D.P.E. Dickson

Synthesis and Structure of an Iron(III) Sulfide-Ferritin Bioinorganic Nanocomposite.

Amorphous iron sulfide minerals containing either 500 or 3000 iron atoms in each cluster have been synthesized in situ within the nanodimensional cavity of horse spleen ferritin. Iron-57 M�ssbauer spectroscopy indicated that most of the iron atoms in the 3000-iron atom cores are trivalent, whereas in the 500-iron atom clusters, approximately 50 percent of the iron atoms are Fe(III), with the remaining atoms having an effective oxidation state of about +2.5. Iron K-edge extended x-ray absorption fine structure data for the 500-iron atom nanocomposite are consistent with a disordered array of edge-shared FeS4 tetrahedra, connected by Fe(S)2Fe bridges with bond lengths similar to those of the cubane-type motif of iron-sulfur clusters. The approach used here for the controlled synthesis of bioinorganic nanocomposites could be useful for the nanoscale engineering of dispersed materials with biocompatible and bioactive properties.

Mössbauer spectroscopic studies of human haemosiderin and ferritin

Ferritin and haemosiderin isolated from iron-overloaded human spleens have been investigated by 57Fe Mössbauer spectroscopy at temperatures between 1.3 and 200 K and also in applied magnetic fields. Virtually identical spectra were obtained from both materials at the high and low-temperature ends of this range, and also at 4.2 K in an applied magnetic field of 10 T; this indicates that both must contain iron in a closely similar chemical form. The difference between the two materials lies in the temperature dependence of their Mössbauer spectra in the intermediate temperature range, between 10 and 100 K. The temperature dependence of the Mössbauer spectra is characteristic of superparamagnetic behaviour, which occurs when a magnetically ordered material is present in the form of small particles. The details of this temperature dependence are related to the distribution of particle sizes and the magnetic anisotropy constant of each substance. Electron microscopy shows the haemosiderin cores to be markedly smaller on average than those of ferritin. Combining the Mössbauer spectroscopy and electron microscopy data we have shown that the magnetic anistropy constant of haemosiderin is considerably larger than that of ferritin. This is thought to result from the smaller core size and less symmetrical protein shell of the former. These data are consistent with the proposal that haemosiderin is derived from ferritin.

Mössbauer spectroscopic studies of the cores of human, limpet and bacterial ferritins

Ferritin cores from human spleen, limpet (Patella vulgata) haemolymph and bacterial (Pseudomonas aeruginosa) cells have been investigated using 57Fe Mössbauer spectroscopy. The Mössbauer spectra were recorded over a range of temperatures from 1.3 to 78 K, all the spectra are quadrupole-split doublets with similar quadrupole splittings and isomer shifts, characteristic of iron(III), while at sufficiently low temperatures the spectra of all the samples show well-resolved magnetic splitting. At intermediate temperatures, the spectra from the human ferritin exhibit typical superparamagnetic behaviour, while those from the bacterial ferritin show behaviour corresponding to a transition from a magnetically ordered to a paramagnetic state. The spectra from the limpet ferritin show a complex combination of the two effects. The results are discussed in terms of the magnetic behaviour of small particles. The data are consistent with magnetic ordering temperatures of about 3 and 30 K for the bacterial and limpet ferritin cores, respectively, while the data indicate that the magnetic ordering temperature for the human ferritin cores must be above 50 K. These differences are interpreted as being related to different densities of iron in the cores and to variations in the composition of the cores. The human ferritin cores are observed to have a mean superparamagnetic blocking temperature of about 40 K, while that of the limpet ferritin cores is about 25 K. This difference is interpreted as being due not only to different mean numbers of iron atoms in the two types of core but also to the higher degree of crystallinity in the cores of the human ferritin.

Determination of f0 for fine magnetic particles

In this paper we have determined a value for the pre-exponential factor in the Neel-Arhennius equation for the iron oxyhydroxide particles found in the protein ferritin. The data were obtained using a combination of zero field magnetic and Mossbauer spectroscopy studies yielding a value for f0 of (9.5±2.7)X1011 Hz. This value is significantly different to that of 2.8 X 109 Hz commonly used and in closer agreement to that of 1013 Hz obtained for iron particles using an analogous technique. Using our experimental value for f0 gives a revised superparamagnetic criterion for DC magnetic measurements on a 100 second time-scale of KV < 32 kT and a Mossbauer spectroscopy criterion for a measurement time-scale of 10-9 s of KV < 8 kT. Our results together with other published data would suggest that a more appropriate estimate for the value of f0 would lie in the range 1012 to 1013 Hz.

