Nickel(II) binding to glycylglycyl-L-tyrosine-N-methyl amide, a peptide mimicking the NH2-terminal nickel(II)-binding site of dog serum albumin: a 1H- and 13C-nuclear magnetic resonance investigation

The nonspecificity of dog serum albumin (DSA) for Ni(II) is mimicked by the simplest tripeptide, glycylglycyl-L-tyrosine-N-methyl amide, which forms a planar complex at high pH. In this study, the 1H and 13C nuclear magnetic resonance (nmr) spectra of the free and complexed peptide are reported. As the pH is increased for the free peptide, the deprotonation of the terminal amino group (pKa = 7.94) is reflected most strongly by the chemical shift changes of the NH2-terminal -CH2CO- unit. Large upfield and downfield shifts for the tyrosine C xi, C epsilon and C gamma carbon resonances occur on the ionization of the phenolic hydroxyl group. The planar Ni(II) complex is in slow exchange on the nmr time scale and is of 1:1 stoichiometry. The greater chemical shift changes on Ni(II) coordination are observed from the protons nearest the peptide and amino nitrogens:amide CH3 (-0.704), Tyr(3) alpha-CH (-0.667), Gly(1) alpha-CH2 (-0.382), and Gly(2) alpha-CH2 (-0.519, -0.487). In the 13C spectrum, the Gly(1) C alpha (+7.58) is most affected. The Ni(II) ion is therefore at the center of four coordinating nitrogens. Changes in the coupling constants for the Tyr(3) -CH-CH2- moiety suggests a mainly gauche conformation with the tyrosyl ring positioned above the plane of coordination and a weak bonding interaction with the Ni(II) ion is indicated. These results provide structural information regarding the reduced affinity of DSA for Ni(II).