Crystallographic studies of magnetite and titanomaghemite

The crystal structure of natural magnetite has been investigated on the basis of previously published X-ray intensity data and a newly acquired, more extensive data base. Both investigations show that the structure does not conform to the centrosymmetrical space group Fd3m, as is normally assumed, but the non-centrosymmetrical space group F43m. The structure refinement provides values for the atom positions, anisotropic thermal parameters and bond lengths. A study of Friedel related pairs of X-ray intensities shows that Friedel's law is violated in magnetite, further confirming that the space group is non-centrosymmetrical. It was found that the octahedral site cations in magnetite do not occupy special positions at the centres of the octahedral interstices as they should under the space group Fd3m, but are displaced along <111 > directions leading to F43m symmetry. A mechanism is known for the origin of these displacements and the likelihood of similar displacements occurring in other natural and synthetic spinels is discussed. The crystal structure of a natural titanomaghemite was determined by a combination of X-ray diffraction and Mõssbauer spectroscopy. This was confirmed as possessing a primitive cubic Bravais lattice with the space group P4332 and the structural formula: Fe3+.0.96 0 0.04 [Fe2+0.23 Fe3+0.99 Ti4+0.42 0 0.37 ] 042 - where 0 represents a cation vacancy. As the above formula shows, there are cation vacancies on both tetrahedral arrl octahedral sites, the majority being restricted to octahedral sltes. No tetrahedral site Fe2+ or Ti4+ was observed. Values for the atom positions, anisotropic thermal parameters and bond lengths have been determined for this particular specimen.

Magnetite-doped polydimethylsiloxane (PDMS) for phosphopeptide enrichment

Reversible phosphorylation plays a key role in numerous biological processes. Mass spectrometry-based approaches are commonly used to analyze protein phosphorylation, but such analysis is challenging, largely due to the low phosphorylation stoichiometry. Hence, a number of phosphopeptide enrichment strategies have been developed, including metal oxide affinity chromatography (MOAC). Here, we describe a new material for performing MOAC that employs a magnetite-doped polydimethylsiloxane (PDMS), that is suitable for the creation of microwell array and microfluidic systems to enable low volume, high throughput analysis. Incubation time and sample loading were explored and optimized and demonstrate that the embedded magnetite is able to enrich phosphopeptides. This substrate-based approach is rapid, straightforward and suitable for simultaneously performing multiple, low volume enrichments. © the Partner Organisations 2014.