) at each temperature were evaluated according to the McMillan−Mayer theory. The hxx values of α-alanine and α-proline enantiomers are all positive (hLL > hDD > 0), while those of α-serine enantiomers are all negative (hLL < hDD < 0). In both cases, the hxx values increase uniformly and gradually with elevating temperatures. A slight but significant enthalpic discrimination effect is found in the homochiral pairwise interactions of these α-amino acid enantiomers. The positive growth of hxx values indicates that increasing temperature promotes the predominance of hydrophobic interactions in the binary aqueous solutions studied.

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been explored.22−34 In the present work, we have measured the dilution enthalpies of six enantiomers of typical α-amino acids (Scheme 1) in pure water at different temperatures by

INTRODUCTION As is well-known, the stability of biological macromolecules like proteins is controlled by various weak nonbonding interactions between α-amino acid residues and other biochemical components in aqueous solutions.1−3 Homotactic or heterotactic enthalpic pairwise interaction coefficients (hxx or hxy) obtained from the framework of the McMillan−Mayer theory are particularly useful for understanding solute−solute and solute−solvent noncovalent interactions.4,5 Over the past several decades, great efforts have been made by many research groups to gain insight into the thermodynamics of complex nonbonding interactions in biological model solution systems.6−14 However, most of the studies have been carried out only at T = 298.15 K, and therefore the temperature dependences of hxx or hxy coefficients and the relevant mechanisms have remained unknown or not very clear, though some attempts have been made for this problem.15−20 Considering that the first derivative of enthalpic pairwise interaction coefficient (h2) on temperature T ((∂h2/∂T)p) equals the second virial coefficient of isobaric heat capacity (cp,2), and the latter seems to be more sensitive to the structural change of water in the hydration shell of solutes,21 it is of greater interest to investigate the temperature dependence of h2. Recently, we have paid much attention to the solvent and substituent effects on homotactic enthalpic pairwise interactions of small biochemical compounds in aqueous mixed solvents containing highly polar cosolvents like dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) at 298.15 K, and the phenomena of enthalpic discrimination in these systems has © 2015 American Chemical Society

Scheme 1. Structures of Six Enantiomers of α-Alanine, αSerine, and α-Proline

isothermal titration calorimetry (ITC), and the temperature dependences and enthalpic discrimination of homochiral pairwise interactions are discussed mainly. We hope to gain insight into how the studied α-amino acids play key roles on the stability of proteins like collagens.35 Received: September 14, 2014 Accepted: March 19, 2015 Published: May 1, 2015 1242

DOI: 10.1021/je500825a J. Chem. Eng. Data 2015, 60, 1242−1249

Journal of Chemical & Engineering Data

Article

Table 1. Provenance and Mole Fraction Purities of the Samples Used chemical name

source

D-proline dimethyl sulfoxide (DMSO) N,N-dimethyllformamide (DMF)

J&K J&K J&K J&K Sigma Sigma Sigma-Aldrich Sigma-Aldrich

L-alanine D-alanine L-serine D-serine L-proline

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initial mole fraction purity 99 99 99 99 99 99 99 99

% % % % % % % %

purification method drying drying drying drying drying drying redistilled redistilled

