Sets with the e set lower as shown in figure 75 the

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sets, with the e set lower as shown in Figure 7.5. The small tetrahedral splitting causes the 3ds Fe 3+ ion to have five unpaired electrons, (e)2(t2)3, 6A 1 . Consistent with this configura- tion, the magnetic susceptibility of rubredoxin gives a f..Leff of 5.85 Bohr mag- netonsY No spin-allowed ligand-field transitions are expected, and the red color is caused by S ~ Fe charge-transfer transitions in the visible region. 38 ,39 In contrast, the 3d 6 Fe 2 + state, with one additional electron, has four un- paired electrons, as confirmed by its magnetic moment of 5.05 Bohr magnetons. In exact tetrahedral symmetry, a single, low-energy, low-intensity d-d absorp- tion of designation 5E ~ 5T [(e) 3(t 2 ) 3 ~ (e)2(t 2 )4] is expected for the high-spin ferrous site (Figure 7.5). Indeed, reduced rubredoxin displays a band in the near-infrared region at 6,250 cm -I that arises as a component of the 5E ~ sT 2 transition. 40 This band stands out particularly vividly in the low-energy circular dichroism (CD) spectrum of reduced rubredoxin. 41 Moreover, magnetic circular dichroism (MCD) has proven valuable in dissecting electronic transitions in sev- eral rubredoxins and metal-sulfide proteins. 38,39,42,43 t 2 -cD-CD-CD- -cD-CD-CD- e ----<D-CD- ---<D-®-- (A) (B) Figure 7.5 Splitting of the 3d orbitals of Fe by the tetrahedral ligand field of four coordinated cysteine residues: (A) Fe 3 +; (B) Fe 2+ .
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374 7 / FERREDOXINS, HYDROGENASES, AND NITROGENASES: METAL-SULFIDE PROTEINS The EPR spectrum of oxidized rubredoxin (Figure 7.6) shows characteristic peaks at g = 4.31 and 9.42 (P. oleovorans) , which have been assigned 44 to transitions within excited and ground-state Kramers doublets, respectively, of a nearly completely rhombic 5 = i site, with D = 1.8 and E = 0.5 cm -I. These values for the mononuclear Fe 3 + ion stand in sharp contrast to those for other iron-sulfur proteins, which are usually 5 = ! (when reduced) and have g values close to 2. The even-electron Fe 2 + state (5 = 2) in reduced rubredoxin has no detectable EPR when conventional instruments are used. * Mossbauer spectroscopy has proven to be a particularly powerful comple- mentary tool to EPR in probing the iron sites in Fe-S proteins. 3 ,37,51,52 It is a nuclear spectroscopy that can give valuable information not available from other techniques. t Unlike EPR, where only paramagnetic centers are "seen," every 57Fe atom in the sample will contribute to the Mossbauer spectrum. For rubre- doxin, the high-spin nature of the ferric and ferrous sites are clearly seen in the Mossbauer spectra. 53 The high-spin Fe 3+ sites show a small quadrupole split- ting of roughly 0,7-0.8 mm/s due to the almost spherical distribution of the five d electrons in the five d orbitals (Figure 7,7 A). In contrast, the high-spin Fe 2+ ion with an additional d electron has a significant asymmetry, and thus displays * But see Reference 45. EPR spectroscopy uses magnetic fields to split the electron spin states into levels that differ by energy in the microwave region of the spectrum. For an S = !
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