Copper atomic spectra

FIGURE 7.14 The absorption spectrum, in a narrow portion of the ultraviolet region, of gaseous copper atoms. 

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... 

The a isomer (deep blue, triclinic) of the dmaep complex is also dinuclear but six-coordinate. The IR spectrum of this form shows very intense peaks at 1145,1128, and 1072 cm-1, which have been assigned to bidentate perchlorate. This deduction has been confirmed by crystallographic evidence (10). The perchlorates are positioned directly above and below the planar central dimeric [(dmaep)Cu(/t-OH)2Cu-(dmaep)]2+ unit (Fig. 11), each linked to the two copper atoms via two... 

VT 13C and 31P NMR studies have shown that the C2 unit undergoes a rapid fluxional process, assigned to a rotation within the Cu4 array with associated flipping of the dppm ligands. Coalescence of the 31P resonances found at 8 —9.6 (P attached to n-bonded Cu) and —12.3 (P attached to a-bonded Cu) occurred at 182 K, while the carbon resonances in the 13C-enriched complex coalesced at 208 K to give a nine-line resonance at 8 149.4 [/(CP) 13.3 Hz]. For the two processes, AG values of ca. 38 and 47 kJ mol-1 were measured. The relatively facile process is indicative of a weak interaction between the C2 ligand and the four copper atoms, which was confirmed by DFT calculations (calc. AG 32.9 kJ mol-1). The UV-vis spectrum contains bands at 262 (dppm intra-ligand transfer), 374 nm (C2 —> Cu4 LMCT), while excitation (X > 350 nm) produces intense... 

This example illustrates the problems of interpreting photoemission spectra, since the location of CO on Cu(100) is found to be on a top-site by LEED, while Xa-SW calculations either favor a 4-fold hollow site or are inconclusive. Yu/130/ has carried out calculations for CuCO and Cu5CO clusters with CO bonded to the Cu surface through the carbon atom. In CuCO the CO molecule was put directly on top of a Cu atom and in Cu5CO it was located in the four-fold coordinated hollow site, simulating CO adsorbed perpendicular to the (100) surface. The calculated valence density of states for the four-fold adsorption site gives a satisfactory interpretation of the two main peaks observed below the copper d-band in the UV photoemission spectrum (UPS)./131/ An observed weak third peak can be correlated with a minority of CO molecules adsorbed directly on top of a copper atom. 

Included in this latter set is a double mutant protein, a T1D/T2D protein that lacks both the type 1 and the type 2 copper atoms. Only the type 3 binuclear cluster contributes to the nonprotein absorbance in this protein, demonstrating that the shoulder at 330 nm is due to this cluster. This cluster also contributes a broad absorbance centered at 720 nm as the spectrum of this double mutant demonstrates. The absorbance of the wild-type protein at 608 nm is clearly due to the type 1 Cu(II) since it is seen only in protein forms that possess this site. The spin Hamiltonian and absorbance values for the copper sites in FetSp are summarized in Table I. Additional properties of these copper site-depleted Fet3p mutant proteins are discussed below. 

One recombinant FetSp mutant is unique among multicopper oxidase species and has been particularly informative about the structure of the type 3 binuclear cluster in these species. This is the T1D/T2D double mutant that contains only this type 3 site (Blackburn et al., 2000). EXAFS analysis of this protein contains contributions from electron ejection and scattering from only the type 3 copper atoms and thus provides direct structural information about this cluster. The K-edge XAS spectrum for this mutant in its oxidized and reduced states is shown in Fig. 21. The oxidized sample has a nearly featureless edge with a midpoint energy of 8990 eV typical of tetragonally distorted type 2 Cu(ll) centers, i.e., those with predominantly histidine imidazole coordination. The reduced type 3 cluster exhibited a pronounced shoulder at 8984 eV just below the... 

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