Enthalpic Interaction Coefficients Research Articles

Enthalpies of dilution of amino alcohols in aqueous solutions at 298.15 K

The enthalpies of dilution of aqueous solutions of the amino alcohols: 3-amino-1-propanol, (RS)-1-amino-2-propanol, (RS)-3-amino-1,2-propanediol, and 1,3-diamino-2-propanol were measured as a function of concentration at 298.15 K using a Thermal Activity Monitor (TAM). The results of the experimental measurements were treated according to the McMillan-Mayer theory, to obtain the enthalpic homogeneous interaction coefficients. The pair interaction coefficient hxx shows a clear dependence on the position and number of polar groups. The enthalpic coefficients are discussed in terms of the molecular interaction between solvated solute molecules.

Interactions of glycine and its oligomers with the trimethylamine N-oxide and urea in aqueous solution at T = 298.15 K: Enthalpic measurement and computer simulation

Enthalpies of solution of glycine and its oligomers in aqueous solutions of trimethylamine N-oxide (TMAO) or urea have been measured by calorimetry at 298.15 K. The results obtained were used to calculate the heterogeneous enthalpic interaction coefficients between glycine and its oligomers and the molecule of TMAO or urea in water and the transfer enthalpies (ΔtrHm) of the glycine and its oligomers from water to aqueous solutions of TMAO or urea. The results are discussed in terms of solute-solute and solute-solvent interactions. The influence of trimethylamine N-oxide (TMAO) and urea on the structure of water has been investigated by molecular dynamics simulation. The results show that water affected by TMAO forms stronger H-bonds and is more ordered than pure water. The results obtained by molecular dynamics simulation support transfer enthalpic changing trend of glycine and its oligomers in aqueous solutions of trimethylamine N-oxide (TMAO) or urea.

Effects of l-α-amino acids side chains on their interparticle interactions with the dissociated potassium chloride in aqueous solutions

Dissolution enthalpies of several natural amino acids (l-α-asparagine, l-α-glutamine, l-α-aspartic acid, l-α-glutamic acid, l-α-arginine, l-α-lysine and l-α-histidine) were measured in aqueous solutions of potassium chloride at T = 298.15 K. Based on the resulting data the standard dissolution enthalpy of amino acids in aqueous solutions of potassium chloride have been determined. The standard dissolution enthalpy values were subsequently used to calculate the heterogeneous enthalpic pair interaction coefficients (hA-KCl) between zwitterions of l-α-amino acids and dissociated potassium chloride in water. Interparticle interactions in system (amino acid + dissociated KCl + water) were interpreted in terms of the hydrophobic - hydrophilic effects of the side chains of the tested encoded amino acids.

Understanding the interactions of polyhydroxy solutes with ammonium salts in aqueous solutions via calorimetric and spectroscopic studies at different temperatures

Enthalpy change, q of polyhydroxy solutes; (+)-d-xylose, xylitol, (−)-l-sorbose, d-sorbitol, (+)-d-glucose, and (+)-d-maltose monohydrate have been measured in water and in aqueous (0.10, 0.15, 0.25, and 0.35) mol·kg−1 ammonium bromide (NH4Br) and (0.15, 0.25, and 0.35) mol·kg−1 tetraethylammonium bromide (Et4NBr) solutions at T=(288.15, 298.15, 308.15, and 318.15) K by using isothermal titration microcalorimeter. From the data q, the limiting standard enthalpies of dilution (ΔdilH°) and their corresponding transfer parameters (ΔtΔdilH°) have been evaluated. The enthalpic interaction coefficients that characterize the interactions of polyhydroxy solutes with ammonium salts in aqueous solutions were determined from ΔtΔdilH° values. Proton (1H) NMR spectra were recorded for mB=(0.15, 0.25 and 0.35) mol·kg−1 tetraethylammonium bromide solutions and in ternary solutions {polyhydroxy solute+Et4NBr+9:1 (w/w) H2O+D2O}, where the molality of solute has been fixed as mA=0.25mol·kg−1. The comparison of NMR results shows more upfield shift in ternary solutions than in pure alkyl ammonium salt solution, thereby suggesting the predominance of hydrophobic-ionic interactions over hydrophilic-ionic interactions.

