Nuclear Magnetic Resonance Studies of the Solution Chemistry of Metal Complexes. 21. The Complexation of Zinc by Glycylhistidine and Alanyihistidine Peptides

The binding of zinc by the peptides glycyl-L-histidine and L-alanyl-L-histidine has been studied by ¹H nuclear magnetic resonance spectroscopy and potentiometry. These peptides were chosen as models for the N-terminal region of the β chains of hemoglobin. Zinc(II) binding sites were identified by the effect of binding on chemical shifts of resonances for carbon-bonded protons, and formation constants of the complexes were determined from pH titration data. At physiological pH and above, complexes form which are in slow exchange with the free peptide on the NMR time scale. Correlation of pH titration results with the intensities of the slow-exchange resonances indicate that they correspond to complexes from which an extra equivalent of protons has been titrated. The chemical shifts of the resonances for the complexed ligand provide evidence that the extra proton is titrated from the amide group of the histidine residues, with Zn(II) binding to the N-terminal amino nitrogen, the deprotonated histidine amide nitrogen, and the imidazole 1-nitrogen. The results are discussed with reference to the binding of Zn(II) by hemoglobin.