Thermodynamics of Mixed-Ligand Nickel(II) Complex Formation with Ornithine and Histidine in Aqueous Solutions

The formation of various mixed-ligand complexes in the Ni(II)–L-histidine (His)–L-ornithine (Orn) system was studied by potentiometric titration, calorimetry, and spectrophotometry. The thermodynamic parameters (log K , Δ r G 0 , Δ r H , and Δ r S ) of complex formation reactions at 298.15 K and ionic strength I = 0.5 (KNO 3 ) were determined. A comparative analysis of the thermodynamic parameters served as the basis to suggest the most likely coordination mode of amino acid moieties in mixed-ligand complexes.