Stellacyanin copper complexes

Several copper enzymes will be discussed in detail in subsequent sections of this chapter. Information about major classes of copper enzymes, most of which will not be discussed, is collected in Table 5.1 as adapted from Chapter 14 of reference 49. Table 1 of reference 4 describes additional copper proteins such as the blue copper electron transfer proteins stellacyanin, amicyanin, auracyanin, rusticyanin, and so on. Nitrite reductase contains both normal and blue copper enzymes and facilitates the important biological reaction NO) — NO. Solomon s Chemical Reviews article4 contains extensive information on ligand field theory in relation to ground-state electronic properties of copper complexes and the application of... 

The blue copper protein stellacyanin, with a molecular weight of about 20,000, is obtained from the Japanese lacquer tree Rhus vemicifera. The EPR spectrum is described by roughly axial g and ACu hfs tensors and an unusually small a j value. As shown in Fig. 39 a, only the largest copper hf value A u can be directly determined from the EPR spectrum202. This coupling does not lie along the largest g-principal axis, in contrast to the usual behaviour of square planar copper complexes. 

Group 11. Paramagnetic (1H, 15N) NMR spectra were used to study a Cu -IDA (=iminodiacetic acid) complex localised on a protein surface. H and 13C NMR spectra were reported for copper(II) bis-benzimidazole complexes.1225 Variable temperature H NMR spectra of copper complexes of p-octafluorinated triarylcorroles reveal a thermally-accessible paramagnetic excited state, i.e. a Cu11 corrole 7i-cation radical.1226 Copper(II) forms of stellacyanin from Rhus vernicifera were characterised by H NMR.1227... 

The 2,6-dipicolinic acid complex [Co(dipic)2] has recently been used as a lipophilic oxidant to study electron transfer with cytochromes c and cssl and with the blue copper centre in stellacyanin, plastocyanin and azurin penetration of the complex towards the metal centre of the protein is thought to aid electron transfer.257,271... 

The blue copper proteins azurin, plastocyanin, stellacyanin, and umecyanin incorporate Cu bound to a combination of N/thiolate/thioether ligands. An important feature of these metalloenzymes is the facile copper(II)/(I) couple that these species exhibit, which is linked to the highly strained, asymmetric coordination geometry at the metal center. The synthesis of model complexes for these so-called Type 1 copper proteins has been reviewed. ... 

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). 

Stellacyanins are different from other phytocyanins and cupredoxins by the nature of the axial ligand coordinated to the copper the axial ligand is a conserved glutamine instead of methionine. They also exhibit lower reduction potentials (180-280 mV vs. NHE) than other cupredoxins, and undergo redox reactions with small inorganic complexes or at electrodes at unusually fast rates.The stellacyanin subfamily includes stellacyanins from lacquer tree Rhus Vernicifera and cucumber Cucumis sativus, mavicyanin from green zucchini, and umecyanin from horseradish root. 

No association was detected in the comparable reactions of [Co(pdta)] and [Co(cydta)] where the bulky alkyl substituents prevent precursor complex formation by hydrogen-bonding to the reduced protein. The similarity in rate constants and activation parameters for the two reagents suggests that efficiency of electron transfer from stellacyanin is governed by the approach of a carb-oxylate from the ligand to the copper centre rather than the alkylated backbone. 

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