Blue Type 1 Cu Sites

The crystal structures of blue copper proteins show that the T1 site is in a trigonally distorted tetrahedral environment (Fig. la) [34—40]. The trigonal plane has a very short ( 2.1 A) Cu-S(Cys) and two typical ( 2.0 A) Cu-N His bonds. An additional axial thioether ligand is usually present with a long Cu-S(Met) bond ( 2.8 A), although the Met residue is replaced by a non-ligating one, such as Lys or Phe, in some tree and fungal laccases [41-44]. 

One of the unique spectroscopic features of flie T1 site, as mentioned above, is the small compared to that of the normal Cu(II) centers. There are three contributions to account for the metal hyperfine coupling, which are flie Fermi contact (. f), the spin dipolar (.4s), and the orbital dipolar (4l) terms. The Fermi contact term is isotropic (A = 1) and associated wifli unpaired electron spin density at flie nucleus. In a Cu(II) center, this contribution involves the unpaired 3d electron spin polarizing the inner Is, 2s, and 3s core electron pairs (mostly 2s) to produce a net negative spin density at the nucleus. The spin dipolar term is anisotropic (A = -1/24 ) and involves the electron spin, averaged over flie shape of its 3d orbital 

Initially, the small value of the blue Cu sites had been attributed to the Cu 4pz mixing into the Cu 3d 2 orbital due to the distorted tetrahedral structure of the blue copper site. The spin dipolar interaction of the Cu nuclear spin with the electron in the Cu 4p orbital would oppose that of an electron spin vector in the d 2 orbital [47]. The experimental probe of the 4p mixing into the d 2 2 or- 

The highly covalent Cu-S bond is also reflected in the intense absorption band at 600 nm. Low-temperature MCD shows that this 600 nm band is associated with a ligand-to-metal CT transition, and not a d-d transition, as its MCD intensity relative to absorption intensity (i.e., Cq/Dq ratio) is small ( 0.01) compared to that of the d-d transitions ( 0.1) [55]. [Note that the MCD C-term intensity is governed by the spin-orbit coupling parameter, and, therefore, the metal-centered d-d bands typically have a higher Cq/Dq ratio than the ligand-centered CT bands ( [Cu(II)] = 830 cm vs. [O/N] = 60-70 cm ).] This CT band is the Cys S n- Cu d 2 CT 

Upon perturbation of the geometry of the blue copper center, from plastocya-nin to cucumber basic blue to nitrite reductase (a green protein), flie Cys S n — Cu CT intensity decreases while the Cys S cr— Cu CT intensity increases 

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