We have studied the superconducting phase diagram of NaxCoO2 ·yH2O as a function of electronic doping, characterizing our samples both in terms of Na content x and the Co valence state. Our findings are consistent with a recent report that intercalation of H3O + ions into NaxCoO2, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal TC is achieved through a Co valence range of 3.24 -3.35, while TC decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO2 sheets as Co valence is varied into the optimal TC region is further evidence for this criticality.
2-hydroxyglutaryl-CoA dehydratase from Acidaminococcus fermentans catalyzes the chemical difficult elimination of water from (R)-2-hydroxyglutaryl-CoA to glutaconyl-CoA. The enzyme consists of two oxygen-sensitive protein components, the homodimeric activator (A) with one [4Fe-4S]1+/2+ cluster and the heterodimeric dehydratase (D) with one nonreducible [4Fe-4S]2+ cluster and reduced riboflavin 5'-monophosphate (FMNH2). For activation, ATP, Mg2+, and a reduced flavodoxin (16 kDa) purified from A. fermentans are required. The [4Fe-4S](1+/2+) cluster of component A is exposed to the solvent since it is accessible to iron chelators. Upon exchange of the bound ADP by ATP, the chelation rate is 8-fold enhanced, indicating a large conformational change. Oxidized component A exhibits ATPase activity of 6 s(-1), which is completely abolished upon reduction by one electron. UV-visible spectroscopy revealed a spontaneous one-electron transfer from flavodoxin hydroquinone (E(0)' = -430 mV) to oxidized component A, whereby the [4Fe-4S]2+ cluster of component A became reduced. Combined kinetic, EPR, and Mössbauer spectrocopic investigations exhibited an ATP-dependent oxidation of component A by component D. Whereas the [4Fe-4S]2+ cluster of component D remained in the oxidized state, a new EPR signal became visible attributed to a d1-metal species, probably Mo(V). Metal analysis with neutron activation and atomic absorption spectroscopy gave 0.07-0.2 Mo per component D. In summary, the data suggest that in the presence of ATP one electron is transferred from flavodoxin hydroquinone via the [4Fe-4S]1+/2+ cluster of component A to Mo(VI) of component D, which is thereby reduced to Mo(V). The latter may supply the electron necessary for transient charge reversal in the unusual dehydration.
The level structure of 196po has been studied using a ~6~176 4n) reaction and a recoil-catcher technique to reduce fission. The results showed a sharp drop off in the first (2 +) and (4 +) energies relative to systematics implying an abrupt increase in the quadrupole collectivity at N = 112. In addition, second (2 +) and (4 +) states were identified at 859 and 1388 keV, respectively, suggesting the discovery of a 4p2h deformed band in Po isotopes which is related to known 2p2h proton bands in several Pb isotopes. A prompt la3W(Z~ 5n) experiment located similar second 2 + and 4 + states at 1039 and 1483 keV in ~98po providing corroboration and extended information on the properties of the new Po deformed bands. A measured halflife of 850 (90) ns for the 2491 keV (11-) state in ~96po implied collective E3 contributions to the extracted B (E3; 11 -~ 8 + ) = 27(5) W.u..
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