Partial Molar Volumes of Amino Acids and Peptides in Aqueous Salt Solutions at 25°C and a Correlation with Stability of Proteins in the Presence of Salts

Partial molar volumes for a homologous series of amino acids and peptides have been measured in aqueous 1M sodium acetate, sodium thiocyanate, and sodium sulfate at 25^°C. These data have been utilized in conjunction with the data in water to deduce partial molar volumes of transfer V _2,m ⁰(tr) from water to these aqueous salt solutions. The volumes of transfer for the amino acids and peptides are found to be positive. The interpretation is that this result arises from the dominant interaction of the sodium salts with the charged centers of amino acids and peptides. Thermal denaturation of the structurally homologous proteins lysozyme and -lactalbumin has been studied in the presence of these salts. Significant thermal stabilization of hen egg-white lysozyme has been observed in the presence of sodium acetate and sodium sulfate. However, the thermal stabilization observed for -lactalbumin is very small in the presence of these salts and sodium thiocyanate leads to a lowering of its thermal denaturation temperature. The rise in the surface tension of aqueous salt solutions with salt concentration has been correlated with the calorimetric and volumetric measurements. The results show that V _2,m ⁰(tr) depends less on the type of electrolyte than on the ionic strength of the solution. The V _2,m ⁰(tr) values correlate very well with the increase in the surface tension of aqueous salt solutions, indicating significant role of surface tension in interactions of amino acids, peptides, or protein with the salts.     

Solubility of amino acids in aqueous poly (ethylene glycol) solutions

The solubilities of amino acids have been measured in water and aqueous poly(ethylene glycol) (PEG) solutions as a function of temperature and PEG concentration. The free energies of transfer from water to aqueous PEG solutions forl-alanine,l-valine,l-isoleucine andl-leucine were positive, while those forl-phenylalanine andl-tryptophan were negative. The corresponding enthalpies of transfer were almost zero for all amino acids. The equilibrium constants of the binding of amino acids to PEG chain were estimated from the solubility data. Amino acids with larger hydrophobicity are bound more strongly to the PEG chain due to the hydrophobic interaction between the methylene groups of PEG and the side chain of amino acid. The equilibrium constants showed a correlation with the dynamic hydration number (n _DHN) which expresses the hydration properties of amino acids in aqueous solution.     

Interactions of Some Amino Acids with Aqueous Osmoprotectant Proline at 298.15 K: Volumetric and Calorimetric Studies

Apparent molar volumes of a homologous series of amino acids in aqueous proline solutions have been obtained from densities at 298.15 K, measured with a vibrating-tube digital densimeter. These data have been used to deduce the partial molar volumes of transfer from water to aqueous proline solutions; these partial molar volumes of transfer are found to be positive for glycine, alanine, α-amino-n-butyric acid and valine, whereas they are negative for leucine. The number of water molecules hydrated to the amino acids was estimated from the partial molar volume data. In order to supplement this information, enthalpies of transfer of aqueous amino acids from water to 0.1, 2.25 and 1 mol⋅dm⁻³ aqueous proline have been determined at 298.15 K using a VP-ITC titration calorimeter. The data on the partial molar volumes and enthalpies of transfer are discussed in terms of various interactions operating in the ternary mixtures of amino acids, water and proline.     

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.     

Sodium sulfate solubilities in high temperature aqueous sodium chloride and sulfuric acid solutions—predictions of solubility,vapor pressure,acidity, and speciation

Experimental measurements of the solubility of sodium sulfate in aqueous solutions containing both sodium chloride and sulfuric acid in the temperature range 250 to 374°C are reported. These measurements have been combined with previous data on the solubility of sodium sulfate in water, in aqueous sodium chloride, and in sulfuric acid solutions to produce a comprehensive model describing the solubility of sodium sulfate in such solutions. Calculations and predictions of solubility, vapor pressure, boiling point elevation, acidity, and speciation are presented. This model is of fundamental interest in itself and also is of importance because the precipitation of sodium sulfate may be a contributing factor in enhancing crevice corrosion in metals exposed to high-temperature water containing chloride and sulfate ions as impurities.     