Nanostructured magnetism in living systems

Biological systems provide a number of examples of magnetic small particles in the nanometre size range. These can be used as model systems for investigations of magnetic behaviour or they can provide a source of novel magnetic materials. Both of these aspects will be considered in this paper, with particular reference to the use of Mossbauer spectroscopy as a technique for both characterisation and magnetic investigation.

Why aren't secondary students interested in physics?

This article describes a questionnaire study to determine why fewer Year 10 school students are interested in physics than in biology. The major general reasons for finding physics uninteresting are that it is seen as difficult and irrelevant. Certain areas within the physics curriculum are considered to be boring by some students, interesting by others. Other physics topics, however, are reported only in terms of being interesting; ‘the universe’ is an example. Males and females offer different reasons for finding physics boring, with males enjoying practical exercises and females valuing where physics can be seen as relevant.

Mössbauer spectroscopy, electron microscopy and electron diffraction studies of the iron cores in various human and animal haemosiderins.

Mössbauer spectroscopy has indicated significant differences in the iron-containing cores of various haemosiderins. In the present study, haemosiderin was isolated from a number of animal species including man. In addition, haemosiderin was isolated from patients with primary idiopathic haemochromatosis or with secondary (transfusional) iron-overload. The iron cores of the animal and normal human haemosiderin appear to be very similar by Mössbauer spectroscopy, and the electron diffraction data indicate a ferrihydrite structure similar to that of ferritin cores. The haemosiderin isolated from secondary iron-overload shows anomalous behaviour in its temperature-dependent Mössbauer spectra. This can be understood in terms of the microcrystalline goethite structure of the cores as indicated by electron diffraction. The haemosiderin cores obtained in the case of primary haemochromatosis have an amorphous Fe(III) oxide structure and show Mössbauer spectra characteristic of a magnetically disordered material, which only orders at very low temperatures.

Structural specificity of haemosiderin iron cores in iron-overload diseases

Haemosiderin iron cores isolated from patients with secondary haemochromatosis have a goethite‐like (α‐FeOOH) crystal structure whereas those from patients with primary haemochromatosis are amorphous Fe (III) oxide. Haemosiderin cores isolated from normal human spleen are crystalline ferrihydrite (5Fe2O3·9H2O). The disease‐specific structures are significantly different from the ferrihydrite structure of associated ferritin cores. The results are important in understanding the biological processing of iron in pathological states and in the clinical treatment of iron‐overload diseases.

Synthesis, structure and magnetic properties of ferritin cores with varying composition and degrees of structural order: models for iron oxide deposits in iron-overload diseases

The cage-like protein ferritin was used to form nanoscale iron-containing mineral particles in vitro with different structures and compositions by reconstituting the metal-free protein (apoferritin) with iron at different temperatures and in the presence of different quantities of phosphate. The products of reconstitution were studied with inductively coupled plasma spectrometry, transmission electron microscopy, electron diffraction, extended X-ray absorption fine structure analysis, and Mossbauer spectroscopy. Reconstitution at 4°C resulted in poorly ordered core structures while reconstitution at 55°C resulted in more ordered structures based on that of the mineral ferrihydrite. The more ordered structure of the 55°C ferritin resulted in stronger magnetic exchange interactions between the iron atoms within each core and a larger magnetic anisotropy energy per core. Incorporation of phosphate within the core structure reduced the core density. This also reduced the strength of the magnetic exchange interactions between the iron atoms. High levels of phosphate within the core resulted in cores with no measurable periodicity within their structure. This in turn caused a reduction in the magnetic anisotropy energy per core. The ability to tailor the degree of structural order and phosphate content of ferritin cores in vitro makes available a range of model materials for a more comprehensive study of the structural and magnetic correlations found in nanoscale iron biominerals in vivo such as native ferritins and haemosiderins deposited in iron-overloaded tissues.

Ultrafine particles of barium ferrite from a citrate precursor

Ultrafine particles of barium ferrite produced by the precursor method have sizes between 5 and 100 nm. These particles have relatively low magnetisations and high coercivities. Mossbauer spectra exhibit different relative areas for the 4fiv and 2b sites compared to the bulk which partly explains the low magnetisations.