Strong polarization enhancement in asymmetric three-component ferroelectric superlattices

Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.     

Evolution of the polarization of the optical afterglow of the gamma-ray burst GRB030329

The association of a supernova with GRB030329 strongly supports the 'collapsar' model of gamma-ray bursts, where a relativistic jet forms after the progenitor star collapses. Such jets cannot be spatially resolved because gamma-ray bursts lie at cosmological distances; their existence is instead inferred from 'breaks' in the light curves of the afterglows, and from the theoretical desire to reduce the estimated total energy of the burst by proposing that most of it comes out in narrow beams. Temporal evolution of the polarization of the afterglows may provide independent evidence for the jet structure of the relativistic outflow. Small-level polarization ( approximately 1-3 per cent) has been reported for a few bursts, but its temporal evolution has yet to be established. Here we report polarimetric observations of the afterglow of GRB030329. We establish the polarization light curve, detect sustained polarization at the per cent level, and find significant variability. The data imply that the afterglow magnetic field has a small coherence length and is mostly random, probably generated by turbulence, in contrast with the picture arising from the high polarization detected in the prompt gamma-rays from GRB021206 (ref. 18).     

Proton-sensing G-protein-coupled receptors

Blood pH is maintained in a narrow range around pH 7.4 mainly through regulation of respiration and renal acid extrusion. The molecular mechanisms involved in pH homeostasis are not completely understood. Here we show that ovarian cancer G-protein-coupled receptor 1 (OGR1), previously described as a receptor for sphingosylphosphorylcholine, acts as a proton-sensing receptor stimulating inositol phosphate formation. The receptor is inactive at pH 7.8, and fully activated at pH 6.8-site-directed mutagenesis shows that histidines at the extracellular surface are involved in pH sensing. We find that GPR4, a close relative of OGR1, also responds to pH changes, but elicits cyclic AMP formation. It is known that the skeleton participates in pH homeostasis as a buffering organ, and that osteoblasts respond to pH changes in the physiological range, but the pH-sensing mechanism operating in these cells was hitherto not known. We detect expression of OGR1 in osteosarcoma cells and primary human osteoblast precursors, and show that these cells exhibit strong pH-dependent inositol phosphate formation. Immunohistochemistry on rat tissue sections confirms the presence of OGR1 in osteoblasts and osteocytes. We propose that OGR1 and GPR4 are proton-sensing receptors involved in pH homeostasis.     

Superoxide anion scavenging effect and superoxide dismutase activity of Ginkgo biloba extract

Ginkgo biloba extract is known to be efficient in diseases associated with free radical generation. The purpose of this work was to study, under in vitro conditions, the action of Ginkgo biloba extract (Gbe) against superoxide anion (O2-.), which is directly or indirectly implicated in cell damage. Gbe appears to have both an O2-. scavenging effect and also a superoxide dismutase activity. Its antiradical effect was demonstrated by low temperature electron spin resonance and in a non-enzymatic system (phenazine methosulfate-NADH), and its enzymatic activity was shown by polarographic determination.     

Structural snapshots along the reaction pathway of ferredoxin-thioredoxin reductase

Oxygen-evolving photosynthetic organisms regulate carbon metabolism through a light-dependent redox signalling pathway. Electrons are shuttled from photosystem I by means of ferredoxin (Fdx) to ferredoxin-thioredoxin reductase (FTR), which catalyses the two-electron-reduction of chloroplast thioredoxins (Trxs). These modify target enzyme activities by reduction, regulating carbon flow. FTR is unique in its use of a [4Fe-4S] cluster and a proximal disulphide bridge in the conversion of a light signal into a thiol signal. We determined the structures of FTR in both its one- and its two-electron-reduced intermediate states and of four complexes in the pathway, including the ternary Fdx-FTR-Trx complex. Here we show that, in the first complex (Fdx-FTR) of the pathway, the Fdx [2Fe-2S] cluster is positioned suitably for electron transfer to the FTR [4Fe-4S] centre. After the transfer of one electron, an intermediate is formed in which one sulphur atom of the FTR active site is free to attack a disulphide bridge in Trx and the other sulphur atom forms a fifth ligand for an iron atom in the FTR [4Fe-4S] centre--a unique structure in biology. Fdx then delivers a second electron that cleaves the FTR-Trx heterodisulphide bond, which occurs in the Fdx-FTR-Trx complex. In this structure, the redox centres of the three proteins are aligned to maximize the efficiency of electron transfer from the Fdx [2Fe-2S] cluster to the active-site disulphide of Trxs. These results provide a structural framework for understanding the mechanism of disulphide reduction by an iron-sulphur enzyme and describe previously unknown interaction networks for both Fdx and Trx (refs 4-6).