Trinuclear copper cluster

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

Figure 1. (a) A schematic representation of the overall organization of the molecule of human ceruloplasmin. Domains 2,4, and 6 contain mononuclear copper centers, while the trinuclear copper cluster is located at the interface of domains 1 and 6. (b) An a-carbon ribbon diagram of the human ceruloplasmin molecule viewed along the pseudo threefold axis highlighting the triplication of the structure. Domains 1, 3, and 5 are depicted by striped motifs, whereas domains 2, 4, and 6 are dark shaded. The copper... 

In the elucidation of the X-ray structure of hCP by the method of isomorphous replacement, gold and mercury heavy atom derivatives were utilized. In the case of the mercury derivative, p-chloromercury-benzoate, the heavy atom bound to the free sulphydryl residue, C221, but for the gold cyanide derivative the gold atom was found to bind in the vicinity of the trinuclear copper cluster. A realistic explanation of this... 

A blue, copper-containing glycoprotein present in mammalian blood plasma and containing type 1, type 2, and type 3 copper centers. The type 2 and type 3 copper centers are close together, forming a trinuclear copper cluster. Ceruloplasmin has an important role in the transport and storage of copper ions. Thus, it participates in the metabolism of copper-containing enzymes. 

Figure 8. Proposed electron transfer pathway in blue copper proteins. The plastocyanin wave function contours have been superimposed on the blue copper (type 1) site in ascorbate oxidase (40). The contour shows the substantial electron delocalization onto the cysteine Spir orbital that activates electron transfer to the trinuclear copper cluster at 12.5 A from the blue copper site. This low-energy, intense Cys Sp - Cu charge-transfer transition provides an effective hole superexchange mechanism for rapid long-range electron transfer between these sites (2, 3, 28).

The low-temperature MCD and absorption titration studies (Figure 10) have determined that azide binds to both the type 2 and type 3 centers with similar binding constants. A series of chemical perturbations and stoichiometry studies have shown that these effects are associated with the same azide. This demonstrates that one N3 bridges between the type 2 and type 3 centers in laccase. These and other results from MCD spectroscopy first defined the presence of a trinuclear copper cluster active site in biology (89). At higher azide concentration, a second azide binds to the trinuclear site in laccase. Messerschmidt et al. have determined from X-ray crystallography that a trinuclear copper cluster site is also present in ascorbate oxidase (87, 92) and have obtained a crystal structure for a two-azide-bound derivative (87). It appears that some differences exist between the two-azide-bound laccase and ascorbate oxidase derivatives, and it will be important to spectroscopically correlate between these sites. 

Figure 22. Comparison of oxygen intermediates. A Electronic absorption spectra of the peroxy-intermediate in laccase versus oxyhemocyanin and oxytyrosinase. B Proposed structural differences between peroxide binding in oxyhemocyanin and oxytyrosinase relative to the end-on bound hydroperoxide intermediate at the trinuclear copper cluster in laccase.

Of specific relevance to MCOs are biomimetics that attempt to reproduce the intramolecular electron transfer between a type 1 site and a bi- or trinuclear copper cluster, or model the trinuclear cluster itself Three recent reports exemplify these attempts. Lever and his coworkers synthesized the tetracop-per assembly shown in Figure 5(a) and demonstrated that it was reduced stepwise by 3e Cu(II)-Cu2(II,II)-Cu(II) - Cu(I)-Cu2 (II,II)-Cu(II) — Cu(I)-Cu2 (II,II)-Cu(I) / Cu(I)-Cu2(I,II)-Cu(II) — Cu(I)-Cu2(I,II)-Cu(I). Figure 5(b) illustrates the polarogram for this reduction. EPR spectra were consistent with the reduction scheme proposed and indicated that the last of these 1 e electron transfers was associated with the reduction potential of —0.42 V. In summary, this complex... 