Thermodynamic Difference between Protocatechualdehyde and p-Hydroxybenzaldehyde in Aqueous Sodium Chloride Solutions

The enthalpies of dilution of protocatechualdehyde and p-hydroxybenzaldehyde in the aqueous sodium chloride solutions were measured by using a mixing-flow microcalorimeter at 298.15 K. Densities of the ternary homogeneous systems at different temperatures (293.15, 298.15, 303.15, 308.15, and 313.15 K) were also measured with a quartz vibrating-tube densimeter. The homogeneous enthalpic interaction coefficients (h2, h3, and h4) were calculated according to the excess enthalpy concept based on the calorimetric data. The apparent molar volumes (Vϕ) and standard partial molar volumes (Vϕ0) of the investigated system were computed from their density data. The variation trends in h2 and Vϕ0 with increasing salt molality were obtained and discussed in terms of the (solute + solute) and (solute + solvent) interactions. The experimental results showed that the molecular structures of protocatechualdehyde and p-hydroxybenzaldehyde, especially the number of hydroxyl groups, have evident influence on their thermodynamic properties. The thermodynamic data obtained in this work may be helpful for exploring the structure–function relationship of protocatechualdehyde and p-hydroxybenzaldehyde.

Investigations to explore interactions in (polyhydroxy solute + l-ascorbic acid + H2O) solutions at different temperatures: Calorimetric and viscometric approach

Isothermal titration micro-calorimeter has been used to measure the enthalpy change (q) of polyhydroxy solutes [(+)-d-xylose, xylitol, (+)-d-glucose, 2-deoxy-d-glucose, (+)-methyl-α-d-glucopyranoside, and (+)-maltose monohydrate] in water and in (0.05, 0.15, and 0.25)mol·kg−1l-ascorbic acid(aq) solutions at (288.15, 298.15, 308.15, and 318.15)K. Limiting enthalpies of dilution (ΔdilH°) of these solutes were calculated from heat evolved/absorbed during calorimetric experiments. Further thermodynamic quantities such as limiting enthalpies of dilution of transfer (ΔtrΔdilH°), change in heat capacity (ΔdilCop,2,m), and pair (hAB) and triplet (hABB) enthalpic interaction coefficients were also calculated and used to explore the nature of interactions of solutes with cosolute (l-ascorbic acid). The Jones-Dole viscosity B-coefficients for (+)-d-xylose, xylitol, (+)-d-galactose, galactitol, (+)-d-glucose, 2-deoxy-d-glucose, (+)-methyl-α-d-glucopyranoside, and (+)-maltose monohydrate in water and in (0.05, 0.15, 0.25, and 0.35)mol·kg−1l-ascorbic acid(aq) solutions have been determined from viscosity (η) data measured over temperature range (288.15–318.15)K and at pressure, P=101.3kPa. The temperature dependence of B-coefficients (dB/dT), and viscosity B-coefficients of transfer (ΔtrB) of solutes from water to cosolute have also been estimated. These parameters have been discussed in terms of structure-making (kosmotropic) or -breaking (chaotropic) behavior of solutes.

Solvent and pH Dependences of Mixing Enthalpies of N-Glycylglycine with Protocatechuic Acid

Protocatechuic acid (PA) is a natural phenolic compound which has been proven to have chemopreventive property against chemically induced carcinogenesis. The mixing enthalpies of PA with N-glycylglycine in sodium phosphate and potassium phosphate buffer solutions with different pH values have been investigated by mixing-flow isothermal microcalorimetry at T = 298.15 K. The heterotactic enthalpic interaction coefficients (hxy) in the pH range of phosphate buffer solution from 3.0 to 8.0 have been calculated according to the McMillan–Mayer theory. Trends of the enthalpic pairwise interaction coefficients (hxy) with increasing pH in both phosphate buffer solutions were obtained. The solvent and pH dependence of the hxy were discussed in terms of molecular interactions between solvated solute molecules.

Thermodynamics of DL-alalnyl-DL-asparagine dissolution in water–organic mixtures at 298.15 K

Enthalpies of dissolution of DL-α-alanyl-DL-α-asparagine (AlaAsn) in aqueous solutions of acetonitrile, 1,4-dioxane, acetone, and dimethyl sulfoxide are measured via calorimetry at T = 298.15 K and organic solvent concentrations x2 = 0–0.3 mole fractions. Standard values of the enthalpy of dissolution (ΔsolH°), transfer (ΔtrH°) of AlaAsn from water into mixed solvent, and enthalpic pair interaction coefficients (hxy) of AlaAsn with molecules of organic solvents are calculated on the basis of experimental data. The effect the composition of a water–organic mixture has on the enthalpy characteristics of AlaAsn is considered. The correlation between the enthalpy of dissolution and the transfer of AlaAsn and the structure and properties of the solvents used is demonstrated.

