Thermodynamic Study on the Protonation and Complexation of the Neuroleptic Drug, Gabapentin with Na+, Ca2+ and Mg2+ at Various Temperatures and Ionic Strengths

The current work investigates the protonation constants of gabapentin (GP), 2-[1-(aminomethyl)cyclohexyl]acetic acid and the stability constants for the binding of GP to Ca2+ and Mg2+ in a wide range of temperature and ionic strength conditions [283.15 ≤ T/K ≤ 318.15 and ionic strengths of NaCl(aq), 0.12 ≤ I/mol·dm−3 ≤ 4.84]. The pH-potentiometric titration method was applied for gathering experimental data and determination of solution equilibrium constants. The ΔpK method was used for the determination of calcium and magnesium stability constants due to the low values which were predicted. A Debye–Hückel type equation, Specific Ion Interaction Theory, Pitzer and van’t Hoff equations were used for the modeling of ionic strength and temperature effects. Two species, ML and MHL, were found according to the best model for Ca2+ and Mg2+. Both protonation processes are exothermic based on the enthalpy values at 298.15 K and infinite dilution. A case study has been performed taking into account the speciation of GP in seawater.