The Partial Molar Volumes and Heat Capacities of the Arginyl Side-chain of Proteins in Aqueous Solution over the Temperature Range 288.15 to 328.15 K

Solution densities over the temperature range 288.15 to 328.15 K have been measured for aqueous solutions of N-acetylarginamide monotrifluoroacetate and sodium trifluoroacetate, from which the partial molar volumes at infinite dilution, V₂^oV_{2}^{\mathrm{o}}, were determined. The partial molar heat capacities at infinite dilution, C_p,2^oC_{p,2}^{\mathrm{o}}, were also determined for these solutes over the same temperature range. These V₂^oV_{2}^{\mathrm{o}} and C_p,2^oC_{p,2}^{\mathrm{o}} results, along with relevant data taken from the literature, have been used to calculate the contributions of the protonated arginyl side-chain to the thermodynamic properties. These new side-chain values were critically compared with those obtained previously using alternative side-chain model compounds.     

Partial Molar Volumes of Glycine and dl-Alanine in Aqueous Ammonium Sulfate Solutions at 278.15, 288.15, 298.15 and 308.15 K

In this work, the partial molar volumes of glycine and dl-alanine in aqueous solutions of ammonium sulfate at 0.0, 0.1, 0.3, 0.7, and 1.0 mol·kg^?1 are determined between 278.15 and 308.15 K. Transfer volumes were obtained, which are larger for glycine than dl-alanine. On the contrary, the hydration numbers are higher for dl-alanine than glycine, and dehydration of the amino acids is observed with increasing temperature or salt molality. The data suggest that interactions between ion and charged/hydrophilic groups are predominant and, by applying the methodology proposed by Friedman and Krishnan, it was concluded that they are mainly pairwise. A group-contribution scheme has been successfully applied to the pairwise volumetric interaction coefficient. Finally, the dehydration effect on glycine, alanine and serine in the presence of different electrolytes has been rationalized in terms of the charge density and a parameter accounting for the cation’s hydration.     

The Enthalpies of Solution of L-cysteine, L-serine and L-asparagine in Aqueous Solutions of Some Alcohols at 298.15 K

Heats of solution, Δ_sol H _m , of L-cysteine, L-serine and L-asparagine amino acids have been measured at different concentrations of aqueous ethanol, propanol and 2-propanol at 298.15 K using solvation calorimetry. These data are compared with the results reported earlier for L-alanine in ethanol. The enthalpic coefficients, h _xy , of the solute-organic cosolvent pair interaction in water have been obtained from the McMillan-Mayer approach and the data have been interpreted in terms of various interactions and changes in solvent structure.     

Measurements of the Molar Heat Capacities and Excess Molar Heat Capacities for Water + Organic Solvents Mixtures at 288.15 K to 303.15 K and Atmospheric Pressure

Experimental molar heat capacity data (Cp _m) and excess molar heat capacity data (Cp^E_m\mathit{Cp}^{\mathrm{E}}_{\mathrm{m}}) of binary mixtures containing water + (formamide or N,N-dimethylformamide or dimethylsulfoxide or N,N-dimethylacetamide or 1,4-dioxane) at several compositions, in the temperature range 288.15 K to 303.15 K and atmospheric pressure, have been determined using a modified 1455 PAAR solution calorimeter. The excess heat capacities are positive for aqueous solutions containing 1,4-dioxane, N,N-dimethylformamide or dimethylsulfoxide, negative for solutions containing water + formamide and show a sigmoid behavior for mixtures containing water + N,N-dimethylacetamide, over the whole composition range. The experimental excess molar heat capacities are discussed in terms of the influence of temperature and of the organic solvent type present in the binary aqueous mixtures, as well as in terms of the existing molecular interactions and the organic solvent’s molecular size and structure.     

Amino Acid Derivatives as Protein Side-Chain Model Compounds: The Partial Molar Volumes and Heat Capacities of Some N-Acetyl-N′-methyl Amino Acid Amides in Aqueous Solution

The partial molar heat capacities, C ^o _p,2, and partial molar volumes, V ^o ₂, at infinite dilution have been determined for the compounds N-acetyl-N-methylglycinamide, N-acetyl-N-methylalaninamide, N-acetyl-N-methylvalinamide, and N-acetyl-N-methylleucinamide in aqueous solution at the temperatures 15, 25, 40, and 55°C. The C ^o _p,2 and V ^o ₂ results have been used to calculate amino acid side-chain contributions to the thermodynamic properties. These side-chain contributions are compared with those derived in previous work using thermodynamic data for the corresponding acetylated primary amides and with results reported using peptide model compounds.     

