PROTON NUCLEAR MAGNETIC RESONANCE STUDY OF METAL-GLYCINE PEPTIDE COMPLEXES . COPPER(2) AND NICKEL(2) COMPLEXES

Glycine peptides and their complexes with copper(II) and nickel(II) have been studied by aqueous (D2O) proton nmr spectral measurements. For each dissociation step, as the pH is increased, the proton nmr peaks of the ligand methylene group nearest to the site of proton dissociation shift to higher field. In the presence of relatively small amounts of copper(II) or nickel(II) ion, these peaks disappear or broaden and the order of such change with increasing concentration of metal ion suggests the manner in which metal ion coordinates to the ligand and the sequence of coordination under changing solution conditions. Paramagnetic octahedral nickel(II) complexes undergo transition to diamagnetic planar forms with displacement of protons from the peptide linkages present in triglycine and tetraglycine. Direct evidence for the nature of this diamagnetic complex is obtained by proton nmr spectra. Contrary to the line broadening observed for paramagnetic metal complexes, three methylene peaks are observed for the nickel-triglycine complex and four methylene peaks, of which two peaks overlap, are observed for the nickel(II)-tetraglycine complex. © 1969, American Chemical Society. All rights reserved.