Proton Nuclear Magnetic Resonance Studies of the Complexation of Zn(II) with Histidine-Containing Peptides, L-histidyl-L-histidylglycylglycine and L-histidylglycyl-L-histidylglycine

The complexation of Zn(II) with L-histidyl-L-histidylglycylglycine (HisHisGlyGly) and L-histidylglycyl-L-histidylglycine (HisGlyHisGly) was studied by proton nuclear magnetic resonance spectroscopy over the pH range from 3 to 11 at room temperature. In weakly acidic solutions below pH 6, both HisHisGlyGly and HisGlyHisGly coordinated to Zn(II) through amino and imidazole nitrogens at the N-terminal histidyl residue. The imidazole nitrogens of the histidyl residues in the second and third positions which are protonated at this pH could not coordinate to the Zn(II) ion. As the pH was raised, however, the imidazolium protons were released and associated with the metal ion. When the peptide bond between His-His in HisHisGlyGly was deprotonated over pH 7, the imidazole nitrogen of the histidyl residue in the second position was likely to coordinate to the metal ion in place of the imidazole nitrogen at the N-terminal histidly residue. A kinetically stable complex in which the Zn(II) coordinated to three nitrogens from the termianl amino, deprotonated peptide-bond, and imidazole of the histidyl residue in the second position was newly produced. The fourth coordination site of Zn(II) was probably occupied with the imidazole nitrogen of the histidyl residue at the N-terminal of another HisHisGlyGly complex.On the other hand, since HisGlyHisGly did not undergo deprotonation of the peptide bond between His-Gly at the N-terminal, the coordination mode with Zn(II) did not change over pH 6.