EXPERIMENTAL SECTION The purities of eight α-amino acids (L-alanine, D-alanine; Lserine, D-serine; L-proline, D-proline) are all better than 99 % in mass fraction, and the former four were purchased from J&K, and the other four from Sigma (see Table 1). They were used without further purification except for drying over P2O5 in a vacuum desiccator for 48 h. The Milli-Q water (Millipore Elix5/Milli-Q Academic system) was used to prepare solutions by mass on a Sartorius balance with a precision of 0.1 mg. Before use, each solution was degassed by an ultrasonicator and then stored in a thermostatic bath to keep the temperature near to the temperature in the sample cell of ITC. To avoid possible expansion or contraction of titrant in the syringe after injection, the room temperature was controlled as near to the needed experimental temperature as possible by air conditioner only when it is not very far from 298.15 K. Since the studied temperature range is not very extensive (from 288.15 K to 323.15 K), and the titrant volume of each injection is very small (∼2 μL) compared with that of titrate in the sample cell (∼200 μL), the effect of thermal expansion and contraction was neglected in this work. The dilution enthalpies were determined at T = (288.15 to 323.15) K (5 K intervals) and atmospheric pressure (∼101.3 kPa) by an isothermal titration calorimeter (ITC200, MicroCal). Both the sample and reference cells were loaded with about 200 μL of pure water, and the 40 μL syringe was filled with aqueous amino acid solution as titrant. A titration run consisted of consecutive injections (N ≈ 20) of 2 μL aliquot and 5 s duration each, with an interval of 2 min between, from the rotating syringe into the vigorously stirred sample cell (syringe rotation, 1000 rpm). The heat effects per injection were determined by automatic peak integration of thermal power (P/μJ·s−1) vs time (t/min) curves. At each temperature under study, blank titrations were carried out by titrating pure water into pure water. No significant blank heats (QB) were found nearly at room temperature (∼298.15 K); therefore the heat from the friction in injection was considered to be negligible (at higher temperatures, blank corrections are necessary). Demarse36 has proposed chemical standards for calibration and testing of nanowatt isothermal titration calorimeters with overflow reaction vessels. However, considering that the required three calibrated parameters, namely the calorimetric factor (f), the buret injection volume (Vinj), and the reaction vessel effective volume (Vrv), are correlative quantities (independent variables) and therefore can not be obtained synchronously in a single chemical testing, we have not carried out such a calibration in accordance with the recommendation. The reaction cell volume was taken as 203.4 μL according to factory specifications. The feasibility of measuring dilution enthalpy by ITC has been checked carefully by us elsewhere and other authors.22,23,37,38

final mole fraction purity 99.2 99.1 99.1 99.2 99.3 99.2 99.8 99.8

analysis method

% % % % % % % %

HPLC HPLC HPLC HPLC HPLC HPLC GC GC

The value of molar dilution enthalpy per injection in ITC can be calculated by37 ΔH(mN − 1 → mN ) = ΔH(mN − 1 , mN )/nd

(1)

where N indicates the number of injections (N = 1, 2, 3...), mN−1 and mN are the molalities of solutions in the cell before and after the Nth injection, ΔH(mN−1, mN) is the overall dilution enthalpy diluted from mN−1 to mN, and nd is the moles of solute in each injection volume (Vinj/μL), which can be calculated as follows, nd = 10−6Vinjdsolb0

(2) −1

in which dsol and b0 is the density (g·mL ) and the concentration (mol·kg−1) of the solution in the syringe, respectively, and b0 is defined especially as the moles of solute in 1 kg of solution (rather than molality). Provided the solutions used are dilute enough, the densities of them can be assumed to be that of pure water. However, this approximation will lead to small systematic errors of the final calculation results.37 In this work, as the initial molalities of solutions (m0) in the syringe are about 0.6 mol·kg−1, the errors introduced are estimated to be less than 2 %. According to the McMillan−Mayer theory,4,5 the thermodynamic formula applied to deal with the excess enthalpy of a binary solution containing solute X and solvent Y can be expressed as a virial expansion, HE(mx ) = hxxmx + hxxxmx 2 + ...

(3)

in which mx is the molality of the solution, and hxx, hxxx, and so forth are known as pairwise, triplet, and higher-order enthalpic interaction coefficients (enthalpic virial coefficients), respectively. To evaluate these coefficients, dilution enthalpies of a binary solution (X + Y) are needed. Regression analysis of dilution enthalpies is based on the following linear equation.37 ΔH(mN − 1 → mN ) = 2hxxm1N − [hxx(m1 + m0) + hxxxm02] + ... (4)

All of the coefficients (hxx, hxxx) obtained by fitting were adopted for further discussion only when they are significant within 95 % confidence limits.