Effect of Tryptophan and Asparagine Structure on the Enthalpic Characteristics of Their Dissolution in Aqueous Solutions of Sodium Dodecyl Sulfate

The integral enthalpies of dissolution of L-tryptophan and L-asparagine in aqueous solutions of sodium dodecyl sulfate (surfactant) at surfactant concentrations of up to 0.05 mol/kg of the solvent are determined and estimated calorimetrically. Standard values of the enthalpies of dissolution and transfer of amino acids from water to a mixed solvent are calculated. The calculated enthalpy coefficients of pair interactions between amino acids and surfactant molecules have positive values. Hydrophobic interactions between amino acids and surfactants have the dominant effect on the enthalpy characteristics of the interaction in a three-component solution.     

Interactions of some amino acids and glycine peptides with aqueous sodium dodecyl sulfate and cetyltrimethylammonium bromide at T=298.15 K: a volumetric approach

The apparent molar volumes V_φ of glycine, alanine, valine, leucine, and lysine have been determined in aqueous solutions of 0.05, 0.5, 1.0 mol · kg⁻¹ sodium dodecyl sulfate (SDS) and 1.0 mol · kg⁻¹ cetyltrimethylammonium bromide (CTAB) by density measurements at T=298.15 K. The apparent molar volumes have also been determined for diglycine and triglycine in 1 mol · kg⁻¹ SDS and CTAB solutions. These data have been used to calculate the infinite dilution apparent molar volumes V₂⁰ for the amino acids and peptides in aqueous SDS and CTAB and the standard partial molar volumes of transfer (Δ_trV_2,m⁰) of the amino acids and peptides to these aqueous surfactant solutions. The linear correlation of V₂⁰ for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups (NH₃⁺, COO⁻), CH₂ group and other alkyl chains of the amino acids to V₂⁰. The results on the partial molar volumes of transfer from water to aqueous SDS and CTAB have been interpreted in terms of ion–ion, ion–polar and hydrophobic–hydrophobic group interactions. The volume of transfer data suggests that ion–ion or ion–hydrophilic group interactions of the amino acids and peptides are stronger with SDS compared to those with CTAB. Comparison of the hydration numbers of amino acids calculated in the present studies with those in other solvents from literature shows that these numbers are almost the same at 1 mol · kg⁻¹ level of the cosolvent/cosolute. Increasing molality of the cosolvent/cosolute beyond 1 mol · kg⁻¹ lowers the hydration number of the amino acids due to increased interactions with the solvent and reduced electrostriction.     

Solubility and solubility constant of barium sulfate in aqueous sodium sulfate solutions between 0 and 80°C

The solubilities of barium sulfate in aqueous solutions of sodium sulfate were studied at ionic strengths from 0.0005 to 0.02 mol-kg^–1 and at 0, 20, 40, 60, and 80°C. The solubility data were used to calculate the solubility constants using an extrapolation method. The solubility constant obtained were in good agreement with literature data. The mean activity coefficients of barium sulfate in aqueous solutions of sodium sulfate were also studied.     

Thermodynamic Studies of Amino Acid–Denaturant Interactions in Aqueous Solutions at 298.15 K

As proteins and other biomolecules consisting of amino acid residues require external additives for their dissolution and recrystallization, it is important to have information about how such additives interact with amino acids. Therefore we have studied the interactions of simple model amino acids with the additives urea and guanidine hydrochloride in aqueous solutions at 298.15 K, using vapor pressure osmometry. During the measurements, the concentration of urea was fixed as ∼2 mol⋅kg⁻¹ and that of guanidine hydrochloride was fixed as ∼1 mol⋅kg⁻¹ whereas the concentrations of amino acids were varied. The experimental water activity data were processed to get the individual activity coefficients of all the three components in the ternary mixture. Further, the activity coefficients were used to get the excess Gibbs energies of solutions and Gibbs energies for transfer of either amino acids from water to aqueous denaturant solutions or denaturant from water to aqueous amino acid solutions. An application of the McMillan-Mayer theory of solutions through virial expansion of transfer Gibbs energies was made to get pair and triplet interaction parameter whose sign and magnitude yielded information about amino acid–denaturant interactions, relative to their interactions with water. The pair interaction parameters have been further used to obtain salting constants and in turn the thermodynamic equilibrium constant values for the amino acid–denaturant mixing process in aqueous solutions at 298.15 K. The results have been explained in terms of hydrophobic hydration, hydrophobic interactions and amino acid–denaturant binding.     

