Metal ion-assisted weak interactions involving biological molecules. From small complexes to metalloproteins

Noncovalent or weak interactions play important roles in molecular recognit ion 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 muc h attention as a clue to understanding the efficiency and specificity of bi ological reactions. We studied weak interactions around the metal center as factors for mixed ligand (ternary) metal complex formation, molecular reco gnition, and construction of self-organized structures by potentiometric, s pectroscopic, and crystallographic methods. We concluded the electrostatic ligand-ligand interactions within Cu(H) and Pd(II) complexes containing an acidic and a basic amino acid and aromatic ring stacking interactions betwe en the aromatic rings of an aromatic amino acid and a coordinated aromatic ligand such as 2,2'-bipyridine. Selective adduct formation between Pt(H) co mplexes and uncoordinated mononucleotides were found to occur through stack ing and hydrogen bonding. In the systems with aromatic ring stacking, a clo se metal-aromatic ring contact was revealed in the solid state, and the Pt- 195 NMR spectra indicated electron density decrease due to stacking. As an extension of such studies, we investigated metalloprotein-charged peptide e lectrostatic 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 st udies on protein-protein binding. Weak interactions between [Cu(arginine)(2 )](2+) and its counterions, such as benzene-1,3-dicarboxylate, led to contr olled self-organization of the complex ion, giving crystalline products wit h handed single-helical or double-helical structures and layer structures, depending on the anions used.