Structures and Reactivities of Quaternary Metal Complexes as Enzyme Models

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With a view to elucidating the structures and reaction mechanisms of metalloenzyme-cofactor-substrate intermediate complexes, we investigated the structures, stabilities, and reactivities of ternary and quaternary complex systems as biological models.1. Aromatic ring stacking in ternary Cu (II) complexes, [Cu (bpy) (L-Trp) ClO_4]and[Cu (bpy) (L-Tyr)ClO_4]・2H_2O (bpy = 2, 2'-bipyridine ; Trp =tryptophan ; Tyr = tyrosine), has been established in the solid state between bpy and the side chain aromatic ring of Trp and Tyr, respectively, by X-ray crystal structure analysis. Upon phosphorylation of the Tyr phenol OH group, the phenol ring is involved in electrostatic interactions with the charged side group of arginine or lysine coordinated to the same Cu (II) ion. The result suggests the possibility that Tyr phosphorylation in proteins causes the change in the mode of interactions between the side chain groups of amino acid residues and thus gives rise to protein conformational changes.2. … More Adduct formation between platinum DNA intercalators[Pt (L) (en)] (L = bpy, 1, 10-phenanthroline (plien), 3, 4, 7, 8tetramethyl-1, 10-phenanthroline (Me_4phen) ) and mononucleotides (NMP = AMP, GMP, CMP) has been studied by spectroscopic methods. [pt (Me_4phen) (en)]with the largest hydrophobic area has been found to form the most stable adducts with NMP mainly through the stacking between L and the base moiety of NMP. It was concluded that the electrostatic interactions between the NMP phosphate group and Pt^<2+> or the coordinated en NH_2 group contribute to the adduct stability.3. Structures and stabilities of ternary and quaternary Cu (II) complexes involving a pterin cofactor model have been investigated by spectroscopic methods and potentiometry. Cu (bpy) (PC) (PC pterin-6-carboxylate) as a phenylalanine hydroxylase active site model was found to form quaternary complexes with L = imidazole (im), NH_3, or N_3^-, and the stability constants for the Cu (bpy) (PC) (im) system were successfully detemined by pH titrations at 25ﾟC and I = 0.1 M (KNO_3). The redox reactions between Cu (bpy)^<2+> and a tetrahydropterin were followed by spectroscopic and chromatographic methods, which revealed that the pterin is oxided to the dihydro form with, consumption of dioxygen. Less