Copper coordination complexes, models proteins

In their pursuit of modeling Type I copper proteins, Kitajima et al. reported112 a rare, tetrahedrally coordinated complex (105), which displayed an EPR spectrum consistent with the presence of the unpaired electron in the dz2 orbital.1 They also isolated a square-pyramidal DMF adduct (complex (106)). They were successful in providing structural proof of a copper(II) complex (trigonal pyramidal) with C6F5S -coordinated complex (107), with CuN3S chromo-phore.113 The X-ray analysis (poor data set) of a closely similar complex with Ph3CS as the... 

While the very rigid and, with respect to the type of donor groups and their geometric disposition (two trans-disposed pyridine donors and two cis-oriented tertiary amines), enforced and inflexible geometry precludes an accurate structural and spectroscopic modeling of copper proteins, it was especially feature (3.) that lead to the isolation and characterization of novel model complexes with hemocyanine- and catechol oxidase activities properties (81, 192, 196, 213). In the latter case, it was possible to isolate and structurally characterize complexes with coordinated catechol model substrates with structural features, which have been proposed to be of relevance in the enzyme catalysis cycle, but have not been observed before in low molecular weight complexes (192, 213). 

We propose that the blue proteins have a five-coordinate copper (II) site (58). This proposal is based on the fact that the three-band system of stellacyanin, both in band positions and intensities, is very similar to the LF spectra of several low-spin Ni(II) model five-coordinate complexes (59, 60). In going from low-spin four-coordinate to low-spin five-coordinate sites in Ni(II), large enhancements (X 10) in LF band intensities are commonly observed (59). 

This extended series of ligands and their Cu(II) complexes have been prepared as spectroscopic models (observable in visible and near-IR regions) for determining the geometries and ligand coordination of copper proteins. From their results it was possible to propose the following structures for the copper coordination and geometry in (blue) type I copper proteins and for the copper site in oxyhemocyanin (251). 

R. R. Gagne, J. L. Allison, R. S. Gall, and C. A. Koval (1977), Models for copper-containing proteins Structure and properties of novel five-coordinate copper(I) complexes. J. Amer. Chem. Soc. 99, 7170-7178. 

Among the coordination compounds obtained on the basis of polypyrazolyl-borates, it is worth emphasizing the copper chelates 235 which are still the only biomimetic model of blue copper proteins, reproducing all their physical (UV-and EPR-spectral) properties [441,446-448], Compound 236 [449] is also an example of complexes of this kind of system ... 

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