EPR signals of Fe

Figure 1. Characteristic EPR signals of Fe(II)Fe(III) sites in semimethemerythrinj (a), semimethemerythrinQ (b), reduced uteroferrin (c), reduced uteroferrin-molybdate complex (d), reduced bovine spleen purple acid phosphatase (e), reduced component A of methane monooxygenase (f). (Reproduced with permission from ref. 26. Copyright 1987 Elsevier.)...

At low temperatures, the EPR signal of Fe ions and Pb ions in zinc oxide is photosensitive. We observed this signal in relatively low-ohmic single crystals of ZnO. A pronounced photo excitation band shows up. It can be related to the photo ionization of the deep donor Fe ions. The analysis of the form of the photo ionization spectrum allows to calculate the optical ionization energy, which turned out to be equal to 1.4 eV for Fe zn ions and 1.65 eV for Pb zn ions. 

Fig. 4. (A) Top light-minus-dark EPR spectrum of TSF-I particles poised at -625 mV and 9 K in the g=1.78 region ([FeS-X -FeS-X] spectrum) middle and bottom kinetics of flash-induced EPR-signal at g=1.78 on two different time scales. (B) Kinetics of the dark decay of the of the EPR signal of FeS-X" at g=1.79 (top) and of P700 at g=2.0026 (bottom) in an LDS-fractionated PS-1 core complex from spinach. Figure sources (A) Shuvalov, Dolan and Ke (1979) Spectral and kinetic evidence for two early electron acceptors in photosystem I. Proc Nat Acad Sci, USA 76 772 (B) Warden and Golbeck (1986) Photosystem I charge separation in the absence of centers A and B. II. ESR spectral characterization of center X and correlation with optical signal A. Biochim Bioohvs Acta 849 28.

Figure 9.13 compares X-band EPR spectra of Fe-MCM-41 before (a) and after (b) and (c) carotenoid adsorption. The sample with incorporated Car exhibits a signal with g=2.0028 + 0.0002, characteristic of carotenoid radical cation prior to irradiation (Figure 9.13b). Irradiation of the samples at 365 nm (77 K) increases the Car 1 signal intensity (Figure 9.13c). The X-band experiments (Figure...

Even after this initial reduction, the enzyme from A. vinosum remains inactive. When performed at 2°C and pH 6, the redox potential in such an enzyme solution can be lowered to —350 mV without any increase in activity also no EPR signals of nickel-based unpaired spins are detectable. So the active site can shuttle between Nij y and the one-electron reduced state Nif y-S (S stands for EPR silent), without activation of the enzyme. A proton accompanies the electron (apparendy a strict charge compensation is obligatory), as the midpoint potential is dependent on the pH (—60 mV per pH unit). Although the changes in the EPR spectra suggest just a reduction of nickel in both cases, the ETIR spectra reflect clear changes at the Fe site for the ready enzyme, but not for the unready one (Fig. 7.6). 

Since malonate (Structure I) is able to fulfill the role of the anion in transferrin, it seemed reasonable to see whether spin-labeled derivatives of malonate could serve as probes of the active sites. Two such spin-labled derivatives were prepared and tentatively identified as having structures II (N-4-(2,2,6,6-tetramethylpiperidin-l-oxyl)malonamide) and III (N-4-(2,2,6,6-tetramethylpiperidin-l-oxyl)malonate). Similar results were obtained with each (Figure 3). Upon mixing Fe(III), transferrin, and II at low pH, and then raising the pH to near-neutrality with C02-free ammonia, the characteristic orange-red color of the ternary Fe-transferrin-anion complex is promptly displayed. However, the anticipated EPR signal of the nitroxide spin-label is not observed, presumably because it is broadened beyond detectability by its proximity... 

Several authors [129,130,190] reported on Fe modified MCM-41 and HMS. Yuan et al. [190] interpreted their FTIR and EPR data on the basis of Fe incorporation in the silicate "framework". Tuel and Gontier [130] found that the EPR spectrum of Fe-HMS is comprised of three signals one of which corresponds to iron-substituted framework sites. 

The binding of DCMU in the T4 mutant has been found to alter the EPR signal of the QB Fe complex, as shown in Fig. 11. The EPR spectmm-(a) of Fig. 11 was obtained by applying one flash to dark-adapted chromatophores of wild-type Rp. viridis at room temperature, and then by rapidly freezing the sample in the dark. The -185 G-wide, double peak atg=l. 84 is typical ofthe oftheQB Fe complex. 

The titration curve, shown in the right panel of Fig. 8 (A), was constructed from the amplitude of the slowly-decaying signal v. potential. The midpoint potential for the species being chemically reduced and thus eventually unavailable to participate in the photochemical charge separation was estimated to be -530 mV, which is identical to the value obtained by reductive EPR titration of FeS-A. 

Fig. 9. (A) Light-induced EPR changes due to P700 photooxidation and FeS-A photoreduction at 13 K. (B) EPR spectra of P700 and FeS-A" after the PS-1 particles had been illuminated at 13 K for 20 s (top row) and after the illuminated sample had been maintained at 175 K for 6 m and then recooled to 13 K (bottom row). (C) plot of loss of EPR signals of P700 and FeS-A measured after exposure to various temperatures for various amounts oftime [see table (D)]. Figure source Ke, Sugahara, Shaw, Hansen, Hamilton and Beinert (1974) Kinetics of appearance and disappearance of light-induced EPR signals ofPlOCt and Iron-sulfur protein(s) at low temperatures. Biochim Biophys Acta 368 405,406.

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