Simultaneous determination of enthalpies of dilution and dissociation of protocatechuic acid in aqueous salt solutions by calorimetric measurements

Protocatechuic acid, a major metabolite of antioxidant polyphenols found in green tea, has been shown to prevent carcinogenesis or antitumor growth in vitro and in vivo studies. The dilution enthalpies ΔdilHm, dissociation enthalpies ΔdisHm and thermodynamic dissociation constants Ka for protocatechuic acid in aqueous NaCl or KCl solutions were simultaneously determined by mixing-flow microcalorimetry at T=298.15K. In order to verify the reliability of the fitted dissociation parameters, the values of dissociation enthalpies and thermodynamic dissociation constants were also determined by isothermal titration calorimetry and electrical conductivity methods. The Ka values obtained through the proposed method agree well with those reported in literatures and obtained by other techniques. Enthalpic interaction coefficients (h2, h3 and h4) were computed according to the McMillan–Mayer model. The trends of h2 and ΔdisHm for protocatechuic acid with increasing salt molality in both the salt solutions were obtained. The different influence of KCl and NaCl on the values of h2 and ΔdisHm were discussed in terms of (solute+solute) and (solute+solvent) interactions. The results showed that it is possible to perform in a single calorimetric experiment the simultaneous determination of dilution enthalpies, dissociation enthalpies and constants in a given solvent.

The conformational stability of ovalbumin and lysozyme in the aqueous solutions of various cosolvents

A thermodynamic method is reported to monitor the conformational stability of lysozyme and ovalbumin in the presence of various cosolvents. Heats of dilution of the proteins in concentrated aqueous solutions of urea, ethanol or glucose have been determined at 298.15 K by flow microcalorimetry. The pairwise enthalpic interaction coefficients of the proteins in the different solvent media are derived: They allow to gain information about the influence of the cosolvents on the interactions between two interacting hydrated molecules of a protein, hence on its conformational stability. The two proteins behave very differently in the various cosolvents. In glucose, the coefficients for ovalbumin are positive up to 3 mol kg−1 of cosolvent and then negative, while those for lysozyme are negative up to 6 mol kg−1. In urea, coefficients for ovalbumin are positive, while negative for lysozyme up to 7 mol kg1. In ethanol, coefficients for ovalbumin are almost invariant, even at the highest concentrations of cosolvent, underlining that the hydration shell of the protein is such to maintain essentially unaltered the native conformation. For lysozyme, coefficients are negative and almost invariant up to 20 mol kg−1 ethanol: Then, a jump occurs toward much more large and negative values. The observed behaviors are rationalized also on the basis of the results previously obtained for small model molecules in concentrated solutions of urea, ethanol or glucose. The differences between the two proteins are explained in terms of the effects of the cosolvents on hydrophilic and hydrophobic interactions and account for the structural characteristics of each protein. In fact, notwithstanding both are globular proteins, they are differently packed and that could make them to react differently toward the action of a given cosolvent.

Temperature Dependences and Enthalpic Discrimination of Homochiral Pairwise Interactions of Six α-Amino Acid Enantiomers in Pure Water

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.

Energetics of pairwise interaction between glycidol enantiomers in (dimethylformamide + water) mixtures rich in water at T = 298.15 K

Successive dilution enthalpies (ΔH(mN-1→mN)) of glycidol enantiomers (R-(+)-glycidol, S-(−)-glycidol) and the racemate (R/S-(±)-glycidol) in (dimethylformamide (DMF)+water) mixtures rich in water (mass fractions of DMF, wDMF=0 to 0.25) have been determined respectively by isothermal titration calorimetry (ITC) at T=298.15 K. The corresponding homotactic enthalpic pairwise interaction coefficients (hXX) of each compound have been evaluated from the framework of McMillan–Mayer theory. Across the studied composition range of (DMF+water) mixtures the hXX values of these compounds are all positive, and decrease gradually with the mass fraction of DMF by the order hRR>hRS≈hMM>hSS>0 (M represents the racemate R/S-(±)-glycidol, and hRS is the estimated value for the heterochiral pair R–S). The positive values of hXX indicate that pairwise interactions of these compounds are endothermic and unfavourable from the point of view of enthalpy. The interaction of S–S pair is considered to be slightly stronger than M–M, R–S and R–R pairs since the former absorbs less heat than the latter in pairwise interactions. The addition of cosolvent (DMF) is in favour of pairwise interactions of these compounds in the mixtures rich in water.

