Tetragonality copper complexes

The ex model has been elaborated in a number of ways. An electrostatic perturbation was added (33) to account for band splittings in the d-d spectra of tetragonal copper(II) ammine complexes where the simple AOM predicted accidental degeneracy the merits of this refinement will be discussed in 2.5.1. Another development has been the introduction of d—s and d—p mixing, which is apparently necessary to account for the d-d spectra of chlorocuprates(II) (34). This requires the additional parameters e, edpa and edpv. 

We have seen that copper(II) is a slowly relaxing metal ion. Magnetic coupling of copper to a fast relaxing metal ion increases the electron relaxation rate of copper, as clearly shown by the NMRD profiles of tetragonal copper(II) complexes reacting with ferricyanide (105) (Fig. 38). The electron relaxation time, estimated from the relaxation rate of the water protons coordinated to the copper ion, is 3 x 10 ° s, a factor of 10 shorter than in the absence of ferricyanide. 

Figure 25 (a) Copper(II) stereochemical structural pathways and (b) the normal modes of vibration of an elongated tetragonal octahedral complex of copper(II), e g. [Cu(NH3)4(SCN)2]... 

Spectroscopic studies of the enzyme-substrate complex show that the 8-(L-a-aminoadipoyl)-L-cysteinyl-D-valine (ACV) thiolate coordinates to the metal center. First, there is a decrease in the Mossbauer isomer shift of the Fe(II) center from 1.2 to 1.0 mm/sec, indicating a more covalent Fe(II)-ligand environment [195], Second, an intense band appears at 390 nm in the visible spectrum of Cu(II)IPNS upon addition of ACV, which is associated with a thiolate-to-Cu(II) charge transfer transition found for tetragonal copper(II) centers [196], Last, EX-AFS analysis of the Fe(II)IPNS-ACV complex indicates the presence of a sulfur scatterer at ca. 2.3 A, which is a distance typical of Fe(II)-thiolate coordination [197,198], The very recently elucidated crystal structure of the Fe(II)IPNS-ACV complex confirms the thiolate coordination [199],... 

The blue copper site exhibits unique spectral properties when compared with those of normal copper complexes. These spectral features include an unusually small copper h perfine splitting of the EPR signal in thej, region (A < 70x10 cm as compared to Aj = 150x10 cm for normal tetragonal copper) [Figure 1] and an... 

Table 1. EPR parameters for tetragonal copper(II) complexes with biologically relevant ligands...

B Ligand field circular dichroism spectrum of optically active tetragonal copper(II) complex (adapted from Ref. 11 o). C Magnetic circular dichroism of the ligand field region of copper(II) complex in (A) scale normalized to 10 K Gauss (adapted from Ref. 11 n)... 

Importantly, such coordination geometries within the polypeptide matrix can effectively accommodate a copper ion in both its Cu(II) and its Cu(I) states. This fact is in marked contrast to that of inorganic copper complexes, where Cu(II) displays preferentially tetragonal coordination. 

Removal of the type 2 copper according to a reported procedure (20) leads to significant decreases of the absorbance at 330 and 750 nm. These decreases indicate that type 2 copper contributes to the absorbance in these regions. For fungal and tree laccase, structures based on tetragonal six, five, or square-planar four coordination, as found in several low-molecular-weight copper complexes, were proposed (37). 

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