The Diels—Alder reaction in organometallic chemistry VI. Diels—Alder reactions of α-pyrone with group IV element-substituted acetylenes

The Diels—Alder reactions of α-pyrone with Me₃SiCCSiMe₃, Me₃SiCCSiMe₂H, Me₂HSiCCSiMe₂H, Me₃GeCCGeMe₃, Me₃SiCCGeMe₃, Me₃SiCCSnMe₃ and EtCCEt were examined. All except the first two acetylenes gave the expected 1,2-disubstituted benzene product, in line with results obtained previously with Me₃SnCCSnMe₃. The first two acetylenes, Me₃SiCCSiMe₃ and Me₃SiCCSiMe₂H, also yielded benzene products containing substantial amounts of the 1,3-disubstituted benzenes, as well as minor amounts of the 1,4-isomers. This formation of unexpected isomers during these reactions was shown to result from acid-catalyzed rearrangement of the initially formed 1,2-disubstituted products, 1,2-(Me₃Si)₂C₆H₄ and 1-Me₃Si-2-Me₂HSiC₆H₄. The acidic impurities arose from pyrolysis of the bromobenzene solvent used or were introduced as contaminants of the α-pyrone. Such isomerizations were inhibited by addition of small amounts of triethylamine. The fact that no rearrangement took place with the other acetylenes is due to the scavenging of acidic impurities which might cause isomerization by the starting acetylene and the benzene product via metal—carbon bond cleavage processes.     

[η5-cyclopentadien-1-YL-η4-tetraphenylcyclobutadienecobalt]diphenylmethylium hexafluorophosphate: A metal-stabilized carbonium ion salt

Mercuration of η⁵-cyclopentadienyl-η⁴-tetraphenylcyclobutadienecobalt, followed by transmetalation with n-butyllithium and reaction of the lithium derivative with benzophenone gave η⁴-Ph₄C₄Coη⁵-C₅H₄CPh₂OH. Treatment of this alcohol produced the [η⁴-Ph₄C₄CoC₅H₄CP₂]⁺ cation. This species reacted as a carbon electrophile with methanol, monomethylamine and N-methylpyrrole, as a cobalt electrophile with N,N-dimethylaniline and anisole. In the latter process the C₅H₄CPh₂ ligand was displaced and the η⁶-arene-η⁴-tetraphenylcyclobutadienecobalt complexes were formed. Similar reactions with benzene, toluene and mesitylene proceeded only in the presence of aluminum chloride. The bonding in the cation is discussed on the basis of this chemistry and ¹³C NMR studies.     

Analysis of post-translational modification and characterization of the domain structure of dynamin A from Dictyostelium discoideum

The post-translational modifications of the 96 kDa protein dynamin A from Dictyostelium discoideum were analyzed using Q-TOF mass spectrometry. The accurate molecular mass of the intact protein revealed a covalent modification causing an additional mass of 42 Da. The modification could be identified as N-terminal acetylation by tandem mass spectrometry. Extracted ion chromatograms for the a(1) and b(1) ion of the tryptic T1 peptide were used to detect the acetylated peptide within 54 nanoelectrospray ionization tandem mass spectra. Owing to the accurate molecular mass of the intact protein, additional covalent modifications could be excluded. In addition to the covalent modification, the domain structure of dynamin A was determined by applying a combination of limited proteolysis, sodium dodecylsulfate polyacrylamide gel electrophoresis, automated tandem mass spectrometry and protein database searching.     

Intercalation materials for lithium rechargeable batteries

An overview is given of intercalation materials for both the negative and the positive electrodes of lithium batteries, including the results of our own research. As well as lithium metal as a negative electrode, we consider insertion materials based on aluminium alloys. In the case of the positive electrode metal-oxides based on manganese, nickel and cobalt are discussed. Received: 27 May 1997 / Accepted: 30 July 1997     

Mixed correlation functions in systems interacting with Bose fields

For a large class of systems interacting with Bose fields, exact expressions are found for correlation functions containing products of Bose operators with evolution law of the free field and operators of the medium. Hierarchical equations describing the evolution of the system are constructed by means of these expressions. The Born—Markov approximation is investigated. A causality principle associated with the absence of an influence on the evolution of the system of stochastic forces due to the free field at short times is established.Joint Institute for Nuclear Research, Dubna. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 91, No. 1, pp. 142–156, April, 1992.     

