Die innere Struktur von Proton und Neutron

The Science of Nature -     

Finger and hand protection on circular table and panel saws

Several hundred accidents involving the use of circular saws and resulting in injury, to hands or fingers occur each year in Germany.In the presented project, new approaches for the prevention of hand injuries and for the contactless detection of fingers are being investigated for comprehensive protection on circular saws. The basic principles can be applied to other machines with manual loading and/or unloading. This paper describes several principles to distinct human skin and wood and a safety guard that prevents touching the rotating blade. In a first approach a reliable protective device with functional diversity has been developed using a passive infrared sensor in combination with a capacitive field sensor. Second the distinction between skin and wood or other material is done by dedicated kind of spectral analysis in the near infrared region. With a kind of light curtain the intrusion into the dangerous zone near the blade can be prevented.The safety guard protects the operator’s hand within 50 ms. The forces of the protective system peak at no more than 120 N. We are presenting a complete strategy for such different tasks as cutting wedges, stopped cutting and hidden cutting, which have a very high odds ratio.     

Occurrence of perfluorinated organic acids in the North and Baltic Seas. Part 2: distribution in sediments

Purpose  

The distribution of polar perfluorinated compounds (PFCs) in sea water of the North and Baltic Seas has been described in part 1 of this study. In part 2, their occurrence in sediments is described in order to further investigate their distribution routes and possible sinks.     

Stereochemistry of LinB-catalyzed biotransformation of δ-HBCD to 1R,2R,5S,6R,9R,10S-pentabromocyclododecanol

The haloalkane dehalogenase LinB from Sphingobium indicum B90A converts β-hexachlorocyclohexane (β-HCH), the most persistent HCH stereoisomer, to mono- and dihydroxylated metabolites. Recently, we reported that LinB also transforms α-, β- and γ-hexabromocyclododecanes (HBCDs), which are structurally related to HCHs. Here, we show that LinB catalyzes the hydroxylation of δ-HBCD to two pentabromocyclododecanols (PBCDOHs) and two tetrabromocyclododecadiols (TBCDDOHs). The stereochemistry of this enzymatic transformation was deduced from XRD crystal structure data of the substrate δ-HBCD and α₂-PBCDOH, one of the biotransformation products. Five stereocenters of δ-HBCD are unchanged but the one at C6 is converted to an alcohol with inversion from S- to R-configuration in a nucleophilic, S_N2-like substitution reaction. Only α₂-PBCDOH with the 1R,2R,5S,6R,9R,10S-configuration is obtained but not its enantiomer. With only two of the 64 PBCDOHs formed, these transformations indeed are regio- and stereoselective. A conformational analysis revealed that the triple-turn motive, which is predominant in δ-HBCD and in several other HBCD stereoisomers, is also found in the product. This shows that LinB preferentially converted reactive bromine atoms but not those in the conserved triple-turn motive. The widespread contamination with HCHs triggered the bacterial evolution of dehalogenases which acquired the ability to convert these pollutants and their metabolites. We here demonstrate that LinB of S. indicum also transforms HBCDs regio- and stereoselectively following a similar mechanism.     

Regio- and stereoselective isomerization of hexabromocyclododecanes (HBCDs): kinetics and mechanism of beta-HBCD racemization

Hexabromocyclododecanes (HBCDs) are high production volume chemicals currently produced in quantities exceeding 20000ty(-1). They are used as flame retardants for plastics and textiles. HBCDs are thermally labile compounds, rapidly decomposing at temperatures above 250 degrees C to form bromine radicals, which scavenge other radicals formed during pyrolysis. But certain HBCD stereoisomers must reach the environment without decomposition, because their levels in soils, sediments, and biota are increasing worldwide. The fate of individual HBCD stereoisomers during production, product use, disposal, and transformation in the environment remains unclear. Herein we report on the thermally induced, highly selective isomerization of (+) and (-)beta-HBCD. Regio- and stereoselective migration of only two of the six bromine atoms resulted in the racemization of both beta-HBCDs. First order rate constants (k(rac)) increased from 0.005, 0.011, 0.021, to 0.055min(-1) at 130, 140, 150, and 160 degrees C, corresponding to half life times tau(1/2) of 143, 63, 29, and 14min, respectively. From the deduced kinetic model, we conclude that any thermal treatment of enantiomerically enriched beta-HBCDs in the range of 100-160 degrees C will result in a loss of most optical activity within few hours. The simultaneous inversion of two asymmetric centers occurred with perfect stereocontrol. Selectively, vicinal dibromides with the RR- and the SS-configurations migrated at these temperatures. An intramolecular reaction mechanism with a four-center transition state is postulated, based on the obtained stereoisomer pattern and the observed reaction kinetics. Crystal structure analysis revealed that all vicinal dibromides in beta-HBCDs prefer synclinal (gauche) conformations. However, an antiperiplanar (staggered) conformation is assumed to facilitate the concerted 1.2-shifts of both bromine atoms, resulting in an inversion of both neighboring carbon atoms. First experiments with other HBCD stereoisomers suggest that the presented isomerization mechanism is of relevance for those stereoisomers as well.     

Thermal degradation products of spruce needles

Spruce needles are analysed by in-source pyrolysis (Py)-field ionization (FI) mass spectrography and Curie-point Py-gas chromatography/electron ionization mass spectrometry. The identified thermal degradation products allow the interpretation of the FI mass signals. It could be shown that Py-FI mass spectra of plant materials are representative fingerprints which consist primarily of molecular ions of Py products derived from the thermal degradation of the main constituents of spruce needles such as carbohydrates, lignin, lipids and proteins. Primary building blocks of these biopolymers form characteristic Py products of spruce needles. Furthermore, over 500 different molecular ion species of volatilized, thermostable plant constituents are recorded using FI as soft ionization method. The present study shows that Py-FIMS is suitable for the rapid and comprehensive characterization of complex biological materials, without pretreatment of the sample except of drying and milling. The presented results contribute to the basic knowledge for the evaluation of the causes of the present tree damage.     

Crystal structure analysis of enantiomerically pure (+) and (-) beta-hexabromocyclododecanes

The molecular structures of individual HBCD stereoisomers are not elucidated yet. Recently, we isolated 8 of the 16 possible stereoisomers from a technical HBCD mixture and tentatively assigned their relative configurations. Herein we report on the isolation of enantiomerically pure (+) and (-) beta-HBCDs, both obtained from preparative chiral-phase liquid chromatography, and we present their absolute configurations determined from X-ray diffraction analysis. The absolute configuration of (+) beta-HBCD was found to be (1S,2S,5S,6R,9S,10R), while the one of (-) beta-HBCD was assigned to (1R,2R,5R,6S,9R,10S). The given structural information allows, for the first time, the unambiguous identification of these two important HBCD stereoisomers, which are typically found in technical products at proportions of about 3-5% for each enantiomer.