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RESULTS AND DISCUSSION All of the experimental dilution enthalpies of the six α-amino acid enantiomers in pure water at different temperatures T = (288.15 to 323.15) K and other experimental data necessary can be obtained from the Supporting Information appended (see Tables S1, S2, S3, and S4). As an example, the typical titration thermal power−time curve (P−t curve) of L-proline in pure water at T = 293.15 K, and the corresponding fitting plot 1243



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Figure 1. (a) Typical P−t curve of ITC for L-proline (m0 = 0.6035 mol·kg−1) diluted in pure water at T = 293.15 K and under p = 0.1 MPa, and (b) the corresponding linear fitting plot of ΔH(mN−1 → mN) vs N (R2 = 0.9967, SD = 1.0).

Table 2. Homochiral Enthalpic Pairwise Interaction Coefficients (hLL and hDD) of the Six α-Amino Acid Enantiomers in Pure Water at T = (288.15 to 323.15) K and under p = 0.1 MPaa,b,c,d hLL/J·kg·mol−2 T/K 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15

L-proline

L-alanine

229 256 285 291 298 301 303 307

95 (10) 135 (7) 173 (8) 207 (10) 237 (9) 276 (12) 316 (11) 342 (15)

(11) (9) (12) (10) (9) (15) (13) (16)

hDD/J·kg·mol−2 L-serine

−784 −666 −569 −490 −396 −323 −255 −205

(24) (23) (19) (21) (13) (16) (12) (10)

D-proline

D-alanine

201 218 244 248 251 260 272 301

88 (4) 129 (6) 167 (8) 197 (10) 226 (11) 266 (9) 304 (12) 329 (14)

(10) (12) (9) (8) (11) (13) (12) (14)

D-serine

−767 −650 −552 −466 −389 −306 −245 −188

(20) (31) (24) (17) (14) (15) (11) (9)

a The atmospheric pressure was determined by a Fortin barometer (FM-52, Russell Scientific Instruments). bThe values of hxx were calculated by least-squares fitting according to eq 4, and all the squares of correlation coefficients (R2) are better than 0.9900. cThe values in parentheses are the combined standard uncertainties (uc) of hxx values. dThe estimated standard uncertainties (u) are u(T) = 0.01 K, u(p) = 5 kPa.

of ΔH(mN−1 → mN) vs N is shown together in Figure 1. The values of hxx at each temperature for all the compounds were evaluated by regression analysis according to eq 4 and collected in Table 2. Since it is complicated to elucidate the third and higher-order coefficients which depends on two-body and nonadditive multiple-body interactions,39 only the pairwise interaction coefficient (hxx) is discussed here.40−43 From Table 2, we can find three main experimental behaviors of hxx as follows. (1) The hxx values of the α-alanine and αproline enantiomers are all positive (hxx = (88 to 342) J·kg· mol−2), signifying an endothermic and repulsive effect, while those of α-serine entantiomers are all negative (hxx = (−188 to −784) J·kg·mol−2), signifying an exothermic and attractive effect. (2) In both of the cases, the hxx values increase uniformly and gradually with the elevating temperatures. (3) The hxx values of L-alanine and L-proline are all slightly more positive than their D-enantiomers, i.e., hLL > hDD > 0, with a difference (hLL − hDD) of (6 to 47) J·kg·mol−2, while those of L-serine are slightly more negative than its D-enantiomer, i.e., hLL < hDD < 0, with a difference (hDD − hLL) of (7 to 24) J·kg·mol−2. Therefore, from the point of view of absolute value of hxx coefficients, we have the inequality |hLL| > |hDD| for all the three pairs of enantiomers under study. For making a comparison conveniently, the trends of hxx of the six α-amino acids with the temperatures were illustrated together in Figure 2.

Figure 2. Temperature dependences of homochiral enthalpic pairwise interaction coefficients (hxx) of the six α-amino acid enantiomers in pure water at T = (288.15 to 323.15) K and under p = 0.1 MPa.

