Cap43 protein has been tested for metal binding domains. The protein, specifically induced by nickel compounds in cultured human cells, had a new mono-histidinic motif consisting of 10 amino acids repeated three times in the C-terminus. The 20-Ac-TRSRSHTSEG-TRSRSHTSEG (Thr(341)-Arg-Ser-Arg-Ser-His(346)-Thr-Ser-Glu-Gly-Thr-Arg-Ser-Arg-Ser-His(356)-Thr-Ser-Glu-Gly(360) - peptide 1) and the 30-Ac-TRSRSHTSEG-TRSRSHTSEG-TRSRSHTSEG (Thr(341)-Arg-Ser-Arg-Ser-His(346)-Thr-Ser-Glu-Gly-Thr-Arg-Ser-Arg-Ser-His(356)-Thr-Ser-Glu-Gly-Thr-Arg-Ser-Arg-Ser-His(366)-Thr-Ser-Glu-Gly(370) - peptide 2) amino acids sequence has been analyzed as a site for Ni(II) binding. A combined pH-metric and spectroscopic (UV-visible, CD, NMR) studies of Ni(II) binding to both fragments were performed. The 20-amino acid peptide can bind one and two metal ions while the 30-amino acid fragment one, two and three metal ions. At physiological pH, depending on the metal to ligand molar ratio, peptide 1 forms the Ni(2)L species while peptide 2 the NiL, Ni(2)L and Ni(3)L complexes where each metal ion is coordinated to the imidazole nitrogen atom of the histidine residue of the 10-amino acid fragment. Octahedral complexes at pH 8-9 and planar 4N complexes with (N(Im), 3N(-)) bonding mode at pH above 9, are formed. This work supports the existence of an interesting binding site at the COOH-terminal domain of the Cap43 protein.