Thermodynamic properties of peptide solutions: 7. Partial molar isentropic pressure coefficients of some dipeptides in aqueous solution

The partial molar isentropic pressure coefficients at infinite dilution, K _S,2 ^o , have been determined for a number of dipeptides in aqueous solution at 25°C. For a series of dipeptides of sequence gly-X, where X is an amino acid with a neutral side chain, the K _S,2 ^o values are all more negative than that for diglycine. The results are discussed in terms of the hydration of the side chains. There are significant differences in the K _S,2 ^o values for sequence isomeric dipeptides. These differences can be rationalized in terms of the mutual interactions between the side chain and the ionic end groups in the dipeptides. Possible relationships between K _S,2 ^o and V ₂ ^o , the partial molar volume at infinite dilution, were investigated. For the dipeptides of sequence gly-X there is an interesting linear relationship between K _S,2 ^o /V ₂ ^o and V ₂ ^o .     

Thermodynamic properties of peptide solutions 3. Partial molar volumes and partial molar heat capacities of some tripeptides in aqueous solution

Partial molar volumes, V ₂ ^o , and partial molar heat capacities, C _p,2 ^o , of the tripeptides glycylglycylglycine, glycylglycylalanine, glycylalanylglycine and alanylglycylglycine have been determined in aqueous solution at 25°C. For the three alanyl-containing tripeptides, the data indicate that the tripeptide-water interaction is influenced by the side chain position within the molecule. The results have been rationalized in terms of likely solutesolvent interactions. The V ₂ ^o and C _p.2 ^o data have also been used to calculate the contribution to these properties of a-CH₃ side chain.     

Thermodynamic properties of peptide solutions. Part 15. Partial molar isentropic compressibilities of some glycyl dipeptides in aqueous solution at 15 and 35°C

The partial molar isentropic compressibilities at infinite dilution,K _s,2 ^o , have been obtained for eight glycyl dipeptides of sequence gly-X (X is an amino acid) in aqueous solution at the temperatures 15 and 35°C. The results have been combined with those obtained at 25°C, that were reported earlier, to evaluate the temperature dependences ofK _s,2 ^o for the dipeptides in the temperature range 15 to 35°C. TheK _s,2 ^o values for all the dipeptides are negative and increase (become more positive) with an increase in temperature. The slopes of the temperature dependences ofK _s,2 ^o for the dipeptides with typically hydrophobic side-chains are significantly larger than those for dipeptides with hydrophilic side-chains.     

Partial molar volumes of monovalent ions in ethanolamine and water + ethanolamine mixtures at 25°C

Precise densities for sodium of chloride, bromide and iodide and potassium iodide in ethanolamine and water+ethanolamine mixtures (15, 30, 50, 60, 70, 80 and 90 mass% ethanolamine) up to a maximum salt molality of 0.15 mol-kg⁻¹ are reported from measurements at 25°C using a vibrating tube densimeter. The electrolyte apparent molar volumes were calculated and extrapolated to infinite dilution using the Masson equation to yield the limiting electrolyte partial molar volumes. The limiting ionic partial molar volumes V _ion ^o were estimated using Mukerjee's method. A correspondence principle proposed earlier for predicting ionic entropies could be used for the estimation of V _ion ^o for rubidium and cesium salts. The estimates of the contributions from geometric and the electrostrictive effects to V _ion ^o are also reported. The variations in these contributions with the change in solvent composition are discussed in terms of the changes in the solvent structure.     

Dissolved states of ionene polymers in water-acetone and their relation to the ionic partial molar adiabatic compressibility and volume

Solution properties of water-soluble synthetic polymer, 3,3-ionene and 6,6-ionene chloride and bromide disolved in water-acetone mixtures up to acetone content 50 wt% were investigated. Their partial molar volumes V ₂ ^o and partial molar adiabatic compressibilities K _s ^o were determined. Ionic additivity of V ₂ ^o with respect to the cation of backbone polymer chain and the counter-anion was confirmed quantitatively. The ionic additivity of V ₂ ^o is discussed along with the K _s ^o in their relation to the counterion binding of ionene polymers. Effects of ionic sites on the ionene are strong but they don't break the solvation layer.     

