In the all figure 79 redox states of fezsz proteins a

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In the all- Figure 7.9 Redox states of FezSz proteins: (A) reduced; (B) oxidized.
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I. IRON-SULFUR PROTEINS AND MODELS 381 ferric oxidized state, the two Fe 3+ sites are antiferromagnetically coupled; i.e., the spins of the five d electrons on the two iron atoms are oppositely aligned, such that their pairing produces an effective S = 0, diamagnetic ground state. In the reduced form, a single unpaired electron is present, because the S = i Fe 3+ and S = 2 Fe 2+ sites are antiferromagnetically coupled, leaving one net unpaired spin and an S = ! ground state. The profound difference between the electronic properties of rubredoxin and Fe2S2 ferredoxin arises because the latter has two Fe atoms in close proximity, which allows for their magnetic coupling. Strong support for the spin-coupling model in Fe2S2 ferredoxins comes from a detailed analysis of their absorption and circular dichroism spectra. 83 As with rubredoxin (see Figure 7.5), we expect no low-energy spin-allowed dod bands for the ferric site in either the oxidized or the reduced state. Indeed, the oxidized state containing all Fe 3+ shows no low-energy bands; the reduced state contain- ing a single Fe 2+ displays low-energy, low-intensity bands in the region 4,000- 9,000 em 1, in close analogy to the situation in reduced rubredoxin. The com- bined EPR and optical spectra leave little doubt about the structural assignment: two coupled high-spin ferric ions in the oxidized state, and coupled high-spin ferric and ferrous ions in the reduced state. Moreover, the spectra are consistent only with a localized model, i.e., one in which the Fe(II) site is associated with a single iron. 83 ,83a The Fe2S2 site is inherently asymmetric, and inequivalence of the Fe(llI) sites is spectroscopically detectable in the all-ferric oxidized form. 84 In fact, the localized valence trapping is present in reduced model compounds that contain no ligand asymmetry. M6ssbauer spectra provide additional and striking confirmation of the struc- tural assignment. The spectrum of the oxidized ferredoxin (Figure 7.7) resem- bles strongly that of oxidized rubredoxin, indicating the presence of high-spin Fe3+, even though the net spin is zero. In the reduced form, the M6ssbauer spectrum involves the superposition of signals from a high-spin Fe 2 + and a high-spin Fe3+, i.e., a reduced and an oxidized rubredoxin, respectively. Clearly, the simplest interpretation of this result consistent with the S = ! spin state required by the EPR is the localized Fe 2 +-Fe 3 + anti ferromagnetic coupling model discussed above. NMR studies of oxidized Fe2S2 proteins reveal broad isotropically shifted resonances for the CH 2 protons of the cysteine ligands. 85 Despite the coupling of the irons, the net magnetism at room temperature is sufficient to lead to large contact shifts (- 30 to - 40 ppm downfield from TMS). The assignment of the resonance was confirmed with the synthesis and spectroscopic analysis of model compounds.
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