Enthalpic Interactions of HMBA with Glycine, l-Alanine, and l-Serine in Aqueous d-Mannitol Solutions at 298.15 K

The mixing enthalpies of N,N′-hexamethylenebisacetamide (HMBA) with glycine, l-alanine, and l-serine in d-mannitol solutions with different molar fractions have been determined by means of mixing-flow isothermal microcalorimetry at 298.15 K. The heterotactic enthalpic interaction coefficients (hxy, hxxy, and hxyy), in the molar fraction (x) range of d-mannitol from (0 to 0.0159), have been calculated in terms of McMillan–Mayer theory. The trends of hxy with the molar fractions of d-mannitol have been discussed based on the interactions between solute molecules under the participation of solvent molecules. The enthalpic contribution of the functional groups of solute molecules to hxy has been estimated according to the Savage–Wood additivity group (SWAG) approach. The influence of structural difference between glucose and d-mannitol on the hxy values of HMBA with glycine has been elucidated by combining the present results with our previous work.

PH effect on the enthalpy of dilution and volumetric properties of protocatechuic acid at T = 298.15 K

The enthalpies of dilution of protocatechuic acid, a natural anti-cancer substance, in sodium phosphate and potassium phosphate buffer solutions with different pH values were measured by using a mixing-flow microcalorimeter at T=298.15K. Densities of the pseudo binary system (phosphate buffer+protocatechuic acid) were also measured with a quartz vibrating-tube densimeter. The enthalpic interaction coefficients (h2, h3 and h4) were computed according to the McMillan–Mayer model. Apparent molar volumes of the system were calculated from the data of densities, which have been used to deduce limiting partial molar volumes (Vϕ0) of protocatechuic acid at different pH values. The aim of the experiments and data process is to investigate the interaction between the molecules of the important drug and that of the drug molecule with coexistent species in aqueous solutions as well as the influences on these interactions of such factors as pH and ion strength. Change trends of the enthalpic pair wise interaction coefficient h2 and Vϕ0 of protocatechuic acid with pH increasing in the both phosphate buffer solutions were obtained. The thermodynamic properties, h2 and Vϕ0 in potassium phosphate buffer solutions were compared with those in sodium phosphate buffer solutions at identical pH. The trends and the differences have been discussed in terms of the (solute+solute) and (solute+solvent) interactions.

Transfer enthalpies of amino acids and glycine peptides from water to aqueous solutions of trimethylamine N-oxide at 298.15 K

• Enthalpies of solution have been measured by calorimetry at 298.15 K. • The heterogeneous enthalpic interaction coefficients were calculated. • The results are discussed in terms of solute–solute and solute–solvent interactions. • The structure of amino acid has a more important effect on the interaction. Enthalpies of solution of glycine, l -alanine, l -serine, l -threonine, l -valine, diglycine and triglycine in aqueous solutions of trimethylamine N -oxide (TMAO) have been measured by calorimetry at 298.15 K. The results obtained were used to calculate the heterogeneous enthalpic interaction coefficients ( h AT ) between the amino acids or glycine peptides and the molecule of TMAO in water and the transfer enthalpies Δ tr H m of the amino acids or glycine peptides from water to aqueous solutions of TMAO. It has been observed that the transfer enthalpies of the amino acids and glycine peptides from water to aqueous solutions of TMAO are monotonically positive over the investigated concentration ranges. The relative order of h AT and Δ tr H m for amino acids are all l -serine > l -threonine > glycine > l -alanine > l -valine, whereas the sequence of glycine peptides is triglycine > diglycine > glycine. The results are discussed in terms of solute–solute and solute–solvent interactions.

Studies of interparticle interactions among some l-α-amino acids and 1,2 (1,3)-propanediols in aqueous solution at 298.15 K

Mixing enthalpies of aqueous glycine, l-α-alanine, l-α-valine, l-α-serine. l-α-threonine and l-α-proline solutions and aqueous 1,2-propanediol and 1,3-propanediol solutions and their dilution enthalpies have been measured with a Thermometric-2277 Thermal Activity Monitor at 298.15K. Experimental data were treated according to the McMillan–Mayer theory and the heterogeneous enthalpic pairwise interaction coefficients (hxy) of the natural amino acids and propanediol molecules in aqueous solution were obtained. Combining the hxy coefficients of amino acids with glycol and glycerol in aqueous solution reported by our previous works, the effects of number and position of OH groups in polyalcohols on the interactions between them and amino acids are discussed in the terms of solute–solute interactions.