Microrobotics and MEMS-based fabrication techniques for scaffold-based tissue engineering

Scaffold based tissue engineering strategies use cells, biomolecules and a scaffold to promote the repair and regeneration of tissues. Although scaffold-based tissue engineering approaches are being actively developed, most are still experimental, and it is not yet clear what defines an ideal scaffold/cell construct. Solid free form fabrication (SFF) techniques can precisely control matrix architecture (size, shape, interconnectivity, branching, geometry and orientation). The SFF methods enable the fabrication of scaffolds with various designs and material compositions, thus providing a control of mechanical properties, biological effects and degradation kinetics. This paper reviews the application of micro-robotics and MEMS-based fabrication techniques for scaffold design and fabrication. It also presents a novel robotic technique to fabricate scaffold/cell constructs for tissue engineering by the assembly of microscopic building blocks.     

Si-E (E = N, O, F) bonding in a hexacoordinated silicon complex: new facts from experimental and theoretical charge density studies

The concept of hypervalency in molecules, which hold more than eight valence electrons at the central atom, still is a topic of constant debate. There is general interest in silicon compounds with more than four substituents at the central silicon atom. The dispute, whether this silicon is hypervalent or highly coordinated, is enlightened by the first experimental charge density determination and subsequent topological analysis of three different highly polar Si-E (E = N, O, F) bonds in a hexacoordinated compound. The experiment reveals predominantly ionic bonding and much less covalent contribution than commonly anticipated. For comparison gas-phase ab initio calculations were performed on this compound. The results of the theoretical calculations underline the findings of the experiment.     

Purification and characterization of a laccase from the white-rot fungus Trametes multicolor

The wood-degrading fungus Trametes multicolor secretes several laccase isoforms when grown on a simple medium containing copper in the millimolar range for stimulating laccase synthesis. The main isoenzyme laccase II was purified to apparent homogeneity from the culture supernatant by using anion-exchange chromatography and gel filtration. Laccase II is a monomeric glycoprotein with a molecular mass of 63 kDa as determined by sodium dodecylsulfate polyacrylamide gel electrophoresis, contains 18% glycosylation, and has a pI of 3.0. It oxidizes a variety of phenolic substrates as well as ferrocyanide and iodide. The pH optimum depends on the substrate employed and shows a bell-shaped pH activity profile with an optimum of 4.0 to 5.0 for the phenolic substrates, while the nonphenolic substrates ferrocyanide and 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonate) show a monotonic pH profile with a rate decreasing with increasing pH.     

Phytochemical Characterization of Chamomile (Matricaria recutita L.) Roots and Evaluation of Their Antioxidant and Antibacterial Potential

Matricaria recutita L., German chamomile, is one of the most widely used medicinal plants, whose efficacy has been proven in numerous studies. However, its roots have attracted only little interest so far, since mainly above-ground plant parts are used for medicinal purposes. To broaden the knowledge of chamomile roots, a profound phytochemical characterization was performed along with a bioactivity screening of corresponding root extracts. While volatile constituents such as chamomillol and polyynes were detected using GC-MS, HPLC-MSⁿ analyses revealed the occurrence of four coumarin glycosides, more than ten phenolic acid esters and five glyceroglycolipids. Furthermore, the antioxidant activity of the extracts was evaluated. Polar extracts revealed IC₅₀ values ranging from 13 to 57 µg/mL in the DPPH radical scavenging assay, which is in the same range as reported for chamomile flower extracts. In addition, superoxide radical scavenging potential and mild antibacterial effects against S. aureus und B. subtilis were demonstrated. Moreover, to assess interspecies variation in chamomile roots, extracts of M. recutita were compared to those of M. discoidea DC. Interestingly, the latter revealed stronger antioxidant activity. The presented results aim at the valorization of chamomile roots, previously discarded as by-product of chamomile flower production, as a sustainable source of bioactive phytochemicals.