Makina Saito

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...

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.

Relaxation transition in glass-forming polybutadiene as revealed by nuclear resonance X-ray scattering

We investigated the arrest mechanism of molecular motions in a glass forming polybutadiene near the glass transition using a new nuclear resonance synchrotron X-ray scattering technique to cover a wide time range (10(-9) to 10(-5) s) and a scattering vector Q range (9.6-40 nm(-1)), which have never been accessed by other methods. Owing to the wide time and Q ranges it was found for the first time that a transition of the α-process to the slow β-process (or the Johari-Goldstein process) was observed in a Q range higher than the first peak in the structure factor S(Q) at the critical temperature T(c) in the mode coupling theory. The results suggest the important roles of hopping motions below T(c), which was predicted by the recent extended mode coupling theory and the cooperative motions due to the strong correlation at the first peak in S(Q) in the arrest mechanism.

Synchrotron radiation-based Mössbauer spectra of 174Yb measured with internal conversion electrons

A detection system for synchrotron-radiation (SR)-based Mossbauer spectroscopy was developed to enhance the nuclear resonant scattering counting rate and thus increase the available nuclides. In the system, a windowless avalanche photodiode (APD) detector was combined with a vacuum cryostat to detect the internal conversion (IC) electrons and fluorescent X-rays accompanied by nuclear de-excitation. As a feasibility study, the SR-based Mossbauer spectrum using the 76.5 keV level of 174Yb was observed without 174Yb enrichment of the samples. The counting rate was five times higher than that of our previous system, and the spectrum was obtained within 10 h. This result shows that nuclear resonance events can be more efficiently detected by counting IC electrons for nuclides with high IC coefficients. Furthermore, the windowless detection system enables us to place the sample closer to the APD elements and is advantageous for nuclear resonant inelastic scattering measurements. Therefore, this detection system...

Development of 151Eu Time-Domain Interferometry and Its Application for the Study of Slow Dynamics in Ionic Liquids

Time-domain interferometry (TDI) employing 151Eu nuclear resonant scattering was developed for the study of slow dynamics. We measured the relaxation times of the density correlation in super-cooled ionic liquid 1-butyl-3-methylimidazolium iodide (BmimI) by the developed TDI. We found that the temperature dependence of the relaxation times follows the Vogel–Fulcher–Tammann law and observed fragile behavior of BmimI. Furthermore, we discussed the potential of TDI for the study of slow dynamics.

Nuclear resonance vibrational spectroscopic and computational study of high-valent diiron complexes relevant to enzyme intermediates

High-valent intermediates of binuclear nonheme iron enzymes are structurally unknown despite their importance for understanding enzyme reactivity. Nuclear resonance vibrational spectroscopy combined with density functional theory calculations has been applied to structurally well-characterized high-valent mono- and di-oxo bridged binuclear Fe model complexes. Low-frequency vibrational modes of these high-valent diiron complexes involving Fe motion have been observed and assigned. These are independent of Fe oxidation state and show a strong dependence on spin state. It is important to note that they are sensitive to the nature of the Fe2 core bridges and provide the basis for interpreting parallel nuclear resonance vibrational spectroscopy data on the high-valent oxo intermediates in the binuclear nonheme iron enzymes.

Nuclear Resonance Vibrational Spectroscopy and DFT study of Peroxo-Bridged Biferric Complexes: Structural Insight into Peroxo Intermediates of Binuclear Non-heme Iron Enzymes†

Binuclear non-heme iron enzymes utilize O2 to catalyze a variety of reactions, including hydrogen atom abstraction, desaturation, electrophilic aromatic substitution, and so on. In most cases, their catalytic cycles begin with the reductive binding of O2 by biferrous centers to form high-spin antiferromagnetically coupled (AFC) peroxo-bridged biferric intermediates. These peroxo intermediates can either react with substrate or convert to more reactive high-valent species. Because of their transient nature, structural information must be deduced from spectroscopic data, which are rich for some peroxo intermediates, while for others too limited for geometric and electronic structural insight. The peroxo intermediate of W48F/D84E ribonucleotide reductase (RR), referred to as P, does exhibit distinct spectral features. These include electronic absorption (Abs) and resonance Raman (rR) spectra that are equivalent to those of cis m-1,2 end-on peroxo-bridged Fe2 model complexes, thus providing a basis for the computational model of P as a cis m-1,2 peroxo-bridged Fe2 species (with the (Glu)4(His)2 ligand set of this protein active site). However, P is not reactive and must convert to a second-peroxo-level intermediate P’ that does not have Abs spectral features for rR-based structural elucidation. For systems that do not have chromophores or are photoactive, nuclear resonance vibrational spectroscopy (NRVS) is an alternative to rR spectroscopy. NRVS is a synchrotron-based technique that probes vibrational side bands of Fe nuclear transitions. Its spectral intensity is determined by the amount of Fe displacement in each normal mode, thus allowing the specific investigation of the Fe active site with high sensitivity and without the limitation of the selection rules of rR spectroscopy. In this study, we establish the basis for the NRVS analysis of peroxo-bridged Fe2 intermediates, based on structurally well-characterized synthetic model complexes. We have measured the NRVS spectra of [Fe2(mOH)(mO2)(6Me2-BPP)2] + (1) and [Fe2(mO)(mO2)(6Me2-BPP)2] (2 ; Figure 1; 6Me2-BPP = N,N-bis(6methyl-2-pyridylmethyl)-3-aminopropionate). These complexes are cis m-1,2 peroxo-bridged species, the former with an additional hydroxo bridge and the latter with an oxo

