153 research outputs found
First-principles study of the inversion thermodynamics and electronic structure of FeM2X4 (thio)spinels (M = Cr, Mn, Co, Ni; X = O, S)
FeM2X4 spinels, where M is a transition metal and X is oxygen or sulfur, are candidate materials for spin filters, one of the key devices in spintronics. We present here a computational study of the inversion thermodynamics and the electronic structure of these (thio)spinels for M = Cr, Mn, Co, Ni, using calculations based on the density functional theory with on-site Hubbard corrections (DFT+U). The analysis of the configurational free energies shows that different behaviour is expected for the equilibrium cation distributions in these structures: FeCr2X4 and FeMn2S4 are fully normal, FeNi2X4 and FeCo2S4 are intermediate, and FeCo2O4 and FeMn2O4 are fully inverted. We have analyzed the role played by the size of the ions and by the crystal field stabilization effects in determining the equilibrium inversion degree. We also discuss how the electronic and magnetic structure of these spinels is modified by the degree of inversion, assuming that this could be varied from the equilibrium value. We have obtained electronic densities of states for the completely normal and completely inverse cation distribution of each compound. FeCr2X4, FeMn2X4, FeCo2O4 and FeNi2O4 are half-metals in the ferrimagnetic state when Fe is in tetrahedral positions. When M is filling the tetrahedral positions, the Cr-containing compounds and FeMn2O4 are half-metallic systems, while the Co and Ni spinels are insulators. The Co and Ni sulfide counterparts are metallic for any inversion degree together with the inverse FeMn2S4. Our calculations suggest that the spin filtering properties of the FeM2X4 (thio)spinels could be modified via the control of the cation distribution through variations in the synthesis conditions
Identification of SmtB/ArsR cis elements and proteins in archaea using the Prokaryotic InterGenic Exploration Database (PIGED)
Microbial genome sequencing projects have revealed an apparently wide
distribution of SmtB/ArsR metal-responsive transcriptional regulators
among prokaryotes. Using a position-dependent weight matrix approach,
prokaryotic genome sequences were screened for SmtB/ArsR DNA binding
sites using data derived from intergenic sequences upstream of
orthologous genes encoding these regulators. Sixty SmtB/ArsR operators
linked to metal detoxification genes, including nine among various
archaeal species, are predicted among 230 annotated and draft
prokaryotic genome sequences. Independent multiple sequence alignments
of putative operator sites and corresponding winged helix-turn-helix
motifs define sequence signatures for the DNA binding activity of this
SmtB/ArsR subfamily. Prediction of an archaeal SmtB/ArsR based upon
these signature sequences is confirmed using purified
Methanosarcina acetivorans C2A protein and
electrophoretic mobility shift assays. Tools used in this study have
been incorporated into a web application, the Prokaryotic InterGenic
Exploration Database (PIGED;
http://bioinformatics.uwp.edu/~PIGED/home.htm),
facilitating comparable studies. Use of this tool and establishment of
orthology based on DNA binding signatures holds promise for
deciphering potential cellular roles of various archaeal winged
helix-turn-helix transcriptional regulators
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