RESUMEN
The nitridoberylloaluminate Ba2 [BeAl3 N5 ]:Eu2+ and solid solutions Sr2-x Bax [BeAl3 N5 ]:Eu2+ (x=0.5, 1.0, 1.5) were synthesized in a hot isostatic press (HIP) under 50â MPa N2 atmosphere at 1200 °C. Ba2 [BeAl3 N5 ]:Eu2+ crystallizes in triclinic space group P 1 â¾ (no. 2) (Z=2, a=6.1869(10), b=7.1736(13), c=8.0391(14)â Å, α=102.754(8), ß=112.032(6), γ=104.765(7)°), which was determined from single-crystal X-ray diffraction data. The lattice parameters of the solid solution series have been obtained from Rietveld refinements and show a nearly linear dependence on the atomic ratio Sr : Ba. The electronic properties and the band gaps of M2 [BeAl3 N5 ] (M=Sr, Ba) have been investigated by a combination of soft X-ray spectroscopy and density functional theory (DFT) calculations. Upon irradiation with blue light (440-450â nm), the nitridoberylloaluminates exhibit intense orange to red luminescence, which can be tuned between 610 and 656â nm (fwhm=1922-2025â cm-1 (72-87â nm)). In contrast to the usual trend, the substitution of the smaller Sr2+ by larger Ba2+ leads to an inverse-tunable luminescence to higher wavelengths. Low-temperature luminescence measurements have been performed to exclude anomalous emission.
RESUMEN
The biosynthesis of organofluorine compounds by Streptomyces cattleya NRRL 8057 was examined using 19F NMR spectroscopy. The organism produced 1.2 mM fluoroacetate and 0.5 mM 4-fluorothreonine as secondary metabolites when cultured for 28 d on a chemically defined medium containing 2 mM fluoride. Cell suspensions from batch cultures harvested at the growth maximum of 4 d were not capable of fluoride uptake or fluorometabolite biosynthesis, but by 6 d had developed an efficient fluoride-uptake system and biosynthesized the two fluorometabolites in almost equal proportions. As the harvest age increased, the proportion of fluoroacetate to 4-fluorothreonine formed by cell suspensions rose progressively so that 16-d-old cells showed a ratio of 76:26 for the two compounds. Fluoride uptake and fluorometabolite production by cell suspensions were highly dependent on pH, with both processes showing a maximum rate at pH 6.0 but declining rapidly at higher pH values. This decrease was particularly marked in the case of fluoroacetate biosynthesis which was barely detectable at pH 7.5. Fluoroacetate and 4-fluorothreonine showed only low levels of interconversion by cell suspensions, suggesting that the carbon skeleton of neither was derived by metabolism of the other. The limited interconversion observed is explicable in terms of a small degree of biological defluorination occurring with each compound, followed by reincorporation of the resulting fluoride ion into the organic form by the active fluorinating system, a phenomenon also noted on incubation of cell suspensions with a number of other fluorinated biochemical intermediates.(ABSTRACT TRUNCATED AT 250 WORDS)