Makoto Seto

Elucidation of the Fe(iv)=O intermediate in the catalytic cycle of the halogenase SyrB2

SUMMARY Mononuclear non-haem iron (NHFe) enzymes catalyse a wide variety of oxidative reactions including halogenation, hydroxylation, ring closure, desaturation, and aromatic ring cleavage. These are highly important for mammalian somatic processes such as phenylalanine metabolism, production of neurotransmitters, hypoxic response, and the biosynthesis of natural products.1–3 The key reactive intermediate in the catalytic cycles of these enzymes is an S = 2 FeIV=O species, which has been trapped for a number of NHFe enzymes4–8 including the halogenase SyrB2, the subject of this study. Computational studies to understand the reactivity of the enzymatic NHFe FeIV=O intermediate9–13 are limited in applicability due to the paucity of experimental knowledge regarding its geometric and electronic structures, which determine its reactivity. Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) is a sensitive and effective method that defines the dependence of the vibrational modes of Fe on the nature of the FeIV=O active site.14–16 Here we present the first NRVS structural characterisation of the reactive FeIV=O intermediate of a NHFe enzyme. This FeIV=O intermediate reacts via an initial H-atom abstraction step, with its subsquent halogenation (native) or hydroxylation (non-native) rebound reactivity being dependent on the substrate.17 A correlation of the experimental NRVS data to electronic structure calculations indicates that the substrate is able to direct the orientation of the FeIV=O intermediate, presenting specific frontier molecular orbitals (FMOs) which can activate the selective halogenation versus hydroxylation reactivity.Mononuclear non-haem iron (NHFe) enzymes catalyse a broad range of oxidative reactions, including halogenation, hydroxylation, ring closure, desaturation and aromatic ring cleavage reactions. They are involved in a number of biological processes, including phenylalanine metabolism, the production of neurotransmitters, the hypoxic response and the biosynthesis of secondary metabolites. The reactive intermediate in the catalytic cycles of these enzymes is a high-spin S = 2 Fe(iv)=O species, which has been trapped for a number of NHFe enzymes, including the halogenase SyrB2 (syringomycin biosynthesis enzyme 2). Computational studies aimed at understanding the reactivity of this Fe(iv)=O intermediate are limited in applicability owing to the paucity of experimental knowledge about its geometric and electronic structure. Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) is a sensitive and effective method that defines the dependence of the vibrational modes involving Fe on the nature of the Fe(iv)=O active site. Here we present NRVS structural characterization of the reactive Fe(iv)=O intermediate of a NHFe enzyme, namely the halogenase SyrB2 from the bacterium Pseudomonas syringae pv. syringae. This intermediate reacts via an initial hydrogen-atom abstraction step, performing subsequent halogenation of the native substrate or hydroxylation of non-native substrates. A correlation of the experimental NRVS data to electronic structure calculations indicates that the substrate directs the orientation of the Fe(iv)=O intermediate, presenting specific frontier molecular orbitals that can activate either selective halogenation or hydroxylation.

Spin Ordering in LaFeAsO and Its Suppression in Superconductor LaFeAsO0.89F0.11 Probed by Mössbauer Spectroscopy

57 Fe Mossbauer spectroscopy was applied to an iron-based layered superconductor LaFeAsO 0.89 F 0.11 with a transition temperature of 26 K and to its parent material LaFeAsO. Throughout the temperature range from 4.2 to 298 K, a singlet pattern with no magnetic splitting was observed in the Mossbauer spectrum of the F-doped superconductor. Furthermore, no additional internal magnetic field was observed for the spectrum measured at 4.2 K under a magnetic field of 7 T. On the other hand, magnetically split spectra were observed in the parent LaFeAsO below 140 K, and this temperature is slightly lower than that of a structural phase transition from tetragonal to orthorhombic phase, which accompanies the electrical resistivity anomaly at around 150 K. The magnetic moment is estimated to be ∼0.35 µ B /Fe from the internal magnetic field of 5.3 T at 4.2 K in the orthorhombic phase, and the spin disorder appears to remain in the magnetically ordered state even at 4.2 K. The lack of a magnetic transition in LaFeA...

