Proteins, Blue Copper Electronic Spectra

Electronic spectra of metalloproteins find their origins in (i) internal ligand absorption bands, such as n->n electronic transitions in porphyrins (ii) transitions associated entirely with metal orbitals (d-d transitions) (iii) charge-transfer bands between the ligand and the metal, such as the S ->Fe(II) and S ->Cu(II) charge-transfer bands seen in the optical spectra of Fe-S proteins and blue copper proteins, respectively. Figure 6.3a presents the characteristic spectrum of cytochrome c, one of the electron-transport haemoproteins of the mitochondrial... 

Figure 97 Blue copper protein (a) electronic spectrum (b) ESR spectrum (-------------- blue copper ------- normal copper) ...

During electron transfer, the Cua site alternates between the fully reduced and the mixed-valence (Cu +Cu ) forms. Interestingly, the unpaired electron in the mixed-valence form seems to be delocalised between the two copper ions. Several theoretical investigations of the electronic structure and spectrum of the Cua dimer have been published [138-144]. In similarity to the blue copper proteins, it has been suggested that the structure and the properties of the Cua site is determined by protein strain. More precisely, it has been proposed [136] that Cua in its natural state is similar to an inorganic model studied by Tolman and coworkers [145]. This complex has a long Cu-Cu bond (293 pm) and short axial interactions (-212 pm). The protein is said to enforce weaker axial interactions, which is conpensated by shorter bonds to the other ligands and the formation of a Cu-Cu bond. This should allow the protein to modulate the reduction potential of the site [136,146]. 

Electronic difference spectra of the Hg(II)-substituted blue copper protein plastocyanin have been interpreted in terms of an unusually low energy charge transfer from a cysteine S atom to the central Hg(II) atom (188). The Hg(II)-plastocyanin affords a unique opportunity for investigating the coordination geometry via the UV spectrum since the three-dimensional structure of the Hg(II)-protein complex is known to high resolution (46), as is the structure of the native copper protein (48, 74). In plastocyanin. 

Finally, we will discuss the electronic spectra of blue copper proteins. The absorption spectrum of plastocyanin, the best studied blue copper protein, is dominated by a bright band at 16,700 cm (600 nm), giving rise to its bright blue color. However, a more thorough investigation of the experimental spectrum identifies at least six more absorption bands below 22,000 cm , as is shown in Table 4. Several different methods have been used to interpret this spectrum, ranging from the semi-empirical CNDO/S method, over various DFT methods (Xa and time-dependent... 

Calculations of EPR parameters were also performed on some of the complexes. Experimental EPR spectra are either axial (gx = gy-, axial type 1 copper proteins) or rhombic (other blue copper proteins). The results indicate that the geometry is more important than the electronic structure for the rhom-bicity of the spectrum the optimized trigonal structure of Cu(imidazole)2(SCH3)(S(CH3)2) and the crystal structure of plastocyanin both give an axial spectrum, while both the crystal structure of nitrite reductase and the other optimized model of Cu(imidazole)2(SCH3)(S(CH3)2)" give a rhombic spectrum, although the latter structure is mainly n bonded with... 

The blue color of these "type 1" copper proteins is much more intense than are the well known colors of the hydrated ion Cu(H20)42+ or of the more strongly absorbing Cu(NH3)42+. The blue color of these simple complexes arises from a transition of an electron from one d orbital to another within the copper atom. The absorption is somewhat more intense in copper peptide chelates of the type shown in Eq. 6-85. However, the -600 nm absorption bands of the blue proteins are an order of magnitude more intense, as is illustrated by the absorption spectrum of azurin (Fig. 23-8). The intense blue is thought to arise as a result of transfer of electronic charge from the cysteine thiolate to the Cu2+ ion.520 521... 

Type I copper enzymes are called blue proteins because of their intense absorbance (s 3000 M-1 cm- ) in the electronic absorption spectrum around... 

Gaussian curves (normal distribution functions) can sometimes be used to describe the shape of the overall envelope of the many vibrationally induced subbands that make up one electronic absorption band, e.g., for the absorption spectrum of the copper-containing blue protein of Pseudomonas (Fig. 23-8) Gaussian bands are appropriate. They permit resolution of the spectrum into components representing individual electronic transitions. Each transition is described by a peak position, height (molar extinction coefficient), and width (as measured at the halfheight, in cm-1). However, most absorption bands of organic compounds are not symmetric but are skewed... 

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