Apparent molar heat capacities and volumes of some alkylated derivatives of uracil and adenine in aqueous solution at 25°C

Densities and apparent molar heat capacities of some alkylated derivatives of uracil and adenine: 1-methyluracil, 1,3-dimethyluracil, 1,3-diethylthymine, 5,6-trimethylene-1,3-dimethyluracil, 5,6-tetramethylene-1,3-dimethyluracil, 5,6-pentamethylene-1,3-dimethyluracil, 2,9-dimethyladenine, 2-ethyl-9-methyladenine, 2-propyl-9-methyladenine, 8-ethyl-9-methyladenine, 6,8,9-trimethyladenine and 8-ethyl-6,9-dimethyladenine were determined using flow calorimetry and flow densimetry at 25°C. It was found that the partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH ₂ -as well as to the number of hydrogen atoms, n _H , belonging to the skeleton of the molecule. In the case of alkylated uracils a difference was observed in the values at infinite dilution V ₂ ^o and C _p2 ^o , depending on the substitution of alkyl and cyclooligomethylene groups.     

Apparent molar heat capacities of aqueous solutions of acetic,propanoic and succinic acids,sodium acetate and sodium propanoate from 300 to 525 K and a pressure of 28 MPa

The apparent molar heat capacities of dilute aqueous solutions of acetic, propanoic and succinic acid and sodium salts of the two monofunctional acids were measured at 300 Kp,2 ^o . After subtracting the heat capacity of a point mass, the remaining heat capacity was successfully decomposed into functional group contributions at all temperatures. Together with the results of our previous paper on alcohols and diols the heat capacity contributions of the CH₂, CH₃, OH, COOH, (COOH)₂, and COONa groups are now available and these allow reasonably accurate predictions of the heat capacities of all compounds composed of these groups in this temperature range.     

Thermochemistry of aqueous solutions of nucleic acid bases and their alkylated derivatives

Enthalpy of solution, ΔH _sol ^o , enthalpy of sublimation, ΔH _subl ^o , apparent partial molar volume and heat capacities,V ₂ ^o andC _p,2 ^o were determined for aqueous solutions of thirty alkylated derivatives of uracyl and adenine, eight derivatives of cytosine and guanine. Calculated accessible surface areas and molar volumes are presented, too. The values of enthalpy of solution, enthalpy of sublimation can be useful in the studies on the nature of interaction between these compounds and water molecules. Apparent partial molar volume and heat capacity give a new aspect on hydrophob properties of the examined nucleic acid base derivatives.     

The Partial Molar Volume and Heat Capacity of the Glycyl Group in Aqueous Solution at 25^C

The partial molar volumes, V₂ ^{^}, and the partial molar heat capacities, C_p,2 ^{^}, at infinite dilution have been determined for three new peptides of sequence seryl(glycyl)_xglycine, where x=0 to 2, in aqueous solution at 25^{^}C. Values for V₂ ^{^} at 25^°C have also been determined for two neutral peptide derivatives N-acetylglycylglycinamide and N-acetylglycylglycylglycinamide. These V₂ ^°; and C_p,2 ^°; results were used to estimate the partial molar volume and heat capacity of the backbone glycyl group, CH₂CONH, of proteins in aqueous solution at 25^°;C. The results obtained are compared with those calculated using partial molar data for alternative model compounds. The new glycyl group contributions are in excellent agreement with those currently used in our group additivity schemes for the calculation of the partial molar volumes and heat capacities of unfolded proteins.     

Partial molar volumes and partial molar adiabatic compressibilities of bipyridine and phenanthroline complexes with Fe(II), Co(II), Ni(II), and Cu(II) and chlorides of these metals

Densities and ultrasonic velocities were measured at 25°C for aqueous solutions of bipyridine and phenanthroline complexes [M(bpy)₃]Cl₂ and [M(phen)₃]Cl₂ (M=Fe, Co, Ni, and Cu, bpy=2,2-bipyridine, and phen=1,10-phenanthroline), and chlorides of these metals. The partial molar volumes V ₂ ^o and partial molar adiabatic compressibilities K _s ^o were calculated. For the complex ions, [M(bpy)₃]²⁺ and [M(phen)₃]²⁺, electrostatic interactions with the solvent are not nearly as important as effects due to the hydrophobic ligands bpy and phen. The relationship between V ₂ ^o and K _s ^o of the complex ions and common metal ions are examined.     