Enthalpic discrimination of homochiral pairwise interactions: Enantiomers of proline and hydroxyproline in (dimethyl formamide (DMF) + H2O) and (dimethylsulfoxide (DMSO) + H2O) mixtures at 298.15 K

Dilution enthalpies of two pairs of α-amino acid enantiomers, namely l-proline vsd-proline, and l-hydroxyproline vsd-hydroxyproline, in water-rich regions of dimethyl formamide (DMF)+H2O and dimethylsulfoxide (DMSO)+H2O mixtures (mass fractions of cosolvents wCOS=0 to 0.30) have been determined respectively at 298.15K by isothermal titration calorimetry (ITC). The successive values of dilution enthalpy obtained in a single run of ITC determination were used to calculate homochiral enthalpic pairwise interaction coefficients (hxx) at the corresponding composition of mixed solvents according to the McMillan–Mayer’ statistical thermodynamic approach. The sign and magnitude of hxx were interpreted in terms of solute–solute interactions mediated by solvent and cosolvent molecules, and preferential configurations of homochiral pairwise interactions (l–l or d–d pairs) in aqueous solutions. The variations of hxx with wCOS were considered to be dependent greatly on the competition equilibrium between hydrophobic and hydrophilic interactions, as well as the structural alteration of water caused by the two highly polar aprotic cosolvents (DMF and DMSO). Especially, it was found that when one of the two kinds of interactions (hydrophobic or hydrophilic interactions) preponderates over the other in solutions, enthalpic effect of homochiral pairwise interactions is always remarkable, and is characterized by a large absolute value of hxx, positive or negative, which corresponds respectively to the prevailing interactions of hydrophobicity or hydrophilicity. Interestingly, it was also found that the hxx values of l-enantiomers are generally larger than those of d-enantiomers across the whole studied composition range of mixed solvents, i.e. |hLL|>|hDD|, which is defined as enthalpic discrimination effect and attributed to the influence of preferential configuration on homochiral pairwise interactions in aqueous solutions.

Enthalpies of Dilution of Penicillamines in N,N-Dimethylformamide + Water Mixtures at 298.15 K

Dilution enthalpies of two penicillamine enantiomers, namely, d-penicillamine, l-penicillamine, and their racemate d,l-penicillamine, in N,N-dimethylformamide (DMF) + water mixtures of various compositions have been determined at 298.15 K respectively, using an isothermal titration calorimeter (MicroCal ITC200). According to the McMillan–Mayer theory, homochiral enthalpic pairwise interaction coefficients (hXX) of the three penicillamines in DMF + water mixtures of different mass fractions (wDMF = 0 to 0.3) have been calculated regressively. It is found that hXX coefficients of each enantiomer and their racemate change increasingly with wDMF to the maxima at wDMF = 0.20 and then decrease relatively rapidly, which follows the order hLL > hDD ≈ hRR > 0 (R represents the racemate mixture of penicillamine). The results are discussed from the point of view of competitive equilibrium between hydrophobic and hydrophilic interactions in solutions.

Homotactic enthalpic pairwise interactions of four deoxynucleosides (dU, dC, dG, dT) in dimethylformamide (DMF) + water mixtures at 298.15 K

The dilution enthalpies of four 2′-deoxynucleosides, namely 2′-deoxyuridine (dU), 2′-deoxycytidine (dC), 2′-deoxyguanosine (dG) and 2′-deoxythymidine (dT), in dimethylformamide (DMF)+water mixtures have been determined at 298.15K respectively, using an isothermal titration calorimeter (ITC200 MicroCal). On the basis of the McMillan–Mayer theory, the homotactic enthalpic pairwise interaction coefficients (hXX) of the four 2′-deoxynucleosides in DMF+water mixtures of various mass fractions (wDMF=0–0.30) have been calculated. It was found that the values of hXX of the four 2′-deoxynucleosides are all large negative and increase gradually across the whole studied composition range of the mixed solvent, though the variation degrees of them are somewhat different. The results are discussed from the point of view of solute–solute and solute–solvent interactions in solutions.

Enthalpic discrimination of position isomerism: Pairwise interaction of piperidinecarboxylic acid isomers in DMSO + H2O mixtures at 298.15 K

Dilution enthalpies of three piperidinecarboxylic acid isomers, namely 2-piperidinecarboxylic acid, 3-piperidinecarboxylic acid and 4-piperidinecarboxylic acid in dimethylsulfoxide (DMSO)+H2O mixtures (mass fractions of DMSO w=0–0.3) have been determined respectively using an isothermal titration calorimeter (ITC200, MicroCal) at 298.15K. According to the McMillan–Mayer theory, the corresponding homogeneous enthalpic pairwise interaction coefficients (hXX) of the three isomers have been calculated at each composition of mixed solvents. It is found that position isomerization effect of the three piperidinecarboxylic acid isomers can be discriminated completely by enthalpic effects from pairwise interactions between solute molecules. The results were interpreted from the point of view of solute–solute and solute–solvent interactions in these ternary aqueous systems, as well as competition equilibrium between hydrophobic–hydrophobic, hydrophobic–hydrophilic and hydrophilic–hydrophilic interactions.