EPR and Mossbauer studies

Mossbauer studies of e as isolated, oxidized P. denitrificans CCP also reveal the presence of two distinct low-spin ferric hemes, with different values of isomer shift (S) and quadrupole splitting (AEq), reflecting distinct heme environments. The parameters (AEq = 2.10mms and = 0.30mm s ) found for the oxidized HP heme are similar to those reported for several cytochromes with methionine/histidine axial coordination (C. W. Pettigrew, I. Moura, and J. J. G. Moura, unpublished results). The AEq value of 2.50mms found for the LP heme, however, is larger than the usual values for oxidized hemes with bis histidine coordination. The S value of 0.26 mm s obtained for the oxidized LP heme is typical for low-spin ferric 

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EPR and Mossbauer study of two mutants of the mononuclear iron sulfur protein rubredoxin72 compared the results of chemical and cryoradiolytic reduction and concluded that the latter reduction method produces exclusively one reduced species (serine coordinated iron known from the X-ray structure of the ferric protein) because it leaves the coordination of the ferric precursor unaltered. The identity of another spectral form observed in the chemically reduced C42S mutant in solution could not be unambiguously identified. 

The [Fe4 84] + state is isoelectronic with HPqx cores. EPR and Mossbauer studies of the sole isolated synthetic cluster, [Fe484(8tibt)4] , indicate that it closely resembles HPqx in electronic structure. The redox couple exhibited by this and other model compounds, however, show significantly more negative potentials than the protein-bound cluster. 

The [2Fe-2S] ferredoxins (10-20 kDa) are found in plant chloroplasts and mammalian tissue. The structure of Spirulina platensis ferredoxin confirmed earlier suggestions, based on EPR and Mossbauer studies, that the iron atoms are present in a spin-coupled [2Fe-2S] cluster structure. One-electron reduction (E° —420 mV) of the protein results in a mixed-valence dimer (Equation... 

Stopped-flow UV-vis absorption and rapid freeze-quench (RFQ) EPR and Mossbauer studies have shown that the reaction pathway diagrammed in Figure 4 for formation of the tyrosyl radical is essentially accurate except that the diiron(IV) species labeled Q in Figure 4 has never been detected in R2. Instead, an intermediate labeled U (not shown in Figured), occurring prior to X, has properties consistent with a protonated tryptophan cation radical. This radical may shuttle an electron from an external donor to the diiron site in order to reach intermediate In... 

Platinum ammine complexes have been a fertile area for studying transinfluence. Table 3.21 lists data for a range of ammines showing how /(195Pt-15N) depends upon the trans-atom [153]. (A further selection of data can be found in R.V. Parish, NMR, NQR, EPR and Mossbauer Spectroscopy in Inorganic Chemistry, Ellis-Horwood, Chichester, 1991, pp. 76, 87.) Possibly the most detailed study (of complexes of tribenzylphosphine) examined over a hundred neutral and cationic complexes [154] (Table 3.22). 

This key enzyme of the dissimilatory sulfate reduction was isolated from all Desulfovibrio strains studied until now 135), and from some sulfur oxidizing bacteria and thermophilic Archaea 136, 137). The enzymes isolated from sulfate-reducing bacteria contain two [4Fe-4S] clusters and a flavin group (FAD) as demonstrated by visible, EPR, and Mossbauer spectroscopies. With a total molecular mass ranging from 150 to 220 kDa, APS reductases have a subunit composition of the type 012)32 or 02)3. The subunit molecular mass is approximately 70 and 20 kDa for the a and )3 subunits, respectively. Amino-acid sequence data suggest that both iron-sulfur clusters are located in the (3 subunit... 

Caldeira J, R Eeicht, H White, M Teixeira, JJG Mourat, H Simon, 1 Moura (1996) EPR and Mossbauer spectroscopic studies on enoate reductase. J Biol Chem 271 18743-18748. 

EPR and Mossbauer spectroscopies have been successfully used to characterize iron-sulfur clusters. Hydrogenases are no exception. Here, we will describe the knowledge gained from applying these spectroscopies to the study of [NiFe] hydrogenase. 

Spectroscopic studies on the Fe-Mo protein by EPR and Mossbauer spectroscopy have shown six iron atoms each in a distinctive magnetic environment coupled to an overall S=3/2 spin system (6,7,8) and electron nuclear double resonance (ENDOR) studies suggest one molybdenum per spin system (8). The 5 Fe signals (five or six doublets) observed in the ENDOR spectra (8) indicate a rather asymmetric structure for the Fe/Mo/S aggregate in which the iron atoms roughly can be grouped into two sets of trios, each set having very similar hyperfme parameters. 

An unusual cysteine sequence motif together with atypical EPR and Mossbauer properties has been reported earlier for a [Fe2S2] protein denoted FhuF. It is an iron-regulated E. coli protein which is probably involved in the reduction of ferric iron in ferrioxamine B.191 In a recent study the authors could provide evidence for the mixed valence state in the FhuF protein to be capable of... 

A Fe2S2 ferredoxin from the hypertherm ophilic bacterium Aquifex aeolicus, expressed in E. coli, studied by Meyer et al. was shown not to be closely similar to plant or mammalian Fe2S2ferrredoxins.83 The combined EPR, MCD, resonance Raman and Mossbauer study identified an S = state from dithionite reduction with ghvalues of 2.05,1.96 and 1.88. 

Table 1 lists some of the properties of the plant-type iron sulfur-proteins for which extensive study by EPR and Mossbauer spectroscopy has been reported. The physical properties summarized show that the plant-type iron sulfur proteins have molecular weights in the range from 12,000 to 24,000 and have EPR g-values (gx, gy, gz) all of the g = 1.94" type shown in Fig. 6 but with minor variations reflecting axial or nonaxial symmetry of the paramagnetic center. The amino-acid sequences of four plant-type iron-sulfur-proteins are known alfalfa (136), L. glauca (137), Scenedesmus (138), and spinach (139). Each protein has about 97 residues, all in a single peptide chain these are shown in Table 2. 

Schoneboom JC, Neese F, Thiel W. Toward identification of the compound I reactive intermediate in cytochrome P450 chemistry a QM/MM study of its EPR and Mossbauer parameters. J Am Chem Soc. 2005 127 5840-3. 

Figure 5 The heterodinuclear Mn" Fe " cluster of Chlamydia trachomatis RNR R2 protein and its proposed formation mechanism from its Mn"Fe" form described in a recent combined biochemical, EPR, and Mossbauer spectroscopic study (33-36).

The ESO2 complex with 3-(3, 4 -dihydroxyphenyl)propionate (DHPP) as substrate has also been studied with both EPR and Mossbauer spectroscopy20). This complex has a half life of ca. 4 min at 4 °C and can thus be generated easily and then frozen to trap the intermediate. The EPR spectrum exhibited signals at g = 6.7 and S.3, corresponding to an E/D value of 0.03. The expected zero-field splitting diagram is shown below. 

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