Binary Complexes of Aspartic Acid with Some Metal Ions in Aqueous Solution and Water-dioxane Mixtures

The reaction between amino acids and metal ions has been used as a model for transferring the metallic ions to body tissues. How stable these complexes are formed is our concern. The stability constants of the complexes between L-aspartic acid (Asp) and Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Ce(III) metal ions were determined by potentiometric titrations in aqueous solution and in 25-75%(v/v) of water-dioxane mixtures(I = 0.1 M, NaNO3) at 15, 20, 25, 35, and 45°C.The observed increasing stability order for the corresponding complexes in water and water-dioxane mixtures were as follows:

Kf Cd (II)-Asp< Kf Zn (II)-Asp < Kf Co (II)-Asp< Kf Ni (II)-Asp < Kf Ce(III)-Asp< Kf Cu(II)-Asp

Mixed solvents such as water-dioxane provide a better model for in vivo reactions. It was observed that with increase in the percentage of dioxane, the stability constants were elevated. The results showed that the reactions were exothermic and spontaneous.