Copper complexes center types

The complexation of anionic species by tetra-bridged phosphorylated cavitands concerns mainly the work of Puddephatt et al. who described the selective complexation of halides by the tetra-copper and tetra-silver complexes of 2 (see Scheme 17). The complexes are size selective hosts for halide anions and it was demonstrated that in the copper complex, iodide is preferred over chloride. Iodide is large enough to bridge the four copper atoms but chloride is too small and can coordinate only to three of them to form the [2-Cu4(yU-Cl)4(yU3-Cl)] complex so that in a mixed iodide-chloride complex, iodide is preferentially encapsulated inside the cavity. In the [2-Ag4(//-Cl)4(yU4-Cl)] silver complex, the larger size of the Ag(I) atom allowed the inner chloride atom to bind with the four silver atoms. The X-ray crystal structure of the complexes revealed that one Y halide ion is encapsulated in the center of the cavity and bound to 3 copper atoms in [2-Cu4(//-Cl)4(//3-Cl)] (Y=C1) [45] or to 4 copper atoms in [2-Cu4(/U-Cl)4(/U4-I)] (Y=I) and to 4 silver atoms in [2-Ag4(/i-Cl)4(/i4-Cl)] [47]. NMR studies in solution of the inclusion process showed that multiple coordination types take place in the supramolecular complexes. 

The other copper-only binuclear centre to be considered is the CuA or purple copper complex. It is part of the terminal oxidase in mitochondrial respiration, cytochrome c oxidase (COX). Its EPR signature, a seven-line spectrum, has since long been known to be different from the classes type 1 to 3 and arises from two copper ions in a 1.5 valence (or mixed valence) state, first proposed from EPR-analysis of a similar center in nitrous oxide (N20) reductase. There is a close correspondence between the blue and purple states of copper since each of the two copper ions in CuA can be considered as being structurally related to the mononuclear blue site coordination. 

After many attempts with various linkers, we found that 1,10-phenanthroline nuclei connected via their 2-positions by a -(CH2)4- linking unit will indeed form a double helix when complexed to two copper(I) centers. In addition, by introducing appropriate functions at the 9-positions, the strategy of Figure 14 could be followed to achieve the synthesis of a molecular knot of the (D) type. The precursors used and the reactions performed are represented in Figure 16. 

Copper-proteins are wide-spread in both animals and plants and have been related to many metabolic processes, as oxygen transport, electron transfer and hydroxylation Copper-containing sites are usually classified in three different types " the type 1, or blue center is characterized by a combination of properties that has not yet been reproduced in model complexes (an intense absorption band at 600 nm, a very small copper hyperfine coupling constant A and a high positive redox potential for the Cu(II)/ Cu(I) couple) the type 2, or non-blue center has properties comparable to those of low molecular weight cupric complexes the type 3 consists of an antiferromagnetically coupled copper(II) pair. 

A remarkable feature of some copper-proteins is an intensive blue color based on the spectroscopic properties of the type 1 copper center. Type 1 copper centers show a characteristic absorption band at approximately 600 nm and an extinction coefficient exceeding 3000 1 mol-1 cm"1. In comparison, the extinction coefficient of the hexaquacopper(II) complex, [Cu(OH2)6]2+, whose blue color is much less intensive, is only 5-10 1 mol"1 cm"1 [6]. This difference in spectro-... 

Type-2, or normal, Cu has undetectable absorption and the EPR line shape of the low-molecular-mass copper complexes (A >0.014Ocm ). Type-2 copper centers are present in copper-containing oxidases... 

Multicopper oxidases are typically active in the catalytic one-electron oxidation of a variety of diphenolic, polyphenolic, enediolic, and aminophe-nolic substrates 1,53,166,167). The mechanism of these reactions is complex and, as discussed in Section I, it involves a sequence of four one-electron oxidations of substrate molecules. The radical products of these reactions undergo dismutation, as shown in Scheme 21 for the oxidation of ascorbate to semidehydroascorbate radical 168,169). The substrate binds to the enzymes close to type 1 Cu, whereas the trinuclear cluster is only accessible to dioxygen, or other small molecules. This situation is clearly difficult to reproduce in a model system and for this reason the type of model oxidation reactions that have been studied so far using synthetic trinuclear copper complexes is more related to the activity of type 3 Cu enzymes than multicopper oxidases. Nevertheless, such trinuclear complexes open new perspectives in stereoselective catalysis, because one of the metal centers... 

The Type 1 or blue cupric center is present in all the proteins which will be discussed in this review, either alone or in combination with two other types of copper complexes. 

Fig. 14. Electron paramagnetic resonance spectra of low molecular mass copper complexes in the presence and in the absence of bovine serum albumin (BSA). All four copper concentrations are identical. Cu-EDTA served as standard. Cu flonazolac) displays no EPR-signal due the antiferromagnetic coupling of the two copper-centers. After addition of BSA, a signal of the biuret-type is obtained, indicating that the original complex was disrupted. A similar signal is seen after addition of CuSO, to BSA...

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