The validity of an experiment testing the influence of acceleration on time dilation using a rotating Mössbauer absorber and a Synchrotron Mössbauer Source.

Three experiments are reviewed, performed (in 2014-2016) at ID18 of ESRF to measure the influence of acceleration on time dilation by measuring the relative shift between the absorption lines of two states of the same rotating absorber with accelerations anti-parallel and parallel to the incident beam. Statistically significant data for rotation frequencies up to 510 Hz in both directions of rotation were collected. For each run with high rotation, a stable statistically significant `vibration-free' relative shift between the absorption lines of the two states was measured. This may indicate the influence of acceleration on time dilation. However, the measured relative shift was also affected by the use of a slit necessary to focus the beam to the axis of rotation to a focal spot of sub-micrometre size. The introduction of the slit broke the symmetry in the absorption lines due to the nuclear lighthouse effect and affected the measured relative shift, preventing to claim conclusively the influence of acceleration on time dilation. Assuming that this loss of symmetry is of first order, the zero value of the relative shift, corrected for this loss, falls always within the experimental error limits, as predicted by Einstein's clock hypothesis. The requirements and an indispensable plan for a conclusive experiment, once the improved technology becomes available, is presented. This will be useful to future experimentalists wishing to pursue this experiment or a related rotor experiment involving a Mössbauer absorber and a synchrotron Mössbauer source.

Advances in testing the effect of acceleration on time dilation using a synchrotron Mössbauer source.

New results, additional techniques and know-how acquired, developed and employed in a recent HC-1898 experiment at the Nuclear Resonance Beamline ID18 of ESRF are presented, in the quest to explore the acceleration effect on time dilation. Using the specially modified Synchrotron Mössbauer Source and KB-optics together with a rotating single-line semicircular Mössbauer absorber on the rim of a specially designed rotating disk, the aim was to measure the relative spectral shift between the spectra of two states when the acceleration of the absorber is anti-parallel and parallel to the source. A control system was used for the first time and a method to quantify the effects of non-random vibrations on the spectral shift was developed. For several runs where the effect of these vibrations was negligible, a stable statistically significant non-zero relative shift was observed. This suggests the influence of acceleration on time.

Synchrotron radiation Mössbauer spectra of a rotating absorber with implications for testing velocity and acceleration time dilation.

Many Mössbauer spectroscopy (MS) experiments have used a rotating absorber in order to measure the second-order transverse Doppler (TD) shift, and to test the validity of the Einstein time dilation theory. From these experiments, one may also test the clock hypothesis (CH) and the time dilation caused by acceleration. In such experiments the absorption curves must be obtained, since it cannot be assumed that there is no broadening of the curve during the rotation. For technical reasons, it is very complicated to keep the balance of a fast rotating disk if there are moving parts on it. Thus, the Mössbauer source on a transducer should be outside the disk. Friedman and Nowik have already predicted that the X-ray beam finite size dramatically affects the MS absorption line and causes its broadening. We provide here explicit formulas to evaluate this broadening for a synchrotron Mössbauer source (SMS) beam. The broadening is linearly proportional to the rotation frequency and to the SMS beam width at the rotation axis. In addition, it is shown that the TD shift and the MS line broadening are affected by an additional factor assigned as the alignment shift which is proportional to the frequency of rotation and to the distance between the X-ray beam center and the rotation axis. This new shift helps to align the disk's axis of rotation to the X-ray beam's center. To minimize the broadening, one must focus the X-ray on the axis of the rotating disk and/or to add a slit positioned at the center, to block the rays distant from the rotation axis of the disk. Our experiment, using the (57)Fe SMS, currently available at the Nuclear Resonance beamline (ID18) at the ESRF, with a rotating stainless steel foil, confirmed our predictions. With a slit installed at the rotation axis (reducing the effective beam width from 15.6 µm to 5.4 µm), one can measure a statistically meaningful absorption spectrum up to 300 Hz, while, without a slit, such spectra could be obtained up to 100 Hz only. Thus, both the broadening and the alignment shift are very significant and must be taken into consideration in any rotating absorber experiment. Here a method is offered to measure accurately the TD shift and to test the CH.

