Metal Ion-Assisted Weak Interactions Involving Biological Molecules. From Small Complexes to Metalloproteins

Noncovalent or weak interactions play important roles in molecular recognition and structual organization in chemical and biological systems. Hydrogen bonding and aromatic ring stacking are widely recognized for biomolecules, and these and other types of weak interactions are recently attracting much attention as a clue to understanding the efficiency and specificity of biological reactions. We studied weak interactions around the metal center as factors for mixed ligand (ternary) metal complex formation, molecular recognition, and construction of self-organized structures by potentiometric, spectroscopic, and crystallographic methods. We concluded the electrostatic ligand–ligand interactions within Cu(II) and Pd(II) complexes containing an acidic and a basic amino acid and aromatic ring stacking interactions between the aromatic rings of an aromatic amino acid and a coordinated aromatic ligand such as 2,2′-bipyridine. Selective adduct formation between Pt(II) complexes and uncoordinated mononucleotides were found to occur through stacking and hydrogen bonding. In the systems with aromatic ring stacking, a close metal–aromatic ring contact was revealed in the solid state, and the 195Pt NMR spectra indicated electron density decrease due to stacking. As an extension of such studies, we investigated metalloprotein–charged peptide electrostatic interactions and established the redox partner binding site of plastocyanin and its subtle structural change due to the interactions. The results demonstrated the use of charged oligopeptides as models for the studies on protein–protein binding. Weak interactions between [Cu(arginine)2]2+ and its counterions, such as benzene-1,3-dicarboxylate, led to controlled self-organization of the complex ion, giving crystalline products with handed single-helical or double-helical structures and layer structures, depending on the anions used.