1. Hydrophilic−Hydrophobic Equilibrium. The enthalpic pairwise interaction coefficient is the interaction energy between a pair of solute molecules when they approach each other from infinite distance in dilute solutions, accompanying the contribution from rearrangement of solvent molecules in 1244



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solvation shells.44,45 The sign and magnitude of hxx are determined by the competitive balance of all contributions from different interaction factors, mainly including (i) the direct solute−solute interactions caused by the short-range intermolecular forces, (ii) the restructuring of partial solvent molecules arising from the overlapping of solvation shells of solutes, and (iii) the change of solute−solvent interactions, etc.46−48 Therefore, the hxx coefficient is sensitive to the structural alteration of solutes and the perturbations of environmental factors such as medium, temperature, pressure, etc.49−51 Quantitatively, solute−solute interactions characterized by homotactic or heterotactic enthalpic pairwise coefficients (hxx or hxy) can be analyzed by considering the additivity of functional groups (or moieties) in solute molecules, which was first formulated by Savage and Wood and known as the SWAG additivity principle.47 According to the SWAG approach, the features of the second virial coefficients of thermodynamic functions between groups (or moieties) in a pair of interacting solute molecules can be generally classified as follows.52,53 (1) Pairwise interactions between groups (or moieties) with the same actions on water structure, i.e., both are hydrophilic structure-breakers (HIB) or hydrophobic structure-makers (HOM), are thermodynamically favorable. But if both are hydrophilic structure-makers (HIM), the interaction is thermodynamically unfavorable. (a) HIB−HIB interaction: gxx < 0, hxx < Tsxx < 0; (b) HOM−HOM interaction: gxx < 0, Tsxx > hxx > 0; (c) HIM−HIM interaction: gxx > 0, hxx > Tsxx > 0. (2) Pairwise interactions between groups (or moieties) with the opposite actions on water structure, i.e. one is a HIB, and the other is a HOM, are thermodynamically unfavorable. But when one is a HIM, and the other is a HOM, even if both have the same structure-making actions on water, the interaction is also thermodynamically unfavorable. (d) HIB−HOM interaction: gxx > 0, hxx > Tsxx > 0; (e) HIM−HOM interaction: gxx > 0, hxx > Tsxx > 0. Thermodynamic quantities gxx, hxx, and sxx correspond respectively to the second virial interaction coefficients of Gibbs free energy (G), enthalpy (H), and entropy (S), with the validity of relations sxx = −(∂gxx/∂T)p, hxx = −T2[∂(gxx/T)/ ∂T]p and gxx = hxx − Tsxx. It should be pointed out that thermodynamic inequalities of interaction (e) are hypothetical and need supporting experimental evidence. Because of the ubiquity of enthalpy−entropy compensation effects, the hxx coefficient (positive or negative) works directly as an indicator for the energetics (endothermic or exothermic effect) of intermolecular pairwise association in solutions. The positive hxx values of α-alanine and α-proline enantiomers reflect that interactions (b) ∼ (e) are predominant in solutions, while the negative hxx values of α-serine enantiomers reflect that interactions (a) are predominant in solutions. From the point of view of enthalpy, both α-alanine and α-proline are HOM solutes, while α-serine is a HIB solute. For zwitterions of simple α-amino acids in aqueous solutions, the most favorable configuration in pairwise interaction is considered to the sideon association, where the hydrated electropositive amino group (−NH4+) of one amino acid interacts directly with the hydrated electronegative carboxyl group (−COO−) of the second one. Besides the negative contribution from electrostatic interactions between hydrophilic zwitterions, both of the methyl side-chain of α-alanine and the five-membered ring side-chain of α-proline