Electronic and magnetic phase diagram of superconductors, SmFeAsO1?xFx

A crystallographic and magnetic phase diagram of SmFeAsO1−xFx is determined as a function of x in terms of temperature based on electrical transport and magnetization, synchrotron powder x-ray diffraction, 57Fe Mossbauer spectra (MS), and 149Sm nuclear resonant forward scattering (NRFS) measurements. MS revealed that the magnetic moments of Fe were aligned antiferromagnetically at ~144 K (TN(Fe)). The magnetic moment of Fe (MFe) is estimated to be 0.34 μB/Fe at 4.2 K for undoped SmFeAsO; MFe is quenched in superconducting F-doped SmFeAsO. 149Sm NRFS spectra revealed that the magnetic moments of Sm start to order antiferromagnetically at 5.6 K (undoped) and 4.4 K (TN(Sm)) (x=0.069). Results clearly indicate that the antiferromagnetic (AF) Sm sublattice coexists with the superconducting phase in SmFeAsO1−xFx below TN(Sm), while the AF Fe sublattice does not coexist with the superconducting phase.

Mossbauer study of polythiophene and its derivatives

Summary form only given. Polythiophene and its derivatives are highly conducting polymers when doped with appropriate chemical species such as iodine or FeCl/sub 3/. We have investigated the chemical structure of the dopants in these polymers using / sup 129/I and /sup 57/Fe, which are Mossbauer nuclei of good resolution. It has been found that the iodine in these polymers exists as linear forms of triiodide and pentaiodide anions and that the abundance ratio of triiodide decreases with increasing of the doping level. Since the polymers were doped with the iodine of neutral molecules, these anions are created by electron transfer between the polymers and the dopants. Our experimental results are consistent with the proposed mechanism of the electric conductivity which is due to migration of polarons on the polymer chains.

Development of an energy-domain 57Fe-Mössbauer spectrometer using synchrotron radiation and its application to ultrahigh-pressure studies with a diamond anvil cell.

An energy-domain (57)Fe-Mössbauer spectrometer using synchrotron radiation (SR) with a diamond anvil cell (DAC) has been developed for ultrahigh-pressure measurements. The main optical system consists of a single-line pure nuclear Bragg reflection from an oscillating (57)FeBO(3) single crystal near the Néel temperature and an X-ray focusing device. The developed spectrometer can filter the Doppler-shifted single-line (57)Fe-Mössbauer radiation with a narrow bandwidth of neV order from a broadband SR source. The focused incident X-rays make it easy to measure a small specimen in the DAC. The present paper introduces the design and performance of the SR (57)Fe-Mössbauer spectrometer and its demonstrative applications including the newly discovered result of a pressure-induced magnetic phase transition of polycrystalline (57)Fe(3)BO(6) and an unknown high-pressure phase of Gd(57)Fe(2) alloy placed in a DAC under high pressures up to 302 GPa. The achievement of Mössbauer spectroscopy in the multimegabar range is of particular interest to researchers studying the nature of the Earths core.

Influences of Co2+, Cu2+ and Cr3+ ions on the formation of magnetite

The oxidation products of Fe (OH)2 formed in the presence of Co2, Cu2 and Cr3 were characterized by various means. Co2 and Cu2 produced pure Fe3O4 by inhibiting the generation of a-FeOOH, whilst Cr3 promoted the a-FeOOH formation and interfered with the Fe3O4 formation. The Fe3O4 particles produced with Co2 exhibited high Fe2 content, electrical conductivity and stability to oxidation. Mossbauer spectroscopy demonstrated that the excess Fe2 replaced for Fe3 in B-sites of Fe3O4. A small quantity of metal Fe formed in addition to Fe3O4 on adding Co2.

