Zur AAS-Bestimmung von Quecksilber in Zahnwurzeln und Kieferknochen

Zusammenfassung Es wird ein Verfahren zur Bestimmung von Quecksilber in Zahnwurzeln und Kieferknochen beschrieben. Die homogenisierten Proben werden in Teflonbomben unter Druck mit einem Gemisch von HNO₃/HF aufgeschlossen. Die Quecksilberbestimmung erfolgt mit einem Atomabsorptionsspektrophotometer nach dem Kaltdampfverfahren. Unter Einsatz von 200 mg Probe können Quecksilbergehalte bis zu 0,1 g/g bestimmt werden. Die relativen Standardabweichungen liegen im Arbeitsbereich von 10–100 ng Quecksilber zwischen 2,7 und 5,6 %. Es wurden Quecksilbergehalte von <0,1–187 g/g in den Proben gefunden.Die Autoren danken Frau E. G. Harsányi für wertvolle Anregungen und Mithilfe bei der praktischen Arbeit.     

Regular Monomorphisms of Hausdorff Frames

Regular monomorphisms in the category of Hausdorff frames are characterized by means of a naturally defined closure operator; this is used also to characterize the epimorphisms. Further it is shown that for spatial (strongly) Hausdorff frames the regular monomorphisms do not generally coincide with the quotients, and do not generally compose. Also, an additional property (under which regular monomorphisms do compose) is briefly studied.     

Morphology Control in Metallocene Polymerization; Organic Supports vs. Non-aqueous Emulsion Polymerization

Nano-sized latex support in metallocene polymerization is known to be able to avoid fuming or leaching and leads to a powder-like and well-processable polymer. Focus has been put on the fragmentation behaviour of the particles, a key parameter to morphology control. To study the different behaviour of the new systems as classical inorganic supported metallocenes, e.g. SiO₂, a wide range of analytical methods were applied. Fluorescence microscopy, polymerization videomicroscopy, as well as kinetic studies led to a better understanding of the process. The performance of the supports was approved by several phenoxy-imine type catalysts (“FI-Catalyst”), which were combined with a tailored latex support. Ultra high molecular weight polyethylene (UHMWPE) was synthesized without any reactor fouling thereby. A different approach towards the metallocene catalyzed olefin polymerization is also presented. Based on emulsion polymerization, it enables very good control over product morphology. The completely hydrophobic system consists of perfluorinated solvent as a continuous phase and a hydrocarbon solvent as a dispersed phase. In contrast to the already existing water based emulsion polymerization of olefins, very high molecular weights are achieved.     

Blue-to-UVB Upconversion,Solvent Sensitization and Challenging Bond Activation Enabled by a Benzene-Based Annihilator

Several energy-demanding photoreactions require harsh UV light from inefficient light sources. The conversion of low-energy visible light to high-energy singlet states via triplet-triplet annihilation upconversion (TTA-UC) could offer a solution for driving such reactions under mild conditions. We present the first annihilator with an emission maximum in the UVB region that, combined with an organic sensitizer, is suitable for blue-to-UVB upconversion. The annihilator singlet was successfully employed as an energy donor in subsequent FRET activations of aliphatic carbonyls. This hitherto unreported UC-FRET reaction sequence was directly monitored using laser spectroscopy and applied to mechanistic irradiation experiments demonstrating the feasibility of Norrish chemistry. Our results provide clear evidence for a novel blue light-driven substrate or solvent activation strategy, which is important in the context of developing more sustainable light-to-chemical energy conversion systems.     

A New Bacterial Adenosine-Derived Nucleoside as an Example of RNA Modification Damage

The fields of RNA modification and RNA damage both exhibit a plethora of non-canonical nucleoside structures. While RNA modifications have evolved to improve RNA function, the term RNA damage implies detrimental effects. Based on stable isotope labelling and mass spectrometry, we report the identification and characterisation of 2-methylthio-1,N6-ethenoadenosine (ms²ϵA), which is related to 1,N6-ethenoadenine, a lesion resulting from exposure of nucleic acids to alkylating chemicals in vivo. In contrast, a sophisticated isoprene labelling scheme revealed that ms²ϵA biogenesis involves cleavage of a prenyl moiety in the known transfer RNA (tRNA) modification 2-methylthio-N6-isopentenyladenosine (ms²i⁶A). The relative abundance of ms²ϵA in tRNAs from translating ribosomes suggests reduced function in comparison to its parent RNA modification, establishing the nature of the new structure in a newly perceived overlap of the two previously separate fields, namely an RNA modification damage.     

Photocatalyzed Synthesis of 3-Substituted Phthalides: A Key Access to (±)-Herbaric Acid

An efficient organophotocatalyzed protocol was developed for the preparation of 3-substituted phthalides. The presented transformation was performed under particularly mild conditions within 6 h and was ultimately applied to a precursor of the herbaric acid.     

Boundary triples and quasi boundary triples for elliptic operators

The abstract concepts of boundary triples and their recent generalizations are useful tools to parametrize the self-adjoint and maximal dissipative/maximal accumulative extensions of formally symmetric elliptic differential expressions with the help of explicit boundary conditions. In the present note the parametrizations induced by the “natural” quasi boundary triple with the Dirichlet and Neumann trace as boundary maps are compared with the parametrizations induced by a “classical” ordinary boundary triple, where a regularized Neumann trace is used for one of the boundary maps. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

T-shaped Ni0 Systems Featuring Cationic Tetrylenes: Direct Observation of L/Z-type Ligand Duality,and Alkene Hydrogenation Catalysis

We report a facile synthetic method for accessing rare T-shaped Ni⁰ species, stabilised by low-coordinate cationic germylene and stannylene ligands which behave as Z-type ligands toward Ni⁰. An in-depth computational analysis indicates significant Ni^d→E^p donation (E=Ge, Sn), with essentially no E→Ni donation. The tetrylene ligand's Lewis acidity can be modulated in situ through the addition of a donor ligand, which selectively binds at the Lewis acidic tetrylene site. This switches this binding centre from a Z-type to a classical L-type ligand, with a concomitant geometry switch at Ni⁰ from T-shaped to trigonal planar. Exploring the effects of this geometry switch in catalysis, isolated T-shaped complexes 3 a – c and 4 a – c are capable of the hydrogenation of alkenes under mild conditions, whilst the closely related trigonal planar and tetrahedral Ni⁰ complexes 5 , D , and E , which feature L-type chloro- or cationic-tetrylene ligands, are inactive under these conditions. Further, addition of small amounts of N-bases to the catalytic systems involving T-shaped complexes significantly reduces turnover rates, giving evidence for the in situ modulation of ligand electronics for catalytic switching.     

Rapid UV Light‐Triggered Macromolecular Click Conjugations via the Use of o‐Quinodimethanes

Shining a light on click chemistry: The use of UV‐radiation as trigger signal provides a facile means to obtain spatial and temporal control over polymer conjugation reactions in addition to providing a further means of achieving orthogonality in click transformations. In the current contribution, UV‐radiation was employed to induce a highly efficient Diels–Alder conjugation of polymeric building blocks via the photo‐induced in situ formation of highly reactive cis‐dienes from a 2‐methylbenzophenone precursor.