Free electron laser-driven ultrafast rearrangement of the electronic structure in Ti

High-energy density extreme ultraviolet radiation delivered by the FERMI seeded free-electron laser has been used to create an exotic nonequilibrium state of matter in a titanium sample characterized by a highly excited electron subsystem at temperatures in excess of 10 eV and a cold solid-density ion lattice. The obtained transient state has been investigated through ultrafast absorption spectroscopy across the Ti M2,3-edge revealing a drastic rearrangement of the sample electronic structure around the Fermi level occurring on a time scale of about 100 fs.

Reflectivity enhancement in titanium by ultrafast XUV irradiation

The study of highly photo-excited matter at solid state density is an emerging field of research, which is benefitting the development of free-electron-laser (FEL) technology. We report an extreme ultraviolet (XUV) reflectivity experiment from a titanium (Ti) sample irradiated with ultrafast seeded FEL pulses at variable incident photon fluence and frequency. Using a Drude formalism we relate the observed increase in reflectivity as a function of the excitation fluence to an increase in the plasma frequency, which allows us to estimate the free electron density in the excited sample. The extreme simplicity of the experimental setup makes the present approach potentially a valuable complementary tool to determine the average ionization state of the excited sample, information of primary relevance for understanding the physics of matter under extreme conditions.

Axial ligand effects on vibrational dynamics of iron in heme carbonyl studied by nuclear resonance vibrational spectroscopy.

Nuclear resonance vibrational spectroscopy (NRVS) and density functional theory calculation (DFT) have been applied to illuminate the effect of axial ligation on the vibrational dynamics of iron in heme carbonyl. The analyses of the NRVS data of five- (5c) and six-coordinate (6c) heme-CO complexes indicate that the prominent feature of (57)Fe partial vibrational density of state ((57)FePVDOS) at the 250-300 cm(-1) region is significantly affected by the association of the axial ligand. The DFT calculations predict that the prominent (57)FePVDOS is composed of iron in-plane motions which are coupled with porphyrin pyrrole in-plane (ν(49), ν(50), and ν(53)), an out-of-plane (γ(8)) (two of four pyrrole rings include the in-plane modes, while the rest of pyrrole rings vibrate along the out-of-plane coordinate), and out-of-phase carbonyl C and O atom displacement perpendicular to the Fe-C-O axis. Thus, in the case of the 5c CO-heme the prominent (57)FePVDOS shows sharp and intense feature because of the degeneracy of the e symmetry mode within the framework of C(4v) symmetry molecule, whereas the association of the axial imidazole ligand in the 6c complex with the lowered symmetry results in split of the degenerate vibrational energy as indicated by broader and lower intensity features of the corresponding NRVS peak compared to the 5c structure. The vibrational energy of the iron in-plane motion in the 6c complex is higher than that in 5c, implying that the iron in the 6c complex includes stronger in-plane interaction with the porphyrin compared to 5c. The iron in-plane mode above 500 cm(-1), which is predominantly coupled with the out-of-phase carbonyl C and O atom motion perpendicular to Fe-C-O, called as Fe-C-O bending mode (δ(Fe-C-O)), also suggests that the 6c structure involves a larger force constant for the e symmetry mode than 5c. The DFT calculations along with the NRVS data suggest that the stiffened iron in-plane motion in the 6c complex can be ascribed to diminished pseudo-Jahn-Teller instability along the e symmetry displacement due to an increased a(1)-e orbital energy gap caused by σ* interaction between the iron d(z(2)) orbital and the nitrogen p orbital from the axial imidazole ligand. Thus, the present study implicates a fundamental molecular mechanism of axial ligation of heme in association with a diatomic gas molecule, which is a key primary step toward versatile biological functions.