Model Complexes for Ureases: A Dinickel(II) Complex with a Novel Asymmetric Ligand and Comparative Kinetic Studies on Catalytically Active Zinc,Cobalt, and Nickel Complexes

The dinuclear nickel(II) complex of the asymmetric ligand 1‐[N,N‐bis(2‐pyridylmethyl)amino]‐3‐[2‐(3,5dimethyl‐1H‐pyrazol‐1‐yl)ethoxy]‐2‐hydroxypropane (HL1) was prepared as a model for the active site of urease. The novel complex [Ni₂(L1)(MeCOO)(ClO₄)(EtOH)₂](ClO₄) · 0.5 Et₂O ( 1 ) crystallizes in the triclinic space group P 1 with a = 11.639(2) Å, b = 12.571(3) Å, c = 16.341(3) Å, α = 92.29°, β = 106.54°, and γ = 113.73°. The nickel ions (c.n. 6) are bridged by the alkoxy donor substituent of the ligand and an acetate anion. The dinuclear nickel(II), cobalt(II), and zinc(II) complexes of the ligands 1‐[N,N‐bis(2‐benzimidazolylmethyl)amino]‐3‐[2‐(3,5‐dimethyl‐1 H‐pyrazol‐1‐yl)ethoxy]‐2‐hydroxypropane (HL2), N‐methyl‐N,N',N'‐tris(2‐benzimidazolylmethyl)‐2‐hydroxy‐1,3‐diaminopropane (HL3), and N,N,N',N'‐tetrakis(2‐benzimidazolylmethyl)‐2‐hydroxy‐1,3‐diaminopropane (HL4) were investigated for their activity towards the hydrolysis of the test substrate p‐nitrophenyl acetate (npa) in ethanol‐water (1 : 1). The second‐order rate constants for the cleavage of npa were determined for all complexes. The profile of the pH dependence indicates that a hydroxide initially binds to the metal ion. The bound nucleophile subsequently attacks the test substrate. The results are discussed in terms of a refined model for the structure activity relationships of the dinuclear active site of urease.     

Umweltprobenbanken-eine wichtige Säule der Umweltbeobachtung

Natural events as well as human activities are affecting the environment and even with largescale monitoring programs it is not possible to completely assess the state of the environment. But a documentation of the environmental status with respect to its chemical composition can be realized with systematically collected representative environmental samples, which can be stored for decades at very low temperatures almost without degradation. Such ‘banking activity’ allows a current environmental monitoring and also a retrospective analysis for the determination of components which have not been analyzed at the time of sampling because of ta lack in analytical techniques or because they have not been known or considered environmentally relevant. The concept of environmental specimen banking and general procedures are described. Various examples illustrate the many facets of such activities.     

Blue Light Emitting Defective Nanocrystals Composed of Earth-Abundant Elements

Copper-based ternary (I–III–VI) chalcogenide nanocrystals (NCs) are compositionally-flexible semiconductors that do not contain lead (Pb) or cadmium (Cd). Cu-In-S NCs are the dominantly studied member of this important materials class and have been reported to contain optically-active defect states. However, there are minimal reports of In-free compositions that exhibit efficient photoluminescence (PL). Here, we report a novel solution-phase synthesis of ≈4 nm defective nanocrystals (DNCs) composed of copper, aluminum, zinc, and sulfur with ≈20 % quantum yield and an attractive PL maximum of 450 nm. Extensive spectroscopic characterization suggests the presence of highly localized electronic states resulting in reasonably fast PL decays (≈1 ns), large vibrational energy spacing, small Stokes shift, and temperature-independent PL linewidth and PL lifetime (between room temperature and ≈5 K). Furthermore, density functional theory (DFT) calculations suggest PL transitions arise from defects within a CuAl₅S₈ crystal lattice, which supports the experimental observation of highly-localized states. The results reported here provide a new material with unique optoelectronic characteristics that is an important analog to well-explored Cu-In-S NCs.     

Silylium-Ion-Promoted (5+1) Cycloaddition of Aryl-Substituted Vinylcyclopropanes and Hydrosilanes Involving Aryl Migration

A transition-metal-free (5+1) cycloaddition of aryl-substituted vinylcyclopropanes (VCPs) and hydrosilanes to afford silacyclohexanes is reported. Catalytic amounts of the trityl cation initiate the reaction by hydride abstraction from the hydrosilane, and further progress of the reaction is maintained by self-regeneration of the silylium ions. The new reaction involves a [1,2] migration of an aryl group, eventually furnishing 4- rather than 3-aryl-substituted silacyclohexane derivatives as major products. Various control experiments and quantum-chemical calculations support a mechanistic picture where a silylium ion intramolecularly stabilized by a cyclopropane ring can either undergo a kinetically favored concerted [1,2] aryl migration/ring expansion or engage in a cyclopropane-to-cyclopropane rearrangement.     

Discovery,Total Synthesis and Key Structural Elements for the Immunosuppressive Activity of Cocosolide,a Symmetrical Glycosylated Macrolide Dimer from Marine Cyanobacteria

A new dimeric macrolide xylopyranoside, cocosolide ( 1 ), was isolated from the marine cyanobacterium preliminarily identified as Symploca sp. from Guam. The structure was determined by a combination of NMR spectroscopy, HRMS, X‐ray diffraction studies and Mosher's analysis of the base hydrolysis product. Its carbon skeleton closely resembles that of clavosolides A–D isolated from the sponge Myriastra clavosa, for which no bioactivity is known. We performed the first total synthesis of cocosolide ( 1 ) along with its [α,α]‐anomer ( 26 ) and macrocyclic core ( 28 ), thus leading to the confirmation of the structure of natural 1 . The convergent synthesis featured Wadsworth–Emmons cyclopropanation, Sakurai annulation, Yamaguchi macrocyclization/dimerization reaction, α‐selective glycosidation and β‐selective glycosidation. Compounds 1 and 26 potently inhibited IL‐2 production in both T‐cell receptor dependent and independent manners. Full activity requires the presence of the sugar moiety as well as the intact dimeric structure. Cocosolide also suppressed the proliferation of anti‐CD3‐stimulated T‐cells in a dose‐dependent manner.