For agicusod the d d transition is red shifted

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For AgICuSOD, the d-d transition is red-shifted because of the change in the ligand geometry when CUll is moved from the copper site to the zinc site (see text), (From M. W. Pantoliano, L. A. Nafie, and J. S. Valentine, J. Am. Chern. Soc. 104 (1982),6310-6317.)
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VII. COPPER-ZINC SUPEROXIDE DISMUTASE 307 an imidazolate-to-Cu 1I charge transfer, indicating that the imidazolate bridge between Cu II and the metal ion in the native zinc site is present, as observed in the crystal structure of CuZnSOD. Derivatives with the zinc site empty, which therefore cannot have an imidazolate bridge, are lacking this 417 nm shoulder. Small but significant changes in the absorption spectrum are seen when the metal ion is removed from the zinc site, e.g., in copper-only SOD (Figure 5.18B). The visible absorption band shifts to 700 nm (14,300 cm -1), presumably due to a change in ligand field strength upon protonation of the bridging imidazolate. In addition, the shoulder at 417 nm has disappeared, again due to the absence of the imidazolate ligand. The spectroscopic properties due to copper in the native zinc site are best observed in the derivative Ag1CuSOD, which has Ag 1 in the copper site and Cu II in the zinc site (see Figure 5.18C), since the d 10 Ag 1 ion is spectroscopi- cally silent. In this derivative, the dod transition is markedly red-shifted from the visible region of the spectrum into the near-IR, indicating that the ligand environment of Cu II in that site is either tetrahedral or five coordinate. The EPR properties of Cu II in this derivative are particularly interesting (as discussed below). The derivative with CUll bound at both sites, CuCuSOD, has a visible-near IR spectrum that is nearly a superposition of the spectra of CuZnSOD and Ag1CuSOD (see Figure 5.19), indicating that the geometry of CUll in each of these sites is little affected by the nature of the metal ion in the other site. 200 E (J (J) ~ 150 ~ C (J) 'u 100 ~ 8 c .~ 50 c ~ W o " " \ \ ....... \.~ . ... \ ... \ ... \ ~ ... " .... " ..... " .' .... ---- ..... _--_. 600 Wavelength, nm 1,200 Figure 5.19 Comparison of the visible absorption spectrum of CuCuSOD, --------, with that of Cu- ZnSOD, ---, and of Ag1CuSOD, A digital addition of the spectra of Cu- ZnSOD and of Ag1CuSOD generated the other spectrum, -'-'-'-'. Note that the spectrum of CuCuSOD, which has CUll ions in both the copper and the zinc sites, closely resembles a super- position of the spectra of CuZnSOD, which has CUll in the copper site, and Ag1CuSOD, which has CUll in the zinc site. (From M. W. Pantoliano, L. A. Nafie, and J. S. Valentine, J. Am. Chern. Soc. 104 (1982), 6310-6317.)
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308 5 / DIOXYGEN REACTIONS EPR spectroscopy has also proven to be particularly valuable in character- izing the metal environments in CuZnSOD and derivatives. The EPR spectrum of native CuZnSOD is shown in Figure 5.20A. The gil resonance is split by the hyperfine coupling between the unpaired electron on CUll and the I = i nuclear spin of copper. The All value, 130 G, is intermediate between the larger All (A) I I I I I I I 2,600 2,800 3,000 3,200 3,400 3,600 3,800 magnetic field (gauss) (B) I I I I I 2,200 2,600 3,000 3,400 3,800 magnetic field (gauss) (C) I I I I I I I 2,400 2,600 2,800 3,000 3,200 3,400 3,600 magnetic field (gauss) Figure 5.20 Frozen solution EPR spectra of (A) CuZnSOD, (B) copper-only SOD (zinc site empty), and (C) Ag1CuSOD. See text for discussion. (Adapted from References 100 and 101.)
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VII.
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