The binding of Ni(II) ions to hexahistidine as a model system of the interaction between nickel and His-tagged proteins

The aim of this work is to study the binding of nickel ions to hexahistidine (His6) combining potentiometric titrations and spectroscopic (UV–Vis and circular dichroism) determinations in order to establish the species distribution as a function of the pH, their stoichiometry, stability and geometry. For comparative purposes, the same procedure was applied to the Ni–histidine (His) system. His behaves as a tridentate ligand, coordinating the carboxyl group, the imidazole and the amino nitrogen atoms to Ni(II) ions in an octahedral coordination and a bis(histidine) complex is formed at pH higher than 5. For the Ni–His6 system, the complex formation starts at pH 4 and five different species (Ni(His6)H, Ni(His6), Nin(His6)n, Nin(His6)nH−n/2, Nin(His6)nH−n) are formed as a function of the pH. Ni(His6)H involves the coordination of the imidazole nitrogen and a deprotonated amide nitrogen (NIm, N−) resulting in an octahedral geometry. In Ni(His6), an imidazole nitrogen is deprotonated and coordinated (2NIm, N−) to the metal ion with a square planar geometry. The aggregated forms result from the extra Ni–NIm coordination, resulting in a 4N square planar geometry that is stabilized by inter/intramolecular hydrogen bonds. This coordination mode is not altered during the deprotonation steps from Nin(His6)n.