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EPR spectra of mixed-valent forms of 2c (a) and 3a (b) in acetone/acetonitrile glass reduced radiolytically at 77 K after annealing for 3 min and for 20 s, respectively, at 115 K, 100 kHz field modulation amplitude 0.5 mT, gain 5000, 10.6 mW microwave power at 9.46 GHz, temperature 20 K
Source publication
Radiolytic reduction at 77 K of oxo-/hydroxo-bridged dinuclear iron(III) complexes in frozen solutions forms kinetically stabilized, mixed-valent species in high yields that model the mixed-valent sites of non-heme, diiron proteins. The mixed-valent species trapped at 77 K retain ligation geometry similar to the initial diferric clusters. The shape...
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Context 1
... the signals decrease by two-to threefold upon progressive annealing be- cause of dissociation into mononuclear iron complexes, as well as reaction with O 2 or radical species. Annealing 2c at 114 K for 5 min causes a gradual disappearance of the initial EPR signal of Fig. 1c and simultaneous growth of a new g av ~2 signal with lower g-anisotropy (Fig. 4a), resembling those for the mono- bridged oxo complexes, 1a and 1b. This new, mixed- valent species is characterized by slower spin relaxation and can be observed without visible broadening up to 100 K, consistent with hydroxo-bridge cleavage to form a mono-bridged oxo ...
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... complex changes are observed in 3a, where annealing at 115 K for 15-20 s causes the initial rhom- bic signal (Fig. 2a) to be replaced by a new axial signal observable at 77 K with effective g k p1.93 and g II p1.87 ( Fig. 4b and Table 2). This signal is similar to that re- ported for 3a generated by chemical reduction [33]. However, this mixed-valent intermediate proved to be unstable in our solvent. After annealing at 115 K for 3-5 min it converts into a new species with an axial EPR signal (Fig. 5a), characterized by large g-anisotro- py (g II ;2.0, g k ...
Citations
... Mixed-valent diiron centers exhibiting broad EPR signals with high anisotropy have been attributed to hydroxo-bridged centers while narrow signals with low anisotropy have been attributed to oxo-bridged centers. [23][24][25] The major signal of the mixed valent wild-type HLP is broad and exhibits high anisotropy (1.99, 1.83, 1.62; Figure 2D has been recently reported that YtfE is a nitrite reductase. 11 The Y54 ligand of HLP may be the major factor in tuning the reactivity of these diiron centers to perform NO oxidation instead of NO2reduction, perhaps by tuning the reduction potential of the diiron site. ...
Tyrosine-ligated hemerythrin-like proteins (HLPs) are uniquely found in pathogenic mycobacteria. There is evidence that these HLPs have roles in virulence. To test the precise role of this tyrosine ligand, a Y54F HLP variant of Mycobacterium kansasii (Mka) was generated. There are substantial spectroscopic differences for the Y54F and the wild-type HLPs. First, the UV-visible absorbance spectrum of Y54F HLP lacked a 500-nm absorbance feature prevalent in the spectra of wild-type HLP and other Tyr-ligated diiron proteins. In addition, EPR spectroscopy showed that the Y54F HLP was isolated in a mixed-valent (FeII-FeIII) state, whereas wild-type HLP was isolated in the diferric state. In addition, the mixed-valent state of Y54F HLP exhibited a narrow, nearly axial EPR signal with g-values of 1.97, 1.93, and 1.89, a signal that is consistent with a µ-oxo bridged mixed-valent diiron site. Meanwhile, the wild-type counterpart exhibited a broad rhombic signal with simulated g-values of 1.99, 1.83, and 1.62, which is consistent with a µ-hydroxo bridged mixed-valent diiron site. Our results are consistent with Y54 coordination to the diiron site and indicate this ligand lowers the diiron [FeIII]2/FeIIFeIII redox couple and increases the basicity of the diiron solvent bridge. With regards to substrate reactivity, the Y54F mutation had no discernable effect on reductive nitrosylation reactivity, however, it precluded formation of the 520-nm intermediate observed when NO is reacted with wild-type Mka HLP. In addition, the Y54F mutation nearly eliminated catalase activity and greatly perturbed NO peroxidase activity. These results suggest that the Y54 ligand is critical for scavenging of H2O2 and NO, both of which are encountered during infection due to the host defense response.