Partial molar compressibilities of halogeno complexes of platinum and palladium in aqueous solutions at 25°C

Ultrasonic propagation velocities were measured at 25°C for dilute aqueous solutions of K₂[PtX₄], K₂[PtX₆], K₂[PdX₄], and K₂[PdX₆] (X = Cl, Br, or SCN). The data obtained were combined with our previous density data to calculate the adiabatic compressibilities of each solution. Then, the infinite dilution values of partial molar adiabatic compressibilitiesK _o ^s were evaluated. By subtraction of theK _o ^s (K⁺) from those of the complexes, the values ofK _o ^s for the halogeno complex ions [PtX₄]^2-, [PtX₆]^2-, [PdX₄]^2- and [PdX₆]^2- were determined. The values for the thiocyano complex ions [Pt(SCN)₄]^2- and [Pt(SCN)₆]^2- are positive, showing that the interaction of the ligand SCN^- with the solvent water is very weak. Although the V ₂ ^o behavior of the complex ions [PtCl₆]^2- and [PdCl₆]^2- differed only slightly, compressibility measurement has indicated that the ion [PdCl₆]_2- interacts more strongly than [PtCl₆]^2-.     

Thermodynamics of nucleic acid bases and nucleosides in water from 25 to 55°C

Specific heat capacities, apparent molar heat capacities, densities, and apparent molar volumes have been determined for cytosine, uracil, thymine, adenine, cytidine, 2-deoxycytidine, uridine, thymidine and adenosine at temperatures from 25°C to 55°C. The results of these measurements have been used to calculate for the first time, the thermodynamic quantities:C _p,2 ^o , (C _p,2 ^o /T)_p, (² C _p,2 ^o T ²)_p,V ₂ ^o , (V ₂ ^o /T)_p, and (² V ₂ ^o /T ²)_p. The-CH₂-group contribution has been calculated at different temperatures. It has also been observed from the data for the nucleic acid bases and nucleosides that the additivity ruleC _p,2 ^o (nucleoside)-C _p,2 ^o (base) +C _p,2 ^o (water)=C _p,2 ^o (ribose) does not hold in these cases.     

Group contributions to the thermodynamic properties of non-ionic organic solutes in dilute aqueous solution

The thermodynamic properties G _h ^o,H _h ^o, and C _p,h ^oassociated with the transfer of non-ionic organic compounds from gas to dilute aqueous solution and the limiting partial molar properties C _p ^o ,2 and V₂ ² of these compounds in water are described through a simple scheme of group contributions. A distinction is made between groups made only of carbon and hydrogen, and functional groups i.e. groups containing at least one atom different from carbon and hydrogen. Each group is assigned a contribution, for each property, through a least squares procedure which utilizes only molecules containing at most one functional group. Finally, for compounds containing more than one functional group, correction parameters are evaluated as the differences between the experimental values and those calculated by means of the group contributions. The different behavior of hydrophilic compared with hydrophobic groups is discussed for the various properties. A rationale for the correction parameters, i.e. for the effects of the interactions among hydrophilic groups on the thermodynamic properties, is attempted.     

Partial Molar Volumes of Tetraalkylammonium Ions in N, N-Dimethylformamide

The densities of tetraalkylammonium bromide, R₄NBr (R = Et, Pr, Bu, Hex, Hep, Oct), solutions in dimethylformamide have been measured for the composition range (0.05–0.4) mol-kg⁻¹ at 25 ^∘C. Apparent molar V_φ and limiting partial molar volumes ⫫₂^o of the electrolytes have been evaluated. Using the extrapolation values, the limiting partial molar volumes of the tetraalkymammonium ions (⫫_i^o) have been calculated. Analysis of different contributions to the ionic ⫫_i^o indicated partial penetration of solvent molecules into the van der Waal’s volume of tetraalkylammonium (TAA) ions.     

Apparent Molar Heat Capacities and Apparent Molar Volumes of Aqueous Nicotinamide at Different Temperatures

Specific heat capacities and apparent molar heat capacities of aqueous nicotinamide have been determined from 25.0 to 55.0°C using microdifferential scanning calorimetry in the molality range of 0.07433 to 1.50124 mol-kg^–1. Densities and apparent molar volumes have also been determined for aqueous nicotinamide from 10.30 to 34.98°C using a digital densimeter in the molality range 0.07804–2.02435 mol-kg^–1. The results of these measurements have been used to calculate the following partial molar quantities and temperature derivatives for aqueous nicotinamide as a function of temperature: C _p,2,m ^o, (C _p,2,m ^o/T)_p, (²C_p,2,m ^o/T ²)_p, V _2,m ^o, ( V _2,m ^o/T)_p, and (² V _2,m ²/T ²)_p. The results are discussed in terms of the changes in the packing of nicotinamide molecules in the crystal, interactions in the aqueous form, and its structure-promoting ability with rise in temperature.     

