Pentaglycine-Ni^II complex: From kinetics to structure

Binding Ni^II to the terminal amine of a peptide depresses totally its nucleophilic character as measured by the rate constant of the reaction of the peptide with 4-nitrophenyl acetate. The structure of Ni ^II(GGGGG) in aqueous solutions was determined by this technique. The results indicate that the ratio of complexes in which the nickel is bound to four deprotonated peptide nitrogen atoms to those in which the nickel is bound to the terminal amine and three deprotonated peptide nitrogen atoms increases as the pH increases. At pH 9.0, this ratio is approximately 1. We measured the nucleophilic properties of the terminal amine of a Ni(peptide) complex as a tool to elucidate the ligation sites of the cation.