Potentiometric and spectroscopic (absorption, circular dichroism and electron paramagnetic resonance) study on the coordination of two angiotensin II fragments (Asp-Arg-Val-Tyr-Ile-His and Arg-Val-Tyr-Ile-His) to Cu(II) ions has shown that competition between amino and imidazole nitrogens to anchor metal ions is a complicated process and may lead to formation of macrochelate rings. The important factor that influences this competition is the distance between competing His and N-terminal residues (number of spacer residues in a peptide sequence).