Methodische Aspekte von Präzisionsgitterkonstantenbestimmungen an Vielkristallen

Zusammenfassung Wie die vorliegenden Untersuchungen zeigen, sind für die Differenz zwischen den Gitterkonstantenbestimmung verschiedener Autoren an einer Substanz nicht allein gerätetechnische Einflüsse, sondern im ebensolchen Ma\e unterschiedliche Methoden der Eliminierung systematischer Me\fehler verantwortlich zu machen. Für die kritische Bewertung verschiedener rechnerischer Auswerteverfahren sollte statt der Reststreuung die sogenannte Konfidenzschranke, die eine Funktion der vorzugebenden Irrtumswahrscheinlichkeit, der verfügbaren Freiheitsgrade und des mittleren Fehlers der Gitterkonstantenbestimmung ist, Verwendung finden. In vielen Fällen ergab sich eine deutliche Verbesserung der Resultate, wenn statt eines eingliedrigen (linearen) Terms für die systematischen Fehler ein zweigliedriger (linearer + quadratischer) Ausdruck benutzt wurde. Bei der Auswertung von Messungen in einem gro\en Glanzwinkelbereich findet man bei entsprechender Me\genauigkeit und eingliedrigem Fehlerterm häufig dann minimale Konfidenzschranken, wenn man Interferenzen mit kleinen Glanzwinkeln vernachlässigt. Diese Erscheinung ist Ausdruck der beschränkten Gültigkeit des gemachten Ansatzes für die systematischen Me\fehler und kann durch einen zweigliedrigen (quadratischen) Fehlerterm vermieden werden. Vergleiche zwischen Ein- und Vielkristallmessungen an halbleitenden Phosphiden lie\en deutlich den Einflu\ der Vertikaldivergenz auf die Ergebnisse der Vielkristallmessungen erkennen, die stets kleiner als die der Einkristallmessungen ausfielen. Dieser Fehlerquelle sollte zukünftig mehr Aufmerksamkeit gewidmet werden.

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Methodical aspects of the precision lattice parameter determination on polycrystals

     

Local coexistence of different phases

Under intuitively reasonable assumptions it is shown that in two dimensions different phases cannot exist locally. In three dimensions we discuss the possibility of local coexistence of districts with different magnetization for the Heisenberg ferromagnet and show that an interaction that breaks rotational invariance is necessary for this phenomenon.     

Application of non-equilibrium thermodynamics to problems of thermal analysis

On the basis of non-equilibrium thermodynamics, the kinetic equations of thermal analysis are derived for several important chemical consecutive and concurrent reactions. These equations are solved analytically and numerically and the obtained solutions are discussed. Hereby hints are obtained for the further development of thermal methods. It is shown that, in most cases, the kinetics of coupled chemical reactions can only be determined with special selective methods.     

RAFT polymerization: an avenue to functional polymeric micelles for drug delivery

RAFT (Reversible addition fragmentation chain transfer) polymerization is demonstrated as a versatile tool to obtain functional and crosslinked micelles for drug delivery purposes.     

Biosynthesis of the isoprenoid moieties of furanonaphthoquinone I and endophenazine A in Streptomyces cinnamonensis DSM 1042

Streptomyces cinnamonensis DSM 1042 produces the polyketide-isoprenoid compound furanonaphthoquinone I (FNQ I) and isoprenylated phenazines, predominantly endophenazine A. However, the recently identified biosynthetic gene cluster for these compounds only contains a single gene for a mevalonate pathway enzyme, that is, a putative mevalonate kinase gene. This is in strong contrast to all Streptomyces strains examined so far, where all six genes encoding the mevalonate pathway enzymes are clustered in a single operon of 6.8 kb and, thus, raised the question about the biosynthetic origin of the isoprenoid moieties of FNQ I and endophenazine A. In this study, we investigated the incorporation of [13C2]acetate and [2-13C]glycerol into FNQ I and endophenazine A. The results unequivocally prove that the isoprenoid building blocks of both compounds are predominantly formed via the mevalonate pathway (approximately 80%) but that the MEP pathway (approximately 20%) contributes to the biosynthesis of these molecules, too. In actinomycetes, this is the first experimentally proven example of the utilization of both biosynthetic routes for the formation of one single secondary metabolite. The incorporation pattern of [2-13C]glycerol was consistent with a "reverse" prenyl transfer, that is, with the formation of a C-C bond from C-3 of GPP to the polyketide nucleus of FNQ I.     

Glyco-acrylate copolymers for bilayer tethering on benzophenone-modified substrates

Model biological membranes are becoming increasingly important for studying fundamental biophysical phenomena and developing membrane-based devices. To address the anticipated problem of non-physiological interactions between membrane proteins and substrates seen in “solid-supported lipid bilayers” that are formed directly on hydrophilic substrates, we have developed a polymer-tethered lipid bilayer system based on a random copolymer with multiple lipid analogue anchors and a glyco-acrylate backbone. This system is targeted at applications that, most importantly, require stability and robustness since each copolymer has multiple lipid analogues that insert into the bilayer. We have combined this copolymer with a flexible photochemical coupling scheme that covalently attaches the copolymer to the substrate. The Langmuir isotherms of mixed copolymer/free lipid monolayers measured at the air–water interface indicate that the alkyl chains of the copolymer lipid analogues and the free lipids dominate the film behavior. In addition, no significant phase transitions are seen in the isotherms, while hysteresis experiments confirm that no irreversible states are formed during the monolayer compression. Isobaric creep experiments at the air–water interface and AFM experiments of the transferred monolayer are used to guide processing parameters for creating a fluid, homogeneous bilayer. Bilayer homogeneity and fluidity are monitored using fluorescence microscopy. Continuous bilayers with lateral diffusion coefficients of 0.6 μm²/s for both leaflets of the bilayer are observed for a 5% copolymer system.     

Analysis of fatty acids from human lipids by gas chromatography

A rapid, quantitative method is described for the analysis of fatty acids from human lipids, namely serum lipids and lipids from adipose tissue biopsies. The method includes extraction of serum lipids with chloroform--methanol, hydrolysis with tetramethylammonium hydroxide, methylation with methyl iodide and N,N-dimethylformamide and gas chromatographic analysis on a Supelcoport SP-2320 column. Fat biopsies are analysed without extraction. Optimal hydrolysis conditions have been investigated.