... For example, DFT methods [9,15,16] [20]. It is worth noting that neither [FeO 2 ] per nor [FeO 2 ] sup have been detected for wildtype P450cam which may be due to the quick protonation or the proton-coupled reduction of the oxyheme complex (even at 77 K or below) from the distal-pocket proton delivery network, and therefore only the hydroperoxo ferric species S6 was captured [25]. These experimental results suggest that S5 is an elusive intermediate in the catalytic cycle. ...
The substrate bound heme-O2 complexes (ferrous dioxygen, S4 and ferric peroxo, S5) of P450cam (CYP101) have been studied by combined quantum mechanical/molecular mechanical (QM/MM) calculations. The oxyheme (without side chains) is treated with density functional theory and the protein/solvent environment by the CHARMM22 force field. The B3LYP/CHARMM calculations are found to give reasonable descriptions of the oxyheme complexes. An open-shell singlet is predicted to be the ground state for S4 with small energy separations to the excited states. This result is consistent with previous experimental and QM studies. Comparisons with analogous calculations on the isolated QM system in the gas phase show that the protein/solvent environment reduces the open-shell singlet-quintet energy gap which should facilitate the spin inversion upon the binding of the atmospheric oxygen. An intact ferric peroxo complex S5 is found only in the doublet state whereas the quartet and sextet states dissociate upon QM/MM optimization (uncoupling).
... Several studies of the EPR properties of mixed-valent dinuclear iron complexes at low temperature have been reported. The low-temperature reduction of EPR-silent diferric complexes produces paramagnetic species by introducing a single electron without significantly altering the coordination environment of the iron, thus enabling the generation of simple models for the mixed-valent state of proteins [42]. These studies provide a simple tool for distinguishing oxo from hydroxo bridges based on EPR characteristics. ...
The past year has seen significant advances in the understanding of the dioxygen-activating chemistry of non-heme diiron enzymes, such as methane monooxygenase. Recent spectroscopic and structural studies on various biomimetic model compounds have provided new and valuable insights into this enzyme’s mechanism of action and the important dioxygen-activation process.
YtfE, a repair of iron centers protein, plays roles in the response of the bacteria to oxidative and nitrosative stress. The 2.10 Å resolution crystal structure of the isolated Escherichia coli YtfE reveals two‐domain architecture with the L‐shape of a C‐terminal nonheme iron‐containing hemerythrin‐like (Hr‐like) domain linked to an N‐terminal ScdA_N domain via a highly flexible pentapeptide loop. The geometry of the diiron core of YtfE is symmetrical with two five‐coordinate irons. Each iron has a bridging oxygen atom, two bridging carboxylates derived from Glu33/Glu208, and two histidines; His84/His204 are bound to Fe(II) and His129/His160 are bound to Fe(III). Structural analysis of YtfE reveals that the diiron core is connected to the solvent regions by two roughly orthogonal tunnels. Both tunnels have small radii, ranging from 1.2 to 2.4 Å, indicating that the possible substrates (or products) for YtfE are small molecules or ions. The long hydrophobic tunnel can facilitate electron transfer to active site, and the hydrophilic tunnel gated by a negatively glutamate triad may trap proton entry or center iron ion to exit. The mixed‐valence Fe(II)–Fe(III) core of the isolated YtfE bridged by μ‐oxo was characterized by UV–vis, electron paramagnetic resonance (EPR)/electron spin echo envelope modulation (ESEEM) experiments, magnetic circular dichroism (MCD), Mössbauer spectroscopy, resonance Raman, and extended X‐ray absorption fine structure (EXAFS) spectroscopy. In spite of the isolated YtfE exhibiting the Fe(II)–Fe(III) state, the whole‐cell EPR spectrum shows that the YtfE protein exists in the reduced Fe(II)–Fe(II) state under normal cellular conditions. With regard to the repair function, YtfE may act as a ferrous‐ion donor, interacting with IscS or SufS to reassemble the Fe–S cluster of iron–sulfur proteins damaged by oxidative or nitrosative stress. Also, the Fe(II)–Fe(II) core of YtfE serves as an NO‐trapping scavenger to convert NO to N 2 O under the participation of electrons and protons.
