E. R. Bauminger

New multiple magnetic phase transitions and structures in RMn2X2, X = Si or Ge, R = rare earth

Abstract Magnetometry and dilute 57 Fe Mossbauer spectroscopy studies of RMn 2 X 2 (X = Si or Ge, R = La, Ce, Pr, Nd, Sm and Gd) at temperatures 4.2–650 K yield the following results; Fe in RMn 2 X 2 is nonmagnetic. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields. The compounds LaMn 2 Si 2 , LaMn 2 Ge 2 , CeMn 2 Ge 2 , PrMn 2 Ge 2 , NdMn 2 Ge 2 and SmMn 2 Ge 2 , known to be ferromagnets with T C = 300–350 K, are antiferromagnetically ordered above their corresponding T C . Their T N values extend from 385 K (SmMn 2 Ge 2 ) to 470 K (LaMn 2 Si 2 ), similar to the T N values of the antiferromagnetic heavy rare earth compounds. At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In SmMn 2 Ge 2 we find five magnetic phase transitions, T C (Sm) = 30 K and T C (Mn) at 105 and 345 K and T N (Mn) at 155 and 385 K. In this compound, a superposition of two six-line 57 Fe Mossbauer patterns is seen between 90 and 155 K with changing relative intensities, indicating a competition of two easy magnetization axes, with an anisotropic transferred hyperfine field at the Fe nucleus. In NdMn 2 Ge 2 we find four phase transitions, T C (Nd) = 21 K, T C (Mn) = 335 K, T N (Mn) = 415 K, and one more very sharp transition at 210 K, associated with a discontinuity in 57 Fe hyperfine interaction parameters and a sharp drop in bulk magnetization; this seems to be a transition from pure ferromagnetism to canted antiferromagnetism. The results for antiferromagnetic CeMn 2 Si 2 , PrMn 2 Si 2 and GdMn 2 Ge 2 revealed no new phenomena and are in full agreement with previous magnetization studies. In GdMn 2 Ge 2 the transferred hyperfine field at the 57 Fe nucleus is smaller at 4.2 K (below the ordering temperature of Gd) than at 90 K, proving that the transferred hyperfine field from Gd is opposite to that produced by Mn.

Mössbauer spectroscopy of 57Fe in high Tc superconductors YbA2Fe3xCu3(1−x)O7−δ

Abstract Mossbauer spectroscopy of 57 Fe in both tetragonal and othorhombic phases of YBa 2 ( Fe x Cu 1− x ) 3 O 7− δ , with x = 0.01, 0.02 and 0.10, at temperatures 4.2 K, 75 K, 90 K, and 300 K have been performed. In all samples three major subspectra corresponding to iron in different local environments are observed. It is concluded that Fe substitutes mainly Cul. At 4.2 K, samples with x =0.01 in the “quenched” tetragonal phase exhibit magnetic hyperfine structure, due to slow spin relaxation rates, whereas in the orthorhombic superconducting phase, only samples with x =0.1 exhibit magnetic hyperfine structure, in this case probably due to spin glass magnetic order.

Mössbauer spectroscopy of Escherichia coli and its iron-storage protein

57Fe Mössbauer spectra of whole frozen Escherichia coli cells and of an iron storage protein isolated from iron-rich cells of E. coli have been measured over a range of temperatures down to 0.08 K. The spectra of E. coli cells with high iron content and of the iron storage protein were found to be very similar. Above 4 K these spectra consist of a quadrupole split doublet characteristic of Fe3+. Below 3.5 K, the spectra display magnetic hyperfine splitting which is temperature dependent, and point to the existence of an ordered magnetic phase associated with a saturation magnetic hyperfine field of 43 tesla in both samples. The results indicate that the bulk of iron in the iron-rich cells is in the form of aggregates similar in nature to the iron cores in the isolated protein, although the latter account for not more than 1% of the total iron in the cells. The Mössbauer spectra of the isolated protein are different from those observed in ferritin, the iron-storage protein of plants and higher animals, showing that the iron cores in these two proteins are different.

Structure and composition of ferritin cores from pea seed (Pisum sativum)

Iron cores from native pea seed (Pisum sativum) ferritin have been analysed by electron microscopy and Mössbauer spectroscopy and shown to be amorphous. This correlates with their relatively high phosphate content (Fe: P = 2.83; 1800 Fe, 640 P atoms/molecule). Reconstituted cores obtained by adding iron (2000 Fe atoms/molecule) in the absence of phosphate to pea seed apoferritin were crystalline ferrihydrite. In vitro rates of formation of pea-seed ferritin iron cores were intermediate between those of recombinant human H-chain and horse spleen apoferritin and this may reflect the amino-acid residues of its ferroxidase and putative nucleation centres. The high phosphate content of pea-seed ferritin suggests that this molecule could be involved in both phosphorus and iron storage. The high phosphate concentration found within plastids, from which the molecules were isolated, is a possible source of the ferritin phosphate.

