L-histidine-containing peptides as models for the interaction of copper (II) and nickel (II) ions with sperm whale apomyoglobin.

1. The association and ionization constants for nickel (II) and a selection of L-histidine-containing peptides have been computed together with the constants for copper (II) and acetylglycylglycyl-L-histidylglycine. 2. A close parallel between the titration behavior of copper (II) apomyoglobin complexes and model peptides has been obtained on the assumption that 1 of the 4 bound metal ions is situated at the NHz-terminal portion of the polypeptide chain. 3. Visible absorption spectra of nickel (II) and copper (II) apomyoglobin complexes containing 4 metal ions per mole have been measured and found to be consistent with the binding of 1 metal ion at the NHr-terminal locus and 3-metal ions in the interior of the peptide chain at histidine loci. dichroism spectra of complexes of copper (II) and nickel (II) ions with sperm whale apomyoglobin and selected n-histidinecontaining peptides. In particular, it deals with acetylglycylglycyl-n-histidylglycine, which may be regarded as perhaps a better model than those considered hitherto for the histidine environment in a polypeptide chain. As an adjunct to the present study, the titration behavior of the complexes of nickel (II) and n-histidine-containing peptides has been investigated together with the copper (II) complex of acetylglycylglycyl-nhistidylglycine which extends work of this type already reported (4, 5).