The Imidazole Group and Its Stacking Properties in Mixed Ligand Metal Ion Complexes

Abstract

The imidazole group is an important metal ion binding site in biosystems. However, the fact that this group is also able to undergo stacking and hydrophobic interactions with other aromatic or aliphatic residues is less well known. In this Comment several examples are summarized for the solid state, as well as for solutions of low-molecular-weight mixed ligand complexes with an intramolecular ligand-ligand interaction involving the imidazole ring. Such an interaction occurs, e.g., in Cu(histidinate)(AA) complexes, where AA - = tryptophanate, phenylalaninate or valinate, and the formation degree of the intramolecular adduct decreases in this order. In addition, evidence is presented for a purine-imidazole stack in M(adenosine 5′-triphosphate)(imidazole)²- complexes. It is thus becoming obvious that the imidazole group does not only bind metal ions but also has additional and rather significant structuring properties via the possibility to undergo stacking and to form hydrophobic adducts with other suitable residues.

Key Words:

• hydrophobic interactions in metal ion complexes
• stacking of imidazole group
• intramolecular ligand-ligand interactions
• stacking in metal ion complexes
• ternary complex intramolecular interactions