Iron and reactive oxygen species activity in parkinsonian substantia nigra

OBJECTIVES We sought to determine concentrations of total and labile iron in substantia nigra from patients with Parkinson disease and from controls to assess if oxidative stress is triggered by an increased concentration of iron. METHODS Total iron concentration in the whole substantia nigra was evaluated in 17 parkinsonian and 29 control samples. Concentrations of labile iron and copper were assessed in 6 parkinsonian and 8 control samples. The total iron concentration, the Fe(2+)/Fe(3+) ratio, and iron-binding compounds were determined by Mössbauer spectroscopy. Labile iron and copper were measured by electrothermal atomic absorption spectrometry. Activity of reactive oxygen species was evaluated by visible light fluorescence. RESULTS The labile iron concentration was significantly higher and corresponded to significantly higher reactive oxygen species activity in parkinsonian vs control samples. No significant difference was found in the total concentrations of copper or iron in the whole substantia nigra between parkinsonian and control samples. Mössbauer spectroscopy detected no Fe(2+) in any samples. CONCLUSIONS The substantia nigra of parkinsonian patients contained more labile iron compared with that of controls. This labile iron generated higher reactive oxygen species activity. The oxidative stress damage in parkinsonian substantia nigra may be related to an excess of labile iron and not of the total iron in the diseased tissue.

Magnetism in plant and mammalian ferritin

A rich variety of magnetic phenomena is observed in Mössbauer studies of ferritin. Depending on the amount of iron in the horse spleen ferritin core, a paramagnetic relaxation spectrum, or quadrupole split doublet or a magnetically split sextet showing superpara-magnetism, are obtained a 4.1 K. Mössbauer studies of the recently prepared iron loaded concanavalin A yield hyperfine parameters identical to those found previously in mammalian ferritin, yet show the existence of larger iron aggregates. Due to the larger particle size it is possible to follow the magnetic hyperfine field and to obtain the magnetic ordering temperature as 240 K. This is exactly the Neél temperature of ferrihydrite, thus establishing that this is indeed the iron compound in the ferritin core.

Iron overload in cultured rat myocardial cells

In order to characterize the nature of iron deposits associated with iron overload in heart cells, Mössbauer spectroscopy and ultrastructural studies were performed in iron loaded heart cell cultures obtained from newborn rats grown in a medium containing 20 μg iron/ml. Maximal uptake of iron after 24 hrs was about 15%. Not more than 20% of the iron in these cells was stored in ferritin and the rest was found in smaller trivalent iron aggregates. With time there was a shift from smaller to larger aggregates. In chase samples there was only a very limited spontaneous release of iron from heart cells. Desferrioxamine, an iron chelating drug, removed a major part of the smaller aggregates, but did not remove ferritin iron.

Further evidence for the interaction of the antimalarial drug amodiaquine with ferriprotoporphyrin IX

Evidence for complex formation of the antimalarial drug amodiaquine (AD) with ferriprotoporphyrin IX (FP) in aqueous medium is presented, in addition to previous preliminary data. A mole ratio of one between the complex components is determined for the insoluble complex at pH 6.7-6.8. Mössbauer data obtained at pH 7-8 and at higher concentrations in the millimolar range confirm the interactions existing between the complex components. These data are considered to aid in removing previous objections to a mechanism of antimalarial action involving complexes of FP with AD and related drugs.

Does iron play a role in Parkinson's disease?

Mössbauer studies of Parkinsonian and control Substantia Nigra (SN) show that the overall amount of iron in SN is about the same in PD and control. At least 90% of this iron is ferritin-like and Fe2+ and/or neuromelanin iron, if present at all, can constitute only less than 10% of the overall iron. During storage in formalin, iron is slowly removed from ferritin and bound to a chelating agent.

Metal binding at the active centre of the ferritin of Escherichia coli (EcFtnA). A Mössbauer spectroscopic study

Abstract Iron storage in the ferritin of Escherichia coli (EcFtnA) involves (1) the catalytic oxidation of 2Fe(II) at a dinuclear centre (sites A and B) and of a single Fe(II) at a nearby site C; (2) the formation of oxo-bridged Fe(III) dimers (designated ‘b’) at A and B and isolated Fe 3+ ions (‘c’) at site C; (3) the conversion of the oxo-bridged dimers to a second dimer form (‘e’) which is probably hydroxo-bridged; (4) the movement of iron into the iron storage cavity where Fe(III) ferrhydrite clusters (‘a’ and ‘d’) form, and (5) the provision on the surfaces of the mineral clusters of alternative sites for further Fe(II) oxidation as well as for the addition of Fe(III) from the dinuclear centres. Each of the iron species gives rise to a characteristic Mossbauer subspectrum and Mossbauer spectroscopy has been used to study the early stages of iron storage with the aid of competing metal ions Zn 2+ and Tb 3+ . The effects of adding these metal ions both before and after Fe(II) have been examined with both wild type EcFtnA and its site directed variants. These studies have provided evidence that both types of Fe(III) dimers are located at the dinuclear centre and that with time dimers ‘b’ are replaced by dimers ‘e’. The effects of the competing metals are different: Zn(II) has a lower affinity than Fe(III) for site C and does not displace it from C but is able to compete with both Fe(III) and Fe(II) for at least one of the dimer sites. Tb(III) competes with Fe(III) for site C displacing Fe(III) to the cavity to form clusters. The presence of these clusters may accelerate loss of Fe(III) from the dimer sites to the cavity.