Reversible charge-transfer phase transition in [(n-C3H7)4N][FeIIFeIII(dto)3](dto = C2O2S2)

We have investigated the physical properties of [(n-C 3 H 7 ) 4 N][Fe II Fe III (dto) 3 ](dto = C 2 O 2 S 2 ) by means of 57 Fe Mossbauer spectroscopy, ESR, and magnetic susceptibility. From the analysis of 57 Fe Mossbauer spectra, we have discovered a new type of first order phase transition for the title complex at about 120 K, where the charge transfer transition between Fe II and Fe III occurs reversibly. Moreover, we have found the ferromagnetic phase transition at 6 K.

Generation and Application of Ultrahigh Monochromatic X-ray Using High-Quality 57FeBO3 Single Crystal

Ultrahigh monochromatic 14.4 keV X-rays with a narrow bandwidth of 15.4 neV were generated successfully with a high counting rate of 12,000 counts/s at the undulator beamline (BL11XU) of SPring-8. It was achieved by combining an intense X-ray from the third generation synchrotron radiation facility SPring-8 and pure nuclear Bragg scattering of a very high-quality 57FeBO3 perfect single crystal at the Neel temperature. We describe the detailed study of the beam characteristics and some performance test experiments of energy-domain synchrotron radiation Mossbauer spectroscopy, including a high-pressure experiment using a diamond anvil cel.

Structure and properties of magnetite formed in the presence of nickel(II) ions

Abstract Fe3O4 particles prepared by oxidation of Fe(OH)2 precipitated from FeCl2 solutions containing different amounts of Ni2+ from 0 to 30 mol% were examined by various means. The thermogravimetry (TG) results show that the addition of Ni2+ raised the air-oxidation temperature of Fe3O4 particles from ca. 200 to ca. 400°C, which means that the Fe3O4 particles were stabilized to air-oxidation by the doped Ni2+. Fe2+ content of the materials obtained at Ni/(Ni + Fe) ≥ 0.1 mol% was significantly greater than the theoretical Fe2+ content of Fe3O4. Fe57 Mossbauer spectra indicated that Fe3+ ions in B sites of the Ni-doped Fe3O4 are exchanged in part by Fe2+. The dc electrical conductivity was increased from 10−4 to 10−2 S cm−1 by Ni2+ loading.

Geometric and Electronic Structure of the Mn(IV)Fe(III) Cofactor in Class Ic Ribonucleotide Reductase: Correlation to the Class Ia Binuclear Non-Heme Iron Enzyme

The class Ic ribonucleotide reductase (RNR) from Chlamydia trachomatis (Ct) utilizes a Mn/Fe heterobinuclear cofactor, rather than the Fe/Fe cofactor found in the β (R2) subunit of the class Ia enzymes, to react with O2. This reaction produces a stable Mn(IV)Fe(III) cofactor that initiates a radical, which transfers to the adjacent α (R1) subunit and reacts with the substrate. We have studied the Mn(IV)Fe(III) cofactor using nuclear resonance vibrational spectroscopy (NRVS) and absorption (Abs)/circular dichroism (CD)/magnetic CD (MCD)/variable temperature, variable field (VTVH) MCD spectroscopies to obtain detailed insight into its geometric/electronic structure and to correlate structure with reactivity; NRVS focuses on the Fe(III), whereas MCD reflects the spin-allowed transitions mostly on the Mn(IV). We have evaluated 18 systematically varied structures. Comparison of the simulated NRVS spectra to the experimental data shows that the cofactor has one carboxylate bridge, with Mn(IV) at the site proximal to Phe127. Abs/CD/MCD/VTVH MCD data exhibit 12 transitions that are assigned as d-d and oxo and OH(-) to metal charge-transfer (CT) transitions. Assignments are based on MCD/Abs intensity ratios, transition energies, polarizations, and derivative-shaped pseudo-A term CT transitions. Correlating these results with TD-DFT calculations defines the Mn(IV)Fe(III) cofactor as having a μ-oxo, μ-hydroxo core and a terminal hydroxo ligand on the Mn(IV). From DFT calculations, the Mn(IV) at site 1 is necessary to tune the redox potential to a value similar to that of the tyrosine radical in class Ia RNR, and the OH(-) terminal ligand on this Mn(IV) provides a high proton affinity that could gate radical translocation to the α (R1) subunit.