Magnetic and structural studies of the multifunctional material SrFe(0.75)Mo(0.25)O(3-δ).

SrFe0.75Mo0.25O3-δ has been recently discovered as an extremely efficient electrode for intermediate temperature solid oxide fuel cells (IT-SOFCs). We have performed structural and magnetic studies to fully characterize this multifunctional material. We have observed by powder neutron diffraction (PND) and transmission electron microscopy (TEM) that its crystal symmetry is better explained with a tetragonal symmetry (I4/mcm space group) than with the previously reported orthorhombic symmetry (Pnma space group). The temperature dependent magnetic properties indicate an exceptionally high magnetic ordering temperature (TN ∼ 750 K), well above room temperature. The ordered magnetic structure at low temperature was determined by PND to be an antiferromagnetic coupling of the Fe cations. Mössbauer spectroscopy corroborated the PND results. A detailed study, with X-ray absorption spectroscopy (XAS), in agreement with the Mössbauer results, confirmed the formal oxidation states of the cations to be mixed valence Fe(3+/4+) and Mo(6+).

Coexistence of ferromagnetism and superconductivity: magnetization and Mössbauer studies of EuFe2(As1 - xPx)2.

Magnetization and (57)Fe and (151)Eu Mössbauer studies of EuFe(2)(As(1 - x)P(x))(2) (x = 0-1.0) at temperatures (5-300 K) have been performed. The magnetization studies show a decrease of the divalent Eu sublattice antiferromagnetic transition temperature from T(AFM) = 20 K for x = 0 to 16 K at x≈0.2. For x > 0.2, the Eu sublattice is ferromagnetically ordered at T(FM), which increases up to 27 K for x = 1.0. For 0.2 < x < 0.5, the system becomes superconducting. (151)Eu Mössbauer studies in the antiferromagnetic range show a constant saturation hyperfine field of 26.2 T and that the magnetization is almost perpendicular to the c-axis. On the other hand, in the ferromagnetic range, the hyperfine field increases up to 30.8 T (for x = 1) and the easy axis is almost parallel to the c-axis. In both regions the magnetic axis seems to be tilted from the basal plane or the c-axis by ∼ 20°. The (57)Fe Mössbauer studies show no magnetism in the iron site for x > 0.2, yet at 5 K exhibit transferred magnetic hyperfine fields (∼1 T) from the ferromagnetically ordered Eu sublattice, even in the superconducting region. Superconductivity in the presence of ferromagnetism is generally not observable. However, transferred magnetic hyperfine fields in the superconducting state are observed here for the first time.

Stages in iron storage in the ferritin of Escherichia coli (EcFtnA): analysis of Mössbauer spectra reveals a new intermediate.

Iron uptake into the nonheme ferritin of Escherichia coli (EcFtnA) and its site-directed variants have been investigated by Mössbauer spectroscopy. EcFtnA, like recombinant human H chain ferritin (HuHF), oxidized Fe(II) at a dinuclear ferroxidase center situated at a central position within each subunit. As with HuHF, Mössbauer subspectra observed between 1 min and 24 h after Fe(II) addition were assigned to Fe(III) monomers, "c", mu-oxo-bridged dimers, "b", and clusters, "a", the latter showing magnetically split spectra, "d", at 4.1 K. Like those of HuHF, the mu-oxo-bridged dimers were formed at the ferroxidase centers. However, the analysis also revealed the presence of a new type of dimer, "e" (QS1 = 0.38 mm/s, IS1 = 0.51 mm/s and QS2 = 0.72 mm/s, IS2 = 0.50 mm/s), and this was also assigned to the ferroxidase center. Dimers "b" appeared to be converted to dimers "e" over time. Subspectra "e" became markedly asymmetric at temperatures above 90 K, suggesting that the two Fe(III) atoms of dimers "e" were more weakly coupled than in the mu-oxo-bridged dimers "b", possibly due to OH- bridging. Monomeric Fe(III), giving relaxation spectra "c", was assigned to a unique site C that is near the dinuclear center. In EcFtnA all three iron atoms seemed to be oxidized together. In contrast to HuHF, no Fe(III) clusters were observed 24 h after the aerobic addition of 48 Fe(II) atoms/molecule in wild-type EcFtnA. This implies that iron is more evenly distributed between molecules in the bacterial ferritins, which may account for its greater accessibility.

