Investigation of Solution pKa and Thermodynamic Values of Lamivudine and Pefloxacin Drugs by Ab initio and DFT Methods

In this work we investigate the thermodynamic properties and pKa value of lamivudine and pefloxacin drugs, in aqueous solutions, by ab initio and density functional theory (DFT) methods at different temperatures. Molecular structures and solute–solvent effects of the anions, cations, and neutral molecules of lamivudine and pefloxacin were studied by the polarizable continuum model (PCM). The calculation was done at the DFT-B3LYP/6-31+G(d) level of theory using Tomasi’s method to analyze the formation of intermolecular hydrogen bonds (IHB) in aqueous solution. The pKa1 values of lamivudine and pKa2 values of pefloxacin increase with temperature increase. In contrast, the pKa1 values of pefloxacin decrease when the temperature increases. Further, the thermodynamic properties of the ionization processes (∆H, ∆S and ∆G) of the drugs in aqueous solution were determined and discussed. The results of this work are in good agreement with the literature data at 298.15 K.