Copper ligand

FIGURE 21.18 (a) The Cn site of cytochrome oxidase. Copper ligands inclnde two histidine imidazole groups and two cysteine side chains from the protein, (b) The coordination of histidine imidazole ligands to the iron atom in the heme a center of cytochrome oxidase. 

Associated to copper(II) pre-catalysts, bis(oxazolines) also allowed the asymmetric Diels-Alder and hetero Diels-Alder transformations to be achieved in nearly quantitative yield and high diastereo- and enantioselectivities. Optically active sulfoximines, with their nitrogen-coordinating site located at close proximity to the stereogenic sulfur atom, have also proven their efficiency as copper ligands for these asymmetric cycloadditions. Other precursors for this Lewis acid-catalyzed transformation have been described (e.g., zinc salts, ruthenium derivatives, or rare earth complexes) which, when associated to bis(oxazolines), pyridine-oxazolines or pyridine-bis(oxazolines), led to efficient catalysts. 

Maikov et al. [37] prepared a series of C2-symmetric bipyridine-type ligands, the chiral moieties arising from the isoprenoid chiral pool (/3-pinene, 3-carene, 2-carene, or a-pinene, for example). Some representative examples are drawn in Scheme 16 (see 25, 26, 27) and were used as copper ligands of a copper(I) species obtained by an in-situ reduction of Cu(OTf )2 with phenyl-hydrazine. The use of the resulting catalysts in enantioselective cyclopropana-tion proceeded with up to 76% ee (for ligand 27) and high diastereoselectivity (up to 99 1). 

Kwong and Lee [39] prepared various chiral 2,2 6, 2"-terpyridines and tested them as copper ligands for the cyclopropanation of alkenes. High enantioselectivities were obtained, the presence of bulky alkyl groups at the 8-position of the tetrahydroquinoline ring being crucial (structure 29 in Scheme 17). Thus when = Bu, up to 90% ee for the trans and 94% for the cis isomer were obtained by performing the reaction at 0 °C (transIds = 69/31). 

Tang synthesized pseudo-Cs-symmetric tris(oxazofines) in two steps from the corresponding triester and the amino alcohol [78]. They tested them as copper ligand for enantioselective Diels-Alder reaction [79]. 

From all these results, optically active sulfoximines, with their nitrogencoordinating site located at the close proximity to the stereogenic sulfur atom, have thus proven their efficiency as copper-ligands for asymmetric Diels-Alder and hetero Diels-Alder reactions. 

Cuprates with two different copper ligands have been developed. These compounds have important advantages in cases in which one of the substituents... 

Several catalytic systems based on copper can also achieve allylic oxidation. These reactions involve induced decomposition of peroxy esters (see Part A, Section 11.1.4). When chiral copper ligands are used, enantioselectivity can be achieved. Table 12.1 shows some results for the oxidation of cyclohexene under these conditions. 

Figure. The pH titration of the C2-H proton of His35 and the methyl protons of the copper ligand Met83. The discontinuous nature of the spectral shifts establish the existence of two structures which interconvert slowly on the NMR time-scale. Note that the same pKa is involved, namely that one associated with the protonation of His35. (The spectra were obtained with a Nicolet 8.5T NMR spectrometer, supported by NIH grant RR01077.)...

As an example on the relationship between proton relaxivity, electron relaxation and coordination environment, we report the case of azurin and its mutants. The relaxivity of wild type azurin is very low (Fig. 6) due to a solvent-protected copper site, the closest water being found at a distance of more than 5 A from the copper ion. The fit, performed with the Florence NMRD program, able to take into account the presence of hyperfine coupling with the metal nucleus (Ay = 62 x 0 cm , see Section II.B) indicates Tie values of 8 X 10 s. Although the metal site in azurin is relatively inaccessible, several mutations of the copper ligands open it up to the solvent. The H NMRD profiles indicate the presence of water coordination for the... 

Spectra, but, in general, leaves the copper site the most exposed of the four cupredoxins. The sequence of Cbp is quite similar to that of stella-cyanin. Stellacyanin is a plant protein, also of unknown function, having visible spectra characteristic of type I copper, but lacking the methionine ligand found in all other type I proteins. A disulfide bond has been suggested as a potential copper ligand in stellacyanin the Cbp has both a methionine and the disulfide, so that prior to the structure determina-... 

The location of the copper with respect to the Greek key fold is interesting when compared to that of the cupredoxins. While the copper in the cupredoxins lies in the interior of the /8 barrel bound by three interior-facing residues of the carboxy-terminal loop in the )8 barrel, and by a histidine in an adjacent strand, the copper in SOD lies on the outside of its jS barrel, bound by one residue from the carhoxy-terminal loop and three from the adjacent strand (cf. Figs. 2c-5c with Fig. 8c.) A structural comparison of plastocyanin and SOD, coupled with sequence alignment of plastocyanin and ceruloplasmin (Ryden, 1988), showed that three of the SOD ligands correspond to putative copper ligands in ceruloplasmin. Why this is so will become more evident after the description of the ascorbate oxidase structure and its relationship to ceruloplasmin. 

In the third chapter, Elinor T. Adman presents a comprehensive view of the structures of copper-containing proteins. This view includes the topological folding of many of these proteins, as shown by ribbon drawings, as well as details of copper-ligand interactions. 

A similar connection between copper coordination to peptides and medical consequences exists for the Alzheimer disease.79 The amyloid-p peptide in senile plaque is the site of copper binding and as before there is interest to study details of the coordination and the stability. The neurotoxicity seems to be related to free radical damage and Cu2+ chelators are probed as therapy. So far, there seems to be no investigation at a resolution possible to pinpoint copper ligands as derived from ENDOR or related pulse techniques. We mention some relevant EPR studies as introductory sources into the topic.80 81,82... 

Figure 24 The effect of temperature on the copper-ligand distances of (a) [Cu(methoxyacetate)2(OH2)2] (237) and...

---