Densities, Specific Heat Capacities, Apparent and Partial Molar Volumes and Heat Capacities of Glycine in?Aqueous Solutions of Formamide, Acetamide, and?N,N-Dimethylacetamide at T=298.15?K and?Ambient Pressure

Apparent molar volumes (V _2,φ ) and heat capacities (C _p2,φ ) of glycine in known concentrations (1.0, 2.0, 4.0, 6.0, and 8.0 mol⋅kg⁻¹) of aqueous formamide (FM), acetamide (AM), and N,N-dimethylacetamide (DMA) solutions at T=298.15 K have been calculated from relative density and specific heat capacity measurements. These measurements were completed using a vibrating-tube flow densimeter and a Picker flow microcalorimeter, respectively. The concentration dependences of the apparent molar data have been used to calculate standard partial molar properties. The latter values have been combined with previously published standard partial molar volumes and heat capacities for glycine in water to calculate volumes and heat capacities associated with the transfer of glycine from water to the investigated aqueous amide solutions, D[`(V)]<sub>2,tr</sub><sup>o</sup>\Delta\overline{V}_{\mathrm{2,tr}}^{\mathrm{o}} and D[`(C)]_p2,tr^o\Delta\overline{C}_{p\mathrm{2,tr}}^{\mathrm{o}} respectively. Calculated values for D[`(V)]<sub>2,tr</sub><sup>o</sup>\Delta\overline{V}_{\mathrm{2,tr}}^{\mathrm{o}} and D[`(C)]_p2,tr^o\Delta\overline{C}_{p\mathrm{2,tr}}^{\mathrm{o}} are positive for all investigated concentrations of aqueous FM and AM solutions. However, values for D[`(C)]_p2,tr^o\Delta\overline{C}_{p\mathrm{2,tr}}^{\mathrm{o}} associated with aqueous DMA solutions are found to be negative. The reported transfer properties increase with increasing co-solute (amide) concentration. This observation is discussed in terms of solute + co-solute interactions. The transfer properties have also been used to estimate interaction coefficients.     

Standard Molar Volumes and Heat Capacities of Aqueous Solutions of Trifluoromethanesulfonic Acid at Temperatures up to 573 K and Pressures to 30 MPa

Densities and heat capacities of dilute aqueous solutions (0.025 to 0.4 mol⋅kg⁻¹) of trifluoromethanesulfonic acid (triflic acid) were measured with original high-temperature, high-pressure instruments at temperatures and pressures up to 574 K and 31 MPa, respectively. Standard molar volumes and standard molar heat capacities were obtained via extrapolation of the apparent molar properties to infinite dilution. The evolution of these standard derivative properties of triflic acid with temperature and pressure is qualitatively compared with that of other acids of different strengths.     

Apparent and Transfer Molar Volumes for Aqueous Solution Containing Polyethylene Glycols and Amino Acid Ionic Liquids at 298.15 K

The densities of 1-n-butyl-3-methylimidazolium ([Bmim]) based amino acid ionic liquids (AAILs) prepared from glycine [Gly], alanine [Ala], and valine [Val], namely [Bmim][Gly], [Bmim][Ala] and [Bmim][Val], in aqueous?~?0.2 mol·kg^?1 polyethylene glycol (PEG400, PEG600 or PEG1000) and PEG400 solutions containing?~?(0.0946, 0.1891 and 0.3820) mol·kg^?1 of [Bmim][Gly], have been determined at 298.15 K. The experimental densities were used to evaluate the apparent molar volumes in the mixed solvent system and further used to obtain transfer molar volumes of AAILs for their transfer from water to aqueous PEG solutions and of PEG400 for its transfer from water to aqueous solutions containing (0.0946, 0.1891 and 0.3820) mol·kg^?1 of [Bmim][Gly]. The transfer molar volumes of AAILs and of PEG400 are found to be negative. The effects of alkyl chain-length variation on the anion of AAILs as well as the chain-length of PEG on transfer molar volumes are investigated and discussed in terms of hydrophobic–hydrophilic, hydrophobic–hydrophobic, and ion–hydrophobic interactions.     

Enthalpies of Solution of L–L- and D–D- Isomers of Valine in Aqueous Alkanols at 298.15 K

The enthalpies of solution of L- and D-valines in water-ethanol, water-n-propanol, and water-i-propanol mixtures were measured calorimetrically at 298.15 K at alcohol mole fractions, x ₂, ranging up to 0.4. Enthalpies of transfer, Δ_tr H°, from water to aqueous alkanol were calculated for each of the system studied. The enthalpic coefficients, h _xy , of the solute-cosolvent pair-wise interaction in water proved to be positive and increasing in the series: ethanol, n-propanol, and i-propanol. It was shown that both the nature of the amino acid L–L- and D–D-isomerization and dimensions of linear or branched cosolvent molecules define the energetics of interaction between valine and alkanol molecules.     

