Protolytic Equilibria In Aqueous Solutions Research Articles

Thermodynamic Characteristics of Protolytic Equilibria of L-Alanayl-L-isoleucine in Aqueous Solutions

Potentiometry and calorimetry are used to study protolytic equilibria in aqueous solutions of L‑alanyl-L-isoleucine. The equilibrium constants and heat effects of the dipeptide dissociation reactions are determined. The standard thermodynamic characteristics of the investigated equilibria are calculated. The obtained results are compared to the corresponding data for related compounds.

STANDARD ENTHALPY OF FORMATION OF PARAOXYPHENYLGLYCINE AND PRODUCTS OF ITS DISSOCIATION IN AQUEOUS SOLUTION

Depending on the chemical nature of the side radicals, amino acids can be divided into aromatic and aliphatic, as well as amino acids, the presence of non-polar or polar functional groups in the side radicals. Since the characteristics of individual amino acids in the protein are determined by the nature (physico-chemical properties) of their side radicals, which are in a hydrated state. It is also extremely important to study the thermodynamic characteristics of the hydration of the side radicals of amino acids of different chemical nature. To obtain empirical correlations that would establish a connection between the thermodynamic parameters of the interaction of dissolved compounds with the solvent and the size of the molecules to be dissolved (having different physico-chemical nature), it is necessary to accumulate a sufficient amount of experimental data on the heats of dissolution of amino acids. This will allow you to calculate the contribution of intermolecular interactions for different groups of molecules. By potentiometric method the protolytic equilibria in aqueous solutions of paraoxyphenylglycine were investigated. The measurements were carried out at a temperature of 298.15 K and an ionic strength of 0.25 (against the background of potassium nitrate). Calorimetric measurements were carried out on an ampoule calorimeter, with an isothermal shell, a thermistor temperature sensor KMT-14, and an automatic record of the temperature-time curve. The operation of the unit was verified by the integral enthalpy of dissolution in water of crystalline potassium chloride. The agreement between the experimental enthalpies of dissolution of KCl (cr) with the most reliable literature data indicates that there are no systematic errors in the calorimetric system. Samples of para-oxyphenylglycine were weighed on a VLP-200 scale with an accuracy of 2∙10-4 g. The confidence interval of the mean value of ΔH was calculated with a probability of 0.95. A paraxyphenylglycine preparation, «Reachim», was used in the work. The reagent was used without further purification. The RRSU program was used to determine the equilibrium composition of the solutions. Standard enthalpies of combustion and formation of crystalline paraxyphenylglycine were calculated. Thermal effects of dissolution of crystalline paraxyphenylglycine in water and in solutions of potassium hydroxide at 298.15 K were determined with the direct calorimetric method. Standard enthalpies of formation of amino acids and products of their dissociation in aqueous solution were calculated.

Thermodynamic characteristics of protolytic equilibria in aqueous solutions of glycyl peptides

Protolytic equilibria in aqueous solutions of glycyl-DL-serine, glycyl-DL-threonine, and glycyl-DL-valine are investigated by means of potentiometry and calorimetry. Dissociation constants and heat effects of the above dipeptides are determined. Standard thermodynamic characteristics (pK°, ΔdisG°, ΔdisH°, ΔdisS°) of the investigated equilibria are calculated. The obtained results are compared to corresponding data on relative compounds.

Thermodynamic characteristics of acid-base equilibria of DL-α-alanyl-DL-norleucine in aqueous solutions at 298 K

Protolytic equilibria in aqueous solutions of DL-α-alanyl-DL-norleucine are studied via potentiometry and calorimetry. Measurements are made at 298.15 K and ionic strengths of 0.5, 1.0, and 1.5 (against a background of potassium nitrate). The thermodynamic characteristics (pK, ΔG, ΔH, ΔS) of the stepwise dissociation of the dipeptide both in aqueous-salt solutions and in standard solution are obtained for the first time.

Enthalpies and constants of dissociation for D,L-Alanyl-D,L-Serine at 298 K

Protolytic equilibria in aqueous solutions of D,L-alanyl-D,L-serine are studied by means of potentiometry and calorimetry. The dissociation constants and thermal effects of this reaction of the dipeptide are determined at 298.15 K and ionic strengths of 0.1, 0.3, 0.5, and 1.0 (KNO3). The standard thermodynamic characteristics (pK°, ΔrG°, ΔrH°, and ΔrS°) of the studied equilibria are calculated. The final results are compared with the corresponding data on the related compounds.

Effect of temperature on the thermodynamic characteristics of the protolytic equilibria of DL-α-alanyl-β-alanine in aqueous solutions in the range of 298 to 318 K

Protolytic equilibria in aqueous solutions of DL-α-alanyl-β-alanine are studied by calorimetry. Measurements are performed at temperatures of 298.15, 308.15, and 318.15 K; and at ionic strengths of 0.5, 1.0, and 1.5 (against a background of potassium nitrate). Thermodynamic characteristics (pK, ΔG, ΔH, and ΔS) of the stepwise dissociation of dipeptides both in water-salt and standard solutions are obtained for the first time.

Thermodynamics of Protolytic Equilibria in Aqueous Solutions of Biologically Active Ligands

The protolytic equilibria in aqueous solutions of different amino acids and dipeptides were studied by a direct calorimetric method at 288–328 K. The measurements were made with an isothermal-shell calorimeter with automatic temperature recording. The thermodynamic characteristics of the processes of acid-base equilibria were found from the combined results of thermochemical measurements and equilibrium constant data. Gurney concepts were used to analyse the thermodynamic characteristics of reactions. Comparisons of the temperature-dependent and temperature-independent contributions to the Gibbs energy and enthalpy change afford the criteria of the variability in sign of the ΔH of the process, i.e. they allow prediction of the magnitude of the temperature θ at which ΔH = 0.