behave certainly as stronger hydrophobic domains, and the HOM−HOM interactions make considerably positive contributions to hxx. With regard to α-serine, the hydroxyl sidechain of it serves as a stronger hydrophilic group, and the HIB− HIB interaction makes a considerably negative contribution to hxx. 2. Temperature Dependence of Pairwise Interactions. So far, the exact mechanism of the temperature dependence of hxx (or hxy) has remained unknown. Some authors have found that the hxx (or hxy) coefficients decrease almost linearly as the temperature increases,54−57 and others have found a totally opposite trend or some more complex (anomalous) behaviors.58−60 No matter what the findings are, they allow one to draw a tentative conclusion that both hydrophobic and hydrophilic interactions are responsible for the temperature dependence of enthalpic pairwise interactions. In this work, as can be seen from Figure 2, the hxx values of the six enantiomers all increase gradually with the temperatures. The behavior indicates that the increase in temperature indeed promotes the prevailing of hydrophobic interactions over hydrophilic interactions in the solutions, though for α-serine, hydrophilic interactions are predominant all along and play a decisive role in the temperature range under study. The positive growth trends of hxx of the two α-serine enantiomers are indubitable (Figure 2), so it is reasonable to believe that once the temperature is elevated up to a higher level, the hxx values of αserine would become positive. It has been argued that at elevated temperatures the association of hydrophobic−hydrophobic interactions will be enhanced in order to minimize the penalty of solvation entropy,61,62 which results in an additional positive contribution to hxx. However, it is risky for one to make such a prediction before further experiments which need higher-temperature calorimetry. As a matter of fact, the heat capacity change accompanying the interaction between two apolar groups is always positive, while that between two polar groups is usually negative.62 According to the thermodynamic equation, cp,xx = (∂hxx/∂T)p, the heat capacity pairwise interaction coefficient (cp,xx) can be calculated from the slope of the fitted line, hxx = a + bT. The fitted equations, correlation coefficients (R) and standard deviations (SD) obtained by the method of linear least-squares fitting for the six systems under study are shown in Table 3. Table 3. Linear Fitted Equations, Correlation Coefficients (R), and Standard Deviations (SD) of hxx vs T Curves for the Six α-Amino Acid Enantiomersa α-amino acids L-alanine D-alanine L-serine D-serine L-proline D-proline

fitted equations

R

SD

Δsxx/J·kg·mol−2·K−1

hxx = −1941 (± 36) + 7.1 (± 0.1)T hxx = −1888 (± 42) + 6.9 (± 0.1)T hxx = −5512 (± 225) + 17 (± 1)T hxx = −5461 (± 221) + 16 (± 1)T hxx = −325 (± 126) + 2.0 (± 0.4)T hxx = −494 (± 81) + 2.4 (± 0.3)T

0.9992

3.8

0.81

0.9988

4.5

0.79

0.9941

24

2.1

0.9942

23

1.9

0.8916

13

0.23

0.9661

8.6

0.28

The values in the brackets represent the errors of the fitted coefficients (within the 95 % confidence limits). a

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There exist better linearities between hxx and T in the systems of α-alanine and α-serine enantiomers, but the linearities in the systems of α-proline enantiomers are not very satisfactory (Table 3). To avoid possible measurement errors, we have remeasured the hxx values of the two α-proline enantiomers at T = (298.15 and 303.15) K, respectively for three times, and no significant difference in the results has been found. The cp,xx coefficients (the slopes of the fitted lines) of the six systems are all smaller positive values (cp,xx = (2.0 to 7.1) J·kg·mol−2·K−1) across the studied temperature range, exhibiting smaller temperature dependences toward positive direction. Sun and his co-workers have determined the hxx values of Lcystine in aqueous solutions of 1 mol·kg−1 NaOH, KOH, and HCl, respectively, and found that the smaller negative values of cp,xx range from (−12.6 to −15.3) J·kg·mol−2·K−1.64 It might be better to calculate varying cp,xx coefficients by piecewise linear fitting for the two systems of α-proline enantiomers. The hxx values of each α-proline enantiomer appear to go through a transitional region (or a turning point) at T ≈ 300 K (Figure 2). Such a behavior is somewhat similar to that of the heterotactic interactions (hxy) of urea (x) with two aromatic amino acids (y) (y = L-phenylalanine and L-histidine, respectively) in water, where the hxy coefficients all pass through a maximum at T = (300 to 308) K, and the abnormal change was considered be related to the temperature dependence of heat capacity of pure water.57−59 If this was true; however, all amino acids would show the same effects. Therefore, we prefer to believe that the rigid ring of proline (also phenylalanine and histidine) plays a key role in pairwise interaction, though the assumption needs more experimental evidence. In addition, according to the thermodynamic equation cp,xx = T(∂sxx/∂T)p, we have the relation Δsxx = cp,xx ln(T2/T1), provided that cp,xx is independent of temperature in the studied range. Thus, we can calculate the change of entropic pairwise interaction coefficient (sxx) from T1 (= 288.15 K) to T2 (= 323.15 K). The calculated Δsxx values of the six enantiomers are from (0.23 to 2.1) J·kg·mol−2·K−1 (Table 3). The smaller positive changes of sxx coefficients, i.e., Δsxx > 0, indicate that increasing temperature promotes the enhancement of entropic contribution on pairwise interaction. It should be noticed that for lack of the experimental data of gxx, the values of sxx cannot be calculated directly from the equation sxx = (hxx −gxx)/T. 3. Enthalpic Discrimination of Pairwise Interactions. For chiral recognition of α-amino acids in aqueous solutions, the presence of dipolar ions (zwitterions) is believed to be the determining factor.65−70 It is difficult to explain the differences in enthalpic pairwise interaction coefficients between two optical isomers by a statistical group additivity approach like SWAG.47 A reasonable interpretation for the enantiometric recognition in solutions is based on the model of “preferential configuration” proposed first by Castronuovo’s research group.71 The existence of “preferential configuration” between two hydrated interacting solute molecules allows the best juxtaposition of their hydrophilic and hydrophobic groups (or domains) in solutions. With the help of this model, they have rationalized the enthalpic discrimination (hLL ≡ hDD ≠ hLD) of some chiral molecular systems and the enthalpic pairwise interactions of other molecular systems.72−78 According to this model, HOM−HOM interactions can be strengthened by the interactions between hydrophilic groups in the same pair of molecules. However, among various possible interaction configurations of two homochiral hydrated α-amino acid molecules, only the one formed between the opposite charges

