Formation and Association Constants of MnCl2, NiCl2 and CuCl2 with Glycine, Lysine and Cysteine in Aqueous Solution at 293.15 K

The complexation reactions between different amino acids (Glycine, L-Lysine, and L-Cysteine) and transition metal cations (Ni2+, Cu2+, and Mn2+) were studied conductometrically in water at 293.15 K. The formation constants (Kf) of the resulting complexes were calculated from the computer fitting of the molar conductance (Λ)-mole ratio (L/M) data. Semi-empirical PM3 calculations were also used to predict the structure of the metal complex by calculating the enthalpy of formation, the geometrical parameters, refractivity, dipole moment and polarizabilities of the suggested structures of the formed complexes in the gaseous state. The association constant (Ka) of MnCl2, NiCl2 and CuCl2 in aqueous solution in the absence and in the presence of glycine and lysine amino acids at 293.15 K were also determined conductometrically. The conductance data for the association process were analyzed using Shedlovsky conductance equation. The free energy changes of the complexation and association processes were evaluated from the temperature dependence of formation and association constants. A computer program was used for calculating the molar conductance, the limiting molar conductance (Λ˚), the formation constant, the association constant, the free energy change of formation (ΔGf) and association (ΔGa) processes, Walden product (Λ˚η) and the triple-ion association constant (K3). The results indicate the formation constant of Mn2+ and Cu2+ with glycine is more in general than that with cysteine and lysine. The results also indicate that the association constant of the studied salts increase in the presence of the amino acids under consideration (glycine and lysine) comparing to that in the absence of these amino acids. The effect of glycine on the association of the studied salts was found to be less than that of lysine. Also the formation and the association processes are spontaneous one.