Iron in parkinsonian and control substantia nigra--a Mössbauer spectroscopy study.

We used Mössbauer spectroscopy to study the iron content, the redox state, and the binding site of iron in substantia nigra (SN) from parkinsonian (PD) and control brains. Measurements performed on fresh-frozen, formalin-fixed, and lyophilized samples demonstrated the presence of ferric (Fe3+) iron only, both in PD and control SN. Ferrous iron, if present at all, may represent at most 5% of the total iron. We found no difference in the total amount of iron in SN between PD and control brains. The Mössbauer spectra observed at 4.1 K in fresh (frozen or lyophilized) samples were different from those obtained in formalin-fixed (frozen or lyophilized) samples. In the fresh samples, only ferritin-like iron was observed, whereas in the samples frozen or lyophilized from formalin, non-ferritin iron was detected.

Iron incorporation into ferritins: evidence for the transfer of monomeric Fe(III) between ferritin molecules and for the formation of an unusual mineral in the ferritin of Escherichia coli.

Iron that has been oxidized by H-chain ferritin can be transferred into other ferritin molecules before it is incorporated into mature ferrihydrite iron cores. Iron(III) dimers are formed at the ferroxidase centres of ferritin H chains at an early stage of Fe(II) oxidation. Mössbauer spectroscopic data now show that the iron is transferred as monomeric species arising from dimer dissociation and that it binds to the iron core of the acceptor ferritin. Human H-chain ferritin variants containing altered threefold channels can act as acceptors, as can the ferritin of Escherichia coli (Ec-FTN). A human H-chain ferritin variant with a substituted tyrosine (rHuHF-Y34F) can act as a donor of Fe(III). Since an Fe(III)-tyrosinate (first identified in bullfrog H-chain ferritin) is absent from variant rHuHF-Y34F, the Fe(III) transferred is not derived from this tyrosinate complex. Mössbauer parameters of the small iron cores formed within Ec-FTN are significantly different from those of mammalian ferritins. Analysis of the spectra suggests that they are derived from both ferrihydrite and non-ferrihydrite components. This provides further evidence that the ferritin protein shell can influence the structure of its iron core.

[Mossbauer spectroscopy of iron in substantia nigra in Parkinson disease and controls]. / Spektroskopia Mössbauerowska zaelaza istoty czarnej w chorobie Parkinsona i kontroli.

Mössbauer spectroscopy was used to study iron content, its redox state and binding sites in substantia nigra from parkinsonian and control brains. Measurements performed on fresh frozen samples demonstrated the presence of ferric iron only, both in disease and control. We found no difference in the total amount of iron in substantia nigra between the disease and control. Mössbauer spectra observed at 4.1 K in fresh frozen samples were different from those obtained in formalin fixed samples. In the fresh frozen samples only ferritin like iron was observed, whereas in the formalin fixed samples also non-ferritin iron was detected. It seems that in formalin fixed brains, during years, iron is released from ferritin and bound to an iron chelator or formalin.

Iron (II) oxidation and early intermediates of iron-core formation in recombinant human H-chain ferritin.

The paper describes a study of Fe(II) oxidation and the formation of Fe(III)-apoferritin complexes in recombinant human H-chain ferritin and its variants. The effects of site-directed changes in the conserved residues associated with a proposed ferroxidase centre have been investigated. A change in any of these residues is shown to reduce the rate of Fe(II) oxidation, confirming the importance of the ferroxidase centre in the catalysis of Fe(II) oxidation. Mössbauer and u.v.-difference spectroscopy show that in the wild-type protein Fe(II) oxidation gives rise to Fe(III) monomers, dimers and larger clusters. The formation of Fe(III) mu-oxo-bridged dimers occurs at the ferroxidase centre and is associated with fast oxidation: in three variants in which Fe(II) oxidation is especially slow, no Fe(III) dimers are seen. Within the time scale 0.5-20 min in wild-type human H-chain ferritin, dimer formation precedes that of the monomer and the progression dimer-->monomer-->cluster is observed, although not to completion. In a preliminary investigation of oxidation intermediates using a stopped-flow instrument, an Fe(III)-tyrosine complex reported by Waldo et al. (1993), is attributed to Tyr-34, a residue at the ferroxidase centre. The Fe(III)-Tyr-34 complex, forms in 0.5 s and then decays, as dimer absorbance increases. The relationship between Fe(III)-tyrosinate and the formation of Fe(III) dimers is uncertain.

