Ferrihydrite Mossbauer spectroscopy

On the basis of a number of physico-chemical methods (Mossbauer spectroscopy, electron diffraction, EXAFS) the iron cores of naturally occurring haemosiderins isolated from various iron-loaded animals and man (horse, reindeer, birds and human old age) were consistently shown to have ferrihydrite-like iron cores similar to those of ferritin (Ward et ah, 1992, 2000). In marked contrast, in the tissues of patients with two pathogenic iron-loading syndromes, genetic haemochromatosis and thalassaemia, the haemosiderins isolated had predominantly amorphous ferric oxide and goethite cores, respectively (Dickson etah, 1988 Mann etah, 1988 ... 

Schwertmarm, U. Schulze, D.G. Murad, E. (1982) Identification of ferrihydrite in soils by dissolution kinetics, differential X-ray diffraction and Mossbauer spectroscopy. Soil Sd. Soc. Am. J. 46 869-875... 

In vitro a crystalline iron core can be laid down in apoferritin by the addition of an oxidant, such as O2, to an aqueous solution of a ferrous salt and apoferritin (32, 132, 140). The reconstituted core of horse ferritin prepared in the absence of phosphate and with O2 as oxidant is very similar to the native core in terms of its size and Mossbauer properties (85). Electron microscopy, however, reveals that it is less well ordered. Reconstitution in the presence of phosphate leads to smaller cores. Reconstituted A. vinelandii cores in the absence of phosphate were more ordered than were the native cores, and clearly contained ferrihydrite particles and, in some cases, crystal domains (85). Thus the nature of the core is not determined solely by the protein coat the conditions of core formation are also important. This is also indicated by Mossbauer spectroscopy studies of P. aeruginosa cells grown under conditions different than those employed for the large-scale pu-... 

There is no information on the site of formation of the Fe(III)-0-Fe(III) dimers that have been observed in rHF as well as in horse spleen ferritin (57,112). This may be clarified by Mossbauer spectroscopy on ferritin variants. Possible positions are the putative nucleation center or the double Tb site. In the latter case, either the dimers themselves must move or the iron core nucleus must incorporate iron atoms from the ferroxidase centers. Because ferrihydrite can be deposited both in rLF and in the rHF variant lacking ferroxidase activity, nucleation at this center is not possible for these molecules. Indeed, a specific center may not be required. However, electron micrographs of broken ferritin molecules suggest an attachment of ferrihydrite to the protein shell (113). 

Iron Core Only a small fraction of the iron atoms in ferritin bind directly to the protein. The core contains the bulk of the iron in a polynuclear aggregate with properties similar to ferrihydrite, a mineral found in nature and formed experimentally by heating neutral aqueous solutions of Fe(III)(N03)3. X-ray diffraction data from ferritin cores are best fit by a model with hexagonal close-packed layers of oxygen that are interrupted by irregularly incomplete layers of octahedrally coordinated Fe(III) atoms. The octahedral coordination is confirmed by Mossbauer spectroscopy and by EXAFS, which also shows that the average Fe(III) atom is surrounded by six oxygen atoms at a distance of 1.95 A and six iron atoms at distances of 3.0 to 3.3 A. 

U. Schwertmann and R. M. Taylor, Iron oxides, in Minerals in Soil Environments (J. B. Dixon and S. B. Weed, eds.) Soil Science Society of America, Madison, Wis., 1977. U. Schwertmann, D. G. Schulze, and E. Murad, Identification of ferrihydrite in soils by dissolution kinetics, differential X-ray diffraction, and Mossbauer spectroscopy, Soil Sci. Soc. Am. J. 46 869 (1982). 

Using Mossbauer spectroscopy, three positions of trivalent iron with nonoverlapping ranges of A q were revealed in bacterial ferrihydrite nanoparticles, produced by the bacterium Klebsielh oxytoca in the course of biomineralization of iron salt solutions from a natural medium, after sample heating [60]. The fit of these spectra and interpretation of results seem to be doubtful, as the authors did not take into account a complicated iron core structure in bacterial ferritin and a core size variation while relating their results to merely two structural differences in ferrihydrite. 

Bauminger and Harrison reviewed studies of the process of iron core formation in human and horse spleen ferritins using Mossbauer spectroscopy. It was demonstrated that iron deposition within ferrihydrite core in human and horse spleen ferritin started with Fe(ll) oxidation. This process was associated with ferroxidase center of H-chains. Further, an Fe(lll) compound and Fe(lll) jL-oxo-bridged dimers in ferroxidase centers of H-chains were found, which were intermediate compounds in the process of iron oxyhydroxide core formation in horse spleen ferritin. The steps leading to ferrihydrite core formation in human L- and H-ferritins were also identified and transfer between ferritin molecules was established [M2]. 

It is important to stress that Mossbauer spectroscopy did not detect any divalent iron in all the samples measured. In a study using analytical transmission electron microscopy, the authors suggested that iron within the ferritin core in pathological SN and HP was present mainly as mixed ferric-ferrous iron oxides (magnetite-like or wiistite) and not as ferrihydrite that is the main mineral in control brain ferritin [31 ]. Mossbauer spectra of magnetite or wiistite differ from ferrihydrite spectra, but only ferrihydrite-like spectra were observed by MS in all pathological tissues. If other phases were present, they could be there only in minor quantities. 

The influence of the V " " ion presence during the formation of goethite from ferrihydrite in an alkaline medium was investigated using Mossbauer spectroscopy and other techniques by Kaur et al. [249]. The presence of ions reduced HMF values in the Mossbauer spectra due to a substitution of Fe by V ions (ionic radius of 0.64 A almost equal to the radius of Fe " ") in the goethite structure. Small quantities of hematite and superparamagnetic goethite were also formed. 

Influence of Tetravalent Cations The influence of the Ti ion presence during the formation of goethite from ferric solution in an alkaline medium was investigated by Mossbauer spectroscopy and other techniques [246]. The presence of Ti " " ions partially suppressed the transformation of ferrihydrite to goethite and as a result a quadrupole doublet, corresponding to the low-crystalline phase, emerged in the Mossbauer spectrum. 

Pollard R J, Cardile C M, Lewis D G and Brown L J, Characterization of FeOOH polymorphs and ferrihydrite using low-temperature, applied-field, Mossbauer-Spectroscopy , Clay Miner, 1992 11 (1) 57—71. 

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