Plastocyanin electronic structure

Reorientation of Solomon s description of electronic structure of plastocyanin... 

Additional information has been obtained from single crystal, polarized optical and ESR spectroscopic studies924 on poplar plastocyanin, which have allowed a correlation of the electronic structure of the blue copper active site with its geometric structure. In summary, the three dominant absorption bands at 13 350, 16 490 and 17 870 cm-1 were assigned to CysS- Cu (d 2-,2 charge-transfer transitions. The methionine makes only a small contribution, due to the long Cu—S(Met) bond (2.9 A) and the poor overlap of the methionine sulfur orbitals with the dx y orbital of copper. Histidine-Cu charge transfer contributes to the weaker absorptions at 21 390 and... 

Figure 13. Left ligand field energy-level diagram calculated for plastocyanin. Center contains energies and wavefunctions of the copper site. Energy levels determined after removing the rhombic distortions to give and C symmetries are shown in the left and right columns, respectively (from Ref. 11). Right electronic structural representation of the plastocyanin active site derived from ligand field calculations (from Ref. 11).

Stellacyanin, the plastocyanins, and the azurins are the most widely studied copper-containing metalloproteins of the next active-site class, the Blue Copper sites. These proteins, which generally appear to be involved in redox chemistry, have quite unique spectral features32,33). The potential for complementary interaction between inorganic spectroscopy and protein crystallography is well demonstrated by the roles that they have played in generating fairly detailed geometric and electronic structural pictures of the Blue Copper metal centers. 

Fig. 18. Electronic structural representation of the plastocyanin active site. Note that the Cu-S(Met) bond is only 5° off gz, while the bonds from Cu to N(His 37), N(His 87), and S(Cys) are all less than 15° below the xy plane (from Ref. 44)...

The classic blue copper sites in plastocyanin and azurin exhibit essentially identical EPR spectra, with approximately axial (gj >= gy) EPR signals. This argues that the long 3.0 Cu-0 A carbonyl oxygen makes little contribution to the electronic structure of azurin, consistent with other spectroscopy and the fact that the relatively compact 0 2p orbitals would be expected to have little contribution to bonding at this distance. On the basis of EPR, the perturbed blue copper sites can be divided into 2 classes (1) those which exhibit a rhombic EPR signal (i.e. A gi = g — g, > 0.01, as in cucumber basic protein, nitrite reductase and stellacyanin, Figure 9)159,160 2) those which are perturbed, but still... 

More recent calculations have shown qualitatively similar descriptions of plastocyanin, reproducing the Cu-S(thiolate) 7T bonding interaction in the HOMO, however, the sulfur 3p character in the HOMO shows a large dependence on the functional used. Here it should be emphasized that spectroscopic results have provided a very detailed description of the electronic structure of blue copper sites, which have been important for the calibration of electronic-structure calculations. ... 

Penfield KW, Gay RR, Himmelwright RS, Eickman NC, Norris VA, Freeman HC, Solomon EL 1981. Spectroscopic studies on plastocyanin single crystals a detailed electronic structure determination of the blue copper active site. J Am Chem Soc... 

Penfield KW, Gewirth AA, Solomon EL 1985. Electronic structure and bonding of the blue copper site in plastocyanin. J Am Chem Soc 107 4519 529. 

Gewirlh AA, Solomon El. 1988. Electronic structure of plastocyanin excited state spectral features. JAm Chem Soc 110 3811-3819. 

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... 

Lowery MD, Guckert JA, Gebhard MS, Solomon El. Active-site electronic structure contributions to electron-transfer pathways in rubredoxin and plastocyanin direct versus superexchange. J Am Chem Soc 2002 115 3012-3013. 

Structure and electron transfer reactivity of the blue copper protein, plastocyanin. A. G. Sykes, Chem. Soc. Rev., 1985,14, 283 (117). 

The electrons subsequently pass to plastocyanin (PC), which is a copper-containing protein. The Cu-containing redox center of this 10.5 kD monomer cycles between Cu(I) and Cu(II) oxidation states. The structure of PC shows that... 

Metalloproteins fall into three main structure categories depending on whether the active site consists of a single coordinated metal atom, a metal-porphyrin unit, or metal atoms in a cluster arrangement. In the context of electron-transfer metalloproteins, the blue Cu proteins, cytochromes, and ferre-doxins respectively are examples of these different structure types. Attention will be confined here mainly to a discussion of the reactivity of the blue Cu protein plastocyanin. Reactions of cytochrome c are also considered, with brief mention of the [2Fe-2S] ferredoxin, and high potential Fe/S protein [HIPIP]. 

It is timely to review the reactivity of plastocyanin in the light of recent aqueous solution studies, and the elegant structural work of Freeman and colleagues on both the PCu(I) and PCu(II) forms (1 2) Plastocyanin now ranks alongside cytochrome c (3) as the electron-transfer metalloprotein for which there is most structural information. 

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