Correlation and prediction the activity coefficients and solubility of amino acids and simple peptide in aqueous solution using the modified local composition model

In this work, the modified Wilson model was used to obtain the activity coefficients of amino acids and simple peptides in non-electrolyte aqueous solutions. The Wilson model was modified using the new local mole fraction proposed by Zhao et al. and non-random case for the reference state. The binary interaction parameters (BIP) of the modified Wilson model for amino acid–water pairs were obtained using the experimental data of the activity coefficients for amino acids available in the literature. The modified Wilson model was also used to correlate the solubility of amino acids in water and the values of Δh/R, Δs/R, and Δg/R of the solutions studied were reported. The results obtained showed that the modified Wilson model can accurately correlate the activity coefficients as well as the solubility of amino acids and simple peptides in aqueous solutions. Also the modified Wilson model was coupled with the Pazuki–Rohani model to correlate the mean ionic activity coefficients of electrolytes in aqueous amino acid solutions. The results showed that the proposed model can accurately correlate the activity coefficients of the electrolytes in aqueous amino acid solution.     

Electrophoretic Behaviour of 34 Organic Acids on Impregnated Strips

Abstract

Electrophoretic behaviour of 34 organic acids on calcium sulphate impregnated papers has been studied. The 19 binary separations in aqueous sodium chloride and sodium sulphate solutions and distilled water have been achieved.     

The effect of ammonium sulfate on the solubility of amino acids in water at (298.15 and 323.15) K

Using the analytical gravimetric method the solubility of glycine, dl-alanine, l-isoleucine, l-threonine, and l-serine in aqueous systems of (NH₄)₂SO₄, at (298.15 and 323.15) K, were measured for salt concentrations ranging up to 2.0 molal.In the electrolyte molality range studied the experimental observations showed that ammonium sulfate is a salting-in agent for most of the amino acids studied. Furthermore, the change of the relative solubility with electrolyte concentration shows a maximum, which makes the representation of the data by a simple empirical correlation such as the Setschenow equation difficult. For the development and evaluation of a robust thermodynamic framework that makes it possible to more profoundly understand aqueous amino acid solutions with ammonium sulfate additional experimental information is needed.     

Partial molar volumes of l-alanine,dl-serine,dl-threonine,l-histidine,glycine, and glycylglycine in water,NaCl, and DMSO aqueous solutions at T = 298.15 K

The apparent molar volumes of l-alanine, dl-serine, dl-threonine, l-histidine, glycine, and glycylglycine in water and in the aqueous solutions of NaCl and DMSO with various concentrations at T = 298.15 K have been measured by the precise vibrating-tube digital densimeter. The calculated partial molar volumes at infinite dilution have been used to obtain corresponding transfer volumes from water to various solutions. The experimental results show that the standard partial molar volumes of the above amino acids and peptide at the dilute DMSO aqueous solutions are very close to those in water. However, the volumes show several types of variations with the increase of the concentrations of DMSO due to different types of side chain of amino acids, which should be discussed specifically. The NaCl changes considerably the infinite dilution standard partial molar volumes of the above amino acids and peptide in the aqueous solutions. The infinite dilution standard partial molar volumes of the each amino acids and peptide increase with the concentrations of NaCl. The experimental results have been rationalized by a cosphere overlap model.     

Thermodynamic Studies on Amino Acid Solvation in Aqueous Urea

The present paper discusses the behaviour of transfer free energy of some amino acids from water to 4M, 6M and 8M aqueous urea. Dissection of transfer free energy into cavity term, interaction term and electrical term reveals that cavity forming free energy of transfer ΔG⁰_t (cav) plays an important role in dictating actual interaction of amino acids in aqueous urea. Cavity forming free energy of transfer has been estimated by using Scaled Particle Theory (SPT).     

