Nachweis und Bestimmung der Gelatine

Ohne Zusammenfassung     

Pancyclic orderings of in-tournaments

An in-tournament is an oriented graph such that the negative neighborhood of every vertex induces a tournament. In this paper, pancyclic orderings of a strong in-tournament D are investigated. This is a labeling, say x₁,x₂,…,x_n, of the vertex set of D such that D[{x₁,x₂,…,x_t}] is Hamiltonian for t=3,4,…,n. Moreover, we consider the related problem on weak pancyclic orderings, where the same holds for t4 and x₁ belongs to an arbitrary oriented 3-cycle. We present sharp lower bounds for the minimum indegree ensuring the existence of a pancyclic or a weak pancyclic ordering in strong in-tournaments.     

Manganese(IV) Corroles with σ‐Aryl Ligands

Different pathways for the preparation of organometallic manganese(IV) corroles with σ‐aryl ligands have been evaluated. The treatment of a manganese(III) corrole with Grignard reagents PhMgX (X = Cl, Br), followed by aerial oxidation yields oxidized halogenido complexes [(cor)Mn^IVX] instead of the anticipated organometallic compounds. Reaction of these halogenido species, especially the bromido compound, with excess Grignard reagents or with lithium aryls results in the formation of the desired σ‐aryl compounds via salt metatheses. Three examples of this class of rare complexes have been characterized by means of optical and ¹H NMR spectroscopy, and in two cases single crystal X‐ray diffraction studies have been carried out. In the crystal, the molecular structures of the σ‐phenyl‐ and the σ‐(p‐bromophenyl) derivatives were observed to be very similar, albeit both species pack in different pattern.     

A nickel hydride complex in the active site of methyl-coenzyme m reductase: implications for the catalytic cycle

Methanogenic archaea utilize a specific pathway in their metabolism, converting C1 substrates (i.e., CO2) or acetate to methane and thereby providing energy for the cell. Methyl-coenzyme M reductase (MCR) catalyzes the key step in the process, namely methyl-coenzyme M (CH3-S-CoM) plus coenzyme B (HS-CoB) to methane and CoM-S-S-CoB. The active site of MCR contains the nickel porphinoid F430. We report here on the coordinated ligands of the two paramagnetic MCR red2 states, induced when HS-CoM (a reversible competitive inhibitor) and the second substrate HS-CoB or its analogue CH3-S-CoB are added to the enzyme in the active MCR red1 state (Ni(I)F430). Continuous wave and pulse EPR spectroscopy are used to show that the MCR red2a state exhibits a very large proton hyperfine interaction with principal values A((1)H) = [-43,-42,-5] MHz and thus represents formally a Ni(III)F430 hydride complex formed by oxidative addition to Ni(I). In view of the known ability of nickel hydrides to activate methane, and the growing body of evidence for the involvement of MCR in "reverse" methanogenesis (anaerobic oxidation of methane), we believe that the nickel hydride complex reported here could play a key role in helping to understand both the mechanism of "reverse" and "forward" methanogenesis.     

Boradigermaallyl: (4+3) Cycloaddition-Initiated Boron Insertion into Benzene

The 2π electron 1,3-dipole boradigermaallyl, valence-isoelectronic to an allyl cation, is synthesized from a bis(germylene). It reacts with benzene at room temperature by insertion of a boron atom into the benzene ring. Computational investigation of the mechanism shows the boradigermaallyl reacting with a benzene molecule in a concerted (4+3) or [π4s+π2s] cycloaddition reaction. Thus, the boradigermaallyl acts as a highly reactive dienophile in this cycloaddition reaction with nonactivated benzene as diene unit. This type of reactivity provides a novel platform for ligand assisted borylene insertion chemistry.     

A New Ir‐NHC Catalyst for Signal Amplification by Reversible Exchange in D2O

NMR signal amplification by reversible exchange (SABRE) has been observed for pyridine, methyl nicotinate, N‐methylnicotinamide, and nicotinamide in D₂O with the new catalyst [Ir(Cl)(IDEG)(COD)] (IDEG=1,3‐bis(3,4,5‐tris(diethyleneglycol)benzyl)imidazole‐2‐ylidene). During the activation and hyperpolarization steps, exclusively D₂O was used, resulting in the first fully biocompatible SABRE system. Hyperpolarized ¹H substrate signals were observed at 42.5 MHz upon pressurizing the solution with parahydrogen at close to the Earth's magnetic field, at concentrations yielding barely detectable thermal signals. Moreover, 42‐, 26‐, 22‐, and 9‐fold enhancements were observed for nicotinamide, pyridine, methyl nicotinate, and N‐methylnicotinamide, respectively, in conventional 300 MHz studies. This research opens up new opportunities in a field in which SABRE has hitherto primarily been conducted in CD₃OD. This system uses simple hardware, leaves the substrate unaltered, and shows that SABRE is potentially suitable for clinical purposes.     

Rotation-libration in a hierarchic supramolecular rotor-stator system: arrhenius activation and retardation by local interaction

Fourfold symmetric zinc-octaethylporphyrin (OEP) has been incorporated in the holes of the hexagonal molecular network generated by thermal dehydrogenation of 4,9-diaminoperylene-quinone-3,10-diimine (DPDI) on a Cu(111) surface and displayed hindered rotation; the reorganization between the potential minima, a rotation-libration, which is characterized by an activation energy of ED=0.17+/-0.03 eV, has been monitored in the STM tunnelling currents as a bi-state "switching".