From considerations of these new clusters, as well as other, better-known ones such as those found in cytochrome oxidase (185) and di-and trinuclear copper clusters (186), one can almost get the impression that some lonely metals do not want to be single and need a (redox) partner. This partner could in some cases be an iron ion, and they are linked by a /n-oxo or /x-hydroxo bridge More seriously, as discussed in,... 

Laccase, ascorbate oxidase, and ceruloplasmin are the classical members of the multicopper oxidase family also known as blue oxidases. Recently, a small number of bacterial members of this family have been characterized, including CueO from E. coli a spore-coat laccase (CotA) from Bacillus suhtilis and phenoxazinone synthase from Streptomyces antibioticus The catalyzed reaction of these enzymes except for phenoxazinone synthase is given in Equation (11). A comprehensive overview of the broad and active research on blue copper oxidases is presented in Messerschmidt. Recent results have been included in a review on the reduction of dioxygen by copper-containing enzymes. The nature and number of the different copper sites in blue oxidases has been described in the sections about the type-1 copper site and the trinuclear copper cluster. 

The observed rate of decay of the native intermediate to the resting state of the enzyme and rate of reduction of the trinuclear copper cluster of the latter are both too slow to be consistent with being relevant to the actual enzyme catalytic turnover.Reduction of the native intermediate is however rapid,indicating that this is the most likely catalytically relevant oxidized species and that the usually discussed normal resting state is altogether bypassed in the reaction cycle. 

IRMS indicates that two oxygen atoms are added to the active site upon formation of the TlHg intermediate.Circular dichroism spectra, which probe the T3 site of this intermediate, display strong similarities with that of the oxidized, resting trinuclear copper cluster. Thus, both... 

Stack and co-workers " discovered a new paradigm in copper(I)/02 chemistry, wherein a Cu702 stoichiometry of 3 1 is observed when a copper(I) complex with A-permethylated (li ,2i )cyclohex-anediamine (L) reacts with O2 at —80°C in dichloromethane. In this reaction, O—O cleavage occurs, and a (bis-//3-oxo)-Cu 2Cu cluster forms (see Figure 16). Thus, four electrons are supplied to O2 from three copper ions, similar to what occurs overall in multicopper oxidases, going from the fully reduced to the native intermediate (see Section 8.17.3.4.3). Spectroscopic,structural, " 445 and theoretical " evidence shows that no charge delocalization occurs in this trinuclear copper cluster two metal centers possess a 2+ charge whereas the third is in its 3+ state. 

Scheme 22. Schematic representation of the structural arrangement of the metal ions in the biomimetic models for biological trinuclear copper clusters (structure I), and diagram showing the coupling interaction through hydroxo bridges between one CuA and CuB (structure II).

Mimmi MC, Gullotti M, Santagostini L (2004) Models for biological trinuclear copper clusters. Characterization and enantioselective catalytic oxidation of catechols by the cop-per(I) complexes of a chiral ligand derived from (5)-(—)-l,l binaphthyl-2,2 -diamine. Dalton Trans 2192-2201... 

More recently, a second crystal structure of pMMO from M. trichosporium OB3b was determined, and this had a different metal site composition The mononuclear copper center is absent, and the zinc center is replaced by the copper ion the dinuclear copper center remains (85). For many years now, Chan and co-workers have suggested the presence of at least one trinuclear copper cluster in pMMO based on the observation of an isotropic EPR signal, along with... 

Trinuclear Cu clusters play a central role in catalytic O2 reduction in the multicopper oxidases [1,76]. These enzymes contain at least four Cu centers that are necessary to catalyze the 4e reduction of O2 to H2O with concomitant le oxidations of various substrates (Fig. 6). The electrons are taken up at the T1 blue copper site and transferred 13 A to the trinuclear copper cluster site that is comprised of a T2 and a T3 copper center where O2 reduction occurs [100]. Reaction of the fully reduced enzyme with O2 involves two 2e intermediate stages, where the first 2e reduction step is rate determining (A 2 x 10 M s ) while the second is fast (k > 350 s ), therefore effectively resulting in a single 4e process [101,102]. 

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