Enthalpies of Dilution of N,N′-hexamethylenebisacetamide in Pure Water and Aqueous Solutions of Alkali Halide at 298.15 K

Enthalpies of dilution of N,N′-hexamethylenebisacetamide in water and aqueous alkali halide solutions at the concentration of 0.150 mol⋅kg⁻¹ (approximately the concentration of physiological saline) have been determined by isothermal titration microcalorimetry at 298.15 K. The enthalpic interaction coefficients in the solutions have been calculated according to the excess enthalpy concept based on the calorimetric data. The values of enthalpic pair-wise interaction coefficients (h ₂) of the solute in aqueous solutions of different salts were discussed in terms of the different alkali salt ions and weak interactions of the diluted component with coexistent species as well as the change in solvent structure caused by ions.     

Limiting Partial Molar Excess Heat Capacities and Volumes of Selected Organic Compounds in Water at 25°C

Well-known Picker flow microcalorimeters for the differential measurements of volumetric heat capacities have been employed in conjunction with vibrating tube densimeters to determine the molar heat capacity, volume, and the apparent properties in dilute aqueous solutions for 17 organic solutes of moderate hydrophobicity. The dependence on concentration of the apparent properties allowed the limiting partial molar quantities at infinite dilution to be extrapolated and the limiting partial molar excess quantities to be evaluated. Comparison with available literature data shows good agreement. The application of group contribution rules to the limiting partial properties has been tested using the original method and parameters proposed by Cabani et al. The predicted values of the partial molar volumes are in fair agreement with the present data except for some less common solutes. With partial molar heat capacities, the agreement is less satisfactory. To improve the performance of the method, missing parameters for some types of monofunctional and bifunctional molecules have been evaluated.     

Volumetric Properties of l-Alanine and l-Serine in Aqueous N,N-Dimethylformamide Solutions at 298.15 K

The densities of l-alanine and l-serine in aqueous solutions of N,N-dimethylformamide (DMF) have been measured at 298.15 K with an Anton Paar Model 55 densimeter. Apparent molar volumes $ (V_{\phi } ) $ ( V ? ) , standard partial molar volumes $ (V_{\phi }^{0} ) $ ( V ? 0 ) , standard partial molar volumes of transfer $ (\Updelta_{\text{tr}} V_{\phi }^{0} ) $ ( Δ tr V ? 0 ) and hydration numbers have been determined for the amino acids. The $ \Updelta_{\text{tr}} V_{\phi }^{0} $ Δ tr V ? 0 values of l-serine are positive which suggest that hydrophilic–hydrophilic interactions between l-serine and DMF are predominant. The –CH₃ group of l-alanine has much more influence on the volumetric properties and the $ \Updelta_{\text{tr}} V_{\phi }^{0} $ Δ tr V ? 0 have smaller negative values. The results have been interpreted in terms of the cosphere overlap model.     

Partial Molar Volume,Ionization, Viscosity and Structure of Glycine Betaine in Aqueous Solutions

Densities, partial molar volumes, and viscosities of aqueous solutions of betaine have been measured at 5, 10, 15, 20, 25, 30, 37, and 45 °C over the concentration range 0.05 to 5.0 mol⋅L⁻¹. The partial molar volumes show that betaine exists partly as a monohydrate and partly in its anhydrous form. The proportion of the anhydrous form increases with increasing temperature. Also, an associated form of betaine appears in concentrated betaine solutions, possibly with water as a bridging group. The significance of the viscosity B-coefficient is discussed. The signs of B _st, the increment of the viscosity B-coefficients arising from structural changes of water, are negative and the signs of dB/dT, the temperature derivative of B, are positive. These results show that betaine is a water structure breaker especially at lower temperatures, and this effect decreases to insignificance at higher temperatures. The ionization equilibria of betaine were investigated in aqueous 0.5 mol⋅L⁻¹ and 1.0 mol⋅L⁻¹ NaNO₃ at 5, 15, 25, and 37 °C by a potentiometric method. Using the least-square computer program SUPERQUAD, the complex forms are deduced to be betanium BH, bis(betanium) BHB, and bis(betaine) B₂ or bis(betaine)hydrate BH₂OB.     

Partial Molar Volumes of Uracil,Thymine, Adenine in Water and of Adenine in Aqueous Solutions of Uracil and Thymine

The partial molar volumes of uracil, thymine and adenine in water and adenine in aqueous solutions of uracil and thymine, at fixed composition, were determined over a range of temperatures. The partial molar volumes of adenine in aqueous uracil and thymine are less than in pure water.     

