In fe4s4 centers at each level of oxidation

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In Fe4S4 centers at each level of oxidation, electronic transitions give rise to characteristic visible and UV spectra, although the delocalized nature of the electronic states makes detailed assignment difficult. MCD spectra of clusters in the three states of oxidation are clearly distinguishable from each other and from MCD of Fe2S2 clusters. 43 ,119 MCD, magnetic susceptibility, and M6ssbauer spectra provide evidence that the S = t state, whose EPR signal is so distinct in re- duced ferredoxins, may coexist at higher T with S = i and perhaps even higher spin states. Indeed, recent studies with model systems 121,122 and theoretical treatments 123,124 clearly support the ability of the Fe4S4 cluster to display a num- ber of spin states that are in labile equilibria, which are influenced, perhaps quite subtly, by local structural conditions. The iron protein of nitrogenase also displays this behavior. The M6ssbauer spectra of Fe4S4 centers of ferredoxins reveal the equiva- lence of the Fe sites, and quadrupole splittings and isomer shifts at averaged values for the particular combination of oxidation states present. 3,51 ,52 Represen- tative spectra are shown in Figure 7.7. Magnetic coupling is seen for the para- magnetic states. Resonance Raman spectra (and IR spectra) have been extensively investi- gated in C. pasteurianum ferredoxin and in model compounds. 57 ,125 Selective labeling of either thiolate sulfur or sulfide sulfur with 34S allows modes associ- ated with the Fe4S4 core to be distinguished from modes associated with the Fe- SR ligands. The band at 351 cm - 1 is assigned to Fe-SR stretching, and Fe4S4 modes occur at 248 and 334 cm - I in reduced ferredoxin from C. pasteurianum. There is little difference between the oxidized and reduced spectra, although an 387
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388 7 I FERREDOXINS, HYDROGENASES, AND NITROGENASES: METAL-SULFIDE PROTEINS extra band at 277 cm 1 seems present in the oxidized protein. The Fe4Se4 sub- stituted protein has also been studied. 125 As in the IFe and 2Fe proteins, 1H NMR spectra reveal resonances from contact-shifted -CH r groups of cysteinyl residues. 125a However, unlike the other proteins, where all states are at least weakly magnetic, only the reduced ferre- doxin and the oxidized HiPIP states show contact shifts. 87a,125a,b,c EXAFS studies on proteins and on model compounds clearly identify the Fe-S distance of ~2.35 A and an Fe-Fe distance of 2.7 A. These distances, as expected, vary only slightly with state of oxidation. 125d Judging from the ease with which models of Fe4S4 are prepared under a variety of conditions and their relative stability, the Fe4S42 + core structure seems to be a relatively stable entity, a local thermodynamic minimum in the multitude of possible iron-sulfide-thiolate complexes. The initial preparation and structural characterization 126,127 of the models showed that synthetic chemistry can dupli- cate the biological centers in far-simpler chemical systems, which can be more easily studied in great detail.
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