Effect of temperature on ionic volumes and heat capacities in methanol

A flow densimeter and flow heat capacity calorimeter have been employed to measure precision densities and specific heats of selected electrolytes and nonelectrolytes in methanol at 20, 25, and 40°C. Apparent molar volumes and heat capacities have been calculated and the corresponding standard state functions, V ^o and C _p, ^o , evaluated. The data have been used, along with known volumes and heat capacity data at 25°C for numerous alkanes, to generate volumes and heat capacities of model compounds for organic electrolytes. Comparison of the thermodynamic functions for the model compounds with those of the corresponding electrolytes at the respective temperatures has made it possible to assign single ion values and to establish the temperature effects of ionic charges on the volumes and heat capacities of solutes.     

Apparent Molar Volumes and Adiabatic Compressibilities of Crown Ethers and Glymes in Aqueous Solutions at Various Temperatures

Apparent molar volumes and adiabatic compressibilities of 18-crown-6,15-crown-5, 12-crown-4, tetraglyme, and triglyme were measured at 15, 25, and40°C. Apparent molar expansibilities andK ^o Tvalues were also determined.The contribution of the -CH₂CH₂O- group to the limiting partial molar volumesand compressibilities of cyclic and open-chain ethers are compared. It isconcluded, on the basis of the compressibility results, that there is a subtle differencebetween the hydration of the ethene-oxide group in cyclic and open-chain ethers.     

Thermodynamics of Aqueous Nitrilotriacetic Acid (NTA) Systems: Apparent and Partial Molar Heat Capacities and Volumes of Aqueous HNTA2−, NTA3−, MgNTA−, CoNTA−, NiNTA− and CuNTA− at 25 °C

Apparent molar heat capacities and volumes have been determined for aqueous Na₂HNTA, Na₃NTA, NaMgNTA, NaCoNTA, NaNiNTA and NaCuNTA at 25 °C. The experimental results have been analyzed in terms of Young’s rule with an extended Debye–Hückel equation to obtain standard partial molar heat capacities C _p ^o and volumes V ^o for the species HNTA²⁻(aq), NTA³⁻(aq), MgNTA⁻(aq), CoNTA⁻(aq), NiNTA⁻(aq) and CuNTA⁻(aq), at ionic strengths I = 0 and I = 0.1 mol⋅kg⁻¹. Values of C _p ^o and V ^o were combined with the literature data to estimate the stability constants of the NTA complexes at temperatures up to 100 °C.     

Partial Molar Volumes of Some of α-Amino Acids in Binary Aqueous Solutions of MgSO4·7H2O at 298.15 K

The apparent molar volume, V ^o _{φ, 2}, of glycine, alanine, α-amino-n-butyric acid, valine and leucine have been determined in aqueous solutions of 0.25, 0.5 and 1.0 mol⋅dm⁻³ magnesium sulfate, and the partial specific volume from density measurements at 298.15 K. These data have been used to calculate the infinite dilution apparent molar volume, V ^o _2,m , group contribution of amino acids and partial molar volume of transfer, Δ_tr V _2,m ^o, from water to aqueous magnesium sulfate solutions. The linear correlation of V _2,m ^o for a homologous series of amino acids has been utilized to calculate the contributions of charged end groups (NH₃ ⁺, COO⁻), CH₂ - groups and other alkyl chains of amino acids to V _2,m ^o. The results for Δ_tr V _2,m ^o of amino acids from water to aqueous magnesium sulfate solutions have been interpreted in terms of ion-ion, ion-polar, hydrophilic-hydrophilic and hydrophobic-hydrophobic group interactions. The values of the standard partial molar volume of transfer for the amino acids with different hydrophobic contents, from water to aqueous MgSO₄ are in general positive, indicating the predominance of the interactions of zwitterionic/hydrophilic groups of amino acids with ions of the salt. The hydration number decreases with increasing concentration of salt. The number of water molecules hydrated to amino acids decreases, further strengthening the predominance of ionic/hydrophilic interactions in this system.     

Partial molar heat capacities and volumes of transfer of nucleic acid bases,nucleosides and nucleotides from water to aqueous solutions of sodium and calcium chloride at 25°C

Partial molar heat capacities and volumes of some nucleic acid bases, nucleosides and nucleotides have been measured in 1m aqueous NaCl and CaCl₂ solutions using Picker flow microcalorimeter and a vibrating tube digital densimeter. The partial molar heat capacities of transfer and volumes of transfer from water to the electrolyte solutions were calculated using earlier data for these compounds in water. The values of these transfer parameters are positive. The higher values for transfer to aqueous CaCl₂ solutions reflect the stronger interactions of the constituents of the nucleic acids with Ca⁺² ions than with the Na⁺ ions.