Thermodynamic Properties of Peptide Solutions. Part 17. Partial Molar Volumes and Heat Capacities of the Tripeptides GlyAspGly and GlyGluGly,and Their Salts K[GlyAspGly] and Na[GlyGluGly] in Aqueous Solution at 25 °C

The partial molar volumes, V^o₂, and partial molar heat capacities, C_p,2^o, at infinite dilution have been determined for the two tripeptides glycylaspartylglycine (glyaspgly) and glycylglutamylglycine (glyglugly), and also for their salts K[glyaspgly] and Na[glyglugly], in aqueous solution at 25 °C. The ionization constants at 25 °C for the aspartyl and glutamyl side-chains have also been determined. These new thermodynamic results have been combined with literature data for electrolytes to obtain the volume and heat capacity changes upon ionization of the acidic side-chains of the peptides. The results are compared with those for other carboxylic acid systems. The partial molar heat capacities and volumes have also been used to calculate the contributions of the acidic amino acid side-chains to the thermodynamic properties.     

Limiting Partial Molar Volumes of Water in 1-Hexanol, 1-Octanol, 1-Decanol,and Cyclohexanol at 298.15 K

Summary. From the density of solutions of water in 1-hexanol, 1-octanol, 1-decanol, and cyclohexanol, measured at 298.15K, the limiting partial molar volume and the excess limiting partial molar volume of water was estimated. The limiting partial molar volume of water in alcohols was discussed in terms of the void space created by the addition of water to alcohol and by the packing density of water. On the basis of the Kirkwood-Buff theory and the activity of water in alcohols an average aggregation number of water molecules, as well as the number of the excess alcohol molecules in the surroundings of the water molecule was calculated. The solvation ability of the investigated alcohols was estimated as the difference in the solvation Gibbs energy of an alcohol molecule in solution relative to the pure alcohol. The observed difference was mainly ascribed to an indirect effect caused by water molecules on the alcohol structure and to a lesser extent to the hydrogen bonding of water to alcohol molecules. The limiting partial molar volume of water was also interpreted in terms of scaled particle theory and the various volume contributions arising from dispersion, dipole–dipole, and inductive interactions between water and an alcohol molecule were calculated.     

Volumetric Properties of Amino Acids in Aqueous N-methylacetamide Solutions at 298.15 K

The apparent molar volumes (V _φ ) of glycine, L-alanine and L-serine in aqueous 0 to 4 mol⋅kg⁻¹ N-methylacetamide (NMA) solutions have been obtained by density measurement at 298.15 K. The standard partial molar volumes (V_f⁰)V_{\phi}^{0}) and standard partial molar volumes of transfer (D_trV_f⁰)\Delta_{\mathrm{tr}}V_{\phi}^{0}) have been determined for these amino acids. It has been show that hydrophilic-hydrophilic interactions between the charged groups of the amino acids and the –CONH– group of NMA predominate for glycine and L-serine, but for L-alanine the interactions between its side group (–CH₃) and NMA predominate. The –CH₃ group of L-alanine has much more influence on the value of D_trV_f⁰\Delta_{\mathrm{tr}}V_{\phi}^{0} than that of the –OH group of L-serine. The results have been interpreted in terms of a co-sphere overlap model.     

Partial Molar Volumes and Isentropic Compressions of Cyclic Ethers in Aqueous Solutions from 288.15 to 313.15 K at Atmospheric Pressure

The partial molar volumes and isentropic compressions of aqueous solutions of tetrahydrofuran, tetrahydropyran, 1,4-dioxane, tetrahydropyran-2-methanol, 3-hydroxytetrahydrofuran, and tetrahydrofurfuryl alcohol were measured at 288.15, 298.15, and 313.15 K. Results are analyzed in terms of the effects of group addition to the molar volumes and isentropic compressions. The temperature dependence of the molar volumes and compressions, and their group contributions, are used to characterize changes in hydration.     

Limiting Partial Molar Volumes of Water in 1-Hexanol, 1-Octanol, 1-Decanol, and Cyclohexanol at 298.15?K

From the density of solutions of water in 1-hexanol, 1-octanol, 1-decanol, and cyclohexanol, measured at 298.15K, the limiting partial molar volume and the excess limiting partial molar volume of water was estimated. The limiting partial molar volume of water in alcohols was discussed in terms of the void space created by the addition of water to alcohol and by the packing density of water. On the basis of the Kirkwood-Buff theory and the activity of water in alcohols an average aggregation number of water molecules, as well as the number of the excess alcohol molecules in the surroundings of the water molecule was calculated. The solvation ability of the investigated alcohols was estimated as the difference in the solvation Gibbs energy of an alcohol molecule in solution relative to the pure alcohol. The observed difference was mainly ascribed to an indirect effect caused by water molecules on the alcohol structure and to a lesser extent to the hydrogen bonding of water to alcohol molecules. The limiting partial molar volume of water was also interpreted in terms of scaled particle theory and the various volume contributions arising from dispersion, dipole–dipole, and inductive interactions between water and an alcohol molecule were calculated.