We report two new FeIII complexes [L¹FeIII(H2O)](OTf)2 and [L²FeIII(OTf)], obtained by replacing pyridines by phenolates in a known non‐heme aminopyridine iron complex. While the original, starting aminopyridine [(L5²)FeII(MeCN)](PF6) complex is stable in air, the potentials of the new FeIII/II couples decrease to the point that [L²FeII] spontaneously reduces O2 to superoxide. We used it as an O2 activator in an electrochemical setup, as its presence allows to generate superoxide at a much more accessible potential (>500 mV gain). Our aim was to achieve substrate oxidation via the reductive activation of O2. While L²FeIII(OTf) proved to be a good O2 activator but a poor oxidation system, its association with another complex (TPEN)FeII(PF6)2 generates a complementary tandem couple for electro‐assisted oxidation of substrates, working at a very accessible potential: upon reduction, L²FeIII(OTf) activates O2 to superoxide and transfers it to (TPEN)FeII(PF6)2 leading in fine to the oxidation of thioanisole.
Molecular mechanisms underlying the repair of nitrosylated [Fe-S] clusters by the microbial protein YtfE remain poorly understood. The X-ray crystal structure of YtfE, in combination with EPR, magnetic circular dichroism (MCD), UV, and (17) O-labeling electron spin echo envelope modulation measurements, show that each iron of the oxo-bridged Fe(II) -Fe(III) diiron core is coordinatively unsaturated with each iron bound to two bridging carboxylates and two terminal histidines in addition to an oxo-bridge. Structural analysis reveals that there are two solvent-accessible tunnels, both of which converge to the diiron center and are critical for capturing substrates. The reactivity of the reduced-form Fe(II) -Fe(II) YtfE toward nitric oxide demonstrates that the prerequisite for N2 O production requires the two iron sites to be nitrosylated simultaneously. Specifically, the nitrosylation of the two iron sites prior to their reductive coupling to produce N2 O is cooperative. This result suggests that, in addition to any repair of iron centers (RIC) activity, YtfE acts as an NO-trapping scavenger to promote the NO to N2 O transformation under low NO flux, which precedes nitrosative stress.
Ribonucleotide reductase (RNR) is the enzyme responsible for the conversion of ribonucleotides to 2′-deoxyribonucleotides and thereby provides the precursors needed for both synthesis and repair of DNA. In the recent years, many new crystal structures have been obtained of the protein subunits of all three classes of RNR. This review will focus upon recent structural and spectroscopic studies, which have offered deeper insight to the mechanistic properties as well as evolutionary relationship and diversity among the different classes of RNR. Although the three different classes of RNR enzymes depend on different metal cofactors for the catalytic activity, all three classes have a conserved cysteine residue at the active site located on the tip of a protein loop in the centre of an α/β-barrel structural motif. This cysteine residue is believed to be converted into a thiyl radical that initiates the substrate turnover in all three classes of RNR. The functional and structural similarities suggest that the present-day RNRs have all evolved from a common ancestral reductase. Nevertheless, the different cofactors found in the three classes of RNR make the RNR proteins into interesting model systems for quite diverse protein families, such as diiron-oxygen proteins, cobalamin-dependent proteins, and SAM-dependent iron–sulfur proteins. There are also significant variations within each of the three classes of RNR. With new structures available of the R2 protein of class I RNR, we have made a comparison of the diiron centres in R2 from mouse and Escherichia coli. The R2 protein shows dynamic carboxylate, radical, and water shifts in different redox forms, and new radical forms are different from non-radical forms. In mouse R2, the binding of iron(II) or cobalt(II) to the four metal sites shows high cooperativity. A unique situation is found in RNR from baker's yeast, which is made up of heterodimers, in contrast to homodimers, which is the normal case for class I RNR. Since the reduction of ribonucleotides is the rate-limiting step of DNA synthesis, RNR is an important target for cell growth control, and the recent finding of a p53-induced isoform of the R2 protein in mammalian cells has increased the interest for the role of RNR during the different phases of the cell cycle.
