Application of 57Fe-enriched synthetic ferrihydrite to speciate the product of bacterial reduction

We have sampled a clay lens with evidence of sulfide reduction from a texturally stratified sandy aquifer at Rømø, Denmark. A minor amount of synthetic, pure ⁵⁷Fe ferrihydrite was added to this sample and allowed to react for up to three months. The initial sample, the ⁵⁷Fe ferrihydrite, and samples taken from the reaction mixture were investigated by Mössbauer spectroscopy at temperatures between 15 and 298 K as sampled and following exposure to oxygen. The initial sample only contained Fe(II) (33% of the iron) and Fe(III) in silicates. The Fe(III) in the ferrihydrite is reduced to Fe(II) as evidenced by an increase of this component by bacterial activity. The Fe(II) component remains paramagnetic at temperatures down to 15 K. Similarly to naturally reduced sediments the new-formed Fe(II) is extremely reactive towards molecular oxygen. Following oxidation the reformed Fe(III) is found as ferrihydrite. The bonding of the Fe(II) is by electrostatical bonding (adsorbed) to the layer silicates as evidenced by a temperature scanning of the sample between 80 and 270 K.     

Synthesis of polyimides and segmented block copolyimides by transimidization

The transimidization reaction has been successfully utilized to prepare a series of segmented block copolyimides. The synthesis and polymerization of an AX‐type amino imide monomer containing the tetrahydro[5]helicene unit were accomplished. The AX‐type amino imide monomer is stable during isolation and purification, owing to its inert X (e.g., N‐pyridyl) group, but yet readily underwent a self‐transimidization reaction and produced polyimide. Because of the presence of two reactive ends, such an AX‐type polyimide could be incorporated into a series of block copolyimides by reaction with commercially available dianhydrides and diamines. All the copolymers showed two distinct glass‐transition temperatures, typically around 250 and 430 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3991–3996, 2000     

Soluble alternating copolyimides containing the tetrahydro[5]helicene unit

A series of novel copolyimides have been synthesized from three commercially available dianhydrides and two diamine monomers containing the tetrahydro[5]helicene unit. Some of the copolyimides were soluble in common organic solvents such as 1,1,2,2-tetrachloroethane, N,N-dimethylacetamide, and N-methyl-2-pyrrolidinone. Molecular modeling was used to explain the enhanced solubility of the presented polyimides, based on structural features of the tetrahydro[5]helicene monomeric units. The onset temperatures for 5% weight loss for these polyimides, as assessed by thermogravimetry, were above 430°C in nitrogen. Inherent viscosities of the obtained polyimides were above 0.36 dL/g. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1349–1353, 1998     

Neuropeptide delivery to the brain: a von Willebrand factor signal peptide to direct neuropeptide secretion

Background  

Multiple neuropeptides, sometimes with opposing functions, can be produced from one precursor gene. To study the roles of the different neuropeptides encoded by one large precursor we developed a method to overexpress minigenes and establish local secretion.     

Structure of rotational bands in 253No

In-beam gamma-ray and conversion electron spectroscopic studies have been performed on the ²⁵³ No nucleus. A strongly coupled rotational band has been identified and the improved statistics allows an assignment of the band structure as built on the $\ensuremath 9/2^-[734]_{\nu}$ ground state. The results agree with previously known transition energies but disagree with the tentative structural assignments made in earlier work.     

Correlated,Dual‐Beam Optical Gating in Coupled Organic–Inorganic Nanostructures

An optical switch with two distinct resonances is formed by combining PbS nanocrystals and the conductive polymer poly[sodium 2‐(2‐ethynyl‐4‐methoxyphenoxy)acetate] (PAE) into a hybrid thin film. Infrared excitation of the nanocrystals invokes charge transfer and consecutive polaron formation in the PAE, which activates the switch for excited‐state absorption at visible frequencies. The optical modulation of the photocurrent response of the switch exhibits highly wavelength‐selective ON/OFF ratios. Transient absorption spectroscopy shows that the polaron formation is correlated with the excited state of the nanocrystals, opening up new perspectives for photonic data processing. Such correlated activated absorption can be exploited to enhance the sensitivity for one optical signal by a second light source of different frequency as part of an optical amplifier or a device with AND logic.