of zwitterions is hypothesized to be favorable and prevailing in aqueous solutions, resulting in the depression of the cooperativity of hydrophobic interaction. The possibility of detecting chiral recognition in solutions of small molecules by microcalorimetry has been our recent ongoing interest.22−34 We have found that, when homochiral pairwise interaction occurs in aqueous solutions containing highly polar aprotic cosolvents like dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), a small but significant energetic difference between S−S and R−R (or L−L and D−D) enantiomeric pairs can be found, i.e., |hSS| > |hRR| or |hLL| > |hDD|.22−34 In the present case, though the largest difference of hxx between L−L and D−D pairs of each α-amino acid is only up to 47 J·kg· mol−2, and the existence of possible experimental errors cannot be ruled out completely, we still believe that the enthalpic discrimination (i.e., |hLL| > |hDD|) is a real phenomenon for the systems under study. The L−L pair is considered to be the more preferentially selected configuration than the D−D pair because of the best juxtaposition of hydrophobic or hydrophilic side-chains on L-α-carbons, in coordination with electrostatic interaction between the “heads” of α-amino acids. Such a cooperative preferential configuration of L−L association pair in solution might be a holdover (molecular memory) from its pure solid or liquid state,79,80 and it should be deserving of further investigation.

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CONCLUSION Homochiral pairwise interactions of three enantiomeric pairs of α-amino acids in pure water at T = (288.15 to 323.15) K have been studied by isothermal titration calorimetry. The enthalpic pairwise interaction coefficient (hxx) of each enantiomer increases gradually with the elevated temperature, and the absolute values of L-enantiomer are all slightly larger than those of D-enantiomer across the whole studied temperature range. The temperature dependence of hxx is considered to be decided by the competitive equilibrium between hydrophobic and hydrophilic interactions. The increase in temperature promotes the prevailing of hydrophobic interactions over hydrophilic interactions in the solutions studied. The discrimination effect of hxx is attributed to be the result of cooperative preferential configuration of homochiral association pairs. The exact mechanisms of temperature dependence and chiral discrimination of hxx should be investigated further in future by more sensitive and effective methods in detail.

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ASSOCIATED CONTENT

S Supporting Information *

Tables SI-1, SI-2, and SI-3 include all the experimental dilution enthalpies of the six enantiomers of α-amino acids under study in pure water at T = (288.15 to 323.15) K respectively, as well as other necessary experimental data for the subsequent calculation. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ je500825a.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86 (0577) 86596022. Funding

The authors thank the National Natural Science Foundation of China (No. 21073132) for financial support. 1246



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Notes

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The authors declare no competing financial interest.

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