The potential of iron molybdates as catalysts in the Formox process stimulates research on aggregated but molecular iron-molybdenum oxo compounds. In this context, [(Me3TACN)Fe](OTf)2 was reacted with (nBu4N)2[MoO4], which led to an oxo cluster, [[(Me3TACN)Fe][μ-(MoO4-κ(3)O,O',O″)]]4 (1, Fe4Mo4) with a distorted cubic structure, where the corners are occupied by (Me3TACN)Fe(2+) and [Mo═O](4+) units in an alternating fashion, being bridged by oxido ligands. The cyclic voltammogram revealed four reversible oxidation waves that are assigned to four consecutive Fe(II) → Fe(III) transfers and motivated attempts to isolate compounds containing the respective cations. Indeed, a salt with a Fe(II)2Fe(III)2Mo(VI)4 constellation, [Fe4Mo4](TCNQ)2 (2), could be isolated after treatment with TCNQ. The Fe(II)Fe(III)3Mo(VI)4 stage could be reached via oxidation with DDQ or 3 equiv of thianthrenium hexafluorophosphate (ThPF6), giving [Fe4Mo4](DDQ)3 (4) or [Fe4Mo4](PF6)3 (5), respectively. The fully oxidized Fe(III)4Mo(VI)4 state was generated through oxidation with 4 equiv of ThPF6, leading to [Fe4Mo4](PF6)4, which showed a unique behavior: upon storage, one of the [Mo═O](4+) corners inverts, so that the terminal oxido ligand is located in the interior of the cage, leading to the formation of [[(Me3TACN)Fe]4[μ-([MoO4]3[MoO4(MeCN-κN)])-κ(3)O,O',O″)](PF6)4 (7). In this form, the compound could no longer be employed to enter the cyclic voltammogram recorded for 1, 3, and 5 from the oxidized side; no discrete redox events were observed. Compounds 1-3 and 7 were characterized structurally and 1, 3, and 7 additionally by SQUID measurements and Mössbauer spectroscopy. The data reveal a high degree of charge delocalization. (16)O/(18)O exchange experiments with labeled water performed with 1 revealed an interesting parallel with the Formox catalyst: water-(18)O exchanges its label with all of the oxido ligands (bridging and terminal). This property relates to the ion mobility being held responsible for the activity of iron molybdate catalysts compared to neat MoO3 or Fe2O3.
The reaction between [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) yields the novel tetranuclear complex [(TPA)Fe(μ-Cp*MoO3)]2(OTf)2, , with a rectangular [Mo-O-Fe-O-]2 core containing high-spin iron(ii) centres. proved to be an efficient initiator/(pre)catalyst for the autoxidation of cis-cyclooctene with O2 to give cyclooctene epoxide. To test, which features of are essential in this regard, analogues with zinc(ii) and cobalt(ii) central atoms, namely [(TPA)Zn(Cp*MoO3)](OTf), , and [(TPA)Co(Cp*MoO3)](OTf), , were prepared, which proved to be inactive. The precursor compounds of , [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) as well as Cp2*Mo2O5, were found to be inactive, too. Reactivity studies in the absence of cyclooctene revealed that reacts both with O2 and PhIO via loss of the Cp* ligands to give the triflate salt of the known cation [((TPA)Fe)2(μ-O)(μ-MoO4)](2+). The cobalt analogue reacts with O2 in a different way yielding [((TPA)Co)2(μ-Mo2O8)](OTf)2, , featuring a Mo2O8(4-) structural unit which is novel in coordination chemistry. The compound [(TPA)Fe(μ-MoO4)]2, , being related to , but lacking Cp* ligands failed to trigger autoxidation of cyclooctene. However, initiation of autoxidation by Cp* radicals was excluded via experiments including thermal dissociation of Cp2*.
