Calculation of acidic dissociation constants of glycylglycine in water at different temperatures using ab initio methods

In this study, acidic dissociation constants of glycylglycine were determined using ab initio and density functional theory (DFT) methods at 25, 30, 35, 40 and 45 °C in aqueous solution. To explain the pKa values obtained, the molecular conformations and solute-solvent interactions of the anions, cations, and neutrals molecules of glycylglycine were investigated. Several ionization reactions and equilibria in water, which possess a high hydrogen-band-donor ability were proposed. The mentioned reactions and equilibria constituted the necessary theoretical basis to calculate the acidity dissociation constants of glycylglycine. Basis sets at the B3LYP/6-31+G(d) level of theory were used for calculations. Tomasi's method was also used to analyze the formation of intermolecular hydrogen bonds between the existent species and water molecules. It was concluded that in alkaline aqueous solutions, the neutral, anion and cation species of glycylglycine are solvated with some molecules of water. The thermodynamic parameters have been obtained for dissociation reactions of glycylglycine from the pKa values and their temperature dependence. There is a good agreement between the experimentally determined pKa values for the acid-base reactions obtained by potentiometric with those calculated theoretically.