Nuclear Magnetic Resonance Studies of the Solution Chemistry of Metal Complexes. 22. Complexation of Zinc by the Growth-Modulating Tripeptide Glycylhistidyllysine

The binding of zinc by the tripeptide glycyl-L-histidyl-L-lysine (GHL) has been studied by ¹H NMR spectroscopy and by potentiometric titration. At physiological pH, resonances are observed for two kinetically stable complexes, and binding sites in these complexes have been identified from the chemical shifts of carbon-bonded protons of complexed GHL. Formation constants have been determined for Zn(II)-GHL complexes from pH titration data. Proton balance considerations indicate that an extra proton is titrated from the Zn(II)-GHL complexes, while the NMR results indicate that the extra proton is titrated from an amide group. It is concluded from chemical shift data that the GHL in one of the two kinetically stable complexes is tridentate, with Zn(II) coordinated by the deprotonated glycyl amino nitrogen, the deprotonated amide nitrogen of the glycyl-histidyl peptide bond, and the imidazole 1-nitrogen, while the amino group of the lysine side chain is protonated. The results are discussed with reference to the synergistic alteration of growth patterns of hepatoma cells by GHL and Zn(II).⁵.