Metal Binding Selectivity of Oligopeptides

Abstract

Oligopeptides provide one of the major groups of organic substances that binds metal ions. As a consequence, a huge number of papers has already been published on the transition metal peptide complexes and the most important observations were reviewed by several authors [1–3]. Complex formation processes between transition elements and peptide molecules have been studied by a number of different techniques and it has been well-identified that terminal amino and subsequent amide nitrogens are the main metal binding sites. Among metal ions palladium(II), copper(II) and nickel(II) were reported to form especially stable complexes even with simple dipeptides. In these, the nitrogen of the deprotonated amide group act as the strongest donor, but terminal carboxylic and amino groups, as well as the carbonyl oxygen being also implicated in metal binding. In case of oligoglycines the complex formation involves the succesive formation of IN-, 2N-, 3N- or 4N-coordinated species with increasing pH. The values pK = 2, 4 and 8 were reported for the metal ion induced deprotonation and coordination of peptide nitrogens in the palladium(II), copper(II) and nickel(II) complexes, respectively. The deprotonation and coordination of the amide groups, however, depend on the additional donor groups present in the side chains of peptide molecules. The effects of imidazole nitrogen, carboxylate oxygen and various sulfur donor atoms are discussed in this study.