Structures and Properties of Metal Binding Sites in Biological and Model Systems

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With a view to obtaining basic information regarding the metal complex in biological systems, we investigated the modes of biomoleculemetal ion interactions and the structures of the resulting complexes.1. Aromatic Ring Stacking and Electrostatic Interactions between Coordinated Ligands. X-ray crystal structure analysis of ternary copper(II) complexes containing histamine and phenylalanine or tyrosine disclosed the aromatic ring stacking between coordinated imidazole and the side chain aromatic ring of the amino acids. Aromatic ring stacking has also been detected between uncoordinated indole and copper(II)-coordinated 1,10-phenanthroline. Intramolecular electrostatic ligand-ligand interactions in ternary copper(II) complexes of ligands with oppositely charged side chains were concluded for the complexes in solution on the basis of the intermolecular interactions established in the solid state between the carboxylate group and the guanidinium group from two adjacent molecules.2. Coordi … More nation Structures and Properties of Copper(II)-Pterin Complexes. Folic acid, a pterin cofactor, has been found to react with copper(II)-2,2'-bipyridine to give a ternary complex containing 2,2'-bipyridine and pterin-6-carboxylate as a result of the Cu(II)-catalyzed oxidative cleavage of the side chain of folic acid. The structure of the complex revealed by X-ray analysis shows that the pterin ring coordinates to copper(II) through the N(5) atom with its ring perpendicular to the coordination plane. An equatorial position is occupied by a water molecule, which was found to be easily displaced by imidazole to give a quaternary complex. This suggests that the position may be the substrate binding site.3. Palladium(II)-Indole and Platinum(II) Complex-Indole Interactions. Tetrachloropalladate(II) reacted with alkylindoles and indole-3-acetate to give complexes where the indole ring coordinates to palladium(II) through its nitrogen in the 3H-indole form. Platinum(II) complexes with coordinated aromatic diamines interacted with indole-3-acetate etc. to form adducts, whose stability constants and thermodynamic parameters showed that the adduct formation is mainly due to stacking between the coordinated aromatic diamine and the indole ring. Less