Calorimetric studies of ternary complexes of Ni(II) and Cu(II) nitrilotriacetic acid and N-acetyloligohistidines

The thermodynamics of Cu2+- and Ni2+-NTA binding to polypeptides containing 2-6 histidine residues in neutral buffered aqueous solutions were characterized by isothermal titration calorimetry (ITC). The apparent dissociation constants of ternary complexes of N-acetylhexahistidine- Ni2+-NTA and N-acetylhexahistidine-Cu2+-NTA were 69.5 lMand 27.1 mu M, respectively. Decreased number of histidine residues in the oligohistidine fragment caused gradual decrease in the stability of the mixed metal complexes and a change in the binding stoichiometry from similar to 1:1 for N-acetylhexahistidine and N-acetylpentahistidine to similar to 0.5:1 for N-acetyltetrahistidine and similar to 0.3:1 for N-acetyltrihistidine, N-acetyldihistidine and N-acetylhistidineligands. In all cases, the binding interactions are shown to be enthalpically favored. Other metal cations formed less stable mixed metal complexes with stability constants in the order of Cu2+ > Ni2+ > Mg2+, Co2+, Zn2+ >> Mn2+, Ca2+. The significance of the data is discussed in terms of important practical applications. (C) 2015 Elsevier B.V. All rights reserved.