THE THEORETICAL CALCULATIONS AND EXPERIMENTAL MEASUREMENTS OF ACID DISSOCIATION CONSTANT AND THERMODYNAMIC PROPERTIES OF GLYCYL-ASPARTIC ACID IN AQUEOUS SOLUTION AT DIFFERENT TEMPERATURES
Shahidi, Sayad Ahmad
In this research work, a potentiometric technic was used to measure the acidic dissociate constants (pKa,s) for glycyl aspartic acid (GLY-ASP) at temperatures (298.15, 303.15, 313.15, and 318.15) K and in 0.1 mol/l ionic strength of chloride sodium. Using this data, we calculated the thermodynamic properties (changes of enthalpy, ΔH, changes of entropy, ΔS, and changes of Gibbs free energy, ΔG) for acidic dissociation reaction of GLY-ASP. All analyses of data were studied in pH = 1.5-11 and in the aqueous solution. In addition, the value of the acid dissociation constants (pKa1, pKa2, and pKa3), the optimized structure, and the thermodynamic properties of GLY-ASP were calculated in aqueous solution at various temperatures by ab initio and DFT methods. Density function theory (DFT) has been used based on the B3LYP/6-31+G(d) theory to explain the obtained acid dissociation constants of GLY-ASP as well as interactions between solvent and solvated cation, anion, and neutral species of GLY-ASP. Thomasi’s method was used to analyze the formation of intermolecular hydrogen bonding between the water molecule and various species of GLY-ASP. In addition, the energy gap of anionic, cationic, and neutral species of GLY-ASP were obtained for dissociation reactions of GLY-ASP. Finally, for GLY-ASP, the theoretically calculated and experimentally determined pKa,s were compared together and a good agreement was observed between them in the first, second, and third ionization constant of GLY-ASP.