Mikrochemische Apparate

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Flachswachs

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Ein verbesserter Trockenkasten

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Photometrische Bestimmung von NH4OH in NH4F-L?sungen und NH4F-HF-Puffergemischen

Zusammenfassung Zur reproduzierbaren Herstellung von NH₄F-HF-Ätzlösungen muß der Gehalt an NH₄OH in den käuflichen 40% igen NH₄F-Lösungen ermittelt werden. Hierzu wurde eine colorimetrische Methode unter Verwendung von pH-Farbindicatoren der Phenolgruppe ausgearbeitet, die geringe pH-Wertabweichungen (0,007 pH) vom Soll (0% NH₄OH) zu messen erlaubt, wobei der Sollwert durch stabile Citronensäure-Phosphatpuffer simuliert wird. Durch Extinktionsmessung gelingt es, 0,01% NH₄OH in NH₄F-Lösungen zu erfassen. Die gefundenen Werte liegen zwischen 0,1–0,3%.

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Photometric determination of NH₄OH in NH₄F solutions and NH₄F-HF buffer mixtures

To obtain NH₄F-HF etching agents with constant etching behaviour the concentration of NH₄OH in commercially available 40% NH₄F solutions has to be measured. By application of pH-indicators of the phenol type very small pH-deviations (0.007 pH) from the specified value (0% NH₄OH) can be measured. The specified value is simulated by the stable McIlvaine buffer. By determining the differences in optical absorption 0.01% NH₄OH can be measured in NH₄F solutions. Values found in practice are between 0.1– 0.3%.

     

The NDP-sugar co-substrate concentration and the enzyme expression level influence the substrate specificity of glycosyltransferases: cloning and characterization of deoxysugar biosynthetic genes of the urdamycin biosynthetic gene cluster

BACKGROUND: Streptomyces fradiae is the principal producer of urdamycin A. The antibiotic consists of a polyketide-derived aglycone, which is glycosylated with four sugar components, 2x D-olivose (first and last sugar of a C-glycosidically bound trisaccharide chain at the 9-position), and 2x L-rhodinose (in the middle of the trisaccharide chain and at the 12b-position). Limited information is available about both the biosynthesis of D-olivose and L-rhodinose and the influence of the concentration of both sugars on urdamycin biosynthesis. RESULTS: To further investigate urdamycin biosynthesis, a 5.4 kb section of the urdamycin biosynthetic gene cluster was sequenced. Five new open reading frames (ORFs) (urdZ3, urdQ, urdR, urdS, urdT) could be identified each one showing significant homology to deoxysugar biosynthetic genes. We inactivated four of these newly allocated ORFs (urdZ3, urdQ, urdR, urdS) as well as urdZ1, a previously found putative deoxysugar biosynthetic gene. Inactivation of urdZ3, urdQ and urdZ1 prevented the mutant strains from producing L-rhodinose resulting in the accumulation of mainly urdamycinone B. Inactivation of urdR led to the formation of the novel urdamycin M, which carries a C-glycosidically attached D-rhodinose at the 9-position. The novel urdamycins N and O were detected after overexpression of urdGT1c in two different chromosomal urdGT1c deletion mutants. The mutants lacking urdS and urdQ accumulated various known diketopiperazines. CONCLUSIONS: Analysis of deoxysugar biosynthetic genes of the urdamycin biosynthetic gene cluster revealed a widely common biosynthetic pathway leading to D-olivose and L-rhodinose. Several enzymes responsible for specific steps of this pathway could be assigned. The pathway had to be modified compared to earlier suggestions. Two glycosyltransferases normally involved in the C-glycosyltransfer of D-olivose at the 9-position (UrdGT2) and in conversion of 100-2 to urdamycin G (UrdGT1c) show relaxed substrate specificity for their activated deoxysugar co-substrate and their alcohol substrate, respectively. They can transfer activated D-rhodinose (instead of D-olivose) to the 9-position, and attach L-rhodinose to the 4A-position normally occupied by a D-olivose unit, respectively.     

A strong two-photon induced phosphorescent Golgi-specific in vitro marker based on a heteroleptic iridium complex

A new heteroleptic iridium complex demonstrated low cytotoxicity and near-infrared excitation (via two-photon absorption) for target-specific in vitro Golgi imaging in various cell lines (HeLa and A549 cells) with two-photon absorption cross section (~350 GM) in DMSO.