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 (His(6)) 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-His(6) system, the complex formation starts at pH 4 and five different species (Ni(His(6))H, Ni(His(6)), Ni(n)(His(6))(n), Ni(n)(His(6))(n)H(-n/2), Ni(n)(His(6))(n)H(-n)) are formed as a function of the pH. Ni(His(6))H involves the coordination of the imidazole nitrogen and a deprotonated amide nitrogen (N(Im), N(-)) resulting in an octahedral geometry. In Ni(His(6)), an imidazole nitrogen is deprotonated and coordinated (2N(Im), N(-)) to the metal ion with a square planar geometry. The aggregated forms result from the extra Ni-N(Im) 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 Ni(n)(His(6))(n).