Defining the roles of the threefold channels in iron uptake, iron oxidation and iron-core formation in ferritin: a study aided by site-directed mutagenesis.

This paper aims to define the role of the threefold intersubunit channels in iron uptake and sequestration processes in the iron-storage protein, ferritin. Iron uptake, measured as loss of availability of Fe(II) to ferrozine (due to oxidation), has been studied in recombinant human H-chain ferritins bearing amino acid substitutions in the threefold channels or ferroxidase centres. Similar measurements with recombinant horse L-chain ferritin are compared. It is concluded that significant Fe(II) oxidation occurs only at the H-chain ferroxidase centres and not in the threefold channels, although this route is used by Fe(II) for entry. Investigations by Mössbauer and u.v.-difference spectroscopy show that part of the iron oxidized by H-chain ferritin returns to the threefold channels as Fe(III). This monomeric Fe(III) can be displaced by addition of Tb(III). Fe(III) also moves into the cavity for formation of the iron-core mineral, ferrihydrite. Iron incorporated into ferrihydrite becomes kinetically inert.

Mössbauer spectroscopic investigation of structure-function relations in ferritins.

Ferritin plays an important role in iron metabolism and our aim is to understand the mechanisms by which iron is sequestered within its protein shell as the mineral ferrihydrite. We present Mössbauer spectroscopic data on recombinant human and horse spleen ferritin from which we draw the following conclusions: (1) that apoferritin catalyses Fe(II) oxidation as a first step in ferrihydrite deposition, (2) that the catalysis of Fe(II) oxidation is associated with residues situated within H chains, at the postulated 'ferroxidase centre' and not in the 3-fold inter-subunit channels previously suggested as the initial Fe(II) binding and oxidation site; (3) that both isolated Fe(III) and Fe(III) mu-oxo-bridged dimers found previously by Mössbauer spectroscopy to be intermediates in iron-core formation in horse spleen ferritin, are located on H chains; and (4) that these dimers form at ferroxidase centres. The importance of the ferroxidase centre is suggested by the conservation of its ligands in many ferritins from vertebrates, invertebrates and plants. Nevertheless iron-core formation does occur in those ferritins that lack ferroxidase centres even though the initial Fe(II) oxidation is relatively slow. We compare the early stages of core formation in such variants and in horse spleen ferritin in which only 10-15% of its chains are of the H type. We discuss our findings in relation to the physiological role of isoferritins in iron storage processes.

Iron (III) can be transferred between ferritin molecules.

The iron-storage molecule ferritin can sequester up to 4500 Fe atoms as the mineral ferrihydrite. The iron-core is gradually built up when FeII is added to apoferritin and allowed to oxidize. Here we present evidence, from Mössbauer spectroscopic measurements, for the surprising result that iron atoms that are not incorporated into mature ferrihydrite particles, can be transferred between molecules. Experiments were done with both horse spleen ferritin and recombinant human ferritin. Mössbauer spectroscopy responds only to 57Fe and not to 56Fe and can distinguish chemically different species of iron. In our experiments a small number of 57FeII atoms were added to two equivalent apoferritin solutions and allowed to oxidize (1-5 min or 6 h). Either ferritin containing a small iron-core composed of 56Fe, or an equal volume of NaCl solution, was added and the mixture frozen in liquid nitrogen to stop the reaction at a chosen time. Spectra of the ferritin solution to which only NaCl was added showed a mixture of species including 57FeIII in solitary and dinuclear sites. In the samples to which 150 56FeIII-ferritin had been added the spectra showed that all, or almost all, of the 57FeIII was in large clusters. In these solutions 57FeIII initially present as intermediate species must have migrated to molecules containing large clusters. Such migration must now be taken into account in any model of ferritin iron-core formation.