Model complexes for the active sites of reduced and oxidized sites of hemocyanin and tyrosinase. Structures of binuclear Cu(I) and Cu(II) complexes and characterization of a model copper monooxygenase reaction

Abstract

It has been established that the dioxygen carrying copper containing protein hemocyanin and the copper monooxygenase tyrosinase both contain electronically coupled binuclear copper centers. In the deoxy protein, copper centers are thought to be two or three coordinate, while in their oxidized form, tetragonally coordinated Cu(II) ions are bridged by an endogenous protein group which is thought to be phenolate from tyrosine. Coordinated dioxygen (as peroxide) is also known to bridge the two coppers. The binding of other exogenous ligands (azide, chloride, etc.) to the binuclear copper centers has been used to probe the active site structures and some of these may also bridge the Cu(II) ions in methemocyanin or tyrosinase derivatives [1]. Extensive coordination model studies are being carried out [2–4] in order to 1) determine the nature of the ligand donors and coordination geometries of these active site copper centers 2) mimick and understand the coordination chemistry of relevant Cu(I) systems; 3) mimick the Cu(II) binuclear centers in order to relate the spectroscopic and chemical properties of these models and their derivatives (with appropriate probes) to those of the proteins and 4) understand those factors necessary and the mechanism(s) for reversible dioxygen binding and ‘activation’ of dioxygen ( i.e. specific hydroxylation of substrates by the copper monooxygenases). As part of our own extensive investigations into the chemistry of binuclear Cu(I) and Cu(II) moieties, we have employed the binucleating ligand m-XYLpy, I (py = 2-pyridyl), where two tridentate donor groups are separated by a meta-xylyl bridge [5]. A novel binuclear cuprous complex, II, ( See next column) containing well separated trigonally coordinated Cu(I) ions is isolated by the reaction of I with two equivalents of Cu(CH 3CN) 4PF 6 [6]. The coordination geometry and the type of ligands in II makes it a good model complex for the deoxy-sites in the proteins. In addition, II reacts with O 2 in a manner analogous to the copper monooxygenase tyrosinase. Specific hydroxylation of the xylyl ligand occurs producing the phenoxy- and hydroxy- (R = H) doubly bridged binuclear Cu(II) complex III. Manometric O 2 uptake experiments with II and mass spectrometric analyses of the oxidation product III utilizing isotopically labeled dioxygen demonstrate that both atoms of oxygen in III are derived from 18O 2[6, 7]: ▪ The structures of compounds II and III have been proven by X-ray crystallographic studies [5–7]. Compounds III contain tetragonally coordinated Cu(II) ions which are phenolate bridged (Cu⋯Cu = ∼3.1 Å), making them excellent structural models for the proposed active sites in hemocyanin and tyrosinase. Reaction of III (R = H) with HX (X = N 3 − , Cl − results in the replacement of the bridging ligand (-OH) by X with the formation of complexes IV. These have also been structurally characterized and they exhibit coordination geometries very similar to III. In the case of the azide complex (X = N 3 − ), a novel μ-1,1-azido type of bridging is observed [7]. This mode of coordination has not been previously proposed to take place in the protein copper centers. Its possible occurrence should be considered in studies where azide ligand binding is used as a spectral probe of the protein copper centers.