Synthese und Konformationsanalyse von Pyranophanonen und Pyrylophanium-Verbindungen mit intraannularen Substituenten

Synthesis and Conformational Analysis of Pyranophanones and Pyrylophanium Compounds with Intraannular Substituents The synthesis of [3.3]dithiapyranophanone 6 and 11 is accomplished by use of the two-components-dilution-principle. Pyrolysis of their bis(sulfones) 7 and 12 gives the [2.2]pyranophanones 8 , 9 and 13 . Under preservation of conformation the intraannular carbonyl-function is used for the synthesis of methylpyranophanoles 14 , 16 and 17 . The synthesis of pyrylophanium compounds 15 and 18 is possible by elimination in trifluoroacetic acid. 6 exhibits anti-conformation within its crystal-structure and like 7 reveals temperature-dependent behavior in solution. Using 6 as an example, a combination of ¹³C-NMR-spectroscopy, forcefield-calculation and computer-simulation is applied for the first time to give evidence for molecular-dynamic processes of cyclophanes. 8 and 9 are the syn- and anti-conformers of the desired product, as shown by X-ray structural analysis. 13 reveals anti-conformation within its crystal structure as well as in solution. The conformational analysis of other new phanes described here is based on the ¹H-NMR-spectra of these pyrolysis products. As expected the intraannular substituents of Pyrylophanium-lons 15 and 18 show the characteristic upfield-shift within their ¹H-NMR-spectra.     

Catalytic role of the active site histidine of porcine pancreatic phospholipase A2 probed by the variants H48Q, H48N and H48K

The catalytic contribution of His48 in the active site of porcinepancreatic phospholipase A2 was examined using site-directedmutagenesis. Replacement of His48 by lysine (H48K) gives riseto a protein having a distorted lipid binding pocket. Activityof this variant drops below the detection limit which is 10⁷-foldlower than that of the wild-type enzyme. On the other hand,the presence of glutamine (H48Q) or asparagine (H48N) at thisposition does not affect the structural integrity of the enzymeas can be derived from the preserved lipid binding propertiesof these variants. However, the substitutions H48Q and H48Nstrongly reduce the turnover number, i.e. by a factor of 10⁵.Residual activity is totally lost after addition of a competitiveinhibitor. We conclude that proper lipid binding on its ownaccelerates ester bond hydrolysis by a factor of 10². With theselected variants, we were also able to dissect the contributionof the hydrogen bond between Asp99 and His48 on conformationalstability, being 5.2 kJ/mol. Another hydrogen bond with His48is formed when the competitive inhibitor (R)-2-dodecanoylamino-hexanol-1-phosphoglycolinteracts with the enzyme. Its contribution to binding of theinhibitor in the presence of an interface was found to be 5.7kJ/mol.     

Engineering model of a passive planar air breathing fuel cell cathode

The behavior of an air breathing fuel cell (ABFC) operated on dry-hydrogen in dead-ended mode is studied using theoretical analysis. A one-dimensional, non-isothermal, combined heat and mass transport model is developed that captures the coupling between water generation, oxygen consumption, self-heating and natural convection at the air breathing cathode. The model is validated against planar ABFC experimental measurements over a range of ambient temperatures. The model confirms the strong effect of self-heating on the water balance within passive ABFCs. Model analysis provides several conclusions: (1) thermal runaway caused by inadequate heat rejection predominantly limits ABFC performance. (2) The natural convection boundary layer represents a significant barrier to cathode mass and heat transfer. (3) Because the mass and heat transport numbers associated with natural convection are small, even slight forced convection dramatically affects cell behavior. (4) Performance optimization requires maximizing heat rejection while minimizing flooding. Decoupling the latter two phenomena is challenging due to the exponential relationship between water vapor saturation and temperature.     

Short-term maternal psychological stress in the post-conception period in ewes affects fetal growth and gestation length

Fetal development can be influenced by maternal environment in the peri-conceptional period. This study investigated the effect of maternal feed intake and psychological stress within the first 6 days after conception on embryo development and fetal growth. Superovulated ewes (n=40) were artificially inseminated with semen from one ram. Ewes were then divided into four groups (n=10): group 1 (control) was fed at maintenance level, group 2 (high) at 2x maintenance, and group 3 (low) at 0.5x maintenance on days 2-6 after conception. Group 4 (stress) was fed at maintenance level and then an intense physical and psychological stress challenge was applied for 1 h only on days 2 and 3 after conception. Embryos were recovered at day 6. A total of 113 transferable grade embryos were transferred singly into synchronized untreated recipients, while the remaining embryos (n=165) were fixed and stained for cell counts. Post-conception maternal stress or feed intake did not alter the cell count or grade of day 6 embryos. Fetuses from the stress group had longer crown-rump lengths at day 30 and longer femur length at day 58. Fetuses from the stressed and high feed groups had greater abdominal circumferences at day 85. Subsequent birth weights were not significantly different. Ewes carrying lambs from the stress treatment had shorter gestation lengths. These results show that short-term perturbations of the post-conception maternal environment have measurable effects on fetal development and gestation length.     

Thermal stabilities of nanoporous metallic electrodes at elevated temperatures

Currently Pt-based metals are the best catalytic electrodes for fuel cells at operating temperatures below 500 °C. Pure platinum electrodes suffer degradation of microstructure over time at elevated temperatures due to Ostwald ripening. In this paper, better thermal stability of Pt–Ni nanoporous thin films relative to pure Pt is reported. Based on ab initio calculations, it was found that both the surface energy of a Pt_0.7Ni_0.3 cluster and the energy change of the Pt–Ni alloy cluster upon ripening on yttria stabilized zirconia (YSZ) solid electrolyte were lower than pure Pt. This suggested a better thermal stability of Pt_0.7Ni_0.3 than Pt. In addition, annealing impacts on microstructures and properties of nanoporous Pt and Pt–Ni alloy thin films were examined experimentally. SEM images show dramatic porosity reduction for pure Pt after annealing at temperatures of 400–600 °C but insignificant microstructure change for Pt–Ni nanoporous thin films. As a result, in solid oxide fuel cells using nanoporous Pt–Ni cathodic catalysts instead of pure Pt, better stability, lower electrode impedances, and higher power densities were achieved at elevated operating temperatures (350–500 °C).