Esterification of C2−C16 dicarboxylic acids to n-butyl esters in aqueous solutions for their gas chromatographic analysis

Summary C₂–C₁₆ dicarboxylic acids were esterified in aqueous solution in the presence of sulfuric acid. Esterification in water/n-butanol mixtures with mole ratios between 0.02 and 2.53 can be utilized for the quantitative determination of the dicarboxylic acids by gas chromatography. The presence of water does not interfere at water/n-butanol mole ratios below 0.27. For mole ratios above 0.27 anhydrous sodium sulfate has been used for binding the water. The mole ratio range was 0.25–0.75 for anhydrous sodium sulfate/water, and 0.32–1.3 for sulfuric acid/anhydrous sodium sulfate.     

Solubility of proline–leucine dipeptide in water and in aqueous sodium chloride solutions from T = (288.15 to 313.15) K

Solubility of proline–leucine dipeptide, in water and in aqueous sodium chloride solutions, was measured at T = (288.15, 298.15, 308.15 and 313.15) K as a function of electrolyte concentration m = (0.1, 0.25, 0.5, 0.75 and 1) mol · kg⁻¹ of water. Solubility data has been evaluated from density measurements using a vibrating tube densimeter. It has been observed that sodium chloride renders the dipeptide proline–leucine more soluble in water. Salting-in coefficients and standard free energies of transfer of proline–leucine, from water to aqueous sodium chloride solutions, have been calculated from the solubility data. Standard enthalpies and entropies of transfer have also been estimated and interpreted in terms of electrostatic and hydrophobic perturbed domains in the hydration shells of the dipeptide and of the cation and anion of the salt, as a function of temperature and salt concentration.     

Paper Chromatography of 34 Organic Acids on Calcium Carbonate and Calcium Sulphate Impregnated Papers

Abstract

Paper chromatographic separations of 34 organic acids on calcium carbonate and calcium sulphate impregnated paper has been developed. Tricarboxylic acids, dicarboxylic acids, amino acids, and keto acids and their derivatives can be separated in simple solvents such as acetone, ethanol, water, aqueous sodium chloride, ammonium chloride and calcium nitrate. Important separations achieved are: Adipic from maleic and malonic, cis-aconitic from I-cystine and nicotinic, citric or tartaric from ascorbic, gallic, malic and salicylic, I-aspartic from arginine-HCl, fumaric and glutamic, barbituric from hippuric and α-ketoglutaric, benzoic from ρ- & m-nitrobenzoic and cis-aconitic from trans-aconitic.     

Volumetric and compressibility behaviour of poly(propylene glycol) – Amino acid aqueous solutions at different temperatures

Precise density and sound velocity measurements have been carried out for aqueous solutions of PPG725 in the absence and presence of (0.2 and 0.5) mol · kg⁻¹ amino acids: alanine, glycine, serine and proline, and also for aqueous solutions of these amino acids in the absence and presence of 0.01 w/w PPG725 at T = (288.15, 293.15, 298.15, 303.15 and 308.15) K. From the experimental density and sound velocity values, the apparent molar volume and isentropic compressibility have been obtained and extrapolated to infinite dilution. The infinite dilution apparent molar properties for transfer of PPG from water to aqueous amino acids solutions and also those for transfer of amino acids from water to aqueous PPG solutions have been studied. Temperature dependency of the infinite dilution apparent molar volume was utilised to determine structure-breaker or structure-maker effects of the solutes. Hydration numbers of the amino acids in the investigated aqueous solutions have been evaluated from the volumetric and compressibility properties. All results are discussed based on the salting-out aptitude of the amino acids (hydrophilic + hydrophobic) interactions and (hydrophobic + hydrophobic) interactions occurred between PPG and the investigated amino acids.     

Gas chromatographic analysis of C2−C16 dicarboxylic acids esterified in aqueous solutions as n-propyl esters

Summary C₂−C₁₆ dicarboxylic acids were esterified in aqueous solution in the presence of sulfuric acid. Esterification in water/n-propanol mixtures with mole ratios between 0.03 and 2.21 can be utilized for the quantitative determination of dicarboxylic acids by gas chromatography. The presence of water does not interfere at water/n-propanol mole ratios below 0.45. For mole ratios above 0.45 anhydrous sodium sulfate has been used for binding the water. The mole ratio range tested was 0.12–0.99 for anhydrous sodium sulfate/water, and 0.16–2.55 for sulfuric acid/anhydrous sodium sulfate.